Characterizing full-phase chlorine species emissions from domestic coal combustion in China: Implications for significant impacts on air pollution and ozone-layer depletion

The distribution and emission characteristics of chlorine in different phases are important for evaluating the impact of chlorine chemistry on the stratospheric and tropospheric environment. This study developed a measurement method of full-phase chlorine and firstly reported full-phase chlorine emissions characteristics of domestic coal combustion. The chlorine content of bituminous coal is 252.5 μg/g, with 87 % released into the gaseous, and the concentrations of particulate chloride (pCl), hydrogen chloride (HCl), chlorine gas (Cl2), and organic chlorides (Cl-VOCs) of 0.57 ± 0.16, 5.96 ± 1.05, 0.38 ± 0.06, and 0.03 ± 0.01 mg Cl/m3, respectively. In addition, unexpectedly high concentrations of very short-lived substances (VSLS), such as CH3Cl, CH2Cl2, C2H5Cl, C2H3Cl, and C2H2Cl2, and ozone-depleting substances (ODS), such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), were measured, with concentration of 270.71 ppb in total. Total emission of chlorine from domestic coal combustion was 33.51 tons in 2013 and declines to 18.47 tons in 2021 in China. Our results indicate that the emissions of CFC-11-eq and CO2-eq were relatively low compared with those of other species controlled by the Montreal Protocol on Substances that Deplete the Ozone Layer; however, subsequent controls should give greater consideration to coal combustion because of its widespread sources.

Significant chlorine emissions from biomass burning affect the long-term atmospheric chemistry in Asia

Biomass burning (BB) is a major source of trace gases and particles in the atmosphere, influencing air quality, radiative balance, and climate. Previous studies have mainly focused on the BB emissions of carbon and nitrogen species with less attention on chlorine. Reactive chlorine chemistry has significant effects on atmospheric chemistry and air quality. However, quantitative information on chlorine emissions from BB, particularly the long-term trend and associated atmospheric impacts, is limited both on regional and global scales. Here, we report a long-term (2001-2018) high-resolution BB emission inventory for the major chlorine-containing compounds (HCl, chloride, and CH3Cl) in Asia based on satellite observations. We estimate an average of 730 Gg yr-1 chlorine emitted from BB activity in Asia, with China contributing the largest share at 24.2% (177 Gg yr-1), followed by Myanmar at 18.7% and India at 18.3%. Distinct seasonal patterns and significant spatial and interannual variability are observed, mainly driven by human-mediated changes in agricultural activity. By incorporating the newly developed chlorine emission inventory into a global chemistry-climate model (CAM-Chem), we find that the BB-chlorine emissions lead to elevated levels of HCl and CH3Cl (monthly average up to 2062 and 1421 parts per trillion by volume (pptv), respectively), subsequently resulting in noticeable changes in oxidants (up to 3.1% in O3 and 17% in OH radicals). The results demonstrate that BB is not only a significant source of air pollutants but also of oxidants, suggesting a larger role of BB emissions in the atmospheric chemistry and oxidation process than previously appreciated. In light of the projected increase in BB activity toward the end of the century and the extensive control of anthropogenic emissions worldwide, the contribution of BB emissions may become fundamental to air quality composition in the future.

Insights into the role of the H-abstraction reaction kinetics of amines in understanding their degeneration fates under atmospheric and combustion conditions

Amines, a class of prototypical volatile organic compounds, have garnered considerable interest within the context of atmospheric and combustion chemistry due to their substantial contributions to the formation of hazardous pollutants in the atmosphere. In the current energy landscape, the implementation of carbon-neutral energy and strategic initiatives leads to generation of new amine sources that cannot be overlooked in terms of the emission scale. To reduce the emission level of amines from their sources and mitigate their impact on the formation of harmful substances, a comprehensive understanding of the fundamental reaction kinetics during the degeneration process of amines is imperative. This perspective article first presents an overview of both traditional amine sources and emerging amine sources within the context of carbon peaking and carbon neutrality and then highlights the importance of H-abstraction reactions in understanding the atmospheric and combustion chemistry of amines from the perspective of reaction kinetics. Subsequently, the current experimental and theoretical techniques for investigating the H-abstraction reactions of amines are introduced, and a concise summary of research endeavors made in this field over the past few decades is provided. In order to provide accurate kinetic parameters of the H-abstraction reactions of amines, advanced kinetic calculations are performed using the multi-path canonical variational theory combined with the small-curvature tunneling and specific-reaction parameter methods. By comparing with the literature data, current kinetic calculations are comprehensively evaluated, and these validated data are valuable for the development of the reaction mechanism of amines.

Aerosolomics based approach to discover source molecular markers: A case study for discriminating residential wood heating vs garden green waste burning emission sources

Biomass burning is a significant source of particulate matter (PM) in ambient air and its accurate source apportionment is a major concern for air quality. The discrimination between residential wood heating (RWH) and garden green waste burning (GWB) particulate matter (PM) is rarely achieved. The objective of this work was to evaluate the potential of non-targeted screening (NTS) analyses using HRMS (high resolution mass spectrometry) data to reveal discriminating potential molecular markers of both sources. Two residential wood combustion appliances (wood log stove and fireplace) were tested under different output conditions and wood moisture content. GWB experiments were carried out using two burning materials (fallen leaves and hedge trimming). PM samples were characterized using NTS approaches with both LC- and GC-HRMS (liquid and gas chromatography-HRMS). The analytical procedures were optimized to detect as many species as possible. Chemical fingerprints obtained were compared combining several multivariate statistical analyses (PCA, HCA and PLS-DA). Results showed a strong impact of the fuel nature and the combustion quality on the chemical fingerprints. 31 and 4 possible markers were discovered as characteristic of GWB and RWH, respectively. Complementary work was attempted to identify potential molecular formulas of the different potential marker candidates. The combination of HRMS NTS chemical characterization with multivariate statistical analyses shows promise for uncovering organic aerosol fingerprinting and discovering potential PM source markers.

Estimating morning and evening commute period O3 concentration in Taiwan using a fine spatial-temporal resolution ensemble mixed spatial model with Geo-AI technology

Elevated levels of ground-level ozone (O3) can have harmful effects on health. While previous studies have focused mainly on daily averages and daytime patterns, it's crucial to consider the effects of air pollution during daily commutes, as this can significantly contribute to overall exposure. This study is also the first to employ an ensemble mixed spatial model (EMSM) that integrates multiple machine learning algorithms and predictor variables selected using Shapley Additive exExplanations (SHAP) values to predict spatial-temporal fluctuations in O3 concentrations across the entire island of Taiwan. We utilized geospatial-artificial intelligence (Geo-AI), incorporating kriging, land use regression (LUR), machine learning (random forest (RF), categorical boosting (CatBoost), gradient boosting (GBM), extreme gradient boosting (XGBoost), and light gradient boosting (LightGBM)), and ensemble learning techniques to develop ensemble mixed spatial models (EMSMs) for morning and evening commute periods. The EMSMs were used to estimate long-term spatiotemporal variations of O3 levels, accounting for in-situ measurements, meteorological factors, geospatial predictors, and social and seasonal influences over a 26-year period. Compared to conventional LUR-based approaches, the EMSMs improved performance by 58% for both commute periods, with high explanatory power and an adjusted R2 of 0.91. Internal and external validation procedures and verification of O3 concentrations at the upper percentile ranges (in 1%, 5%, 10%, 15%, 20%, and 25%) and other conditions (including rain, no rain, weekday, weekend, festival, and no festival) have demonstrated that the models are stable and free from overfitting issues. Estimation maps were generated to examine changes in O3 levels before and during the implementation of COVID-19 restrictions. These findings provide accurate variations of O3 levels in commute period with high spatiotemporal resolution of daily and 50m * 50m grid, which can support control pollution efforts and aid in epidemiological studies.

Metabolite cross-feeding enables concomitant catabolism of chlorinated methanes and chlorinated ethenes in synthetic microbial assemblies

Isolate studies have been a cornerstone for unraveling metabolic pathways and phenotypical (functional) features. Biogeochemical processes in natural and engineered ecosystems are generally performed by more than a single microbe and often rely on mutualistic interactions. We demonstrate the rational bottom-up design of synthetic, interdependent co-cultures to achieve concomitant utilization of chlorinated methanes as electron donors and organohalogens as electron acceptors. Specialized anaerobes conserve energy from the catabolic conversion of chloromethane or dichloromethane to formate, H2, and acetate, compounds that the organohalide-respiring bacterium Dehalogenimonas etheniformans strain GP requires to utilize cis-1,2-dichloroethenene and vinyl chloride as electron acceptors. Organism-specific qPCR enumeration matched the growth of individual dechlorinators to the respective functional (i.e. dechlorination) traits. The metabolite cross-feeding in the synthetic (co-)cultures enables concomitant utilization of chlorinated methanes (i.e. chloromethane and dichloromethane) and chlorinated ethenes (i.e. cis-1,2-dichloroethenene and vinyl chloride) without the addition of an external electron donor (i.e. formate and H2). The findings illustrate that naturally occurring chlorinated C1 compounds can sustain anaerobic food webs, an observation with implications for the development of interdependent, mutualistic communities, the sustenance of microbial life in oligotrophic and energy-deprived environments, and the fate of chloromethane/dichloromethane and chlorinated electron acceptors (e.g. chlorinated ethenes) in pristine environments and commingled contaminant plumes.

Significant Impact of Reactive Chlorine on Complex Air Pollution Over the Yangtze River Delta Region, China

The chlorine radical (Cl) plays a crucial role in the formation of secondary air pollutants by determining the total atmospheric oxidative capacity (AOC). However, there are still large discrepancies among studies on chlorine chemistry, mainly due to uncertainties from three aspects: (a) Anthropogenic emissions of reactive chlorine species from disinfectant usage are typically overlooked. (b) The heterogeneous reaction uptake coefficients used in air quality models resulted in certain differences. (c) The co-effect of anthropogenic and natural emissions is rarely investigated. In this study, the Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) modeling system (updated with 21 new reactions and a comprehensive emissions inventory) was used to simulate the combined impact of chlorine emissions on the air quality of a coastal city cluster in the Yangtze River Delta (YRD) region. The results indicate that the new emissions of reactive chlorine and the updated gas-phase and heterogeneous chlorine chemistry can significantly enhance the AOC by 21.3%, 8.7%, 43.3%, and 58.7% in spring, summer, autumn, and winter, respectively. This is more evident in inland areas with high Cl concentrations. Our updates to the chlorine chemistry also increases the monthly mean maximum daily 8-hr average (MDA 8) O3 mixing ratio by 4.1-7.0 ppbv in different seasons. Additionally, chlorine chemistry promotes the formation of fine particulate matter (PM2.5), with maximum monthly average enhancements of 4.7-13.3 μg/m3 in different seasons. This study underlines the significance of adding full chlorine emissions and updating chlorine chemistry in air quality models, and demonstrates that chlorine chemistry may significantly impact air quality over coastal regions.

Interpretation of the effects of anthropogenic chlorine on nitrate formation over northeast Asia during KORUS-AQ 2016

The Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model, implemented with anthropogenic chlorine (Cl) emissions, was evaluated against ground and NASA DC-8 aircraft measurements during the Korea-United States Air Quality (KORUS-AQ) 2016 campaign. The latest anthropogenic Cl emissions, including gaseous HCl and particulate chloride (pCl-) emissions from the Anthropogenic Chlorine Emissions Inventory of China (ACEIC-2014) (over China) and a global emissions inventory (Zhang et al., 2022) (over outer China), were used to examine the impacts of Cl emissions and the role of nitryl chloride (ClNO2) chemistry in N2O5 heterogeneous reactions on secondary nitrate (NO3-) formation across the Korean Peninsula. The model results against aircraft measurements clearly showed significant Cl- underestimations due mainly to the high gas-particle (G/P) partitioning ratios at aircraft measurement altitudes such as 700-850 hPa, but the ClNO2 simulations were reasonable. Several simulations of CMAQ-based sensitivity experiments against ground measurements indicated that although addition of Cl emission did not significantly alter NO3- formation, the activated ClNO2 chemistry with Cl emissions showed the best model performance with the reduced normalized mean bias (NMB) of 18.7 % compared to a value of 21.1 % for the Cl emissions-free case. In our model evaluation, ClNO2 accumulated during the night but quickly produced Cl radical due to ClNO2 photolysis at sunrise, which modulated other oxidation radicals (e.g., ozone [O3] and hydrogen oxide radicals [HOx]) in the early morning. In the morning hours (0800-1000 LST), the HOx were the dominant oxidants, contributing 86.6 % of the total oxidation capacity (sum of major oxidants such as O3 and HOx species), while oxidability was enhanced by up to ∼6.4 % (increase in 1 h HOx average of 2.89 × 106 molecules·cm-3) in the early morning mainly due to the changes in OH (+7.2 %), hydroperoxyl radical (HO2)(+10.0 %), and O3 (+4.2 %) over the Seoul Metropolitan Area, during the KORUS-AQ campaign. Our results improve understanding of the atmospheric changes in the PM2.5 formation pathway caused by ClNO2 chemistry and Cl emissions over northeast Asia.

Global environmental implications of atmospheric methane removal through chlorine-mediated chemistry-climate interactions

Atmospheric methane is both a potent greenhouse gas and photochemically active, with approximately equal anthropogenic and natural sources. The addition of chlorine to the atmosphere has been proposed to mitigate global warming through methane reduction by increasing its chemical loss. However, the potential environmental impacts of such climate mitigation remain unexplored. Here, sensitivity studies are conducted to evaluate the possible effects of increasing reactive chlorine emissions on the methane budget, atmospheric composition and radiative forcing. Because of non-linear chemistry, in order to achieve a reduction in methane burden (instead of an increase), the chlorine atom burden needs to be a minimum of three times the estimated present-day burden. If the methane removal target is set to 20%, 45%, or 70% less global methane by 2050 compared to the levels in the Representative Concentration Pathway 8.5 scenario (RCP8.5), our modeling results suggest that additional chlorine fluxes of 630, 1250, and 1880 Tg Cl/year, respectively, are needed. The results show that increasing chlorine emissions also induces significant changes in other important climate forcers. Remarkably, the tropospheric ozone decrease is large enough that the magnitude of radiative forcing decrease is similar to that of methane. Adding 630, 1250, and 1880 Tg Cl/year to the RCP8.5 scenario, chosen to have the most consistent current-day trends of methane, will decrease the surface temperature by 0.2, 0.4, and 0.6 °C by 2050, respectively. The quantity and method in which the chlorine is added, its interactions with climate pathways, and the potential environmental impacts on air quality and ocean acidity, must be carefully considered before any action is taken.

Natural short-lived halogens exert an indirect cooling effect on climate

Observational evidence shows the ubiquitous presence of ocean-emitted short-lived halogens in the global atmosphere1-3. Natural emissions of these chemical compounds have been anthropogenically amplified since pre-industrial times4-6, while, in addition, anthropogenic short-lived halocarbons are currently being emitted to the atmosphere7,8. Despite their widespread distribution in the atmosphere, the combined impact of these species on Earth's radiative balance remains unknown. Here we show that short-lived halogens exert a substantial indirect cooling effect at present (-0.13 ± 0.03 watts per square metre) that arises from halogen-mediated radiative perturbations of ozone (-0.24 ± 0.02 watts per square metre), compensated by those from methane (+0.09 ± 0.01 watts per square metre), aerosols (+0.03 ± 0.01 watts per square metre) and stratospheric water vapour (+0.011 ± 0.001 watts per square metre). Importantly, this substantial cooling effect has increased since 1750 by -0.05 ± 0.03 watts per square metre (61 per cent), driven by the anthropogenic amplification of natural halogen emissions, and is projected to change further (18-31 per cent by 2100) depending on climate warming projections and socioeconomic development. We conclude that the indirect radiative effect due to short-lived halogens should now be incorporated into climate models to provide a more realistic natural baseline of Earth's climate system.

The role of sources and meteorology in driving PM2.5-bound chlorine

The role of chloride in atmospheric chemistry received increased attention over recent years. Given the primary and chemical-active nature of PM_2.5-bound chlorine (p-Cl^-), it makes sense to get to know the sources and processes of p-Cl^-. The temporal behavior of observed p-Cl^- concentration based on 1-h high resolution exhibited seasonal variation of high in winter, low in summer and diurnal variation of high in the morning, low in afternoon. Meteorological normalization technique based on random forest was used to disentangle the effects of emission changes which affected the seasonal variation and meteorology which was related to diurnal variation on p-Cl^-. Generalized additive model (GAM) identified RH and temperature as the key meteorological factors of p-Cl^- generation, and p-Cl^- pollution was serious under the condition of low temperature and high RH. Dispersion-normalized positive matrix factorization (DN-PMF) was used to apportion the p-Cl^- to its sources, finding that coal combustion was the main source of p-Cl^-, followed by biomass burning and industrial process emissions. Our results will provide the basis for further analysis the causes of p-Cl^- pollution and composite air pollution control strategies.

NASA GEOS Composition Forecast Modeling System GEOS-CF v1.0: Stratospheric Composition

The NASA Goddard Earth Observing System (GEOS) Composition Forecast (GEOS-CF) provides recent estimates and 5-day forecasts of atmospheric composition to the public in near-real time. To do this, the GEOS Earth system model is coupled with the GEOS-Chem tropospheric-stratospheric unified chemistry extension (UCX) to represent composition from the surface to the top of the GEOS atmosphere (0.01 hPa). The GEOS-CF system is described, including updates made to the GEOS-Chem UCX mechanism within GEOS-CF for improved representation of stratospheric chemistry. Comparisons are made against balloon, lidar, and satellite observations for stratospheric composition, including measurements of ozone (O3) and important nitrogen and chlorine species related to stratospheric O3 recovery. The GEOS-CF nudges the stratospheric O3 toward the GEOS Forward Processing (GEOS FP) assimilated O3 product; as a result the stratospheric O3 in the GEOS-CF historical estimate agrees well with observations. During abnormal dynamical and chemical environments such as the 2020 polar vortexes, the GEOS-CF O3 forecasts are more realistic than GEOS FP O3 forecasts because of the inclusion of the complex GEOS-Chem UCX stratospheric chemistry. Overall, the spatial patterns of the GEOS-CF simulated concentrations of stratospheric composition agree well with satellite observations. However, there are notable biases-such as low NO x and HNO3 in the polar regions and generally low HCl throughout the stratosphere-and future improvements to the chemistry mechanism and emissions are discussed. GEOS-CF is a new tool for the research community and instrument teams observing trace gases in the stratosphere and troposphere, providing near-real-time three-dimensional gridded information on atmospheric composition.

Reactive halogens increase the global methane lifetime and radiative forcing in the 21st century

CH₄ is the most abundant reactive greenhouse gas and a complete understanding of its atmospheric fate is needed to formulate mitigation policies. Current chemistry-climate models tend to underestimate the lifetime of CH₄, suggesting uncertainties in its sources and sinks. Reactive halogens substantially perturb the budget of tropospheric OH, the main CH₄ loss. However, such an effect of atmospheric halogens is not considered in existing climate projections of CH₄ burden and radiative forcing. Here, we demonstrate that reactive halogen chemistry increases the global CH₄ lifetime by 6-9% during the 21st century. This effect arises from significant halogen-mediated decrease, mainly by iodine and bromine, in OH-driven CH₄ loss that surpasses the direct Cl-induced CH₄ sink. This increase in CH₄ lifetime helps to reduce the gap between models and observations and results in a greater burden and radiative forcing during this century. The increase in CH₄ burden due to halogens (up to 700 Tg or 8% by 2100) is equivalent to the observed atmospheric CH₄ growth during the last three to four decades. Notably, the halogen-driven enhancement in CH₄ radiative forcing is 0.05 W/m² at present and is projected to increase in the future (0.06 W/m² by 2100); such enhancement equals ~10% of present-day CH₄ radiative forcing and one-third of N₂O radiative forcing, the third-largest well-mixed greenhouse gas. Both direct (Cl-driven) and indirect (via OH) impacts of halogens should be included in future CH₄ projections.

Hydrogen chloride (HCl) at ground sites during CalNex 2010 and insight into its thermodynamic properties

Gas phase hydrogen chloride (HCl) was measured at Pasadena and San Joaquin Valley (SJV) ground sites in California during May and June 2010 as part of the CalNex study. Observed mixing ratios were on average 0.83 ppbv at Pasadena, ranging from below detection limit (0.055 ppbv) to 5.95 ppbv, and were on average 0.084 ppbv at SJV with a maximum value of 0.776 ppbv. At both sites, HCl levels were highest during midday and shared similar diurnal variations with HNO3. Coupled phase partitioning behavior was found between HCl/Cl- and HNO3/NO3 - using thermodynamic modelling and observations. Regional modeling of Cl- and HCl using CMAQ captures some of the observed relationships but underestimates measurements by a factor of 5 or more. Chloride in the 2.5-10 μm size range in Pasadena was sometimes higher than sea salt abundances, based on co-measured Na+, implying that sources other than sea salt are important. The acid-displacement of HCl/Cl- by HNO3/NO3 - (phase partitioning of semi-volatile acids) observed at the SJV site can only be explained by aqueous phase reaction despite low RH conditions and suggests the temperature dependence of HCl phase partitioning behavior was strongly impacted by the activity coefficient changes under relevant aerosol conditions (e.g., high ionic strength). Despite the influence from activity coefficients, the gas-particle system was found to be well constrained by other stronger buffers and charge balance so that HCl and Cl- concentrations were reproduced well by thermodynamic models.

Development of Continuous Gas Generation Method for Hydrogen Chloride Using Azeotropic Hydrochloric Acid System

Standard gases are often prepared using high-pressure gas cylinders. However, it is difficult to accurately prepare a known concentration of hydrogen chloride (HCl) gas using this method because HCl is highly corrosive and adsorptive. In this study, a simple method for the continuous generation of HCl gas was developed using a diffusion tube containing hydrochloric acid and a nitrogen carrier gas. The concentration of HCl produced from this system was almost unstable, but constant gas generation was realized for several hours when azeotropic hydrochloric acid (20.6% HCl in water) and a temperature near the azeotropic point (108.5 °C) were used, resulting in the generation of 103.6 ppm (mean, n = 5) of HCl gas with a relative standard deviation (RSD) of 2.34%. In this case, the percentage of HCl present in the entire gas mixture of HCl and water vapor was 22.5%, which is almost equivalent to the HCl content in the azeotropic hydrochloric acid (20.6%). The HCl concentration could also be controlled by changing the flow rate of the carrier gas. This work demonstrates a simple technique based on the diffusion theory that allows for the constant, controllable generation of a known concentration of HCl gas using an azeotropic hydrochloric acid system.

HprSR is a Reactive Chlorine Species-Sensing, Two-Component System in Escherichia coli

Two-component systems (TCS) are signalling pathways that allow bacterial cells to sense, respond and adapt to fluctuating environments. Among the classical TCS of Escherichia coli, HprSR has recently been shown to be involved in the regulation of msrPQ, which encodes the periplasmic methionine sulfoxide reductase system. In this study, we demonstrate that hypochlorous acid (HOCl) induces the expression of msrPQ in an HprSR-dependant manner, whereas H₂O₂, NO and paraquat (a superoxide generator) do not. Therefore, HprS appears to be an HOCl-sensing histidine kinase. Using a directed mutagenesis approach, we show that Met residues located in the periplasmic loop of HprS are important for its activity: as HOCl preferentially oxidizes Met residues, we provide evidence that HprS could be activated via the reversible oxidation of its methionine residues, meaning that MsrPQ plays a role in switching HprSR off. We propose that the activation of HprS by HOCl could occur through a Met redox switch. HprSR appears to be the first characterized TCS able to detect reactive chlorine species (RCS) in E. coli. This study represents an important step towards understanding the mechanisms of RCS resistance in prokaryotes. IMPORTANCE Understanding how bacteria respond to oxidative stress at the molecular level is crucial in the fight against pathogens. HOCl is one of the most potent industrial and physiological microbiocidal oxidants. Therefore bacteria have developed counterstrategies to survive HOCl-induced stress. Over the last decade, important insights into these bacterial protection factors have been obtained. Our work establishes HprSR as a reactive chlorine species-sensing, two-component system in Escherichia coli MG1655, which regulates the expression of MsrPQ, a repair system for HOCl-oxidized proteins. Moreover we provide evidence suggesting that HOCl could activate HprS through a methionine redox switch.

Photochemistry of Monohydrated Chloromethane: Formation of Free and Hydrated Cl- and CH3+ Ions from a Solvent-Shared Semi-Ion-Pair

The effect of water molecule on the excited states of CH₃Cl(H₂O), as compared to those of the isolated chloromethane, has been studied at the multireference configuration interaction with singles and doubles (MR-CISD), including extensivity corrections. Eight new Rydberg states are due to the water molecule but the common states of both systems are not severely altered. Potential energy curves of 23 singlet states along the C-Cl coordinate have also been computed at the MR-CISD level. The dissociation energy of the C-Cl bond decreases from ∼0.4 to 0.5 eV due to the water molecule. As for CH₃Cl (de Medeiros, V. C., J. Am. Chem. Soc. 2016, 138, 272-280), a stable ion-pair has also been characterized. However, for CH₃Cl(H₂O), this ion-pair is better described as a solvent-shared semi-ion-pair, CH₃^+δ(H₂O)Cl^-δ. This species is connected with three ionic dissociation channels, with two being due to the water molecule. The presence of these new ionic channels, particularly the lowest energy one, [H₃C-O]⁺ + Cl^-, raises a very important question of atmospheric relevance: can the interaction of chloroalkanes with water decrease its deleterious effect on the ozone layer? Several potentially new competing dissociation channels are also studied. The latter results can help to set up the most important states to be included in nonadiabatic dynamic calculations to study how the yields of the ionic channels change due to the water molecule.

Source Apportionment of Aerosol at a Coastal Site and Relationships with Precipitation Chemistry: A Case Study over the Southeast United States

This study focuses on the long-term aerosol and precipitation chemistry measurements from colocated monitoring sites in Southern Florida between 2013 and 2018. A positive matrix factorization (PMF) model identified six potential emission sources impacting the study area. The PMF model solution yielded the following source concentration profiles: (i) combustion; (ii) fresh sea salt; (iii) aged sea salt; (iv) secondary sulfate; (v) shipping emissions; and (vi) dust. Based on these results, concentration-weighted trajectory maps were developed to identify sources contributing to the PMF factors. Monthly mean precipitation pH values ranged from 4.98 to 5.58, being positively related to crustal species and negatively related to SO4 2-. Sea salt dominated wet deposition volume-weighted concentrations year-round without much variability in its mass fraction in contrast to stronger seasonal changes in PM2.5 composition where fresh sea salt was far less influential. The highest mean annual deposition fluxes were attributed to Cl-, NO3 -, SO4 2-, and Na+ between April and October. Nitrate is strongly correlated with dust constituents (unlike sea salt) in precipitation samples, indicative of efficient partitioning to dust. Interrelationships between precipitation chemistry and aerosol species based on long-term surface data provide insight into aerosol-cloud-precipitation interactions.

Multiphase Photochemistry of Iron-Chloride Containing Particles as a Source of Aqueous Chlorine Radicals and Its Effect on Sulfate Production

Photolysis of iron chlorides is a well-known photolytic source of Cl^• in environmental waters. However, the role of particulate chlorine radicals (Cl^• and Cl₂^•-) in their multiphase oxidative potential has been much less explored. Herein, we examine the effect of Cl^•/Cl₂^•- produced from photolysis of particulate iron chlorides on atmospheric multiphase oxidation. As a model system, experiments on multiphase oxidation of SO₂ by Cl^•/Cl₂^•- were performed. Fast sulfate production from SO₂ oxidation was observed with reactive uptake coefficients of ∼10^-5, comparable to the values necessary for explaining the observations in the haze events in China. The experimental and modeling results found a good positive correlation between the uptake coefficient, γ_SO2, and the Cl^• production rate, d[Cl^•]/dt, as γ_SO2 = 5.3 × 10^-6 × log(d[Cl^•]/dt) + 4.9 × 10^-5. When commonly found particulate dicarboxylic acids (oxalic acid or malonic acid) were added, sulfate production was delayed due to the competition of Fe³⁺ between chloride and the dicarboxylic acid for its complexation at the initial stage. After the delay, comparable sulfate production was observed. The present study highlights the importance of photochemistry of particulate iron chlorides in multiphase oxidation processes in the atmosphere.

Indoor acids and bases

Numerous acids and bases influence indoor air quality. The most abundant of these species are CO2 (acidic) and NH3 (basic), both emitted by building occupants. Other prominent inorganic acids are HNO3 , HONO, SO2 , H2 SO4 , HCl, and HOCl. Prominent organic acids include formic, acetic, and lactic; nicotine is a noteworthy organic base. Sources of N-, S-, and Cl-containing acids can include ventilation from outdoors, indoor combustion, consumer product use, and chemical reactions. Organic acids are commonly more abundant indoors than outdoors, with indoor sources including occupants, wood, and cooking. Beyond NH3 and nicotine, other noteworthy bases include inorganic and organic amines. Acids and bases partition indoors among the gas-phase, airborne particles, bulk water, and surfaces; relevant thermodynamic parameters governing the partitioning are the acid-dissociation constant (Ka ), Henry's law constant (KH ), and the octanol-air partition coefficient (Koa ). Condensed-phase water strongly influences the fate of indoor acids and bases and is also a medium for chemical interactions. Indoor surfaces can be large reservoirs of acids and bases. This extensive review of the state of knowledge establishes a foundation for future inquiry to better understand how acids and bases influence the suitability of indoor environments for occupants, cultural artifacts, and sensitive equipment.

The role of anthropogenic chlorine emission in surface ozone formation during different seasons over eastern China

Anthropogenic chlorine emission is an important source of Cl radicals, which plays an important role in the oxidative chemistry of the troposphere. However, its seasonal impacts on surface ozone levels in China have yet been comprehensively explored. In this study, we conducted numerical simulations for January, April, July and October 2015 by using the Community Multiscale Air Quality (CMAQ) modeling system with updated heterogeneous reactions of nitrogen oxides with particulate chlorine and updated Anthropogenic Chlorine Emission Inventory for China (ACEIC). Two experiments with and without ACEIC in the model were established, and their results were compared with each other. The model can faithfully reproduce the magnitudes and variations of meteorological parameters and air pollutant concentrations. Cl radicals were generated by the photolysis of ClNO₂, ClNO and Cl₂, HCl oxidation by OH radicals, and the heterogeneous reactions of NO₃ with particulate Cl^-. ClNO₂ and ClNO were mainly produced from the heterogeneous reactions of N₂O₅ and NO₂ with particulate Cl^-, respectively. The spatial and seasonal variations ofz these chlorinated species and their responses to the implementation of ACEIC were revealed in this study. Our results suggested that besides N₂O₅, the heterogeneous reactions of NO₂ and NO₃ with particulate Cl^- could be important sources of Cl radicals. Anthropogenic chlorine emission increased the Cl radical concentration through enhancing the photolysis of ClNO, Cl₂, and ClNO₂. The implementation of ACEIC in the model increased the degradation of Volatile Organic Compounds (VOCs) not only by Cl radicals but also by OH radicals. Although the seasonal variation of AECIE was insignificant, the larger formation of Cl radicals caused by higher levels of NO_x in January was counteracted by the larger loss of them due to more VOCs degradations, resulting in a lower increase in Cl radicals due to the implementation of ACEIC compared with other months. The anthropogenic chlorine emissions increased the monthly mean maximum daily 8-hour average (MDA8) O₃ mixing ratio by up to 4.9 ppbv, and increased the 1-hour O₃ mixing ratio by up to 34.3 ppbv. The impact of ACEIC was the most significant in January and the least in July due to the high emissions of NO_x and VOCs and adverse meteorological conditions in winter. It indicated that although the ozone concentration was low, the anthropogenic chlorine emission significantly contributed to the atmospheric oxidation capacity and increase ozone concentrations in winter.

Experimental study on HCl uptake by MgCl2 and sea salt under humid conditions

We studied HCl uptake by MgCl2 and sea salt over a relative humidity (RH) range from 0% to 34% at 278-313 K using a differential bead-filled flow tube coupled to a high-pressure chemical ionization mass spectrometer. The results showed that dry MgCl2 and sea salt are essentially inert to gaseous HCl with a probability of less than 10-6 . However, under humid conditions, HCl was found to be efficiently taken up by wet inorganic surfaces. The HCl uptake coefficient for MgCl2 and sea salt increased squarely with RH, reaching a value of 0.00123 and 0.00171, respectively, at 29% RH and 298 K. Such wetting behavior is even enhanced at elevated temperatures, with the coefficient reaching 0.00208 and 0.00239, respectively, at 313 K. Based on the study of the dependence of γHCl on the initial HCl concentration, we estimate γHCl as 0.012 at 24% RH at a typical HCl concentration in the troposphere. In addition, the observation of the remarkable enhancement in the OH uptake by the HCl-treated salts agrees with the results of our previous investigation, which suggested that water absorption on salts enhances γOH by lowering the surface pH. The proposed mechanism of HCl uptake by sea salt aerosol has implications for ozone production in the marine boundary layer.

Potential Effect of Halogens on Atmospheric Oxidation and Air Quality in China

Air pollution has been a hazard in China over recent decades threatening the health of half a billion people. Much effort has been devoted to mitigating air pollution in China leading to a significant reduction in primary pollutants emissions from 2013 to 2017, while a continuously worsening trend of surface ozone (O₃, a secondary pollutant and greenhouse gas) was observed over the same period. Atmospheric oxidation, dominated by daytime reactions involving hydroxyl radicals (OH), is the critical process to convert freshly-emitted compounds into secondary pollutants, and is underestimated in current models of China's air pollution. Halogens (chlorine, bromine, and iodine) are known to profoundly influence oxidation chemistry in the marine environment; however, their impact on atmospheric oxidation and air pollution in China is unknown. In the present study, we report for the first time that halogens substantially enhance the total atmospheric oxidation capacity in polluted areas of China, typically 10% to 20% (up to 87% in winter) and mainly by significantly increasing OH level. The enhanced oxidation along the coast is driven by oceanic emissions of bromine and iodine, and that over the inland areas by anthropogenic emission of chlorine. The extent and seasonality of halogen impact are largely explained by the dynamics of Asian monsoon, location and intensity of halogen emissions, and O₃ formation regime. The omission of halogen emissions and chemistry may lead to significant errors in historical re-assessments and future projections of the evolution of atmospheric oxidation in polluted regions.

Understanding the role of Cl and NO3 radicals in initiating atmospheric oxidation of fluorene: A mechanistic and kinetic study

The photooxidation of volatile organic compounds (VOCs) initiated by Cl and NO₃ radicals has been investigated for decades to assess the atmospheric fates of pollutants. Gas-phase fluorene is one of the most abundant polycyclic aromatic hydrocarbons (PAHs) that can be oxidized by activated radicals. In this study, we used quantum chemical calculation to study the atmospheric degradation of fluorene initiated by Cl and NO₃ radicals. The results showed that the Cl radical initiated reaction of fluorene mainly produces 9-fluorene radical that has significant potential to form secondary pollutants with more persistent toxic properties. The NO₃ radical initiated reaction of fluorene leads to the formation of oxygenated PAHs (OPAHs) and nitrated PAHs (NPAHs) including nitrooxyfluorene, nitrooxyfluorenone and 1,4-fluorenequinone. The rate constants and branch ratios of elementary reactions were determined based on Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The atmospheric lifetime of fluorene determined by NO₃ radical is deduced to be 1.52 days according to the calculated overall rate constant, 1.52 × 10^-14 cm³ molecule^-1 s^-1. The derivatives produced from the atmospheric degradation of fluorene initiated by Cl and NO₃ radicals increase the environmental risks of fluroene. Combined with previous experimental and theoretical findings, this work can help to clarify the atmospheric fate and assess the environmental risks of fluorene.

Wet deposition of sulfur and nitrogen at Mt. Emei in the West China Rain Zone, southwestern China: Status, inter-annual changes, and sources

The West China Rain Zone (WCRZ) is of ecological importance and thus, anthropogenic impacts on its vitality are of concern. To ascertain if China's SO₂ and NO_x emissions reductions in the recent years are reflected in reduced impacts, sulfur and nitrogen wet deposition at Mt. Emei was observed during 2017-2019. The source contributions to sulfur and nitrogen wet deposition were estimated using the positive matrix factorization (PMF) and the source-oriented Community Multi-scale Air Quality (CMAQ) model. The annual Volume Weighted Mean (VWM) pH of precipitation increased from 4.01 to 4.75 in the 1980s-2000s to 5.39 in 2017-2019. However, 12% of the samples during 2017-2019 had pH < 4.50. The VWM SO₄^2- concentration decreased from 80.9 to 26.0 μeq L^-1 from 2007-2009 to 2017-2019. The VWM NO₃^- concentration (18.3 μeq L^-1) in 2017-2019 was close to that measured in 2007-2009. Although the dry deposition fluxes onto forest canopy were under-estimated, the mean annual total wet and dry deposition flux of nitrogen (24.3 kg N ha^-1 yr^-1) was higher than the critical load (CL: 10.0-15.0 kg N ha^-1 yr^-1), while the flux of sulfur (12.1 kg S ha^-1 yr^-1) was lower than the CL (16.0-32.0 kg S ha^-1 yr^-1). The annual wet deposition fluxes of SO₄^2- and NO₃^- were mainly from industries (54% and 43%, respectively) and power plants (21% and 25%, respectively), and that of NH₄⁺ was mostly from agriculture (88%). Emissions within and outside the Sichuan Basin (SCB) were both important sources of the wet deposition fluxes of sulfur (45% and 39%, respectively) and nitrogen (68% and 29%, respectively), and other unidentified sources accounted for 16% and 3% of the sulfur and nitrogen fluxes, respectively. This study suggests that to protect Mt. Emei's ecosystems, efforts are needed to further control sulfur and nitrogen deposition through reducing emissions within and outside the SCB.

Atmospheric oxidation mechanism and kinetics of isoprene initiated by chlorine radicals: A computational study

The reaction with chlorine radicals (·Cl) has been considered to be one of indispensable sinks for isoprene. However, the mechanism of ·Cl initiated isoprene reaction was not fully understood. Herein, the reaction of isoprene with ·Cl, and ensuing reactions of the resulting isoprene relevant radicals were investigated by combined quantum chemistry calculations and kinetics modeling. The results indicate that ·Cl addition to two terminal C-atoms of two double bonds of isoprene, forming IM_1-1 and IM_1-4, are more favorable than H-abstractions from isoprene. Interestingly, the predicted reaction rate constant for the direct H-abstraction pathway is much lower than that of the indirect one, clarifying a direct H-abstraction mechanism for previously experimental observation. The IM_1-1 and IM_1-4 have distinct fate in their subsequent transformation. The reaction of IM_1-1 ends after the one-time O₂ addition. However, IM_1-4 can follow auto-oxidation mechanism with two times O₂ addition to finally form highly oxidized multi-functional molecules (HOMs), C₅H₇ClO₃ and ·OH. More importantly, the estimated contribution of ·Cl on HOMs (monomer only) formation from isoprene is lower than that of ·OH in addition pathway, implying overall HOMs yield from atmospheric isoprene oxidation could be overestimated if the role of ·Cl in transforming isoprene is ignored.

Mineralization versus fermentation: evidence for two distinct anaerobic bacterial degradation pathways for dichloromethane

Dichloromethane (DCM) is an anthropogenic pollutant with ozone destruction potential that is also formed naturally. Under anoxic conditions, fermentation of DCM to acetate and formate has been reported in axenic culture Dehalobacterium formicoaceticum, and to acetate, H₂ and CO₂ in mixed culture RM, which harbors the DCM degrader 'Candidatus Dichloromethanomonas elyunquensis'. RM cultures produced 28.1 ± 2.3 μmol of acetate from 155.6 ± 9.3 μmol DCM, far less than the one third (i.e., about 51.9 µmol) predicted based on the assumed fermentation model, and observed in cultures of Dehalobacterium formicoaceticum. Temporal metabolite analyses using gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy revealed that no ¹³C-labeled acetate was formed in ¹³C-DCM-grown RM cultures, indicating acetate was not a direct product of DCM metabolism. The data were reconciled with DCM mineralization and H₂ consumption via CO₂ reduction to acetate and methane by homoacetogenic and methanogenic partner populations, respectively. In contrast, Dehalobacterium formicoaceticum produced ¹³C-labeled acetate and formate from ¹³C-DCM, consistent with a fermentation pathway. Free energy change calculations predicted that organisms with the mineralization pathway are the dominant DCM consumers in environments with H₂ <100 ppmv. These findings have implications for carbon and electron flow in environments where DCM is introduced through natural production processes or anthropogenic activities.

Impact of halogen chemistry on summertime air quality in coastal and continental Europe: application of the CMAQ model and implications for regulation

Halogen (Cl, Br, and I) chemistry has been reported to influence the formation of secondary air pollutants. Previous studies mostly focused on the impact of chlorine species on air quality over large spatial scales. Very little attention has been paid to the effect of the combined halogen chemistry on air quality over Europe and its implications for control policy. In the present study, we apply a widely used regional model, the Community Multiscale Air Quality Modeling System (CMAQ), incorporated with the latest halogen sources and chemistry, to simulate the abundance of halogen species over Europe and to examine the role of halogens in the formation of secondary air pollution. The results suggest that the CMAQ model is able to reproduce the level of O₃, NO₂, and halogen species over Europe. Chlorine chemistry slightly increases the levels of OH, HO₂, NO₃, O₃, and NO₂ and substantially enhances the level of the Cl radical. Combined halogen chemistry induces complex effects on OH (ranging from -0.023 to 0.030 pptv) and HO₂ (in the range of -3.7 to 0.73 pptv), significantly reduces the concentrations of NO₃ (as much as 20 pptv) and O₃ (as much as 10 ppbv), and decreases NO₂ in highly polluted regions (as much as 1.7 ppbv); it increases NO₂ (up to 0.20 ppbv) in other areas. The maximum effects of halogen chemistry occur over oceanic and coastal regions, but some noticeable impacts also occur over continental Europe. Halogen chemistry affects the number of days exceeding the European Union target threshold for the protection of human beings and vegetation from ambient O₃. In light of the significant impact of halogen chemistry on air quality, we recommend that halogen chemistry be considered for inclusion in air quality policy assessments, particularly in coastal cities.

Significant impact of heterogeneous reactions of reactive chlorine species on summertime atmospheric ozone and free-radical formation in north China

Heterogeneous reactions of N₂O₅, O₃, OH, ClONO₂, HOCl, ClNO₂, and NO₂, with chlorine-containing particles are incorporated in the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of heterogeneous reactions of reactive chlorine species on ozone and free radicals. Changes of summertime ozone and free radical concentrations due to the additional heterogeneous reactions in north China were quantified. These heterogeneous reactions increased the O₃, OH, HO₂ and RO₂ concentrations by up to 20%, 28%, 36% and 48% for some regions in the Beijing-Tianjin-Hebei (BTH) area. These areas typically have a larger amount of NOx emissions and a lower VOC/NOx ratio. The zero-out method evaluates that the photolysis of ClNO₂ and Cl₂ are the major contributors (42.4% and 57.6%, respectively) to atmospheric Cl in the early morning hours but the photolysis of Cl₂ is the only significant contributor after 10:00 am. The results highlight that heterogeneous reactions of reactive chlorine species are important to atmospheric ozone and free-radical formation. Our study also suggests that the on-going NOx emission controls in the NCP region with a goal to reduce both O₃ and secondary nitrate can also have the co-benefit of reducing the formation Cl from ClNO₂ and Cl₂, which may also lead to lower secondary organic aerosol formation and thus the control of summertime PM_2.5 in the region.

Spatial Distributions and Sources of Inorganic Chlorine in PM2.5 across China in Winter

Chlorine is an important atmospheric photochemical oxidant, but few studies have focused on atmospheric chlorine. In this study, PM2.5 samples were collected from urban and rural sites across China in January 2018, and concentrations of Cl− and other water-soluble ions in PM2.5 were analyzed. The size-segregated aerosol Cl− data measured across Chinese cities by other studies were compiled for comparison. The observed data demonstrated that the Cl− concentrations of PM2.5 in northern cities (5.0 ± 3.7 µg/m3) were higher than those in central (1.9 ± 1.2 µg/m3) and southern cities (0.84 ± 0.54 µg/m3), suggesting substantial chlorine emissions in northern cities during winter. The concentrations of Cl− in aerosol were significantly higher in urban regions (0.11–26.7 µg/m3) compared to than in rural regions (0.03–0.61 µg/m3) across China during winter, implying strong anthropogenic chlorine emission in cities. Based on the mole ratios of Cl−/Na+, Cl−/K+ and Cl−/ SO 4 2 − and the PMF model, Cl− in northern and central cities was mainly sourced from the coal combustion and biomass burning, but in southern cities, Cl− in PM2.5 was mainly affected by the equilibrium between gas-phase HCl and particulate Cl−. The size-segregated statistical data demonstrated that particulate Cl− had a bimodal pattern, and more Cl− was distributed in the fine model than that in the coarse mode in winter, with the opposite pattern was observed in summer. This may be attributed to both sources of atmospheric Cl− and Cl− involved in chemical processes. This study reports the concentrations of aerosol Cl− on a national scale, and provides important information for modeling the global atmospheric reactive chlorine distribution and the effects of chlorine on atmospheric photochemistry.

Individual stages of bacterial dichloromethane degradation mapped by carbon and chlorine stable isotope analysis

The fractionation of carbon and chlorine stable isotopes of dichloromethane (CH₂Cl₂, DCM) upon dechlorination by cells of the aerobic methylotroph Methylobacterium extorquens DM4 and by purified DCM dehalogenases of the glutathione S-transferase family was analyzed. Isotope effects for individual steps of the multi-stage DCM degradation process, including transfer across the cell wall from the aqueous medium to the cell cytoplasm, dehalogenase binding, and catalytic reaction, were considered. The observed carbon and chlorine isotope fractionation accompanying DCM consumption by cell supensions and enzymes was mainly determined by the breaking of CCl bonds, and not by inflow of DCM into cells. Chlorine isotope effects of DCM dehalogenation were initially masked in high density cultures, presumably due to inverse isotope effects of non-specific DCM oxidation under conditions of oxygen excess. Glutathione cofactor supply remarkably affected the correlation of variations of DCM carbon and chlorine stable isotopes (Δδ¹³C/Δδ³⁷Cl), increasing corresponding ratio from 7.2-8.6 to 9.6-10.5 under conditions of glutathione deficiency. This suggests that enzymatic reaction of DCM with glutathione thiolate may involve stepwise breaking and making of bonds with the carbon atom of DCM, unlike the uncatalyzed reaction, which is a one-stage process, as shown by quantum-chemical modeling.

Formation and emission of hydrogen chloride in indoor air

To improve our understanding of chlorine chemistry indoors, reactive chlorine species such as hydrogen chloride (HCl) must be analyzed using fast time-response measurement techniques. Although well studied outdoors, sources of HCl indoors are unknown. In this study, mixing ratios of gaseous HCl were measured at 0.5 Hz in the indoor environment using a cavity ring-down spectroscopy (CRDS) instrument. The CRDS measurement rate provides a major advance in observational capability compared to other established techniques. Measurements of HCl were performed during three types of household activities: (a) floor exposure to bleach, (b) chlorinated and nonchlorinated detergent use in household dishwashers, and (c) cooking events. Surface application of bleach resulted in a reproducible increase of 0.1 ppbv in the affected room. Emissions of HCl from automated dishwashers were observed only when chlorinated detergents were used, with additional HCl emitted during the drying cycle. Increased mixing ratios of HCl were also observed during meal preparation on an electric element stovetop. These observations of HCl derived from household activities indicate either direct emission or secondary production of HCl via chlorine atoms is possible. Calculations of photolysis rate constants of chlorine atom precursors provide evidence that photolysis may contribute to indoor HCl levels.

Effects of halogens on European air-quality

Halogens (Cl, Br) have a profound influence on stratospheric ozone (O₃). They (Cl, Br and I) have recently also been shown to impact the troposphere, notably by reducing the mixing ratios of O₃ and OH. Their potential for impacting regional air-quality is less well understood. We explore the impact of halogens on regional pollutants (focussing on O₃) with the European grid of the GEOS-Chem model (0.25° × 0.3125°). It has recently been updated to include a representation of halogen chemistry. We focus on the summer of 2015 during the ICOZA campaign at the Weybourne Atmospheric Observatory on the North Sea coast of the UK. Comparisons between these observations together with those from the UK air-quality network show that the model has some skill in representing the mixing ratios/concentration of pollutants during this period. Although the model has some success in simulating the Weybourne ClNO₂ observations, it significantly underestimates ClNO₂ observations reported at inland locations. It also underestimates mixing ratios of IO, OIO, I₂ and BrO, but this may reflect the coastal nature of these observations. Model simulations, with and without halogens, highlight the processes by which halogens can impact O₃. Throughout the domain O₃ mixing ratios are reduced by halogens. In northern Europe this is due to a change in the background O₃ advected into the region, whereas in southern Europe this is due to local chemistry driven by Mediterranean emissions. The proportion of hourly O₃ above 50 nmol mol^-1 in Europe is reduced from 46% to 18% by halogens. ClNO₂ from N₂O₅ uptake onto sea-salt leads to increases in O₃ mixing ratio, but these are smaller than the decreases caused by the bromine and iodine. 12% of ethane and 16% of acetone within the boundary layer is oxidised by Cl. Aerosol response to halogens is complex with small (∼10%) reductions in PM_2.5 in most locations. A lack of observational constraints coupled to large uncertainties in emissions and chemical processing of halogens make these conclusions tentative at best. However, the results here point to the potential for halogen chemistry to influence air quality policy in Europe and other parts of the world.

Atmospheric Oxidation of Piperazine Initiated by ·Cl: Unexpected High Nitrosamine Yield

Chlorine radicals (·Cl) initiated amine oxidation plays an important role for the formation of carcinogenic nitrosamine in the atmosphere. Piperazine (PZ) is considered as a potential atmospheric pollutant since it is an alternative solvent to monoethanolamine (MEA), a benchmark solvent in a leading CO₂ capture technology. Here, we employed quantum chemical methods and kinetics modeling to investigate ·Cl-initiated atmospheric oxidation of PZ, particularly concerning the potential of PZ to form nitrosamine compared to MEA. Results showed that the ·Cl-initiated PZ reaction exclusively leads to N-center radicals (PZ-N) that mainly react with NO to produce nitrosamine in their further reaction with O₂/NO. Together with the PZ + ·OH reaction, the PZ-N yield from PZ oxidation is still lower than that of the corresponding MEA reactions. However, the nitrosamine yield of PZ is higher than the reported value for MEA when [NO] is <5 ppb, a concentration commonly encountered in a polluted urban atmosphere. The unexpected high nitrosamine yield from PZ compared to MEA results from a more favorable reaction of N-center radicals with NO compared to O₂. These findings show that the yield of N-center radicals cannot directly be used as a metric for the yield of the corresponding carcinogenic nitrosamine.

Chloromethane formation and degradation in the fern phyllosphere

Chloromethane (CH₃Cl) is the most abundant halogenated trace gas in the atmosphere. It plays an important role in natural stratospheric ozone destruction. Current estimates of the global CH₃Cl budget are approximate. The strength of the CH₃Cl global sink by microbial degradation in soils and plants is under discussion. Some plants, particularly ferns, have been identified as substantial emitters of CH₃Cl. Their ability to degrade CH₃Cl remains uncertain. In this study, we investigated the potential of leaves from 3 abundant ferns (Osmunda regalis, Cyathea cooperi, Dryopteris filix-mas) to produce and degrade CH₃Cl by measuring their production and consumption rates and their stable carbon and hydrogen isotope signatures. Investigated ferns are able to degrade CH₃Cl at rates from 2.1 to 17 and 0.3 to 0.9μgg_dw^-1day^-1 for C. cooperi and D. filix-mas respectively, depending on CH₃Cl supplementation and temperature. The stable carbon isotope enrichment factor of remaining CH₃Cl was -39±13‰, whereas negligible isotope fractionation was observed for hydrogen (-8±19‰). In contrast, O. regalis did not consume CH₃Cl, but produced it at rates ranging from 0.6 to 128μgg_dw^-1day^-1, with stable isotope values of -97±8‰ for carbon and -202±10‰ for hydrogen, respectively. Even though the 3 ferns showed clearly different formation and consumption patterns, their leaf-associated bacterial diversity was not notably different. Moreover, we did not detect genes associated with the only known chloromethane utilization pathway "cmu" in the microbial phyllosphere of the investigated ferns. Our study suggests that still unknown CH₃Cl biodegradation processes on plants play an important role in global cycling of atmospheric CH₃Cl.

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

In the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a comprehensive overview of our current understanding of potential exoplanet biosignatures, including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required to maintain short-lived biogenic gases as atmospheric signatures. We focus particularly on advances made since the seminal review by Des Marais et al. The purpose of this work is not to propose new biosignature strategies, a goal left to companion articles in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward. Key Words: Exoplanets-Biosignatures-Habitability markers-Photosynthesis-Planetary surfaces-Atmospheres-Spectroscopy-Cryptic biospheres-False positives. Astrobiology 18, 663-708.

Abundance and origin of fine particulate chloride in continental China

Particulate chloride can be converted to nitryl chloride (ClNO₂) through heterogeneous reactions with dinitrogen pentoxide (N₂O₅), and photolysis of ClNO₂ affects atmospheric oxidative capacity. However, the characteristics and sources of chloride, especially those with an anthropogenic origin, are poorly characterized, which makes it difficult to evaluate the effects of ClNO₂ on radical chemistry and air quality in polluted regions. Aerosol composition data from the literature were compiled to derive the spatial distributions of particulate chloride across China, and hourly aerosol composition data collected at a highly polluted inland urban site in eastern China and at a coastal site in southern China were analysed to gain further insights into non-oceanic sources of chloride. The results show that particulate chloride is concentrated mainly in fine particles and that high chloride loadings are observed in the inland urban areas of northern and western China with higher Cl^-/Na⁺ mass ratios (2.46 to 5.00) than sea water (1.81), indicative of significant contributions from anthropogenic sources. At the inland urban site, the fine chloride displays distinct seasonality, with higher levels in winter and summer. Correlation analysis and positive matrix factorization (PMF) results indicate that coal combustion and residential biomass burning are the main sources (84.8%) of fine chloride in winter, and open biomass burning is the major sources (52.7%) in summer. The transport of plumes from inland polluted areas leads to elevated fine chloride in coastal areas. A simulation with WRF-Chem model confirmed a minor contribution of sea-salt aerosol to fine chloride at the inland site during summer with winds from the East Sea. The widespread sources of chloride, together with abundant NOx and ozone, suggest significant ClNO₂ production and subsequent enhanced photochemical processes over China.

"New" Reactive Nitrogen Chemistry Reshapes the Relationship of Ozone to Its Precursors

Tropospheric ozone pollution has been a major environmental issue, and mitigation of this persistent problem requires a comprehensive understanding of the sensitivity of ozone to its precursors, i.e., nitrogen oxides (NO _x) and volatile organic compounds (VOCs). Recent studies have proposed several "new" reactive nitrogen chemical processes, including additional sources of nitrous acid, heterogeneous uptake of dinitrogen pentoxide, and production of nitryl chloride. These processes significantly affect the budgets of radicals and NO _x and hence the formation of ozone. In present study, we aim to investigate to what extent these processes alter the relationships between ozone and its precursors. A revised Weather Research and Forecasting model coupled with Chemistry incorporating the "new" nitrogen chemistry was adopted to simulate the ozone sensitivity regime in China in summer. The results showed that nitrogen chemistry changed the ozone sensitivity regime for approximately 40% of the simulated area with human influence, mostly from VOC-sensitive or NO _x-sensitive regimes to mixed-sensitive regime. The nitrogen chemistry changed the isopleth plots of the ozone peak values for major cities, suggesting a different strategy for controlling ozone pollution. This study underscores the need to consider unconventional nitrogen chemistry in air quality models used in the design of ozone control strategies.

Chloromethane Degradation in Soils: A Combined Microbial and Two-Dimensional Stable Isotope Approach

Chloromethane (CHCl, methyl chloride) is the most abundant volatile halocarbon in the atmosphere and involved in stratospheric ozone depletion. The global CHCl budget, and especially the CHCl sink from microbial degradation in soil, still involves large uncertainties. These may potentially be resolved by a combination of stable isotope analysis and bacterial diversity studies. We determined the stable isotope fractionation of CHCl hydrogen and carbon and investigated bacterial diversity during CHCl degradation in three soils with different properties (forest, grassland, and agricultural soils) and at different temperatures and headspace mixing ratios of CHCl. The extent of chloromethane degradation decreased in the order forest > grassland > agricultural soil. Rates ranged from 0.7 to 2.5 μg g dry wt. d for forest soil, from 0.1 to 0.9 μg g dry wt. d for grassland soil, and from 0.1 to 0.4 μg g dry wt. d for agricultural soil and increased with increasing temperature and CHCl supplementation. The measured mean stable hydrogen enrichment factor of CHCl of -50 ± 13‰ was unaffected by temperature, mixing ratio, or soil type. In contrast, the stable carbon enrichment factor depended on CHCl degradation rates and ranged from -38 to -11‰. Bacterial community composition correlated with soil properties was independent from CHCl degradation or isotope enrichment. Nevertheless, increased abundance after CHCl incubation was observed in 21 bacterial operational taxonomical units (OTUs at the 97% 16S RNA sequence identity level). This suggests that some of these bacterial taxa, although not previously associated with CHCl degradation, may play a role in the microbial CHCl sink in soil.

Anthropogenic Emissions of Hydrogen Chloride and Fine Particulate Chloride in China

Particulate chloride (Cl^-) can be transformed to nitryl chloride (ClNO₂) via heterogeneous reaction with nitrogen pentoxide (N₂O₅) at night. Photolysis of ClNO₂ and subsequent reactions of chlorine radical with other gases can significantly affect the atmospheric photochemistry. In China, the only available integrated anthropogenic chloride emission inventory was compiled in the 1990s with low spatial resolution, which hinders assessment of impact of ClNO₂ on current air quality. In this study, we developed an up-to-date and high-resolution anthropogenic inventory of hydrogen chloride (HCl) and fine particulate Cl^- emissions in China for 2014 with 0.1° × 0.1° resolution. Detailed local data and county-level activity data were collected and complied. The anthropogenic emissions of HCl and fine particulate Cl^- in 2014 were estimated to be 458 and 486 Gg, respectively. Biomass burning was the largest contributor, accounting for 75% of fine particulate Cl^- emission and 32% of HCl emission. Northeast China and North China Plain were the largest chloride emitters. The monthly distribution varied in different regions, due to different agricultural activities and climate conditions. This work updates the chloride emission information and improves its spatial and temporal resolution, which enables better quantification of the ClNO₂ production and its impact over China.

Airborne Observations of Reactive Inorganic Chlorine and Bromine Species in the Exhaust of Coal-Fired Power Plants

We present airborne observations of gaseous reactive halogen species (HCl, Cl₂, ClNO₂, Br₂,BrNO₂, and BrCl), sulfur dioxide (SO₂), and nonrefractory fine particulate chloride (pCl) and sulfate(pSO₄) in power plant exhaust. Measurements were conducted during the Wintertime INvestigation of Transport, Emissions, and Reactivity campaign in February-March of 2015 aboard the NCAR-NSF C-130 aircraft. Fifty air mass encounters were identified in which SO₂ levels were elevated ~5 ppb above ambient background levels and in proximity to operational power plants. Each encounter was attributed to one or more potential emission sources using a simple wind trajectory analysis. In case studies, we compare measured emission ratios to those reported in the 2011 National Emissions Inventory and present evidence of the conversion of HCl emitted from power plants to ClNO₂. Taking into account possible chemical conversion downwind, there was general agreement between the observed and reported HCl: SO₂ emission ratios. Reactive bromine species (Br₂, BrNO₂, and/or BrCl) were detected in the exhaust of some coal-fired power plants, likely related to the absence of wet flue gas desulfurization emission control technology. Levels of bromine species enhanced in some encounters exceeded those expected assuming all of the native bromide in coal was released to the atmosphere, though there was no reported use of bromide salts (as a way to reduce mercury emissions) during Wintertime INvestigation of Transport, Emissions, and Reactivity observations. These measurements represent the first ever in-flight observations of reactive gaseous chlorine and bromine containing compounds present in coal-fired power plant exhaust.

Chloromethane emissions in human breath

Chloromethane (CH₃Cl), currently the most abundant chlorinated organic compound in the atmosphere at around ~550 parts per trillion by volume (pptv), is considered responsible for approximately 16% of halogen-catalyzed stratospheric ozone destruction. Although emissions of CH₃Cl are known to occur from animals such as cattle, formation and release of CH₃Cl from humans has not yet been reported. In this study a pre-concentration unit coupled with a gas chromatograph directly linked to a mass spectrometer was used to precisely measure concentrations of CH₃Cl at the pptv level in exhaled breath from 31 human subjects with ages ranging from 3 to 87years. We provide analytical evidence that all subjects exhaled CH₃Cl in the range of 2.5 to 33 parts per billion by volume, levels which significantly exceed those of inhaled air by a factor of up to 60. If the mean of these emissions was typical for the world's population, then the global source of atmospheric CH₃Cl from humans would be around 0.66Ggyr^-1 (0.33 to 1.48Ggyr^-1), which is less than 0.03% of the total annual global atmospheric source strength. The observed endogenous formation of a chlorinated methyl group in humans might be of interest to biochemists and medical scientists as CH₃Cl is also known to be a potent methylating agent and thus, could be an important target compound in future medical research diagnostic programs.

Impact of Wildfire Emissions on Chloride and Bromide Depletion in Marine Aerosol Particles

This work examines particulate chloride (Cl^-) and bromide (Br^-) depletion in marine aerosol particles influenced by wildfires at a coastal California site in the summers of 2013 and 2016. Chloride exhibited a dominant coarse mode due to sea salt influence, with substantially diminished concentrations during fire periods as compared to nonfire periods. Bromide exhibited a peak in the submicrometer range during fire and nonfire periods, with an additional supermicrometer peak in the latter periods. Chloride and Br^- depletions were enhanced during fire periods as compared to nonfire periods. The highest observed %Cl^- depletion occurred in the submicrometer range, with maximum values of 98.9% (0.32-0.56 μm) and 85.6% (0.56-1 μm) during fire and nonfire periods, respectively. The highest %Br^- depletion occurred in the supermicrometer range during fire and nonfire periods with peak depletion between 1.8-3.2 μm (78.8% and 58.6%, respectively). When accounting for the neutralization of sulfate by ammonium, organic acid particles showed the greatest influence on Cl^- depletion in the submicrometer range. These results have implications for aerosol hygroscopicity and radiative forcing in areas with wildfire influence owing to depletion effects on composition.

Continental-scale variation in chloride/bromide ratios of wet deposition

Chlorine and bromine play crucial roles in atmospheric element cycles and are important environmental tracers in catchment investigations, so understanding their distribution at the Earth's near-surface is imperative for deciphering their behaviour. This study presents the first continental-scale analysis of Cl^- and Br^- concentrations of wet deposition, based on six and half years of weekly samples collected across North America. A recently developed imputation algorithm was applied to estimate the high proportion of censored Br^- values, as well as the other eight analytes, based on the multivariate relationships of nine analytes. The results are consistent with previous studies that have found that the concentrations of these two ions in wet deposition, and the Cl^-/Br^- ratios of wet deposition decrease with distance inland. Close to the coast, Cl^-/Br^- ratios of wet deposition are similar to the ratio found in seawater (~288 by mass), rapidly decrease to approximately a third of the seawater ratio at ~200km inland, and then decrease at a much lesser rate to reach mass-ratios of 20 to 10 at ~1500km inland, following a logarithmic regression. The Niagara Falls and the Great Salt Lake are identified as localised sources of atmospheric solutes based on elevated Cl^-/Br^- ratios of wet deposition at proximal sites. Our observations provide further confidence in the findings presented in previous studies that have shown that Cl^-/Br^- ratios systematically decrease with increasing distance from the coast, despite the potential confounding impact of other processes, such as weather patterns, chemical behaviour and anthropogenic activity. Our results provide improved estimates of Cl^-/Br^- ratios of wet deposition source water in the absence of site-specific data.