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

Typhoon- and pollution-driven enhancement of reactive bromine in the mid-latitude marine boundary layer

Tropospheric reactive bromine is important for atmospheric chemistry, regional air pollution, and global climate. Previous studies have reported measurements of atmospheric reactive bromine species in different environments, and proposed their main sources, e.g. sea-salt aerosol (SSA), oceanic biogenic activity, polar snow/ice, and volcanoes. Typhoons and other strong cyclonic activities (e.g. hurricanes) induce abrupt changes in different earth system processes, causing widespread destructive effects. However, the role of typhoons in regulating reactive bromine abundance and sources remains unexplored. Here, we report field observations of bromine oxide (BrO), a critical indicator of reactive bromine, on the Huaniao Island (HNI) in the East China Sea in July 2018. We observed high levels of BrO below 500 m with a daytime average of 9.7 ± 4.2 pptv and a peak value of ∼26 pptv under the influence of a typhoon. Our field measurements, supported by model simulations, suggest that the typhoon-induced drastic increase in wind speed amplifies the emission of SSA, significantly enhancing the activation of reactive bromine from SSA debromination. We also detected enhanced BrO mixing ratios under high NOx conditions (ppbv level) suggesting a potential pollution-induced mechanism of bromine release from SSA. Such elevated levels of atmospheric bromine noticeably increase ozone destruction by as much as ∼40% across the East China Sea. Considering the high frequency of cyclonic activity in the northern hemisphere, reactive bromine chemistry is expected to play a more important role than previously thought in affecting coastal air quality and atmospheric oxidation capacity. We suggest that models need to consider the hitherto overlooked typhoon- and pollution-mediated increase in reactive bromine levels when assessing the synergic effects of cyclonic activities on the earth system.

Pollution-Derived Br2 Boosts Oxidation Power of the Coastal Atmosphere

The bromine atom (Br^•) has been known to destroy ozone (O₃) and accelerate the deposition of toxic mercury (Hg). However, its abundance and sources outside the polar regions are not well-known. Here, we report significant levels of molecular bromine (Br₂)─a producer of Br^•─observed at a coastal site in Hong Kong, with an average noontime mixing ratio of 5 ppt. Given the short lifetime of Br₂ (∼1 min at noon), this finding reveals a large Br₂ daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br₂ is generated by the photodissociation of particulate nitrate (NO₃^-) and that the reactive uptake of dinitrogen pentoxide (N₂O₅) on aerosols is an important nighttime source. Model-calculated Br^• concentrations are comparable with that of the OH radical─the primary oxidant in the troposphere, accounting for 24% of the oxidation of isoprene, a 13% increase in net O₃ production, and a nearly 10-fold increase in the production rate of toxic Hg^II. Our findings reveal that reactive bromines play a larger role in the atmospheric chemistry and air quality of polluted coastal and maritime areas than previously thought. Our results also suggest that tightening the control of emissions of two conventional pollutants (NO_x and SO₂)─thereby decreasing the levels of nitrate and aerosol acidity─would alleviate halogen radical production and its adverse impact on air quality.

Global Emissions of Hydrogen Chloride and Particulate Chloride from Continental Sources

Gaseous and particulate chlorine species play an important role in modulating tropospheric oxidation capacity, aerosol water uptake, visibility degradation, and human health. The lack of recent global continental chlorine emissions has hindered modeling studies of the role of chlorine in the atmosphere. Here, we develop a comprehensive global emission inventory of gaseous HCl and particulate Cl- (pCl), including 35 sources categorized in six source sectors based on published up-to-date activity data and emission factors. These emissions are gridded at a spatial resolution of 0.1° × 0.1° for the years 1960 to 2014. The estimated emissions of HCl and pCl in 2014 are 2354 (1661-3201) and 2321 (930-3264) Gg Cl a-1, respectively. Emissions of HCl are mostly from open waste burning (38%), open biomass burning (19%), energy (19%), and residential (13%) sectors, and the major sources classified by fuel type are combustion of waste (43%), biomass (32%), and coal (25%). Emissions of pCl are mostly from biofuel (29%) and open biomass burning processes (44%). The sectoral and spatial distributions of HCl and pCl emissions are very heterogeneous along the study period, and the temporal trends are mainly driven by the changes in emission factors, energy intensity, economy, and population.

An unexpected large continental source of reactive bromine and chlorine with significant impact on wintertime air quality

Halogen atoms affect the budget of ozone and the fate of pollutants such as hydrocarbons and mercury. Yet their sources and significances in polluted continental regions are poorly understood. Here we report the observation of unprecedented levels (averaging at 60 parts per trillion) of bromine chloride (BrCl) at a mid-latitude site in North China during winter. Widespread coal burning in rural households and a photo-assisted process were the primary source of BrCl and other bromine gases. BrCl contributed about 55% of both bromine and chlorine atoms. The halogen atoms increased the abundance of 'conventional' tropospheric oxidants (OH, HO₂ and RO₂) by 26%-73%, and enhanced oxidation of hydrocarbon by nearly a factor of two and the net ozone production by 55%. Our study reveals the significant role of reactive halogen in winter atmospheric chemistry and the deterioration of air quality in continental regions where uncontrolled coal combustion is prevalent.

Quantifying wintertime O3 and NOx formation with relevance vector machines

This paper uses a machine learning model called a relevance vector machine (RVM) to quantify ozone (O₃) and nitrogen oxides (NO_x) formation under wintertime conditions. Field study measurements were based on previous work described by Olson et al. (2019), where continuous measurements were reported from a wintertime field study in Utah. RVMs were formulated using either O₃ or nitrogen dioxide (NO₂) as the output variable. Values of the correlation coefficient (r²) between predicted and measured concentrations were 0.944 for O₃ and 0.931 for NO₂. RVMs are constructed from the observed measurements and result in sparse model formulations, meaning that only a subset of the data is used to approximate the entire dataset. For this study, the RVM with O₃ as the output variable used only 20% of the measurement data while the RVM with NO₂ used 16%. RVMs were then used as a predictive model to assess the importance of individual precursors. Using O₃ as the output variable, increases in three species resulted in increased O₃ concentrations: hydrogen peroxide (H₂O₂), dinitrogen pentoxide (N₂O₅), and molecular chlorine (Cl₂). For the two termination products measured during the study, nitric acid (HNO₃) and formic acid (CH₂O₂), no change in O₃ concentration was observed. Using NO₂ as the output variable, only increases in N₂O₅ resulted in increased NO₂ concentrations.

Multiphase Oxidation of Sulfur Dioxide in Aerosol Particles: Implications for Sulfate Formation in Polluted Environments

Atmospheric oxidation of sulfur dioxide (SO₂) forms sulfate-containing aerosol particles that impact air quality, climate, and human and ecosystem health. It is well-known that in-cloud oxidation of SO₂ frequently dominates over gas-phase oxidation on regional and global scales. Multiphase oxidation involving aerosol particles, fog, and cloud droplets has been generally thought to scale with liquid water content (LWC) so multiphase oxidation would be negligible for aerosol particles due to their low aerosol LWC. However, recent field evidence, particularly from East Asia, shows that fast sulfate formation prevails in cloud-free environments that are characterized by high aerosol loadings. By assuming that the kinetics of cloud water chemistry prevails for aerosol particles, most atmospheric models do not capture this phenomenon. Therefore, the field of aerosol SO₂ multiphase chemistry has blossomed in the past decade, with many oxidation processes proposed to bridge the difference between modeled and observed sulfate mass loadings. This review summarizes recent advances in the fundamental understanding of the aerosol multiphase oxidation of SO₂, with a focus on environmental conditions that affect the oxidation rate, experimental challenges, mechanisms and kinetics results for individual reaction pathways, and future research directions. Compared to dilute cloud water conditions, this paper highlights the differences that arise at the molecular level with the extremely high solute strengths present in aerosol particles.

Kinetic Measurements of Cl Atom Reactions with C5–C8 Unsaturated Alcohols

The reactions of five structurally similar unsaturated alcohols, i.e., (Z)-2-penten-1-ol, (E)-2-hexen-1-ol, (E)-3-hexen-1-ol, (Z)-3-hexen-1-ol, and 1-octen-3-ol, with Cl atoms in the gas phase, were investigated at 296 ± 2 K and 1 atm by the relative-rate kinetic technique using a 600-L Teflon reaction chamber. Selected ion flow tube mass spectrometry (SIFT-MS) was used simultaneously to monitor the decay of the alcohols of interest and selected reference compounds. Tetrahydrofuran (THF), propan-1-ol, and octane were used as reference compounds. Chlorine atoms were produced by the photolysis of molecular chlorine (Cl2) using broadband actinic lamps near 365 nm. The estimated rate constant values (in 10−10 cm3∙molecule−1∙s−1) followed the order 2.99 ± 0.53 ((Z)-2-penten-1-ol) < 3.05 ± 0.59 ((E)-3-hexen-1-ol) < 3.15 ± 0.58 ((Z)-3-hexen-1-ol) < 3.41 ± 0.65 ((E)-2-hexen-1-ol) < 4.03 ± 0.77 (1-octen-3-ol). The present work provides the first value of the rate constant for the reaction of 1-octen-3-ol with Cl atoms. The results are discussed and interpreted in relation to other studies where literature data are available. The structure–activity relationship and the atmospheric implications are discussed as well.

ClNO2 Production from N2O5 Uptake on Saline Playa Dusts: New Insights into Potential Inland Sources of ClNO2

Nitryl chloride (ClNO₂), formed when dinitrogen pentoxide (N₂O₅) reacts with chloride-containing aerosol, photolyzes to produce chlorine radicals that facilitate the formation of tropospheric ozone. ClNO₂ has been measured in continental areas; however, the sources of particulate chloride required to form ClNO₂ in inland regions remain unclear. Dust emitted from saline playas (e.g., dried lakebeds) contains salts that can potentially form ClNO₂ in inland regions. Here, we present the first laboratory measurements demonstrating the production of ClNO₂ from playa dusts. N₂O₅ reactive uptake coefficients (γ_N2O5) ranged from ∼10^-3 to 10^-1 and ClNO₂ yields (φ_ClNO2) were >50% for all playas tested except one. In general, as the soluble ion fraction of playa dusts increases, γ_N2O5 decreases and φ_ClNO2 increases. We attribute this finding to a transition from aerosol surfaces dominated by silicates that react efficiently with N₂O₅ and produce little ClNO₂ to aerosols that behave like deliquesced chloride-containing salts that generate high yields of ClNO₂. Molecular bromine (Br₂) and nitryl bromide (BrNO₂) were also detected, highlighting that playas facilitate the heterogeneous production of brominated compounds. Our results suggest that parameterizations and models should be updated to include playas as an inland source of aerosol chloride capable of efficiently generating ClNO₂.

Enhanced Chlorine and Bromine Atom Activation by Hydrolysis of Halogen Nitrates from Marine Aerosols at Polluted Coastal Areas

Detailed multiphase chemistry box model studies are carried out, investigating halogen radical activation at polluted coastal areas. Simulations are performed for a nonpermanent cloud and a cloud-free scenario and reveal that ClNO₂ photolysis and ICl photolysis are crucial for gas-phase Cl atom activation. In the cloud scenario, the integrated ClNO₂ and ICl photolysis rates are 3.7 × 10⁷ and 3.1 × 10⁷ molecules cm^-3 s^-1. In the cloud-free scenario, the integrated ClNO₂ and ICl photolysis rates are 8.1 × 10⁷ and 3.6 × 10⁷ molecules cm^-3 s^-1. The simulations show larger contributions of ClNO₂ photolysis in the morning and higher ones of ICl photolysis during afternoon. Throughout the simulation, average contributions to Cl atom activation in the cloud and cloud-free scenarios by ClNO₂ photolysis are 42% and 62% and by ICl photolysis 35% and 28%, respectively. ICl is formed through an aqueous-phase reaction of HOI with chloride. Two thirds of the formed ICl is released into the gas phase. The residual third reacts with bromide, creating IBr. Overall, the simulations emphasize the crucial role of INO₃ hydrolysis for Cl and Br atom activation in polluted coastal areas. Therefore, it needs to be considered in chemical transport models to improve air quality predictions.