Photochemical oxidation of VOCs and their source impact assessment on ozone under de-weather conditions in Western Taiwan

The application of statistical models has excellent potential to provide crucial information for mitigating the challenging issue of ozone (O3) pollution by capturing its associations with explanatory variables, including reactive precursors (VOCs and NOX) and meteorology. Considering the large contribution of O3 in degrading the air quality of western Taiwan, three-year (2019-2021) hourly concentration data of VOC, NOX and O3 from 4 monitoring stations of western Taiwan: Tucheng (TC), Zhongming (ZM), Taixi (TX) and Xiaogang (XG), was evaluated to identify the effect of anthropogenic emissions on O3 formation. Owing to the high-ambient reactivity of VOCs on the underestimation of sources, photochemical oxidation was assessed to calculate the consumed VOC (VOCcons) which was followed by the source identification of their initial concentrations. VOCcons was observed to be highest in the summer season (16.7 and 22.7 ppbC) at north (TC and ZM) and in the autumn season (17.8 and 11.4 ppbC) in southward-located stations (TX and XG, respectively). Results showed that VOCs from solvents (25-27%) were the major source at northward stations whereas VOCs-industrial emissions (30%) dominated in south. Furthermore, machine learning (ML): eXtreme Gradient Boost (XGBoost) model based de-weather analysis identified that meteorological factors favor to reduce ambient O3 levels at TC, ZM and XG stations (-67%, -47% and -21%, respectively) but they have a major role in accumulating the O3 (+38%) at the TX station which is primarily transported from the upwind region of south-central Taiwan. Crucial insights using ML outputs showed that the finding of the study can be utilized for region-specific data-driven control of emission from VOCs-sources and prioritized to limit the O3-pollution at the study location-ns as well as their accumulation in distant regions.

Secondary aerosol formation in incense burning particles by O3 and OH oxidation via single particle mixing state analysis

Incense burning is a common religious activity that emits abundant gaseous and particulate pollutants into the atmosphere. During their atmospheric lifetime, these gases and particles are subjected to oxidation, leading to the formation of secondary pollutants. We examined the oxidation of incense burning plumes under O₃ exposure and dark condition using an oxidation flow reactor connected to a single particle aerosol mass spectrometer (SPAMS). Nitrate formation was observed in incense burning particles, mainly attributable to the ozonolysis of nitrogen-containing organic compounds. With UV on, nitrate formation was significantly enhanced, likely due to HNO₃/HNO₂/NOx uptake triggered by OH chemistry, which is more effective than ozone oxidation. The extent of nitrate formation is insensitive to O₃ and OH exposure, possibly due to the diffusion limitation on interfacial uptake. The O₃-UV-aged particles are more oxygenated and functionalized than O₃-Dark-aged particles. Oxalate and malonate, two typical secondary organic aerosol (SOA) components, were found in O₃-UV-aged particles. Our work reveals that nitrate, accompanied by SOA, can rapidly form in incense-burning particles upon photochemical oxidation in the atmosphere, which could deepen our understanding of air pollution caused by religious activities.

Evolution characteristic of atmospheric black carbon particles at a coastal site in the Pearl River Delta, China

The mixing of black carbon (BC) with secondary materials is a major uncertainty source in assessing its radiative forcing. However, current understanding of the formation and evolution of various BC components is limited, particularly in the Pearl River Delta, China. This study measured submicron BC-associated nonrefractory materials and the total submicron nonrefractory materials using a soot particle aerosol mass spectrometer and a high-resolution time-of-flight aerosol mass spectrometer, respectively, at a coastal site in Shenzhen, China. Two distinct atmospheric conditions were also identified to further explore the distinctive evolution of BC-associated components: polluted period (PP) and clean period (CP). Comparing the components of two particles, we found that more-oxidized organic factor (MO-OOA) prefers to form on BC during PP rather CP. The formation of MO-OOA on BC (MO-OOA_BC) was affected by both enhanced photochemical processes and nocturnal heterogeneous processes. Enhanced photo-reactivity of BC, photochemistry during the daytime, and heterogeneous reaction at nighttime were potential pathways for MO-OOA_BC formation during PP. The fresh BC surface was favorable for the formation of MO-OOA_BC. Our study shows the evolution of BC-associated components under different atmospheric conditions, which should be considered in regional climate models to improve the assessment of the climate effects of BC.

Secondary organic aerosol formation from atmospheric reactions of anisole and associated health effects

Anisole (methoxybenzene) represents an important marker compound of lignin pyrolysis and a starting material for many chemical products. In this study, secondary organic aerosols (SOA) formed by anisole via various atmospheric processes, including homogeneous photooxidation with varying levels of OH• and NO_x and subsequent heterogeneous NO₃• dark reactions, were investigated. The yields of anisole SOA, particle-bound organoperoxides, particle-induced oxidative potential (OP), and cytotoxicity were characterized in view of the atmospheric fate of the anisole precursor. Anisole SOA yields ranged between 0.12 and 0.35, depending on the reaction pathways and aging degrees. Chemical analysis of the SOA suggests that cleavage of the benzene ring is the main reaction channel in the photooxidation of anisole to produce low-volatility, highly oxygenated small molecules. Fresh anisole SOA from OH• photooxidation are more light-absorbing and have higher OP and organoperoxide content. The high correlation between SOA OP and organoperoxide content decreases exponentially with the degree of OH• aging. However, the contribution of organoperoxides to OP is minor (<4%), suggesting that other, non-peroxide oxidizers play a central role in anisole SOA OP. The particle-induced OP and particulate organoperoxides yield both reach a maximum value after ∼2 days' of photooxidation, implicating the potential long impact of anisole during atmospheric transport. NO_x-involved photooxidation and nighttime NO₃• reactions facilitate organic nitrate formation and enhance particle light absorption. High NO_x levels suppress anisole SOA formation and organoperoxides yield in photooxidation, with decreased aerosol OP and cellular oxidative stress. In contrast, nighttime aging significantly increases the SOA toxicity and reactive oxygen species (ROS) generation in lung cells. These dynamic properties and the toxicity of anisole SOA advocate consideration of the complicated and consecutive aging processes in depicting the fate of VOCs and assessing the related effects in the atmosphere.

Secondary organic aerosol formation at an urban background site on the coastline of South China: Precursors and aging processes

Understanding the formation mechanisms of secondary organic aerosols (SOA) is an arduous task in atmospheric chemistry. In November 2018, a sampling campaign was conducted at an urban background site in Hong Kong for characterization of secondary air pollution. A high-resolution time-of-flight aerosol mass spectrometer was used to monitor the compositions of non-refractory submicron particulate matters (NR-PM₁), and multiple online instruments provided us with comprehensive auxiliary data. Organic aerosol (OA) constituted the largest fraction (43.8%) of NR-PM₁, and 86.5% of the organics was contributed by the oxygenated OA (OOA, secondary components). Formation mechanisms of a dominant and more variable component of the less-oxidized OOA (labelled as LO-OOA1 in this study) and the more-oxidized OOA (MO-OOA) were explored. Based on the multilinear regression with molecular markers of OA (e.g., hydroxybenzonic acids and 2,3-dihydroxy-4-oxopentanoic acid), we presumed that anthropogenic organic compounds, especially aromatics, were the most likely precursors of LO-OOA1. MO-OOA correlated well with odd oxygen (O_x), and its concentration responded positively to the increase of liquid water content (LWC) in NR-PM₁, indicating that the formation of MO-OOA involved photochemical oxidation and aqueous processes. It exhibited the best correlation with malic acid which can be formed through the oxidation of various precursors. Moreover, it was plausible that LO-OOA1 was further oxidized to MO-OOA through aqueous processes, as indicated by the consistent diurnal variations of MO-OOA to LO-OOA1 ratio and LWC. This study highlights the important roles of anthropogenic emissions and aqueous processes in SOA formation in coastal areas downwind of cities.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO₄^2- on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO₄^2- in simulated AMD environments, and the effects of SO₄^2- concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO₄^2- could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO₄^2-, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite-As(III) complex along with the formation of reactive oxygen species in the presence of O₂. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO₄^2- concentration. In the pH range of 2.0-4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO₄^2-. This study reveals how SO₄^2- promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

Atmospheric pollutants response to the emission reduction and meteorology during the COVID-19 lockdown in the north of Africa (Morocco)

Climate and air quality change due to COVID-19 lockdown (LCD) are extremely concerned subjects of several research recently. The contribution of meteorological factors and emission reduction to air pollution change over the north of Morocco has been investigated in this study using the framework generalized additive models, that have been proved to be a robust technique for the environmental data sets, focusing on main atmospheric pollutants in the region including ozone (O₃), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), particulate matter (PM_2.5 and PM₁₀), secondary inorganic aerosols (SIA), nom-methane volatile organic compounds and carbon monoxide (CO) from the regional air pollution dataset of the Copernicus Atmosphere Monitoring Service. Our results, indicate that secondary air pollutants (PM_2.5, PM₁₀ and O₃) are more influenced by metrological factors and the other air pollutants reported by this study (NO₂ and SO₂₎. We show a negative effect for PBHL, total precipitation and NW10M on PM (PM_2.5 and PM₁₀ ), this meteorological parameters contribute to decrease in PM_2.5 by 9, 2 and 9% respectively, before LCD and 8, 1 and 5% respectively during LCD. However, a positive marginal effect was found for SAT, Irradiance and RH that contribute to increase PM_2.5 by 9, 12 and 18% respectively, before LCD and 17, 54 and 34% respectively during LCD. We found also that meteorological factors contribute to O₃, PM_2.5, PM₁₀ and SIA average mass concentration by 22, 5, 3 and 34% before LCD and by 28, 19, 5 and 42% during LCD respectively. The increase in meteorological factors marginal effect during LCD shows the contribution of photochemical oxidation to air pollution due to increase in atmospheric oxidant (O₃ and OH radical) during LCD, which can explain the response of PM to emission reduction. This study indicates that PM (PM_2.5, PM₁₀) has more controlled by SO₂ due to the formation of sulfate particles especially under high oxidants level. The positive correlation between westward wind at 10 m (WW10M), Northward Wind at 10 m (NW10M) and PM indicates the implication of sea salt particles transported from Mediterranean Sea and Atlantic Ocean. The Ozone mass concentration shows a positive trend with Irradiance, Total and SAT during LCD; because temperature and irradiance enhance tropospheric ozone formation via photochemical reaction.This study shows the contribution of atmospheric oxidation capacity to air pollution change.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00477-022-02224-z.

The underappreciated role of monocarbonyl-dicarbonyl interconversion in secondary organic aerosol formation during photochemical oxidation of m-xylene

Photochemical oxidation (including photolysis and OH-initiated reactions) of aromatic hydrocarbon produces carbonyls, which are involved in the formation of secondary organic aerosols (SOA). However, the mechanism of this process remains incompletely understood. Herein, the monocarbonyl-dicarbonyl interconversion and its role in SOA production were investigated via a series of photochemical oxidation experiments for m-xylene and representative carbonyls. The results showed that SOA mass concentration peaked at 113.5 ± 3.5 μg m^-3 after m-xylene oxidation for 60 min and then decreased. Change in the main oxidation products from dicarbonyl (e.g., glyoxal, methylglyoxal) to monocarbonyl (e.g., formaldehyde) was responsible for this decrease. The photolysis of methylglyoxal or glyoxal produced formaldehyde, favoring SOA formation, while photopolymerization of formaldehyde to glyoxal decreased SOA production. The presence of ·OH altered the balance of photolysis interconversion, resulting in greater production of formaldehyde and SOA from glyoxal than methylglyoxal. Both photolysis and OH-initiated transformations of glyoxal to formaldehyde were suppressed by methylglyoxal, while glyoxal accelerated the reaction of ·OH with methylglyoxal to generate products which reversibly converted to glyoxal and methylglyoxal. These interconversion reactions reduced SOA production. The present study provides a new research perspective for the contribution mechanism of carbonyls in SOA formation and the findings are also helpful to efficiently evaluate the atmospheric fate of aromatic hydrocarbons.

Characterization of dicarboxylic acids, oxoacids, and α-dicarbonyls in PM2.5 within the urban boundary layer in southern China: Sources and formation pathways

Low-molecular-weight dicarboxylic acids, which are important components of secondary organic aerosols, have been extensively studied in recent years. Many studies have focused on ground-level observations and literature reports on the vertical distribution of the organic aerosols within the urban boundary layer are limited. In this study, the vertical profiles of dicarboxylic acids and related organic compounds (DCRCs) in PM_2.5 were investigated at altitudinal levels (ground level and 488 m above the ground level) at the Canton Tower in Guangzhou, southern China, to elucidate their primary sources and secondary formation processes. The concentrations of DCRCs at ground level were generally higher than those at 488 m. Oxalic acid (C₂) was the most abundant species, followed by succinic acid (C₄) and malonic acid (C₃) at both heights. The higher ratio of DCRCs-bound carbon to organic carbon (i.e., DCRCs-C/OC) at 488 m (4.8 ± 1.2%) relative to that at ground level (2.7 ± 0.5%) indicated a higher degree of aerosol aging at 488 m. The abundance of C₂ was increased and the conversion of C₄ to C₃ was enhanced due to the photochemical oxidation of its homologues during long-range transport periods. The increase in C₂ was associated with in-cloud processes during pollution periods. Principal component analysis showed that DCRCs were mainly derived from atmospheric secondary processing and biomass burning was also an important source of long-chain carboxylic acids during autumn in Guangzhou. Our results illustrate that secondary processing and biomass burning play prominent roles in controlling the abundance of DCRCs. Furthermore, DCRCs are affected by air masses from regional areas, oxidation of their precursors via vertical transport and in-cloud processes.

Assessing the role of mineral particles in the atmospheric photooxidation of typical carbonyl compound

Mineral particles are ubiquitous in the atmosphere and exhibit an important effect on the photooxidation of volatile organic compounds (VOCs). However, the role of mineral particles in the photochemical oxidation mechanism of VOCs remains unclear. Hence, the photooxidation reactions of acrolein (ARL) with OH radical (OH) in the presence and absence of SiO₂ were investigated by theoretical approach. The gas-phase reaction without SiO₂ has two distinct pathways (H-abstraction and OH-addition pathways), and carbonyl-H-abstraction is the dominant pathway. In the presence of SiO₂, the reaction mechanism is changed, i.e., the dominant pathway from carbonyl-H-abstraction to OH-addition to carbonyl C-atom. The energy barrier of OH-addition to carbonyl C-atom deceases 21.33 kcal/mol when SiO₂ is added. Carbonyl H-atom of ARL is occupied by SiO₂ via hydrogen bond, and carbonyl C-atom is activated by SiO₂. Hence, the main product changes from H-abstraction product to OH-adduct in the presence of SiO₂. The OH-adduct exhibits a thermodynamic feasibility to yield HO₂ radical and carboxylic acid via the subsequent reactions with O₂, with implications for O₃ formation and surface acidity of mineral particles.

The formation and evolution of secondary organic aerosol during summer in Xi'an: Aqueous phase processing in fog-rain days

Secondary organic aerosol (SOA) is an important contributor to organic aerosol (OA), however, the model simulations of SOA concentrations and oxidation states remain significant uncertainties because of inadequate cognition of its formation and aging chemistry. In this study, SOA formation and evolution processes during summer in Xi'an were investigated, based on high-resolution online measurements of non-refractory PM_2.5 (NR-PM_2.5) species and OA source apportionment using positive matrix factorization. The results showed that the total SOA, including less oxidized-oxygenated OA (LO-OOA), more oxidized-oxygenated OA (MO-OOA), and aqueous-phase-processed oxygenated OA (aq-OOA), on average constituted 69% of OA, and 43% of NR-PM_2.5, suggesting the high atmospheric oxidation capacity and the dominance of SOA during summer in Xi'an. Photochemical oxidation processes dominated the summertime SOA formation both during non-fog-rain days and fog-rain days, which were responsible for the formation of both LO-OOA and MO-OOA. Consistently, LO-OOA and MO-OOA in total contributed 59% to OA during non-fog-rain days and 56% to OA during fog-rain days, respectively. On the contrary, aq-OOA was mainly observed during fog-rain days, which increased dramatically from 2% of OA during non-fog-rain days to 19% of OA during fog-rain days with the mass concentration increasing accordingly from 0.3 μg m^-3 to 2.5 μg m^-3. Episodic analyses further highlighted the persistently high RH period with high aerosol liquid water content (ALWC) was the driving factor of aq-OOA formation, and high O_x condition could further enhance its formation. Meanwhile, air masses from east and southeast were much favorable for the formation of long-time fog-rain days, which facilitated aq-OOA production during summer in Xi'an.

Understanding the origin of carbonaceous aerosols during periods of extensive biomass burning in northern India

Post-harvest crop residue burning is extensively practiced in North India, which results in enhanced particulate matter (PM) concentrations. This study explores the PM_2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) emissions during various time periods (pre-monsoon, monsoon, and post-monsoon) over the biomass burning source region in Beas, Punjab. The PM_2.5 concentrations during the pre-monsoon period (106-458 μg m^-3) and the post-monsoon period (184-342 μg m^-3) were similar but much higher than concentrations during the monsoon season (23-95 μg m^-3) due to enhanced wet deposition. However, the carbonaceous aerosol fraction in PM_2.5 was nearly double in the post-monsoon season (∼27%) than the pre-monsoon period (∼15%). A higher contribution of secondary organic carbon (SOC) observed during the pre-monsoon season can be attributed to enhanced photochemical activity in dry conditions. Stable carbon isotope ratio (δ¹³C value) of ambient PM allowed elucidation of contributing sources. δ¹³C_TC correlation with SOC during post-monsoon and pre-monsoon periods suggests significant influence of secondary formation processes during both time periods. The concentrations of carbon fractions in sampled sources and aerosols suggests contribution of biofuels, resulting in enhanced PM concentration at this location. δ¹³C_TC values of pre- and post-monsoon samples show dominance of freshly emitted aerosols from local sources. Impact of biomass and biofuel combustion was also confirmed by biomass burning K⁺_BB tracer, indicating that major agriculture residue burning occurred primarily during nighttime. C₃ plant derived aerosols dominated at the sampling location during the entire sampling duration and contributed significantly during the pre-monsoon season. Whereas, both fossil fuel and C₃ plant combustion contributed to the total mass of carbonaceous aerosols during the post-monsoon and monsoon seasons.

Iron(III)-induced photooxidation of arsenite in the presence of carboxylic acids and phenols as model compounds of natural organic matter

Iron species have essential influence on the environmental/geochemical behaviors of arsenic species in water and soil. Colloidal ferric hydroxide (CFH) induces photooxidation of arsenite (As(III)) to arsenate (As(V)) in water at neutral pH through surface complexation and ligand-to-metal charge transfer (LMCT). However, the effect of the co-existing natural organic matter (NOM) on the complexation-photolysis in this process has remained unclear. In the present work, the photooxidation of As(III) induced by CFH was investigated in the presence of various carboxylic acids and polyphenols as simple model compounds of NOM. Two different light sources of ultraviolet A (UVA) (λ_max = 365 nm) and ultraviolet B (UVB) (λ_max = 313 nm) were used for photooxidation treatment of the experimental ternary system and the control binary system respectively. The obtained results demonstrated that all investigated NOM inhibited the photooxidation of As(III) in the As(III)/CFH system at pH 7. Moreover, the correlation analysis between the pseudo-first order rate constant k_obs and various property parameters of NOM showed that the stable constant for the complexation between Fe(III) and NOM (logK_Fe-NOM) as well as the molecular weight of NOM and the percentages of total acidity of NOM exhibited significant correlations. A simple quantitative structure-activity relationship (QSAR) model was established between k_obs and these three parameters utilizing a multiple linear regression method, which can be employed to estimate the photooxidation efficiency of As(III) in the presence of ferric iron and NOM. Thus, the present work contributes to the understanding of the environmental interactions between NOM and iron.

Formation and transformation of manganese(III) intermediates in the photochemical generation of manganese(IV) oxide minerals

As important natural oxidants and adsorbents, manganese (Mn) oxide minerals affect the speciation, bioavailability and fate of pollutants and nutrient elements. It was found that birnessite-type Mn(IV) oxide minerals can be formed in the presence of NO₃^- and solar irradiation. However, the photochemical formation and transformation processes from Mn²⁺ to Mn(IV) oxide minerals remain unclear. In this work, the Mn(IV) oxide minerals were confirmed to be photochemically formed mainly due to the disproportionation of Mn(III) intermediates generated from the oxidation of Mn²⁺ in the presence of NO₃^- under UV light irradiation. The oxidation rate of Mn²⁺ to Mn(IV) oxide minerals decreased with increasing initial Mn²⁺ concentration due to the lower disproportionation rate. The increase in NO₃^- concentration, pH and temperature promoted Mn²⁺ photochemical oxidation. The photochemical formation rate of Mn(IV) oxide minerals increased with increasing ligand concentrations at low ligand concentrations. Ligands affected the formation of Mn(IV) oxide minerals by promoting the formation and reducing the reactivity of Mn(III) intermediates. Overall, this work reveals the important role of Mn(III) intermediates in the formation of natural Mn oxide minerals.

Problems of conventional disinfection and new sterilization methods for antibiotic resistance control

The problem of bacterial antibiotic resistance has attracted considerable research attention, and the effects of water treatment on antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) are being increasingly investigated. As an indispensable part of the water treatment process, disinfection plays an important role in controlling antibiotic resistance. At present, there were many studies on the effects of conventional and new sterilization methods on ARB and ARGs. However, there is a lack of literature relating to the limitations of conventional methods and analysis of new techniques. Therefore, this review focuses on analyzing the deficiencies of conventional disinfection and the development of new methods for antibiotic resistance control to guide future research. Firstly, we analyzed the effects and drawbacks of conventional disinfection methods, such as chlorine (Cl), ultraviolet (UV) and ozone on antibiotic resistance control. Secondly, we discuss the research progress and shortcomings of new sterilization methods in antibiotic resistance. Finally, we propose suggestions for future research directions. There is an urgent need for new effective and low-cost sterilization methods. Disinfection via UV and chlorine in combination, UV/chlorine showed greater potential for controlling ARGs.

Environmental performance of an industrial biofilter: Relationship between photochemical oxidation and odorous impacts

Biological techniques are widely used to treat gaseous streams derived from waste treatment plants. The generation of volatile organic compounds (VOCs) is one of the principal pollution sources in composting facilities from which nuisance odours are released. In addition, the generation of photochemical smog with other gases such as NO_X can produce ozone at ground level due to their photochemical ozone creation potential (POCP). In this work, the performance of an industrial biofilter was evaluated from an environmental point of view. Specifically, this study evaluated the potential impact in terms of photochemical oxidation and odour emission derived from composting in a vessel under four different aeration conditions. Gas chromatography-time-of-flight mass spectrometry (GC-TOFMS) was used to perform the chemical characterisation of the gaseous streams, while dynamic olfactometry was used to carry out the sensorial analysis. A total of 95 compounds belonging to 12 different families of VOCs were selected. Principal component analysis revealed the influence of each VOC family on each impact category and explained 88% of the total variance. Multivariate regression was used to study the correlation between photochemical oxidation and odour impact, which has never been reported before. The correlations obtained (r ≥ 0.97) evidenced the direct relationship between these two impacts. Photochemical oxidation and odour emission were proven to be important environmental impacts derived from composting facilities, whose abatement might be carried out by biofiltration systems.

Electrospray Photochemical Oxidation of Proteins

Photooxidation of peptides and proteins by pulsed ultraviolet laser irradiation of an electrospray in the ion source of a mass spectrometer was demonstrated. A 193-nm excimer laser at 1.5-mJ pulse energy was focused with a cylindrical lens at the exit of a nanoelectrospray capillary and ions were sampled into a quadrupole time-of-flight mass spectrometer. A solution containing a peptide or protein and hydrogen peroxide was infused into the spray at a flow rate of 1 μL/min using a syringe pump. The laser creates OH radicals directly in the spray which modify biomolecules within the spray droplet. These results indicate that photochemical oxidation of proteins can be initiated directly within electrospray droplets and detected by mass spectrometry.

Characteristics and secondary formation of water-soluble organic acids in PM1, PM2.5 and PM10 in Beijing during haze episodes

Water-soluble organic acids are widely involved in various atmospheric physicochemical processes and appear as an important fraction of atmospheric aerosols. Nineteen water-soluble organic acids in 12-h PM₁, PM_2.5 and PM₁₀ samples collected in urban Beijing during haze episodes in winter and spring of 2017 were identified to investigate their characteristics and secondary formation mechanism. The molecular distributions of water-soluble organic acids as well as the high ratio of phthalic acid (Ph)/azelaic acid (C₉) indicated severe aromatic secondary organic aerosol pollution during the haze episodes, especially in winter. The diurnal patterns, size distributions, and concentration ratios of specific organic acids were investigated to reveal the pollution characteristics and possible sources of major organic acids in particulate matter in Beijing during haze events. Multiple linear regression was used to tentatively quantify the relative contributions of photochemical oxidation and aqueous-phase oxidation to the formation of total water-soluble organic acids in PM₁, PM_2.5 and PM₁₀ during haze episodes. The formation mechanism of sulfate and nitrate was also investigated for comparison. Different from the secondary formation of sulfate, the secondary formation of water-soluble organic acids showed enhanced contribution of gas-phase photochemical oxidation though the aqueous-phase oxidation was the dominant process. CAPSULE: Molecular analyses of organic acids in PM₁, PM_2.5 and PM₁₀ in Beijing during haze periods revealed their pollution characteristics, possible sources and formation mechanism.

Seasonal variation and size distribution of biogenic secondary organic aerosols at urban and continental background sites of China

Size-resolved biogenic secondary organic aerosols (BSOA) derived from isoprene and monoterpene photooxidation in Qinghai Lake, Tibetan Plateau (a continental background site) and five cities of China were measured using gas chromatography/mass spectrometry (GC/MS). Concentrations of the determined BSOA are higher in the cities than in the background and are also higher in summer than in winter. Moreover, strong positive correlations (R²=0.44-0.90) between BSOA and sulfate were found at the six sites, suggesting that anthropogenic pollution (i.e., sulfate) could enhance SOA formation, because sulfate provides a surface favorable for acid-catalyzed formation of BSOA. Size distribution measurements showed that most of the determined SOA tracers are enriched in the fine mode (<3.3μm) except for cis-pinic and cis-pinonic acids, both presented a comparable mass in the fine and coarse (>3.3μm) modes, respectively. Mass ratio of oxidation products derived from isoprene to those from monoterpene in the five urban regions during summer are much less than those in Qinghai Lake region. In addition, in the five urban regions relative abundances of monoterpene oxidation products to SOA are much higher than those of isoprene. Such phenomena suggest that BSOA derived from monoterpenes are more abundant than those from isoprene in Chinese urban areas.

Photooxidation of arsenic(III) to arsenic(V) on the surface of kaolinite clay

As one of the most toxic heavy metals, the oxidation of inorganic arsenic has drawn great attention among environmental scientists. However, little has been reported on the solar photochemical behavior of arsenic species on top-soil. In the present work, the influencing factors (pH, relative humidity (RH), humic acid (HA), trisodium citrate, and additional iron ions) and the contributions of reactive oxygen species (ROS, mainly HO and HO2/O2(-)) to photooxidation of As(III) to As(V) on kaolinite surfaces under UV irradiation (λ=365nm) were investigated. Results showed that lower pH facilitated photooxidation, and the photooxidation efficiency increased with the increase of RH and trisodium citrate. Promotion or inhibition of As(III) photooxidation by HA was observed at low or high dosages, respectively. Additional iron ions greatly promoted the photooxidation, but excessive amounts of Fe(2+) competed with As(III) for oxidation by ROS. Experiments on scavengers indicated that the HO radical was the predominant oxidant in this system. Experiments on actual soil surfaces proved the occurrence of As(III) photooxidation in real topsoil. This work demonstrates that the photooxidation process of As(III) on the soil surface should be taken into account when studying the fate of arsenic in natural soil newly polluted with acidic wastewater containing As(III).

Roles of SO2 oxidation in new particle formation events

The oxidation of SO2 is commonly regarded as a major driver for new particle formation (NPF) in the atmosphere. In this study, we explored the connection between measured mixing ratio of SO2 and observed long-term (duration>3 hr) and short-term (duration<1.5 hr) NPF events at a semi-urban site in Toronto. Apparent NPF rates (J30) showed a moderate correlation with the concentration of sulfuric acid ([H2SO4]) calculated from the measured mixing ratio of SO2 in long-term NPF events and some short-term NPF events (Category I) (R2=0.66). The exponent in the fitting line of J30~[H2SO4]n in these events was 1.6. It was also found that SO2 mixing ratios varied a lot during long-term NPF events, leading to a significant variation of new particle counts. In the SO2-unexplained short-term NPF events (Category II), analysis showed that new particles were formed aloft and then mixed down to the ground level. Further calculation results showed that sulfuric acid oxidized from SO2 probably made a negligible contribution to the growth of >10 nm new particles.

Effect of nitrate ions on the efficiency of sonophotochemical phenol degradation

A sonophotochemical oxidation process has been used for the treatment of an aqueous solution of phenol. The aim of this work is to evaluate the effect of nitrate ions on hydroxyl radical production and on phenol oxidation. It has been demonstrated that ultrasound can produce NOx (nitrate and nitrite), with a production rate of 2.2 μM min(-1). The photolysis of nitrate can significantly improve the hydroxyl radical production. The apparent rate constant for hydroxyl radical production increased from 0.0015 min(-1) to 0.0073 min(-1) while increasing initial nitrate concentration from 0 to 0.5mM. The concentration of hydroxyl radical was directly proportional to the initial nitrate concentration. Using US/UV process, the apparent reaction rate constant of phenol degradation in the presence of nitrate reached 0.020 min(-1), which was relatively lower than the value obtained (0.027 min(-1)) in the absence of nitrate. It appeared that, nitrate ions can inhibit the sonochemical degradation of organic compounds such as phenol.