Atmospheric Oxidation Capacity Research Articles

Physical activity alleviated associations of oxidation capacity of the atmosphere with platelet-based inflammatory indicators: findings from the Henan Rural Cohort Study.

Background: several adverse effects of ozone (O3) and nitrogen dioxide (NO2) are assessed using combined oxidant capacity (Ox) and redox-weighted oxidant capacity (Owtx) as surrogates. However, the associations of oxidant capacity (Ox and Owtx) with platelet-based inflammatory indicators and the potential modifying role of physical activity (PA) remain unclear. Methods: 31 318 participants were selected from the baseline survey of the Henan Rural Cohort Study. The Ox and Owtx were calculated based on O3 and NO2. The International Physical Activity Questionnaire was used to evaluate PA. Platelet-based inflammatory indicators were obtained from the data of physical examination. Generalized linear models were applied to explore associations between atmospheric oxidation capacity indicators (O3, Ox, Owtx, and NO2) and platelet-based inflammatory indicators and whether PA modified these associations. Results: O3, Ox, and Owtx were positively associated with platelet-based inflammatory indicators (PCT, PLT, PLR, SII, MLR and SIRI). The estimated β values and 95% confidence intervals (CIs) for PLT in response to a 5 μg m-3 increment in O3, Ox, and Owtx were 19.267 × 109 L-1 (95% CI: 17.493, 21.041 × 109 L-1), 6.226 × 109 L-1 (95% CI: 5.502, 6.950 × 109 L-1), and 14.664 × 109 L-1 (95% CI: 13.101, 16.227 × 109 L-1), respectively. The corresponding values for SIRI were 0.134 × 109 L-1 (95% CI: 0.111, 0.156 × 109 L-1), 0.064 × 109 L-1 (95% CI: 0.055, 0.073 × 109 L-1), and 0.135 × 109 L-1 (95% CI: 0.115, 0.155 × 109 L-1). And similar results were observed for NO2. Furthermore, we observed positive associations of O3, Ox, Owtx, and NO2 with platelet-based inflammatory indicators attenuated by increased PA levels. Conclusions: exposure to O3, Ox, Owtx, and NO2 was positively associated with platelet-based inflammatory indicators, and these associations were modified by PA. The findings suggested that a healthy lifestyle of PA might be an effective measure against early adverse effects of air pollution.

A Review of Laboratory Studies on the Heterogeneous Chemistry of NO2: Mechanisms and Uptake Kinetics.

NO2 is a significant primary atmospheric pollutant that plays a key role in atmospheric chemistry. It serves as a crucial precursor to photochemical smog, acid rain, and secondary particulate matter and is instrumental in determining the atmospheric oxidation capacity. In this review, we focus on the heterogeneous chemistry of NO2, which has been demonstrated to significantly influence the sources and sinks of various nitrogen-containing species through field measurements and model simulations. We provide a comprehensive summary of laboratory studies investigating the reaction mechanisms and uptake kinetics of NO2 in heterogeneous reactions. NO2 can undergo disproportionation reactions on atmospheric particles. For instance, it may hydrolyze on wetted surfaces to form HONO and HNO3, produce nitrate and NO on mineral dust, or generate nitrate and NOCl on sea salt. Additionally, NO2 can be reduced to HONO on soot and Fe-bearing minerals or photocatalytically reduced to HONO and NO on photosensitive components. Furthermore, NO2 can be photo-oxidized to nitrate or N2O5 on illuminated TiO2. In addition, the synergistic effect of the heterogeneous reactions between NO2 and SO2 is discussed. The uptake coefficients of NO2 on typical particles and the factors influencing these coefficients are also summarized. Finally, based on the current insufficient understanding of heterogeneous reactions of NO2, we propose prospects for future research.

Quantification and source apportionment of atmospheric oxidation capacity in urban atmosphere by considering reactive chlorine species and photochemical loss of VOCs.

Atmospheric oxidation capacity (AOC) reflects the potential of the atmosphere in converting primary pollutants into secondary aerosols and ozone (O3). In this study, the AOC at an urban supersite in Wuhan, a megacity in central China, was quantified by considering the reactions of volatile organic compounds (VOCs) and carbon monoxide (CO) with atmospheric oxidants (OH, NO3, O3, and Cl). Photochemical loss of total VOCs (13.7-23.7%) during transport was accounted for by monitoring the concentration ratio of o-xylene and ethylbenzene, a VOC pair with diverse reaction rate constants with OH. AOC would be underestimated by 9.0-25.2% in the 4 seasons if being estimated from the observed VOCs instead of photochemical-loss corrected VOCs. The atmospheric oxidants were measured or indirectly estimated using well-established parameterizations. In particular, hourly reactive chlorine species were measured using an iodide-based chemical ionization mass spectrometer (I-CIMS). Cl radical concentration was calculated by assuming a steady-state between the production from reactive chlorine species and the removal by O3 and VOCs. The result showed that AOC would be underestimated by 14.5% and 1.9% in winter and spring if Cl radicals were neglected. Based on the above quantification, the composition of AOC was further apportioned to atmospheric oxidants, VOC categories, as well as the sources of VOC and CO. In winter, CNG combustion exhaust, electronics industry, and secondary formation were critical for regulating the AOC. In spring, secondary formation was the most important AOC source, followed by electronics industry and biogenic sources.

Decadal changes in summer aerosol composition and secondary aerosol formation mechanisms in Beijing

Abstract Aerosol chemistry in China has undergone significant transformation due to stringent emission control measures, leading to great shifts in aerosol composition and formation mechanisms. This study investigates the summer chemical evolution of aerosol species in Beijing over the past decade based on two summertime measurements using aerosol chemical speciation monitors. The results reveal a substantial decrease in fine particulate matter concentrations by 72.7% in summer over the past decade, particularly primary species that dropped by 86.3%–95.1%. However, this improvement in particulate matter was accompanied by a worsening of ozone pollution between 2011 and 2022. In contrast, secondary components such as sulfate and secondary organic aerosol (SOA) exhibited significant increases in their contributions, rising from 18.2%–25.5% to 21.4%–41%. The varying responses of aerosol species to emission reductions are closely tie to changes in emission sources, aerosol chemistry, and meteorology. By decoupling the influence of meteorology through machine learning, our analysis highlights the crucial role of emission reductions in improving air quality, though with different impacts on aerosol chemistry. The dominant formation mechanisms of secondary components varied between the two summers, likely influenced by shifts in aerosol liquid water content and atmospheric oxidation capacity due to NOx reductions. Compared to the summer of 2011, the formation of sulfate and SOA in summer 2022 was primarily driven by photochemical processes related to ozone, with less impacts from aqueous-phase formation, while nitrate was predominantly formed via N2O5 heterogeneous hydrolysis. Considering the complex nature of secondary aerosol formation, future summer pollution control strategies should prioritize stricter collaborative regulation of precursors for both secondary aerosol and ozone.

Increasing Contribution of Chlorine Chemistry to Wintertime Ozone Formation Promoted by Enhanced Nitrogen Chemistry.

Chlorine (Cl) radicals strongly affect atmospheric oxidation and the fate of pollutants. Despite several observations, the potential impacts of nitrogen chemistry associated with NO2 on Cl chemistry are poorly understood. Here, we provided direct field evidence that the nitrogen chemistry enhancements triggered by the increased NO2 drove daytime nitrate (NO3-) photolysis and nighttime NO3-N2O5 reactions, significantly promoting the increases in the concentrations of ClNO2 and Cl2 after the Chinese Spring Festival. The enhancement in the Cl chemistry facilitated the elevations of both O3 and atmospheric oxidation capacity during the winter daytime. Our findings highlighted the importance of nitrogen chemistry induced by the increased NO2 in enhanced Cl chemistry.

The atmospheric oxidizing capacity in China – Part 2: Sensitivity to emissions of primary pollutants

Abstract. Despite substantial reductions in anthropogenic emissions, ozone (O3) pollution remains a severe environmental problem in urban China. These reductions affect ozone formation by altering levels of O3 precursors, intermediates, and the oxidation capacity of the atmosphere. However, the underlying mechanisms driving O3 changes are still not fully understood. Here, we employ a regional chemical transport model to quantify ozone changes due to a specified emission reduction (50 %) for winter and summer conditions in 2018. Our results indicate that reductions in nitrogen oxide (NOx) emissions increase surface O3 concentrations by 15 %–33 % on average across China in winter and by up to 17 % in volatile organic compound (VOC)-limited areas during summer. These ozone increases are associated with a reduced NOx titration effect and higher levels of OH radicals. Reducing NOx emissions significantly decreases the concentration of particulate nitrate, which enhances ozone formation through increased HO2 radical levels due to reduced aerosol uptake and diminished aerosol extinction. Additionally, an enhanced atmospheric oxidative capacity, driven by larger contributions from the photolysis of oxidized VOCs (OVOCs) and OH-related reactions, also favors urban ozone formation. With additional reductions in anthropogenic VOC emissions, increases in summertime ozone (VOC-limited areas) can be offset by reduced production of radicals from VOC oxidations. To effectively mitigate ozone pollution, a simultaneous reduction in the emission of NOx and specific VOC species should be applied, especially regarding alkenes, aromatics, and unsaturated OVOCs, including methanol and ethanol.

Black Carbon Radiative Impacts on Surface Atmospheric Oxidants in China with WRF-Chem Simulation

Black carbon (BC) changes the radiative flux in the atmosphere by absorbing solar radiation, influencing photochemistry in the troposphere. To evaluate the seasonal direct radiative effects (DREs) of BC and its influence on surface atmospheric oxidants in China, the WRF-Chem model was utilized in this study. The simulation results suggested that the average annual mean values of the clear-sky DREs of BC at the top of the atmosphere, in the atmosphere and at the surface over China are +2.61, +6.27 and −3.66 W m−2, respectively. Corresponding to the seasonal variations of BC concentrations, the relative changes of the mean surface photolysis rates (J[O1D], J[NO2] and J[HCHO]) in the four seasons range between −3.47% and −6.18% after turning off the BC absorption, which further leads to relative changes from −4.27% to −6.82%, −2.14% to −4.40% and −0.47% to −2.73% in hydroxyl (OH) radicals, hydroperoxyl (HO2) radicals and ozone (O3), respectively. However, different from the relative changes, the absolute changes in OH and HO2 radicals and O3 after turning off BC absorption show discrepancies among the different seasons. In the North China Plain (NCP) region, O3 concentration decreases by 1.79 ppb in the summer, which is higher than the magnitudes of 0.24–0.88 ppb in the other seasons. In southern China, the concentrations of OH and HO2 radicals reach the maximum decreases in the spring and autumn, followed by those in the summer and winter, which is due to the enhancement of solar radiation and the summer monsoon. Thus, BC inhibits the formation of atmospheric oxidants, which further weakens the atmospheric oxidative capacity.

ACEIC: a comprehensive anthropogenic chlorine emission inventory for China

Abstract. Chlorine species play a crucial role as precursors to Cl radicals, which can significantly impact the atmospheric oxidation capacity and influence the levels of trace gases related to climate and air quality. Several studies have established a chlorine emission inventory in China in recent years, but the emission remains uncertain and requires further investigation. The Anthropogenic Chlorine Emission Inventory for China (ACEIC) was the first chlorine emission inventory for China based on local data developed in our previous study, which only includes the emissions from coal combustion and waste incineration. In this study, we updated this inventory to include data for a more recent year (2019) and expanded the range of species considered (HCl, fine particulate Cl−, Cl2, and hypochlorous acid (HOCl)) and the number of anthropogenic sources (41 specific sources). Compared with previous studies, this updated inventory considered more anthropogenic sources, used more localized emission factors, and adopted more refined estimation methods. The total emissions of HCl, fine particulate Cl−, Cl2, and HOCl in mainland China for the year 2019 were estimated to be 361 (−18 % to 27 %), 174 (−27 % to 59 %), 18 (−10 % to 15 %), and 79 (−12 % to 18 %) Gg, respectively. To facilitate analysis, we aggregated the chlorine emissions from various sources into five economic sectors: power, industry, residential, agriculture, and biomass burning. HCl emissions were primarily derived from the industry (43 %), biomass burning (38 %), and residential (13 %) sectors. The biomass burning and industry sectors accounted for 74 % and 19 % of the fine particulate Cl− emissions, respectively. Residential and industry sectors contributed 61 % and 29 % of the total Cl2 emissions. HOCl emissions were predominantly from the residential sector, constituting 90 % of the total emissions. Notably, the usage of chlorine-containing disinfectants was identified as the most significant source of Cl2 and HOCl emissions in the residential sector. Geographically, regions with high HCl and fine particulate Cl− emissions were found in the North China Plain, northeastern China, central China, and the Yangtze River Delta, whereas the Pearl River Delta, Yangtze River Delta, and Beijing–Tianjin–Hebei regions exhibited elevated levels of Cl2 and HOCl emissions. Regarding monthly variation, emissions of HCl and fine particulate Cl− were higher during early spring (February to April) and winter (December to January) due to intensified agricultural activities, while Cl2 and HOCl emissions were higher in the summer months due to increased demand for water disinfection. We incorporated this emission inventory into the chemical transport model and found the simulated concentrations of chlorine species agreed reasonably well with the observations, which suggested the relatively faithful estimations of their emissions. This updated inventory contributes to a better understanding of anthropogenic sources of chlorine species and can aid in the formulation of emission control strategies to mitigate secondary pollution in China.

Troposphere–stratosphere-integrated bromine monoxide (BrO) profile retrieval over the central Pacific Ocean

Abstract. Bromine monoxide (BrO) is relevant to atmospheric oxidative capacity, affecting the lifetime of greenhouse gases (i.e., methane, dimethylsulfide) and mercury oxidation. However, measurements of BrO radical vertical profiles are rare, and BrO is highly variable. As a result, the few available aircraft observations in different regions of the atmosphere are not easily reconciled. Autonomous multi-axis differential optical absorption spectroscopy (MAX-DOAS) instruments placed at remote mountaintop observatories (MT-DOAS) present a cost-effective alternative to aircraft, with the potential to probe the climate-relevant yet understudied free troposphere more routinely. Here, we describe an innovative full-atmosphere BrO and formaldehyde (HCHO) profile retrieval algorithm using MT-DOAS measurements at Mauna Loa Observatory (MLO – 19.536° N, 155.577° W; 3401 m a.s.l.). The retrieval is based on time-dependent optimal estimation and simultaneously inverts 190+ individual BrO (and formaldehyde, HCHO) SCDs (slant column densities; SCD = dSCD + SCDRef) from solar stray light spectra measured in the zenith and off-axis geometries at high and low solar zenith angles (92° > SZA > 30°) to derive BrO concentration profiles from 1.9 to 35 km with 7.5 degrees of freedom (DoFs). Two case study days are characterized by the absence (26 April 2017, base case) and presence of a Rossby-wave-breaking double tropopause (29 April 2017, RW-DT case). Stratospheric-BrO vertical columns are nearly identical on both days (VCD = (1.5 ± 0.2) × 1013 molec. cm−2), and the stratospheric-BrO profile peaks at a lower altitude during the RW-DT (1.6–2.0 DoFs). Tropospheric-BrO VCDs increase from (0.70 ± 0.14) × 1013 molec. cm−2 (base case) to (1.00 ± 0.14) × 1013 molec. cm−2 (RW-DT) owing to a 3-fold increase in BrO in the upper troposphere (1.7–1.9 DoFs). BrO at MLO increases from (0.23 ± 0.03) pptv (base case) to (0.46 ± 0.03) pptv (RW-DT) and is characterized by an added time resolution (∼ 3.8 DoFs). Up to (0.9 ± 0.1) pptv BrO is observed above MLO in the lower free troposphere in the absence of the double tropopause. We validate the retrieval using aircraft BrO profiles and in situ HCHO measurements aboard the NSF/NCAR GV aircraft above MLO (11 January 2014) that establish BrO peaks around 2.4 pptv above 13 km in the upper troposphere–lower stratosphere (UTLS) during a similar RW-DT event (0.83 × 1013 molec. cm2 tropospheric-BrO VCD above 2 km). The tropospheric-BrO profile measured using MT-DOAS (RW-DT case) and using the aircraft agree well (after averaging-kernel smoothing). Furthermore, these tropospheric-BrO profiles over the central Pacific Ocean are found to closely resemble those over the eastern Pacific Ocean (2–14 km) and are in contrast to those over the western Pacific Ocean, where a C-shaped tropospheric-BrO profile shape has been observed.

Implications of 1.5 K climate warming on warm-season ozone exposure and atmospheric oxidation capacity in China

Surface ozone (O3) poses significant threats to public health, agricultural crops, and plants in natural ecosystems. Global warming is likely to increase future O3 mainly by altering atmospheric photochemical reactions and enhancing biogenic volatile organic compound (BVOC) emissions. To assess the impacts of the future 1.5 K climate target on O3 concentrations and ecological O3 exposure in China, numerical simulations were conducted using the CMAQ (Community Multiscale Air Quality) model during April–October 2018. Ecological O3 exposure was estimated using six indices (i.e., M7, M24, N100, SUM60, W126, and AOT40f). The results show that the temperature rise increases the MDA8 O3 (maximum daily eight-hour average O3) concentrations by ∼3 ppb and the number of O3 exceedance days by 10–20 days in the North China Plain (NCP), Yangtze River Delta (YRD), and Sichuan Basin (SCB) regions. All O3 exposure indices show substantial increases. M24 and M7 in eastern and southern China will rise by 1–3 ppb and 2–4 ppb, respectively. N100 increases by more than 120 h in the surrounding regions of Beijing. SUM60 increases by greater than 9 ppm h−1, W126 increases by greater than 15 ppm h−1 in Shaanxi and SCB, and AOT40f increases by 6 ppm h−1 in NCP and SCB. The temperature increase also promotes atmospheric oxidation capacity (AOC) levels, with the higher AOC contributed by OH radicals in southern China but by NO3 radicals in northern China. The change in the reaction rate caused by the temperature increase has a greater influence on O3 exposure and AOC than the change in BVOC emissions.摘要地表臭氧(O₃)对公众健康, 农作物以及自然生态系统构成重大威胁. 全球变暖会增强大气光化学反应以及增加生物源挥发性有机化合物(BVOC)排放, 从而导致 O₃浓度增加. 为了评估未来 1.5 K 气候目标对中国 O₃浓度以及生态 O₃暴露的影响, 在 2018 年 4 月至 10 月期间使用 CMAQ模型进行了数值模拟. 使用六个指标(即 M7, M24, N100, SUM60, W126 和 AOT40f)估算生态 O₃暴露. 结果表明, 在华北平原,长江三角洲和四川盆地地区, 温度升高使每日最大8 小时平均 O₃浓度增加约 3 ppb, O₃超标天数增加 10–20 天. 所有 O₃暴露指标均显著增加. 中国东部和南部的 M24 和 M7 将分别增加 1–3 ppb 和 2–4 ppb. 北京周边地区的 N100 增加超过 120 小时. 陕西和四川盆地的 SUM60 增加超过 9 ppm h⁻¹, W126 增加超过 15 ppm h⁻¹, 华北平原和四川盆地的 AOT40f 增加 6 ppm h⁻¹. 温度升高还提升了大气氧化能力(AOC)水平, 在中国南部较高的 AOC 由羟基自由基贡献, 而在中国北部则由硝基自由基贡献. 由温度升高引起的反应速率变化对 O₃暴露和 AOC 的影响比 BVOC 排放增加带来的贡献更大.

Spontaneous Molecular Bromine Production in Sea-Salt Aerosols.

Bromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea-salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br-) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br-, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton-electron transfer process at the air-water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br- and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm-2 ⋅ s-1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br- activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br- activation. Our findings highlight the crucial role of the proton-electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br- by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Carbonyl Compounds Regulate Atmospheric Oxidation Capacity and Particulate Sulfur Chemistry in the Coastal Atmosphere.

Carbonyl compounds play a crucial role in the formation of ozone (O3) and secondary aerosols, with recent studies particularly highlighting formaldehyde (HCHO) as a significant contributor to the missing particulate sulfur. However, evaluations based on field observations are limited, especially in clean marine environments. Utilizing observation data from a coastal mountain site in May 2021 in Qingdao, northern China, we reveal the important regulating effect of carbonyls in atmospheric oxidation capacity and particulate sulfur chemistry using detailed chemical box models. Photolysis of gaseous carbonyls accounted for >90% and >60% of the primary sources of HO2 and RO2, respectively, contributing 38% of net O3 production. Notably, HCHO alone constituted 80% of the primary HO2 and 15% of net O3 production. Using a multiphase model with updated HCHO-related chemistry, we determine that HCHO chemistry can account for up to 30% of total particulate sulfur (the sum of hydroxymethanesulfonate and sulfate) and address more than one-third of the simulated sulfate gap. The emission-based multiphase model indicates that the HCHO-related pathway remains significant and can account for 20% of the particulate sulfur under clean marine conditions. These findings underscore the importance of carbonyls, particularly HCHO, in regulating the atmospheric oxidation capacity and particulate sulfur chemistry in the marine atmosphere, urging further laboratory studies on chemical kinetics and field measurements of particle-phase carbonyls.

Exploration of radical chemistry, precursor sensitivity and O3 control strategies in a provincial capital city, northwestern China

As the core city in northwestern China, Xi'an has suffered from serious O3 pollution in recent years. An observational model, coupled with the Master Chemical Mechanism (V3.3.1) and comprehensive data, analyzed atmospheric oxidation capacity (AOC), O3 formation mechanism, precursor sensitivity and control strategies in Xi'an. The study examined two periods (period I and II), with notable temperature and humidity differences. Despite lower precursor levels in period I, an O3 episode (peak value: 102.68 ppb) occurred only in period I, highlighting the crucial influence of meteorological conditions. Higher removal rate of primary pollutants occurs in period I, but meteorological conditions may not alter the dominant factor of AOC. Period I exhibited markedly high OH reactivity, with aromatics, alkenes, and NOx being the main contributors. HONO photolysis (50.78%) in the morning and O3 photolysis (34.13%) in the afternoon were the main HOx sources in period I, while HONO photolysis (62.86–80.68%) was significant at daytime for period II. Higher temperatures in period I accelerated all reaction rates involved in radical cycling, especially emphasizing RO2 + NO → RO + NO2. The results of sensitivity analysis showed that O3 sensitivity regime is VOC-limited in Xi'an, but the specific reactive species target VOC reductions emissions exhibited discrepancies for two distinct periods. A 20% decrease in VOCs and a 50% decrease in NOx (VOCs/NOx = 0.4) was proposed to be the optimal solution to achieve the O3 control target in period I. This study enhances our understanding of O3 formation under different meteorological conditions and provides scientific evidence for O3 pollution abatement policies in northwestern China, with potential applicability to other cities in this region and similar countries.

Unclassical Radical Generation Mechanisms in the Troposphere: A Review.

Hydroxyl (OH) and hydroperoxyl (HO2) radicals, collectively known as HOx radicals, are crucial in removing primary pollutants, controlling atmospheric oxidation capacity, and regulating global air quality and climate. An imbalance between radical observations and simulations has been identified based on radical closure experiments, a valuable tool for accessing the state-of-the-art chemical mechanisms, demonstrating a deviation between the existing and actual tropospheric mechanisms. In the past decades, researchers have attempted to explain this deviation and proposed numerous radical generation mechanisms. However, these newly proposed unclassical radical generation mechanisms have not been systematically reviewed, and previous radical-related reviews dominantly focus on radical measurement instruments and radical observations in extensive field campaigns. Herein, we overview the unclassical generation mechanisms of radicals, mainly focusing on outlining the methodology and results of radical closure experiments worldwide and systematically introducing the mainstream mechanisms of unclassical radical generation, involving the bimolecular reaction of HO2 and organic peroxy radicals (RO2), RO2 isomerization, halogen chemistry, the reaction of H2O with O2 over soot, epoxide formation mechanism, mechanism of electronically excited NO2 and water, and prompt HO2 formation in aromatic oxidation. Finally, we highlight the existing gaps in the current studies and suggest possible directions for future research. This review of unclassical radical generation mechanisms will help promote a comprehensive understanding of the latest radical mechanisms and the development of additional new mechanisms to further explain deviations between the existing and actual mechanisms.

Spontaneous Molecular Bromine Production in Sea‐Salt Aerosols

AbstractBromine chemistry is responsible for the catalytic ozone destruction in the atmosphere. The heterogeneous reactions of sea‐salt aerosols are the main abiotic sources of reactive bromine in the atmosphere. Here, we present a novel mechanism for the activation of bromide ions (Br−) by O2 and H2O in the absence of additional oxidants. The laboratory and theoretical calculation results demonstrated that under dark conditions, Br−, O2 and H3O+ could spontaneously generate Br and HO2 radicals through a proton–electron transfer process at the air–water interface and in the liquid phase. Our results also showed that light and acidity could significantly promote the activation of Br− and the production of Br2. The estimated gaseous Br2 production rate was up to 1.55×1010 molecules cm−2 ⋅ s−1 under light and acidic conditions; these results showed a significant contribution to the atmospheric reactive bromine budget. The reactive oxygen species (ROS) generated during Br− activation could promote the multiphase oxidation of SO2 to produce sulfuric acid, while the increase in acidity had a positive feedback effect on Br− activation. Our findings highlight the crucial role of the proton‐electron transfer process in Br2 production; here, H3O+ facilitates the activation of Br− by O2, serves as a significant source of atmospheric reactive bromine and exerts a profound impact on the atmospheric oxidation capacity.

Important yet Overlooked HONO Source from Aqueous-phase Photochemical Oxidation of Nitrophenols.

Nitrites (NO2-/HONO), as the primary source of hydroxyl radicals (•OH) in the atmosphere, play a key role in atmospheric chemistry. However, the current understanding of the source of NO2-/HONO is insufficient and therefore hinders the accurate quantification of atmospheric oxidation capacity. Herein, we highlighted an overlooked HONO source by the reaction between nitrophenols (NPs) and •OH in the aqueous phase and provided kinetic data to better evaluate the contribution of this process to atmospheric HONO. Three typical NPs, including 4-nitrophenol (4NP), 2-nitrophenol (2NP), and 4-nitrocatechol (4NC), underwent a denitration process to form aqueous NO2- and gaseous HONO through the •OH oxidation, with the yield of NO2-/HONO varied from 15.0 to 33.5%. According to chemical composition and structure analysis, the reaction pathway, where the ipso addition of •OH to the NO2 group on 4NP generated hydroquinone, can contribute to more than 61.9% of the NO2-/HONO formation. The aqueous photooxidation of NPs may account for HONO in the atmosphere, depending on the specific conditions. The results clearly suggest that the photooxidation of NPs should be considered in the field observation and calculation to better evaluate the HONO budget in the atmosphere.

Trends of peroxyacetyl nitrate and its impact on ozone over 2018–2022 in urban atmosphere

Peroxyacetyl nitrate (PAN) is an important photochemical product and affects ozone (O3) formation in the troposphere. Yet, the long-term observation of PAN remains scarce, limiting the full understanding of its impacts on photochemical pollution. Here, we observed PAN from 2018 to 2022 in urban Fuzhou, Southeastern China. We found that, in contrast to upward trend of O3, PAN concentrations shown a significant decreasing trend at an average rate of −0.07 ppb/year. NO2, CO, UVB, and T contributed to the decreasing trend of PAN according to Machine learning analyses, while the effect of O3-represented atmospheric oxidation capacity on PAN was fluctuating from year to year. Chemical box model revealed active PA production and depletion in Fuzhou. Thus, despite the decreasing PAN concentration, PAN chemistry effectively promoted O3 formation by rising ROx levels, leading to increases of 2.18%–58.4% in net O3 production rate in different years. Our results provide valuable insights into the evolution of photochemical pollution in urban environments.

Effects of heat waves on ozone pollution in a coastal industrial city: Meteorological impacts and photochemical mechanisms

Surface ozone (O3) pollution triggered by extreme weather conditions is attracting increasing attention. Heat waves in summer of 2022 were chosen to explore the photochemical and meteorological impacts on O3 formation in the southeastern coastal industrial city of Quanzhou in China. The machine-learning (ML)-based weather normalization showed that the de-weathered O3 concentrations (increased by 6.69 μg m−3) in 2022 were higher than those from 2015 to 2021. Temperature is the most important variable (33.5%), followed by solar radiation (23.5%) and RH (10.1%), differing from the O3 pollution in inland cities. The Observation-Based Model (OBM) analysis results showed that hydroxyl radical (OH) was the predominant oxidant for daytime atmospheric oxidation capacity (AOC). And oxygenated volatile organic compounds (OVOCs), NO2, and CO were the dominant contributors to OH reactivity, accelerating the recycling of ROx radicals and O3 formation. The daytime reaction rate of HO2+NO during the O3 pollution episodes was 20.0 ppb h−1, accounting for 65% of the total O3 production. The contribution of RO2+NO to the O3 production enhanced the possibility of the MDA8h O3 exceeding the Air Quality Standard of China. This study improves the understanding of O3 formation mechanisms in a coastal industrial city during heat waves, and the elevated contributions of meteorological conditions to O3 pollution become more challenge for the reduction of anthropogenic emissions controll.

Potential environmental impact of the chlorine-containing disinfectants usage during the COVID-19

Chlorine radicals (Cl) play a crucial role in atmospheric chemistry. The COVID-19 (coronavirus disease 2019) pandemic significantly increased the use of chlorine-containing disinfectants in China, leading to enhanced emissions of chlorine gas (Cl2) and hypochlorous acid (HOCl). In this study, we use the Community Multiscale Air Quality (CMAQ) model with updated chlorine chemistry to evaluate the potential air quality impact of chlorine emissions from disinfectant usage during February 2020, a month with a large outbreak of COVID-19 in mainland China. Results indicate that during the pandemic, there was a sharp increase of reactive chlorine emissions, with Cl2 and HOCl emissions reaching 773.9 t and 5913.1 t, making 3 times increase. The emissions of chlorine enhanced atmospheric oxidation capacity (AOC) through the oxidation of VOC by Cl and OH, with the average enhancement in Shanghai, Beijing and Wuhan areas reaching 4.6% ± 1.8%, 10.3% ± 6.6% and 7.4% ± 4.6%, respectively. Consequently, the use of chlorine-containing disinfectants may have contributed to observed increases in the monthly average of the maximum daily 8-h average ozone (MDA8 O3) and fine particulate matter (PM2.5) concentrations by up to 1.5 ppbv (5%) and 1.7 μg/m3 (1%), respectively. Especially, the maximum hourly increase values of O3 and PM2.5 concentrations were 4.8 ppbv and 11.4 μg/m³, 2.5 ppbv and 4.5 μg/m³ in Beijing and Wuhan, respectively. These results indicate that chlorine emissions from the widespread use of disinfectants have had a significant impact on air quality in certain regions (Beijing and Wuhan) and during specific periods (9:00∼12:00).

Isoprene emission dynamics in Chaetoceros curvisetus: Insights from transcriptome analysis under light/dark cycles

Isoprene from marine emissions significantly influences the atmospheric oxidative capacity and secondary pollutants in the marine boundary layer. While marine algae are recognized as primary contributors to marine emissions of isoprene, the long-term and dark-phase isoprene production from these organisms remains underexplored, potentially lead to inaccuracies in estimating marine isoprene emissions. In this study, we conducted a time series investigation of isoprene emission from Chaetoceros curvisetus with a growth cycle duration of 19 days and examined the influence of environmental factors. Our findings revealed that C. curvisetus emits isoprene during its whole growth cycle, with peak emissions (2.82 × 10−11 μmol cells−1 h−1) recorded in the stationary phase. Under nitrogen, light and temperature stress, significant release of isoprene is found. During both the light and dark phases, isoprene emission was observed with comparable intensities. Transcriptome analysis showed that 41 differentially expressed transcripts were involved in the synthesis of volatile organic compounds. Notably, downregulation of genes associated with pyruvate synthesis in the glycolysis pathway suggests its importance for dark isoprene release. The study also highlighted downregulation of isopentenyl phosphate kinase and Gene Ontology enrichment in organelles housing mevalonate (MVA) pathways, suggesting that exchange involving the transport of intermediate metabolites between MVA and methylerythritol phosphate (MEP) pathway may contribute to the released isoprene in dark phase. Furthermore, the synthesis of substrates and downstream products is related to the dark release of isoprene. In conclusion, our study illuminates the pivotal roles of substrate availability, interplay between MVA and MEP pathways, and glycolysis pathway in governing isoprene emissions under light/dark cycles.