Atmospheric Oxidation Capacity Elevated during 2020 Spring Lockdown in Chengdu, China: Lessons for Future Secondary Pollution Control.

This study presents the measurement of photochemical precursors during the lockdown period from January 23, 2020, to March 14, 2020, in Chengdu in response to the coronavirus (COVID-19) pandemic. To derive the lockdown impact on air quality, the observations are compared to the equivalent periods in the last 2 years. An observation-based model is used to investigate the atmospheric oxidation capacity change during lockdown. OH, HO2, and RO2 concentrations are simulated, which are elevated by 42, 220, and 277%, respectively, during the lockdown period, mainly due to the reduction in nitrogen oxides (NOx). However, the radical turnover rates, i.e., OH oxidation rate L(OH) and local ozone production rate P(O3), which determine the secondary intermediates formation and O3 formation, only increase by 24 and 48%, respectively. Therefore, the oxidation capacity increases slightly during lockdown, which is partly attributed to unchanged alkene concentrations. During the lockdown, alkene ozonolysis seems to be a significant radical primary source due to the elevated O3 concentrations. This unique data set during the lockdown period highlights the importance of controlling alkene emission to mitigate secondary pollution formation in Chengdu and may also be applicable in other regions of China given an expected NOx reduction due to the rapid transformation to electrified fleets in the future.

Modeling the molecular composition of secondary organic aerosol under highly polluted conditions: A case study in the Yangtze River Delta Region in China.

A near-explicit mechanism, the master chemical mechanism (MCMv3.3.1), coupled with the Community Multiscale Air Quality (CMAQ) model (CMAQ-MCM-SOA), was applied to investigate the characteristics of secondary organic aerosol (SOA) during a pollution event in the Yangtze River Delta (YRD) region in summer 2018. Model performances in predicting explicit volatile organic compounds (VOCs), organic aerosol (OA), secondary organic carbon (SOC), and other related pollutants in Taizhou, as well as ozone (O3) and fine particulate matter (PM2.5) in multiple cities in this region, were evaluated against observations and model predictions by the CMAQ model coupled with a lumped photochemical mechanism (SAPRC07tic, S07). MCM and S07 exhibited similar performances in predicting gaseous species, while MCM better captured the observed PM2.5 and inorganic aerosols. Both models underpredicted OA concentrations. When excluding data during biomass burning events, SOC concentrations were underpredicted by the CMAQ-MCM-SOA model (-28.4 %) and overpredicted by the CMAQ-S07 model (134.4 %), with better agreement with observations in the trend captured by the CMAQ-MCM-SOA model. Dicarbonyl SOA accounted for a significant fraction of total SOA in the YRD, while organic nitrates originating from aromatics were the most abundant species contributing to the SOA formation from gas-particle partitioning. The oxygen-to­carbon ratio (O/C) for SOA and OA were 0.68-0.75 and 0.20-0.65, respectively, indicating a higher oxidation state in the areas influenced by biogenic emissions. Finally, the phase state of SOA was examined by calculating the glass transition temperature (Tg) based on its molecular composition. It was found that semi-solid state characterized SOA in the YRD, which could potentially impact their chemical transformation and lifetimes along with those of their precursors.

Reducing Soil-Emitted Nitrous Acid as a Feasible Strategy for Tackling Ozone Pollution.

Severe ozone (O3) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NOx) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions (FHONO) enhanced by nitrogen fertilizer application at an agricultural site. The observed FHONO significantly improves model performance in predicting atmospheric HONO and leads to regional O3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NOx, by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O3 pollution and climate change.

Biogenic volatile organic compounds dominated the near-surface ozone generation in Sichuan Basin, China, during fall and wintertime.

The complex air pollution driven by both Ozone (O3) and fine particulate matter (PM2.5) significantly influences the air quality in the Sichuan Basin (SCB). Understanding the O3 formation during autumn and winter is necessary to understand the atmospheric oxidative capacity. Therefore, continuous in-site field observations were carried out during the late summer, early autumn and winter of 2020 in a rural area of Chongqing. The total volatile organic compounds (VOCs) concentration reported by a Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) were 13.66 ± 9.75 ppb, 5.50 ± 2.64 ppb, and 9.41 ± 5.11 ppb in late summer, early autumn and winter, respectively. The anthropogenic VOCs (AVOCs) and biogenic VOCs (BVOCs) were 8.48 ± 7.92 ppb and 5.18 ± 2.99 ppb in late summer, 3.31 ± 1.89 ppb and 2.19 ± 0.93 ppb in autumn, and 6.22 ± 3.99 ppb and 3.20 ± 1.27 ppb in winter. A zero-dimensional atmospheric box model was employed to investigate the sensitivity of O3-precursors by relative incremental reactivity (RIR). The RIR values of AVOCs, BVOCs, carbon monoxide (CO), and nitrogen oxides (NOx) were 0.31, 0.71, 0.09, and -0.36 for late summer, 0.24, 0.59, 0.22, and -0.38 for early autumn, and 0.30, 0.64, 0.33 and -0.70 for winter, and the results showed that the O3 formation of sampling area was in the VOC-limited region, and O3 was most sensitive to BVOCs (with highest RIR values, > 0.6). This study can be helpful in understanding O3 formation and interpreting the secondary formation of aerosols in the winter.

Reactive aldehyde chemistry explains the missing source of hydroxyl radicals.

Hydroxyl radicals (OH) determine the tropospheric self-cleansing capacity, thus regulating air quality and climate. However, the state-of-the-art mechanisms still underestimate OH at low nitrogen oxide and high volatile organic compound regimes even considering the latest isoprene chemistry. Here we propose that the reactive aldehyde chemistry, especially the autoxidation of carbonyl organic peroxy radicals (R(CO)O2) derived from higher aldehydes, is a noteworthy OH regeneration mechanism that overwhelms the contribution of the isoprene autoxidation, the latter has been proved to largely contribute to the missing OH source under high isoprene condition. As diagnosed by the quantum chemical calculations, the R(CO)O2 radicals undergo fast H-migration to produce unsaturated hydroperoxyl-carbonyls that generate OH through rapid photolysis. This chemistry could explain almost all unknown OH sources in areas rich in both natural and anthropogenic emissions in the warm seasons, and may increasingly impact the global self-cleansing capacity in a future low nitrogen oxide society under carbon neutrality scenarios.

Revisiting the Ultraviolet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget.

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultraviolet (UV) absorption cross section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. In this study, we performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometry and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultrapure standard HONO source were utilized for laboratory intercomparisons. Results show a consistent ∼22% negative bias in measurements of the IBBCEAS compared with a SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation of the absolute UV absorption cross sections through careful analysis of possible uncertainties. We quantified the absorption cross section of gaseous HONO (360-390 nm) utilizing a custom-built IBBCEAS instrument, and the results were found to be 22-34% lower than the previously published absorption cross sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross section of HONO and the re-evaluation of the previously reported HONO budgets.

Determination of Urban Formaldehyde Emission Ratios in the Shanghai Megacity.

We analyzed two data sets of atmospheric formaldehyde (FA) at an urban site in the Shanghai megacity during the summer of 2017 and the winter of 2017/18, with the primary objective of determining the emission ratio of formaldehyde versus carbon monoxide (CO). Through the photochemical age method and the minimum R squared (MRS) method, we derived the summer urban formaldehyde release ratios of 3.37 ppbv (ppmv of CO)-1 and 4.04 ppbv (ppmv of CO)-1, respectively. The error of both estimations is within ±20%, indicating the consistency of the results. We recognized the hourly minimum emission ratios determined from the MRS method to be indicative of actual formaldehyde emission ratios. Similarly, the emission ratio in winter is determined to be 2.10 ppbv (ppmv of CO)-1 utilizing the MRS method. The findings provide significant insights into the potential impact of motor vehicle exhaust on formaldehyde emissions in urban areas. This work demonstrates that the formaldehyde emission ratio determined by the MRS method can be used to represent the emissions of the freshest air mass. Formaldehyde photolysis contributed an average of 9% to the free radical primary reaction rate (P(ROx)) as a single chemical species during the daytime in summer, which was lower than the 11% recorded in winter. Formaldehyde emission reduction positively impacts local ozone production, so models describing ozone formation in Shanghai during summer need to reflect these emissions accurately. Evidence of the crucial catalytic role of formaldehyde in particulate matter formation has been confirmed by recent research. A potentially effective way to decrease the incidence of haze days in autumn and winter in the future is therefore to focus on reducing formaldehyde emissions.

Recent Progress in Atmospheric Chemistry Research in China: Establishing a Theoretical Framework for the "Air Pollution Complex".

Atmospheric chemistry research has been growing rapidly in China in the last 25 years since the concept of the "air pollution complex" was first proposed by Professor Xiaoyan TANG in 1997. For papers published in 2021 on air pollution (only papers included in the Web of Science Core Collection database were considered), more than 24 000 papers were authored or co-authored by scientists working in China. In this paper, we review a limited number of representative and significant studies on atmospheric chemistry in China in the last few years, including studies on (1) sources and emission inventories, (2) atmospheric chemical processes, (3) interactions of air pollution with meteorology, weather and climate, (4) interactions between the biosphere and atmosphere, and (5) data assimilation. The intention was not to provide a complete review of all progress made in the last few years, but rather to serve as a starting point for learning more about atmospheric chemistry research in China. The advances reviewed in this paper have enabled a theoretical framework for the air pollution complex to be established, provided robust scientific support to highly successful air pollution control policies in China, and created great opportunities in education, training, and career development for many graduate students and young scientists. This paper further highlights that developing and low-income countries that are heavily affected by air pollution can benefit from these research advances, whilst at the same time acknowledging that many challenges and opportunities still remain in atmospheric chemistry research in China, to hopefully be addressed over the next few decades.

Impact of aerosol in-situ peroxide formations induced by metal complexes on atmospheric H2O2 budgets.

Hydrogen peroxide (H2O2), hydroxyl radicals (OH), hydroperoxyl (HO2), and superoxide (O2-) radicals interacting with aerosol particles significantly affect the atmospheric pollutant budgets. A multiphase chemical kinetic box model (PKU-MARK), including the multiphase processes of transition metal ions (TMI) and their organic complexes (TMI-OrC), was built to numerically drive H2O2 chemical behaviors in the aerosol particle liquid phase using observational data obtained from a field campaign in rural China. Instead of relying on fixed uptake coefficient values, a thorough simulation of multiphase H2O2 chemistry was performed. In the aerosol liquid phase, light-driven TMI-OrC reactions promote OH, HO2/O2-, and H2O2 recycling and spontaneous regenerations. The in-situ generated aerosol H2O2 would offset gas-phase H2O2 molecular transfer into the aerosol bulk phase and promote the gas-phase level. When combined with the multiphase loss and in-situ aerosol generation involving TMI-OrC mechanism, the HULIS-Mode significantly improves the consistency between modeled and measured gas-phase H2O2 levels. Aerosol liquid phase could be a pivotal potential source of aqueous H2O2 and influence the multiphase budgets. Our work highlights the intricate and significant effects of aerosol TMI and TMI-OrC interactions on the multiphase partitioning of H2O2 when assessing atmospheric oxidant capacity.

Evaluating cost and benefit of air pollution control policies in China: A systematic review.

China has put great efforts into air pollution control over the past years and recently committed to its most ambitious climate target. Cost and benefit analysis has been widely used to evaluate the control policies in terms of past performance, future reduction potential, and direct and indirect impacts. To understand the cost and benefit analysis for air pollution control in China, we conducted a bibliometric review of more than 100 studies published over the past two decades, including the current research progress, most commonly adopted methods, and core findings. The control target in cost and benefit analysis has shifted in three stages, from individual and primary pollution control, moving to joint prevention of multiple and secondary pollutants, and then towards synergistic control of air pollution and carbon. With the expansion of the research scope, the integrated assessment model has gradually demonstrated the necessity for long-term ex-anti policy simulation, especially for dealing with complex factors. To ensure long-term air quality, climate, public health, and sustainable economic development, substantial evidence from published studies has suggested that China needs to continue its efforts in the upstream adjustment of the energy system and industrial structure with multi-regional and -sector collaboration. This cost and benefit review paper provides decision-makers with the fundamental information and knowledge gaps in air pollution control strategies in China, and direction for facing future challenges.

Compilation of reaction kinetics parameters determined in the Key Development Project for Air Pollution Formation Mechanism and Control Technologies in China.

A compilation of new advances made in the research field of laboratory reaction kinetics in China's Key Development Project for Air Pollution Formation Mechanism and Control Technologies was presented. These advances are grouped into six broad, interrelated categories, including volatile organic compound (VOC) oxidation, secondary organic aerosol (SOA) formation, new particle formation (NPF) and gas-particle partitioning, ozone chemistry, model parameters, and secondary inorganic aerosol (SIA) formation, highlighting the laboratory work done by Chinese researchers. For smog chamber applications, the current knowledge gained from laboratory studies is reviewed, with emphasis on summarizing the oxidation mechanisms of long-chain alkanes, aromatics, alkenes, aldehydes/ketones in the atmosphere, SOA formation from anthropogenic emission sources, and oxidation of aromatics, isoprene, and limonene, as well as SIA formation. For flow tube applications, atmospheric oxidation mechanisms of toluene and methacrolein, SOA formation from limonene oxidation by ozone, gas-particle partitioning of peroxides, and sulfuric acid-water (H2SO4-H2O) binary nucleation, methanesulfonic acid-water (MSA-H2O) binary nucleation, and sulfuric acid-ammonia-water (H2SO4-NH3-H2O) ternary nucleation are discussed.

Progress in quantitative research on the relationship between atmospheric oxidation and air quality.

Atmospheric oxidizing capacity (AOC) is an essential driving force of troposphere chemistry and self-cleaning, but the definition of AOC and its quantitative representation remain uncertain. Driven by national demand for air pollution control in recent years, Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research. This paper will give a brief review of these developments. First, AOC indexes were established that represent apparent atmospheric oxidizing ability (AOIe) and potential atmospheric oxidizing ability (AOIp) based on aspects of macrothermodynamics and microdynamics, respectively. A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing, and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country. In addition, the detection of ground or vertical profiles for atmospheric OH·, HO2·, NO3· radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments. Moreover, laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O3 and NO2, which are typical oxidants in the surface/interface atmosphere, and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies, multiphase and multi-interface conditions were obtained. Finally, based on the GRAPES-CUACE adjoint model improved by Chinese scholars, simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized. Normalized numerical simulations of AOIe and AOIp were performed, and regional coordination of AOC was adjusted. An optimized plan for controlling O3 and PM2.5 was analyzed by scenario simulation.

A critical review of sulfate aerosol formation mechanisms during winter polluted periods.

Sulfate aerosol contributes to particulate matter pollution and plays a key role in aerosol radiative forcing, impacting human health and climate change. Atmospheric models tend to substantially underestimate sulfate concentrations during haze episodes, indicating that there are still missing mechanisms not considered by the models. Despite recent good progress in understanding the missing sulfate sources, knowledge on different sulfate formation pathways during polluted periods still involves large uncertainties and the dominant mechanism is under heated debate, calling for more field, laboratory, and modeling work. Here, we review the traditional sulfate formation mechanisms in cloud water and also discuss the potential factors affecting multiphase S(Ⅳ) oxidation. Then recent progress in multiphase S(Ⅳ) oxidation mechanisms is summarized. Sulfate formation rates by different prevailing oxidation pathways under typical winter-haze conditions are also calculated and compared. Based on the literature reviewed, we put forward control of the atmospheric oxidation capacity as a means to abate sulfate aerosol pollution. Finally, we conclude with a concise set of research priorities for improving our understanding of sulfate formation mechanisms during polluted periods.

Review of technologies and their applications for the speciated detection of RO2 radicals.

Peroxy radicals (RO2), which are formed during the oxidation of volatile organic compounds, play an important role in atmospheric oxidation reactions. Therefore, the measurement of RO2, especially distinct species of RO2 radicals, is important and greatly helps the exploration of atmospheric chemistry mechanisms. Although the speciated detection of RO2 radicals remains challenging, various methods have been developed to study them in detail. These methods can be divided into spectroscopy and mass spectrometry technologies. The spectroscopy methods contain laser-induced fluorescence (LIF), UV-absorption spectroscopy, cavity ring-down spectroscopy (CRDS) and matrix isolation and electron spin resonance (MIESR). The mass spectrometry methods contain chemical ionization atmospheric pressure interface time-of-flight mass spectrometry (CI-APi-TOF), chemical ionization mass spectrometry (CIMS), CI-Orbitrap-MS and the third-generation proton transfer reaction-time-of-flight mass spectrometer (PTR3). This article reviews technologies for the speciated detection of RO2 radicals and the applications of these methods. In addition, a comparison of these techniques and the reaction mechanisms of some key species are discussed. Finally, possible gaps are proposed that could be filled by future research into speciated RO2 radicals.

Formation mechanism and control strategy for particulate nitrate in China.

Over the past decade, fine particulate matter (PM) pollution in China has been abated significantly, benefiting from strict emission control measures, but particulate nitrate continues to rise. Here, we review the progress in particulate nitrate (pNO3-) pollution characterization, nitrate formation mechanisms, and the proposed control strategies in China. The spatial and temporal distributions of pNO3- are summarized. The current status of knowledge on the chemical mechanism is updated, and the significance of its formation pathways is assessed by various approaches such as field observation and modelling of nitrate production rate, as well as isotopic analysis. The factors impacting pNO3- formation and the corresponding pollution regulation strategies are discussed, in which the importance of atmospheric oxidation capacity and ammonia are addressed. Finally, the challenges and open questions in pNO3- pollution control in China are outlined.

A review of gas-phase chemical mechanisms commonly used in atmospheric chemistry modelling.

The atmospheric chemical mechanism is an essential component of airshed models used for investigating the chemical behaviors and impacts of species. Since the first tropospheric chemical mechanism was proposed in the 1960s, various mechanisms including Master Chemical Mechanism (MCM), Carbon Bond Mechanism (CBM), Statewide Air Pollution Research Center (SAPRC) and Regional Atmospheric Chemistry Mechanism (RACM) have been developed for different research purposes. This work summarizes the development and applications of these mechanisms, introduces their compositions and lumping methods, and compares the ways the mechanisms treat radicals with box model simulations. CBM can reproduce urban pollution events with relatively low cost compared to SAPRC and RACM, whereas the chemical behaviors of radicals and the photochemical production of ozone are described in detail in RACM. The photolysis rates of some oxygenated compounds are low in SAPRC07, which may result in underestimation of radical levels. As an explicit chemical mechanism, MCM describes the chemical processes of primary pollutants and their oxidation products in detail. MCM can be used to investigate certain chemical processes; however, due to its large size, it is rarely used in regional model simulations. A box model case study showed that the chemical behavior of OH and HO2 radicals and the production of ozone were well described by all mechanisms. CBM and SAPRC underestimated the radical levels for different chemical treatments, leading to low ozone production values in both cases. MCM and RACM are widely used in box model studies, while CBM and SAPRC are often selected in regional simulations.

Observation and modeling of organic nitrates on a suburban site in southwest China.

Here we report the measurements of two types of organic nitrates (ONs), peroxy nitrates (PNs) and alkyl nitrates (ANs), in Chengdu, China, during summer 2019. The average concentrations of PNs and ANs were 1.3 ± 1.1 ppbv and 0.5 ± 0.3 ppbv during the day, with peaks of 7.7 ppbv and 1.9 ppbv, respectively, which were in the middle and upper end of the reported levels in China. Much higher PNs and ANs concentrations were found during the photochemical pollution period than during the clean period. Box model simulation was capable of reproducing PNs during photochemical pollution episodes but showed overestimation in other periods, which was likely caused by the simplification of PNs sinks. The OH oxidation of aldehydes and ketones was the most important source of the PNs precursors, PAs (peroxyacyl radicals), except for the thermal decomposition of PNs, which was further confirmed by the relative incremental reactivity (RIR) analysis. The model basically reproduced the observed ANs by the refinement of related mechanisms, with isoprene contributing to its formation by 29.2 %. The observed PNs and total oxidants (Ox = NO2 + O3) showed a good positive correlation, with a ratio of PNs to Ox of 0.079, indicating a strong suppression of PNs chemistry to ozone formation. The model quantified the suppression of PNs chemistry on the peak ozone production rate by 21.3 % on average and inhibited ozone formation up to 20 ppbv in total. The RIR analysis suggests that the production of both O3 and ANs was in the VOC-limited regime and highlights the importance of VOC control (especially aromatics) to mitigate photochemical pollution in Chengdu. The study deepens the understanding of photochemical pollution in urban areas of western China and further emphasizes the impacts of ONs chemistry on ozone pollution.