Accurately Predicting Spatiotemporal Variations of Near-Surface Nitrous Acid (HONO) Based on a Deep Learning Approach.

Gaseous nitrous acid (HONO) is identified as a critical precursor of hydroxyl radicals (OH), influencing atmospheric oxidation capacity and the formation of secondary pollutants. However, large uncertainties persist regarding its formation and elimination mechanisms, impeding accurate simulation of HONO levels using chemical models. In this study, a deep neural network (DNN) model was established based on routine air quality data (O3, NO2, CO, and PM2.5) and meteorological parameters (temperature, relative humidity, solar zenith angle, and season) collected from four typical megacity clusters in China. The model exhibited robust performance on both the train sets [slope = 1.0, r2 = 0.94, root mean squared error (RMSE) = 0.29 ppbv] and two independent test sets (slope = 1.0, r2 = 0.79, and RMSE = 0.39 ppbv), demonstrated excellent capability in reproducing the spatiotemporal variations of HONO, and outperformed an observation-constrained box model incorporated with newly proposed HONO formation mechanisms. Nitrogen dioxide (NO2) was identified as the most impactful features for HONO prediction using the SHapely Additive exPlanation (SHAP) approach, highlighting the importance of NO2 conversion in HONO formation. The DNN model was further employed to predict the future change of HONO levels in different NOx abatement scenarios, which is expected to decrease 27-44% in summer as the result of 30-50% NOx reduction. These results suggest a dual effect brought by abatement of NOx emissions, leading to not only reduction of O3 and nitrate precursors but also decrease in HONO levels and hence primary radical production rates (PROx). In summary, this study demonstrates the feasibility of using deep learning approach to predict HONO concentrations, offering a promising supplement to traditional chemical models. Additionally, stringent NOx abatement would be beneficial for collaborative alleviation of O3 and secondary PM2.5.

Radical chemistry and VOCs-NOx-O3-nitrate sensitivity in the polluted atmosphere of a suburban site in the North China Plain.

In this study, the chemical mechanisms of O3 and nitrate formation as well as the control strategy were investigated based on extensive observations in Tai'an city in the NCP and an observation-constrained box model. The results showed that O3 pollution was severe with the maximum hourly O3 concentration reaching 150 ppb. Higher O3 concentration was typically accompanied by higher PM2.5 concentrations, which could be ascribed to the common precursors of VOCs and NOx. The modeled averaged peak concentrations of OH, HO2, and RO2 were relatively higher compared to previous observations, indicating strong atmospheric oxidation capacity in the study area. The ROx production rate increased from 2.8 ppb h-1 to 5 ppb h-1 from the clean case to the heavily polluted case and was dominated by HONO photolysis, followed by HCHO photolysis. The contribution of radical-self combination to radical termination gradually exceeded NO2 + OH from clean to polluted cases, indicating that O3 formation shifted to a more NOx-limited regime. The O3 production rate increased from 14 ppb h-1 to 22 ppb h-1 from clean to heavily polluted cases. The relative incremental reactivity (RIR) results showed that VOCs and NOx had comparable RIR values during most days, which suggested that decreasing VOCs or NOx was both effective in alleviating O3 pollution. In addition, HCHO, with the largest RIR value, made important contribution to O3 production. The Empirical Kinetic Modeling Approach (EKMA) revealed that synergistic control of O3 and nitrate can be achieved by decreasing both NOx and VOCs emissions (e.g., alkenes) with the ratio of 3:1. This study emphasized the importance of NOx abatement for the synergistic control of O3 and nitrate pollution in the Tai'an area as the sustained emissions control has shifted the O3 and nitrate formation to a more NOx-limited regime.

Evolution of black carbon and brown carbon during summertime in Southwestern China: An assessment of control measures during the 2023 Chengdu Summer World University Games.

The 31st FISU Summer World University Games (SWUG) was held in Chengdu, southwestern China, from July 22 to August 8, 2023. A series of control measures were carried out to ensure good air quality during the SWUG, providing an opportunity to investigate the atmospheric behaviors of light-absorbing aerosols under such a substantial disturbance caused by the control measures. To assess the impacts of emission controls on primary pollutants, a field campaign was conducted at a rural site in Chengdu to investigate the characterization of equivalent black carbon (eBC). The changes of eBC concentrations before, during, and after the SWUG were characterized. The sources of eBC were resolved, and the impacts of atmospheric processes on the absorption capacity were also investigated. During the SWUG, the eBC concentration decreased by 12.1 % and 25.3 % compared with those before and after the SWUG. A fossil fuel combustion (eBCff) and a biomass burning (eBCbb) originated eBC were resolved using the aethalometer model. Both eBCff and eBCbb decreased during the SWUG, indicating the effectiveness of control measures. After the SWUG, the influence of biomass burning emissions became more and more significant, and the contribution of brown carbon (BrC) to light absorption at 370-660 nm increased by 52, 19, 7, 6, and 17 % compared to those during the SWUG. As the biomass burning emitted aerosols aged, the absorption Ångström exponent and babs(BrC370nm) decreased gradually, which was mainly due to the photobleaching of the chromophores during the daytime. eBCff was mainly affected by strong wind, while high eBCbb concentration was mainly attributed to the gradual accumulation of biomass-burning emissions near the observation site. The results show the significant reduction of eBC with the implementation of the air pollution mitigation campaign, and provide insights on the impacts of atmospheric processes on BC optical properties during summertime.

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.

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.

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.

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.

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.

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.