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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, HO 2 , and RO 2 concentrations are simulated, which are elevated by 42, 220, and 277%, respectively, during the lockdown period, mainly due to the reduction in nitrogen oxides (NO x ). However, the radical turnover rates, i.e., OH oxidation rate L(OH) and local ozone production rate P(O 3 ), which determine the secondary intermediates formation and O 3 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 O 3 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 NO x reduction due to the rapid transformation to electrified fleets in the future.

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 (O x = NO 2 + O 3 ) showed a good positive correlation, with a ratio of PNs to O x 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 O 3 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., Competing Interests: Declaration of competing interest The authors declare that they have no conflicts of interest., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Tropospheric ozone (O 3 ) is a harmful gas compound to humans and vegetation, and it also serves as a climate change forcer. O 3 is formed in the reactions of nitrogen oxides and volatile organic compounds (VOCs) with light. In this study, an O 3 pollution episode encountered in Shenzhen, South China in 2018 was investigated to illustrate the influence of aerosols on local O 3 production. We used a box model with comprehensive heterogeneous mechanisms and empirical prediction of photolysis rates to reproduce the O 3 episode. Results demonstrate that the aerosol light extinction and NO 2 heterogeneous reactions showed comparable influence but opposite signs on the O 3 production. Hence, the influence of aerosols from different processes is largely counteracted. Sensitivity tests suggest that O 3 production increases with further reduction in aerosols in this study, while the continued NO x reduction finally shifts O 3 production to an NO x -limited regime with respect to traditional O 3 -NO x -VOC sensitivity. Our results shed light on the role of NO x reduction on O 3 production and highlight further mitigation in NO x not only limiting the production of O 3 but also helping to ease particulate nitrate, as a path for cocontrol of O 3 and fine particle pollution.

We present a comprehensive field campaign conducted in Beijing, September 2016, to elucidate the photochemical smog pollution, i.e. Ozone (O 3 ). The observed daily maximum hydroxyl radical (OH) and hydroperoxy radical (HO 2 ) concentrations were up to 1 × 10 7  cm -3 and 6 × 10 8  cm -3 , respectively, indicating the active photochemistry in autumn Beijing. Photolysis of nitrous acid (HONO) and O 3 contributed 1-2 ppbv h -1 to OH primary production during daytime. OH termination were dominated by the reaction with nitric oxide (NO) and nitrogen dioxide (NO 2 ), which were in general larger than primary production rates, indicating other primary radical sources maybe important. The measurement of radicals facilitates the direct determination of local ozone production rate P (O x ) (O x  = O 3  + NO 2 ). The integrated P(O x ) reached 75 ppbv in afternoon (for 4 h) when planetary boundary layer was well developed. At the same time period, the observed total oxidant concentrations O x , increased significantly by 70 ppbv. In addition, the O x measurement showed compact increase in 12 stations both temporally and spatially in Beijing, indicating that active photochemical production happened homogenously throughout the city. The back-trajectory analysis showed that Beijing was isolated from the other cities during the episode, which further proved that the fast ozone pollution was contributed by local photochemical production rather than regional advection., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Particulate nitrate (pNO 3 - ) has often been found to be the major component of fine particles in urban air-sheds in China, the United States, and Europe during winter haze episodes in recent years. However, there is a lack of knowledge regarding the experimentally determined contribution of different chemical pathways to the formation of pNO 3 - . Here, for the first time, we combine ground and tall-tower observations to quantify the chemical formation of pNO 3 - using observationally constrained model approach based on direct observations of OH and N 2 O 5 for the urban air-shed. We find that the gas-phase oxidation pathway (OH+NO 2 ) during the daytime is the dominant channel over the nocturnal uptake of N 2 O 5 during pollution episodes, with percentages of 74% in urban areas and 76% in suburban areas. This is quite different from previous studies in some regions of the US, in which the uptake of N 2 O 5 was concluded to account for a larger contribution in winter. These results indicate that the driving factor of nitrate pollution in Beijing and different regions of the US is different, as are the mitigation strategies for particulate nitrate.

In contrast to summer smog, the contribution of photochemistry to the formation of winter haze in northern mid-to-high latitude is generally assumed to be minor due to reduced solar UV and water vapor concentrations. Our comprehensive observations of atmospheric radicals and relevant parameters during several haze events in winter 2016 Beijing, however, reveal surprisingly high hydroxyl radical oxidation rates up to 15 ppbv/h, which is comparable to the high values reported in summer photochemical smog and is two to three times larger than those determined in previous observations during winter in Birmingham (Heard et al. Geophys. Res. Lett. 2004, 31, (18)), Tokyo (Kanaya et al. J. Geophys. Res.: Atmos. 2007, 112, (D21)), and New York (Ren et al. Atmos. Environ. 2006, 40, 252-263). The active photochemistry facilitates the production of secondary pollutants. It is mainly initiated by the photolysis of nitrous acid and ozonolysis of olefins and maintained by an extremely efficiently radical cycling process driven by nitric oxide. This boosted radical recycling generates fast photochemical ozone production rates that are again comparable to those during summer photochemical smog. The formation of ozone, however, is currently masked by its efficient chemical removal by nitrogen oxides contributing to the high level of wintertime particles. The future emission regulations, such as the reduction of nitrogen oxide emissions, therefore are facing the challenge of reducing haze and avoiding an increase in ozone pollution at the same time. Efficient control strategies to mitigate winter haze in Beijing may require measures similar as implemented to avoid photochemical smog in summer.

Monoterpenes have been known to have a critical influence on air quality and climate change through their impact on the formation of fine particles. Here we present field evidence that monoterpene oxidations largely enhanced local ozone production in a regional site in eastern China. The observed monoterpene was most likely from biomass burning rather than biogenic emissions, as indicated by the high correlation with CO at night-time, and the observed ratio of these two species was consistent with previously determined values from biomass burning experiments. Fast monoterpene oxidations were determined experimentally based on direct radical measurements, leading to a daily ozone enhancement of 4–18 parts per billion by volume (ppb), which was 6%–16% of the total ozone production, depending on the speciation of monoterpenes. It demonstrates that the previously overlooked anthropogenic monoterpenes make an important contribution to O3 production in eastern China. The role could possibly be important at similar locations across China and other parts of the world that are characterized by massive emissions, especially where there are high NO x levels. Our results highlight that anthropogenic monoterpenes should be taken into account when proceeding with the coordinated mitigation of O3 and particulate matter pollution. [ABSTRACT FROM AUTHOR]