Surface ozone is a severe air pollution problem in the North China Plain, which is home to 300 million people. Ozone concentrations are highest in summer, driven by fast photochemical production of hydrogen oxide radicals (HOx) that can overcome the radical titration caused by high emissions of nitrogen oxides (NOx) from fuel combustion. Ozone has been very low during winter haze (particulate) pollution episodes. However, the abrupt decrease of NOx emissions following the COVID-19 lockdown in January 2020 reveals a switch to fast ozone production during winter haze episodes with maximum daily 8-h average (MDA8) ozone concentrations of 60 to 70 parts per billion. We reproduce this switch with the GEOS-Chem model, where the fast production of ozone is driven by HOx radicals from photolysis of formaldehyde, overcoming radical titration from the decreased NOx emissions. Formaldehyde is produced by oxidation of reactive volatile organic compounds (VOCs), which have very high emissions in the North China Plain. This remarkable switch to an ozone-producing regime in January–February following the lockdown illustrates a more general tendency from 2013 to 2019 of increasing winter–spring ozone in the North China Plain and increasing association of high ozone with winter haze events, as pollution control efforts have targeted NOx emissions (30% decrease) while VOC emissions have remained constant. Decreasing VOC emissions would avoid further spreading of severe ozone pollution events into the winter–spring season.

We quantified aerosol direct radiative effects on surface layer concentrations of aerosols during a heavily polluted event in the North China Plain (NCP, 35.4°N–41.2°N, 113.3°E–119.3°E) during 21–27 February 2014, using the chemistry version of the Weather Research and Forecasting (WRF‐Chem) Model. Comparisons of model results with observations showed that the WRF‐Chem model reproduced the spatial and temporal variations of meteorological variables reasonably well, but overestimated average PM2.5 concentration by 21.7% over the NCP during 21–27 February. The simulated direct radiative effects of total, absorbing, and scattering aerosols reduced the planetary boundary layer (PBL) heights by 111.4 m, 35.7 m, and 70.7 m, respectively, averaged over NCP and 21–27 February. The direct radiative effects of total aerosols induced increases in aerosol concentrations by 11.5% for SO42−, 29.5% for NO3−, 29.6% for NH4+, 28.7% for organic carbon (OC), 26.7% for black carbon (BC), and 20.4% for PM2.5, respectively, averaged over the NCP during 21–27 February 2014. The increase in PM2.5 concentration averaged over the NCP and the haze event was 29.6 μg m−3 (16.8%) due to radiative effect of scattering aerosols, as a result of the decreases in PBL height and changes in secondary aerosol production rates. The corresponding increase in PM2.5 concentration owing to absorbing aerosols was 2.1 μg m−3 (1.0%), resulting from the offsetting impacts of changes in PBL height, wind near the surface, and chemical processes.

Abstract. Fine-particle pollution associated with haze threatens human health, especially in the North China Plain region, where extremely high PM2.5 concentrations are frequently observed during winter. In this study, the Weather Research and Forecasting with Chemistry (WRF-Chem) model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event over the Beijing–Tianjin–Hebei (BTH) region in December 2015; this included an examination of the contributions of local emissions and regional transport to the PM2.5 concentration in the BTH area, and the contributions of each detailed physical or chemical process to the variations in the PM2.5 concentration. The mechanisms influencing aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using process analysis. During the aerosol accumulation stage (16–22 December, Stage 1), the near-surface PM2.5 concentration in the BTH region increased from 24.2 to 289.8 µg m−3, with the contributions of regional transport increasing from 12 % to 40 %, while the contribution of local emissions decreased from 59 % to 38 %. During the aerosol dispersion stage (23–27 December, Stage 2), the average concentration of PM2.5 was 107.9 µg m−3, which was contributed by local emissions (51 %) and regional transport (24 %). The 24 h change (23:00 minus 00:00 LST) in the near-surface PM2.5 concentration was +43.9 µg m−3 during Stage 1 and −41.5 µg m−3 during Stage 2. The contributions of aerosol chemistry, advection, and vertical mixing to the 24 h change were +29.6 (+17.9) µg m−3, −71.8 (−103.6) µg m−3, and −177.3 (−221.6) µg m−3 during Stage 1 (Stage 2), respectively. Small differences in the contributions of other processes were found between Stage 1 and Stage 2. Therefore, the PM2.5 increase over the BTH region during the haze formation stage was mainly attributed to strong production by the aerosol chemistry process and weak removal by the advection and vertical mixing processes. When aerosol radiative feedback was considered, the 24 h PM2.5 increase was enhanced by 4.8 µg m−3 during Stage 1, which could be mainly attributed to the contributions of the vertical mixing process (+22.5 µg m−3), the advection process (−19.6 µg m−3), and the aerosol chemistry process (+1.2 µg m−3). The restrained vertical mixing was the primary reason for the enhancement in the near-surface PM2.5 increase when aerosol radiative forcing was considered.

High levels of secondary air pollutants during COVID-19 in China have aroused great concern. In Beijing, measured daily mean peroxyacetyl nitrate (PAN) concentrations reached 4 ppb over the lockdown period (24 January to 15 February), whose averages were 2-3 times that before lockdown (1-23 January). The lockdown PAN levels also reached a high historical record based on our long-term measurements (2016-2019). Unlike ozone and PM 2.5 , PAN formation depends on less complex photochemistry between NO x and volatile organic compounds (VOCs), providing a novel approach to investigate the wintertime photochemistry during COVID-19. The GEOS-Chem simulations suggest a markedly enhanced photochemistry by a factor of 2 during the lockdown. Change of meteorology featuring with anomalous wind convergence under higher temperatures is the main reason for enhanced photochemical formation of PAN, while chemically nonlinear feedbacks also play a role. Our results suggest implementing targeted VOC emission controls in the context of increasing photochemical pollution over this complex polluted region. Plain Language Summary Outbreaks of the COVID-19 pandemic caused immediate implementation of lockdown policy in China, which drastically decreased emissions of primary air pollutants. Peroxyacetyl nitrate (PAN), as an important photochemical product, is controlled by reactions between NO x and volatile organic compounds (VOCs) that were reduced substantially due to the lockdown. However, observed PAN levels in Beijing during the lockdown were markedly enhanced and were even much higher than the concentrations during the same periods in 2016-2019. Modeling results prove that this increase in PAN is driven by enhanced photochemistry, resulting from anomalous wind convergence under higher temperature and enhanced radical level in response to NO x reduction. Our results suggest the necessity of reducing VOC emissions in controlling photochemical pollution even in the wintertime over China.

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<p><strong>Abstract.</strong> Fine–particle pollution associated with haze threatens human health, especially in the North China Plain, where extremely high PM<sub>2.5</sub> concentrations were frequently observed during winter. In this study, the WRF–Chem model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event happened over Beijing–Tianjin–Hebei (BTH) in December 2015, including examining the contributions of local emission and outside transport to the absolute PM<sub>2.5</sub> concentration in BTH, and the contributions of each detailed physical or chemical process to the variations in the PM<sub>2.5</sub> concentration. The influence mechanisms of aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using the process analysis. During the aerosol accumulation stage (December 20–22, Stage_1), the average near–surface PM<sub>2.5</sub> concentration in BTH was 250.0 µg m<sup>−3</sup>, which was contributed by local emission of 42.3 % and outside transport of 36.6 %. During the aerosol dispersion stage (December 23–27, Stage_2), the average concentration of PM<sub>2.5</sub> was 107.9 µg m<sup>−3</sup>. The contribution of local emission increased to 50.9 %, while the contribution of outside transport decreased to 24.3 %. The 24–h change (23:00LST minus 00:00LST) in the near–surface PM<sub>2.5</sub> concentration was +50.4 µg m<sup>−3</sup> during Stage_1 and −41.5 µg m<sup>−3</sup> during Stage_2. Contributions of aerosol chemistry process and vertical mixing process to the 24–h change were +43.8 (+17.9) µg m<sup>−3</sup> and −161.6 (−221.6) µg m<sup>−3</sup> for Stage_1 (Stage_2), respectively. Small differences in contributions from other processes were found between Stage_1 and Stage_2, such as advection process, cloud chemistry process, and so on. Therefore, the PM<sub>2.5</sub> increase over BTH during haze formation stage (Stage_1) was mainly attributed to strong production by aerosol chemistry process and weak removal by vertical mixing process. When aerosol radiative feedback was considered, the 24–h PM<sub>2.5</sub> increase was enhanced by 9.6 µg m<sup>−3</sup> during Stage_1, which could be mainly attributed to the contributions of vertical mixing process (+39.8 µg m<sup>−3</sup>), advection process (−38.6 µg m<sup>−3</sup>) and aerosol chemistry process (+5.1 µg m<sup>−3</sup>). The restrained vertical mixing could be the primary reason for the enhancement in near–surface PM<sub>2.5</sub> increase when aerosol radiative forcing was considered.</p>

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