SignificanceDrastic air pollution control in China since 2013 has achieved sharp decreases in fine particulate matter (PM2.5), but ozone pollution has not improved. After removing the effect of meteorological variability, we find that surface ozone has increased in megacity clusters of China, notably Beijing and Shanghai. The increasing trend cannot be simply explained by changes in anthropogenic precursor [NOx and volatile organic compound (VOC)] emissions, particularly in North China Plain (NCP). The most important cause of the increasing ozone in NCP appears to be the decrease in PM2.5, slowing down the sink of hydroperoxy radicals and thus speeding up ozone production. Decreasing ozone in the future will require a combination of NOx and VOC emission controls to overcome the effect of decreasing PM2.5.

Abstract. Surface ozone data from the Chinese Ministry of Ecology and Environment (MEE) network show sustained increases across the country over the 2013–2019 period. Despite Phase 2 of the Clean Air Action Plan targeting ozone pollution, ozone was higher in 2018–2019 than in previous years. The mean summer 2013–2019 trend in maximum 8 h average (MDA8) ozone was 1.9 ppb a−1 (p<0.01) across China and 3.3 ppb a−1 (p<0.01) over the North China Plain (NCP). Fitting ozone to meteorological variables with a multiple linear regression model shows that meteorology played a significant but not dominant role in the 2013–2019 ozone trend, contributing 0.70 ppb a−1 (p<0.01) across China and 1.4 ppb a−1 (p=0.02) over the NCP. Rising June–July temperatures over the NCP were the main meteorological driver, particularly in recent years (2017–2019), and were associated with increased foehn winds. NCP data for 2017–2019 show a 15 % decrease in fine particulate matter (PM2.5) that may be driving the continued anthropogenic increase in ozone, as well as unmitigated emissions of volatile organic compounds (VOCs). VOC emission reductions, as targeted by Phase 2 of the Chinese Clean Air Action Plan, are needed to reverse the increase in ozone.

Published by Copernicus Publications on behalf of the European Geosciences Union. 8340 X. Lu et al.: Exploring 2016-2017 surface ozone pollution over ChinaBVOC ozone enhancements) and ozone chemical production, increase the thermal decomposition of peroxyacetyl nitrate (PAN), and further decrease ozone dry deposition velocity. More stringent emission control measures are thus required to offset the adverse effects of unfavorable meteorology, such as high temperature, on surface ozone air quality.

Severe haze pollution in eastern China has caused substantial health impacts and economic loss. Conducive atmospheric conditions are important to affect occurrence of severe haze events, and circulation changes induced by future global climate warming are projected to increase the frequency of such events. However, a potential contribution of an anthropogenic influence to recent most severe haze (December 2015 and January 2013) over eastern China remains unclear. Here we show that the anthropogenic influence, which is estimated by using large ensemble runs with a climate model forced with and without anthropogenic forcings, has already increased the probability of the atmospheric patterns conducive to severe haze by at least 45% in January 2013 and 27% in December 2015, respectively. We further confirm that simulated atmospheric circulation pattern changes induced by anthropogenic influence are driven mainly by increased greenhouse gas emissions. Our results suggest that more strict reductions in pollutant emissions are needed under future anthropogenic warming.

h i g h l i g h t sContributions from five domestic sectors and emissions outside China are simulated. Residential and industry sectors are the largest contributors to BC levels in China. The TOA direct radiative forcing of BC in China is simulated to be 1.22 Wm À2 in 2010. Domestic and non-China emissions contribute 75% and 25% to BC forcing, respectively. a b s t r a c tWe quantify the contributions from five domestic emission sectors (residential, industry, transportation, energy, and biomass burning) and emissions outside of China (non-China) to concentration and direct radiative forcing (DRF) of black carbon (BC) in China for year 2010 using a nested-grid version of the global chemical transport model (GEOS-Chem) coupled with a radiative transfer model. The Hemispheric Transport of Air Pollution (HTAP) anthropogenic emissions of BC for year 2010 are used in this study. Simulated surface-layer BC concentrations in China have strong seasonal variations, which exceed 9 mg m À3 in winter and are about 1e5 mg m À3 in summer in the North China Plain and the Sichuan Basin.Residential sector is simulated to have the largest contribution to surface BC concentrations, by 5 e7 mg m À3 in winter and by 1e3 mg m À3 in summer, reflecting the large emissions from winter heating and the enhanced wet deposition during summer monsoon. The contribution from industry sector is the second largest and shows relatively small seasonal variations; the emissions from industry sector contribute 1e3 mg m À3 to BC concentrations in the North China Plain and the Sichuan Basin. The contribution from transportation sector is the third largest, followed by that from biomass burning and energy sectors. The non-China emissions mainly influence the surface-layer concentrations of BC in western China; about 70% of surface-layer BC concentration in the Tibet Plateau is attributed to transboundary transport. Averaged over all of China, the all-sky DRF of BC at the top of the atmosphere (TOA) is simulated to be 1.22 W m À2 . Sensitivity simulations show that the TOA BC direct radiative forcings from the five domestic emission sectors of residential, industry, energy, transportation, biomass burning, and non-China emissions are 0.44, 0.27, 0.01, 0.12, 0.04, and 0.30 W m À2 , respectively. The domestic and non-China emissions contribute 75% and 25% to BC DRF in China, respectively. These results have important implications for taking measures to reduce BC emissions to mitigate near-term climate warming and to improve air quality in China.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Satellite observations of tropospheric NO&lt;sub&gt;2&lt;/sub&gt; columns are extensively used to infer trends in anthropogenic emissions of nitrogen oxides (NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;&amp;#8201;&amp;#8801;&amp;#8201;NO&amp;#8201;+&amp;#8201;NO&lt;sub&gt;2&lt;/sub&gt;), but this may be complicated by trends in NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; lifetime. Here we use 2004&amp;#8211;2018 observations from the OMI satellite-based instrument (QA4ECV and POMINO v2 retrievals) to examine the seasonality and trends of tropospheric NO&lt;sub&gt;2&lt;/sub&gt; columns over central-eastern China, and we interpret the results with the GEOS-Chem chemical transport model. The observations show a factor of 3 increase in NO&lt;sub&gt;2&lt;/sub&gt; columns from summer to winter, which we explain in GEOS-Chem as reflecting a longer NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; lifetime in winter than in summer (21&amp;#8201;h versus 5.9&amp;#8201;h in 2017). The 2005&amp;#8211;2018 summer trends of OMI NO&lt;sub&gt;2&lt;/sub&gt; closely follow the trends in the Multi-resolution Emission Inventory for China (MEIC), with a rise over the 2005&amp;#8211;2011 period and a 25&amp;#8201;% decrease since. We find in GEOS-Chem no significant trend of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; lifetime in summer, supporting the emission trend reported by MEIC. The winter trend of OMI NO&lt;sub&gt;2&lt;/sub&gt; is steeper than in summer over the entire period, which we attribute to a decrease in NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; lifetime at lower NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; emissions. Half of the NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; sink in winter is from N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; hydrolysis, which counterintuitively becomes more efficient as NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; emissions decrease due to less titration of ozone at night. Formation of organic nitrates also becomes an increasing sink of NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; as NO&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; emissions decrease but emissions of volatile organic compounds (VOCs) do not.&lt;/p&gt;

Abstract. Nadir-viewing satellite observations of tropospheric ozone in the UV have been shown to have some sensitivity to boundary layer ozone pollution episodes, but so far they have not yet been compared to surface ozone observations collected by large-scale monitoring networks. Here we use 2013–2017 surface ozone data from China's new Ministry of Ecology and Environment (MEE) network of ∼ 1000 sites, together with vertical profiles from ozonesondes and aircraft, to quantify the ability of tropospheric ozone retrievals from the Ozone Monitoring Instrument (OMI) and to detect boundary layer ozone pollution in China. We focus on summer when ozone pollution in China is most severe and when OMI has the strongest sensitivity. After subtracting the Pacific background, we find that the 2013–2017 mean OMI ozone enhancements over eastern China have strong spatial correlation with the corresponding multiyear means in the surface afternoon observations (R=0.73), and that OMI can estimate these multiyear means in summer afternoon surface ozone with a precision of 8 ppb. The OMI data show significantly higher values on observed surface ozone episode days (>82 ppb) than on non-episode days. Day-to-day correlations with surface ozone are much weaker due to OMI noise and are stronger for sites in southern China (<34∘ N; R=0.3–0.6) than in northern China (R=0.1–0.3) because of weaker retrieval sensitivity and larger upper tropospheric variability in the north. Ozonesonde data show that much of the variability of OMI ozone over southern China in summer is driven by the boundary layer. Comparison of 2005–2009 and 2013–2017 OMI data indicates that mean summer afternoon surface ozone in southern China (including urban and rural regions) has increased by 3.5±3.0 ppb over the 8-year period and that the number of episode days per summer has increased by 2.2±0.4 (as diagnosed by an extreme value model), generally consistent with the few long-term surface records. Ozone increases have been particularly large in the Yangtze River Delta and in the Hubei, Guangxi and Hainan provinces.

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