<p><strong>Abstract.</strong> In the summer of 2017, heavy ozone pollution swamped most of the North China Plain (NCP), with the maximum regional average of daily maximum 8-h ozone concentration (MDA8) reaching almost 120 ppbv. In light of the continuing reduction of anthropogenic emissions in China, the underlying mechanisms for the occurrences of these regional extreme ozone episodes are elucidated from two perspectives: meteorology and biogenic emissions. The significant positive correlation between MDA8 and temperature, which is amplified during heat waves concomitant with stagnant air and no precipitation, supports the crucial role of meteorology in driving high ozone concentrations. We also find that biogenic emissions are enhanced due to factors previously not considered. During the heavy ozone pollution episodes in June 2017, biogenic emissions driven by high vapor pressure deficit (VPD), land cover change and urban landscape yield an extra mean MDA8 ozone of 3.08, 2.79 and 4.74 ppbv, respectively over the NCP, which together contribute as much to MDA8 ozone as biogenic emissions simulated using the land cover of 2003 and ignoring VPD and urban landscape. In Beijing, the biogenic emission increase due to urban landscape has a comparable effect on MDA8 ozone to the combined effect of high VPD and land cover change between 2003 and 2016. This study highlights the vital contributions of heat waves, land cover change and urbanization to the occurrence of extreme ozone episode, with significant implications for ozone pollution control in a future when heat wave frequency and intensity are projected to increase under global warming.</p>

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Organic matter (OM) constitutes a significant fraction (20%-90%) of fine particles with aerodynamic diameter less than 2.5 (PM 2.5 ) or 1.0 μm (

<p><strong>Abstract.</strong> We illustrate the nonlinear relationships among anthropogenic NO<sub><i>x</i></sub> emissions, NO<sub>2</sub> tropospheric vertical column densities (TVCDs), and NO<sub>2</sub> surface concentrations using model simulations for July 2011 over the contiguous United States (CONUS). The variations of NO<sub>2</sub> surface concentrations and TVCDs are generally consistent and reflect well anthropogenic NO<sub><i>x</i></sub> emission variations for high-anthropogenic-NO<sub><i>x</i></sub> emission regions. For low-anthropogenic-NO<sub><i>x</i></sub> emission regions, however, nonlinearity in the emission-TVCD relationship makes it difficult to use satellite observations to infer anthropogenic NO<sub><i>x</i></sub> emission changes. The analysis is extended to 2003–2017. Similar variations of NO<sub>2</sub> surface measurements and coincident satellite NO<sub>2</sub> TVCDs over urban regions are in sharp contrast to the large variation differences between surface and satellite observations over rural regions. We find a continuous decrease of anthropogenic NO<sub><i>x</i></sub> emissions after 2011 by examining surface and satellite measurements in CONUS urban regions, but the decreasing rate is lower by 9 %–46 % than the pre-2011 period.</p>

Abstract. The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane (MeOH / DCM, 1:1, v/v), MeOH / DCM (1:2, v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ∼95 %), followed by MeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts had significantly (p<0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (>280∘) are less soluble in MeOH (∼80 %) than in DMF (∼90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed consistent temporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 times greater than the MeOH extracts in late spring and summer. The average light absorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p<0.01) than that of MeOH extracts. Source apportionment results indicated that the MeOH solubility of BrC associated with biomass burning, lubricating oil combustion, and coal combustion is similar to their DMF solubility. The BrC linked with unburned fossil fuels and polymerization processes of aerosol organics was less soluble in MeOH than in DMF, which was likely the main reason for the large difference in time series between MeOH and DMF extract absorption. These results highlight the importance of testing different solvents to investigate the structures and light absorption of BrC, particularly for the low-volatility fraction potentially originating from non-combustion sources.