El Niño–Southern Oscillation (ENSO), a phenomenon of periodic changes in sea surface temperature in the equatorial central-eastern Pacific Ocean, is the strongest signal of interannual variability in the climate system with a quasi-period of 2–7 years. El Niño events have been shown to have important influences on meteorological conditions in China. In this study, the impacts of El Niño with different durations on aerosol concentrations and haze days during December–January–February (DJF) in China are quantitatively examined using the state-of-the-art Energy Exascale Earth System Model version 1 (E3SMv1). We find that PM 2.5 concentrations are increased by 1–2  µg m −3 in northeastern and southern China and decreased by up to 2.4  µg m −3 in central-eastern China during El Niño events relative to the climatological means. Compared to long-duration (LD) El Niño events, El Niño with short duration (SD) but strong intensity causes northerly wind anomalies over central-eastern China, which is favorable for aerosol dispersion over this region. Moreover, the anomalous southeasterly winds weaken the wintertime prevailing northwesterly in northeastern China and facilitate aerosol transport from southern and southeast Asia, enhancing aerosol increase in northeastern China during SD El Niño events relative to LD El Niño events. In addition, the modulation effect on haze days by SD El Niño events is 2–3 times more than that by LD El Niño events in China. The aerosol variations during El Niño events are mainly controlled by anomalous aerosol accumulation/dispersion and transport due to changes in atmospheric circulation, while El Niño-induced precipitation change has little effect. The occurrence frequency of SD El Niño events has been increasing significantly in recent decades, especially after the 1940s, suggesting that El Niño with short duration has exerted an increasingly intense modulation on aerosol pollution in China over the past few decades.

We developed a top-down methodology combining the inversed chemistry transport modeling and satellite-derived tropospheric vertical column of NO 2 and estimated the NO x emissions of the Yangtze River Delta (YRD) region at a horizontal resolution of 9 km for January, April, July, and October 2016. The effect of the top-down emission estimation on air quality modeling and the response of ambient ozone (O 3 ) and inorganic aerosols (SO 4 2 - , NO 3 - , and NH 4 + , SNA) to the changed precursor emissions were evaluated with the Community Multi-scale Air Quality (CMAQ) system. The top-down estimates of NO x emissions were smaller than those (i.e., the bottom-up estimates) in a national emission inventory, Multi-resolution Emission Inventory for China (MEIC), for all the 4 months, and the monthly mean was calculated to be 260.0 Gg/month, 24 % less than the bottom-up one. The NO 2 concentrations simulated with the bottom-up estimate of NO x emissions were clearly higher than the ground observations, indicating the possible overestimation in the current emission inventory, attributed to its insufficient consideration of recent emission control in the region. The model performance based on top-down estimate was much better, and the biggest change was found for July, with the normalized mean bias (NMB) and normalized mean error (NME) reduced from 111 % to − 0.4 % and from 111 % to 33 %, respectively. The results demonstrate the improvement of NO x emission estimation with the nonlinear inversed modeling and satellite observation constraint. With the smaller NO x emissions in the top-down estimate than the bottom-up one, the elevated concentrations of ambient O 3 were simulated for most of the YRD, and they were closer to observations except for July, implying the VOC (volatile organic compound)-limited regime of O 3 formation. With available ground observations of SNA in the YRD, moreover, better model performance of NO 3 - and NH 4 + was achieved for most seasons, implying the effectiveness of precursor emission estimation on the simulation of secondary inorganic aerosols. Through the sensitivity analysis of O 3 formation for April 2016, the decreased O 3 concentrations were found for most of the YRD region when only VOC emissions were reduced or the reduced rate of VOC emissions was 2 times of that of NO x , implying the crucial role of VOC control in O 3 pollution abatement. The SNA level for January 2016 was simulated to decline 12 % when 30 % of NH 3 emissions were reduced, while the change was much smaller with the same reduced rate for SO 2 or NO x . The result suggests that reducing NH 3 emissions was the most effective way to alleviate SNA pollution of the YRD in winter.

Fire is an important source of ozone ( O 3 ) precursors. The formation of surface O 3 can cause damage to vegetation and reduce stomatal conductance. Such processes can feed back to inhibit dry deposition and indirectly enhance surface O 3 . Here, we apply a fully coupled chemistry–vegetation model to estimate the indirect contributions of global fires to surface O 3 through O 3 –vegetation feedback during 2005–2012. Fire emissions directly increase the global annual mean O 3 by 1.2  ppbv (5.0 %) with a maximum of 5.9  ppbv (24.4 %) averaged over central Africa by emitting a substantial number of precursors. Considering O 3 –vegetation feedback, fires additionally increase surface O 3 by 0.5  ppbv averaged over the Amazon in October, 0.3  ppbv averaged over southern Asia in April, and 0.2  ppbv averaged over central Africa in April. During extreme O 3 –vegetation interactions, such a feedback can rise to >0.6   ppbv in these fire-prone areas. Moreover, large ratios of indirect-to-direct fire O 3 are found in eastern China (3.7 %) and the eastern US (2.0 %), where the high ambient O 3 causes strong O 3 –vegetation interactions. With the likelihood of increasing fire risks in a warming climate, fires may promote surface O 3 through both direct emissions and indirect chemistry–vegetation feedbacks. Such indirect enhancement will cause additional threats to public health and ecosystem productivity.

Atmospheric CO 2 mole fractions are observed at Beijing (BJ), Xianghe (XH), and Xinglong (XL) in North China using Picarro G2301 cavity ring-down spectroscopy instruments. The measurement system is described comprehensively for the first time. The geographical distances among these three sites are within 200  km , but they have very different surrounding environments: BJ is inside the megacity; XH is in the suburban area; XL is in the countryside on a mountain. The mean and standard deviation of CO 2 mole fractions at BJ, XH, and XL between October 2018 and September 2019 are 448.4±12.8 , 436.0±9.2 , and 420.6±8.2   ppm , respectively. The seasonal variations of CO 2 at these three sites are similar, with a maximum in winter and a minimum in summer, which is dominated by the terrestrial ecosystem. However, the seasonal variations of CO 2 at BJ and XH are more affected by human activities as compared to XL. Using CO 2 at XL as the background, CO 2 enhancements are observed simultaneously at BJ and XH. The diurnal variations of CO 2 are driven by the boundary layer height, photosynthesis, and human activities at BJ, XH, and XL. We also compare the CO 2 measurements at BJ, XH, and XL with five urban sites in the USA, and it is found that the CO 2 mean concentration at BJ is the largest. Moreover, we address the impact of the wind on the CO 2 mole fractions at BJ and XL. This study provides an insight into the spatial and temporal variations of CO 2 mole fractions in North China.

Light absorption and radiative forcing of black carbon (BC) is influenced by both BC itself and its interactions with other aerosol chemical compositions. Although the changes in BC concentrations in response to emission reduction measures have been well documented, the influence of emission reductions on the light absorption properties of BC and its influence on BC-boundary-layer interactions has been less explored. In this study, we used the online coupled WRF-Chem model to examine how emission control measures during the Asia-Pacific Economic Cooperation (APEC) summit affect the mixing state and light absorption of BC, and the associated implications for BC-PBL interactions. We found that both the mass concentration of BC and the BC coating materials declined during the APEC week, which reduced the light absorption and light absorption enhancement ( E ab ) of BC. The reduced absorption aerosol optical depth (AAOD) during APEC was caused by both the decline in the mass concentration of BC itself (52.0 %), and the lensing effect of BC (48.0 %). The reduction in coating materials (39.4 %) contributed the most to the influence of the lensing effect, and the reduced light absorption capability ( E ab ) contributed 3.2 % to the total reduction in AAOD. Reduced light absorption of BC due to emission control during APEC enhanced planetary boundary layer height (PBLH) by 8.2 m. PM 2.5 and O 3 were found to have different responses to the changes in the light absorption of BC. Reduced light absorption of BC due to emission reductions decreased near-surface PM 2.5 concentrations but near-surface O 3 concentrations were enhanced in the North China Plain. These results suggest that current measures to control SO 2 , NO x , etc. would be effective in reducing the absorption enhancement of BC and in inhibiting the feedback of BC on the boundary layer. However, enhanced ground O 3 might be a side effect of current emission control strategies. How to control emissions to offset this side effect of current emission control measures on O 3 should be an area of further focus.

Fire is an important source of ozone ( O 3 ) precursors. The formation of surface O 3 can cause damage to vegetation and reduce stomatal conductance. Such processes can feed back to inhibit dry deposition and indirectly enhance surface O 3 . Here, we apply a fully coupled chemistry–vegetation model to estimate the indirect contributions of global fires to surface O 3 through O 3 –vegetation feedback during 2005–2012. Fire emissions directly increase the global annual mean O 3 by 1.2  ppbv (5.0 %) with a maximum of 5.9  ppbv (24.4 %) averaged over central Africa by emitting a substantial number of precursors. Considering O 3 –vegetation feedback, fires additionally increase surface O 3 by 0.5  ppbv averaged over the Amazon in October, 0.3  ppbv averaged over southern Asia in April, and 0.2  ppbv averaged over central Africa in April. During extreme O 3 –vegetation interactions, such a feedback can rise to >0.6   ppbv in these fire-prone areas. Moreover, large ratios of indirect-to-direct fire O 3 are found in eastern China (3.7 %) and the eastern US (2.0 %), where the high ambient O 3 causes strong O 3 –vegetation interactions. With the likelihood of increasing fire risks in a warming climate, fires may promote surface O 3 through both direct emissions and indirect chemistry–vegetation feedbacks. Such indirect enhancement will cause additional threats to public health and ecosystem productivity.