| With the enactment of Clean Air Action in 2013,concentrations of PM2.5(particulate matter with an aerodynamic equivalent diameter of 2.5?m or less)decreased dramatically while those of ozone(O3)increased in China,accompanying with more frequent O3 pollution events.High concentrations of O3 are not only harmful to human health but also detrimental ecosystems.Concentration of O3 is closely correlated to meteorological conditions and anthropogenic emissions,and is also affected by aerosol-radiation interactions.Aerosol-radiation interactions,including the effects of aerosol-photolysis interaction(API)and aerosol-radiation feedback(ARF),can influence meteorological variables such as photolysis rate,temperature and boundary layer height and then affect O3 concentration.Therefore,it is necessary to explore the influence mechanism of aerosol on O3,especially the respective roles of API and ARF on O3and the response of aerosol-radiation interactions to Clean Air Action and its influence on O3concentration,which is essential for coordinated air pollution control strategies.First,we examined the impacts of aerosol-radiation interactions,including API and ARF,on surface-layer ozone(O3)concentrations during four multi-pollutant air pollution episodes characterized by high O3 and PM2.5levels(PM2.5 concentration?75?g m-3 as well as maximum daily 8 h average ozone concentration?80 ppb)during 28 July to 3 August 2014(Episode1),8-13 July 2015(Episode2),5-11 June 2016(Episode3),and 28 June to 3 July 2017(Episode4)in North China,by using the Weather Research and Forecasting with Chemistry(WRF-Chem)model embedded with an integrated process analysis scheme.Our results show that API and ARF reduced the daytime shortwave radiative fluxes at the surface by 92.4?102.9 W m-2 and increased daytime shortwave radiative fluxes in the atmosphere by 72.8?85.2 W m-2,as the values were averaged over the complex air pollution areas(CAPAs)in each of the four episodes.As a result,the stabilized atmosphere decreased the daytime planetary boundary layer height and 10 m wind speed by 129.0?249.0 m and 0.05?0.15 m s-1,respectively,in CAPAs in the four episodes.Aerosols were simulated to reduce the daytime near-surface photolysis rates of J[NO2]and J[O1D]by 1.8×10-3?2.0×10-3s-1 and 5.7×10-6?6.4×10-6 s-1,respectively,in CAPAs in the four episodes.All the four episodes show the same conclusion that the reduction in O3 by API is larger than that by ARF.API(ARF)was simulated to change daytime surface-layer O3concentrations by-8.5 ppb(-2.9 ppb),-10.3 ppb(-1.0 ppb),-9.1 ppb(-0.9 ppb)and-11.4 ppb(+0.7 ppb)in CAPAs of the four episodes,respectively.Process analysis indicated that the weakened O3 chemical production made the greatest contribution to API effect,while the reduced vertical mixing was the key process for ARF effect.These results suggest that future PM2.5 reductions may lead to O3increases.Secondly,we applied surface-layer observations and a coupled meteorology-chemistry model(WRF-Chem)to quantify the response of aerosol-radiation interactions to Clean Air Action and its contribution to the increased O3 concentrations over eastern China(22?41.5°N,102?123°E)in summer and winter.The model can well reproduce the spatiotemporal changes and the increasing trends of O3 concentrations both in summer and winter over the eastern China.Sensitivity studies show that the daily maximum 8-h average O3 concentrations(MDA8 O3)changes vary spatially and seasonally,and the increase of MDA8 O3 in winter is dominated by emission changes.However,in specific regions in summer,e.g.,south China,southwestern China and Beijing-Tianjin-Hebei region,the impact of meteorology on MDA8 O3is greater than that of emission changes.The enhancement of MDA8 O3 by emission reduction induced weakened aerosol-radiation interactions over the urban areas is 1.77 ppb(0.44 ppb)in summer(winter).Moreover,the weakened aerosol-radiation interaction caused the MDA8 O3 increase in summer(winter)by 0.81 ppb(0.63 ppb)over eastern China,with weakened API and ARF contributing 55.6%(61.9%)and 44.4%(38.1%),respectively.Furthermore,the additional simulations show that the weakened scattering aerosol-photolysis interaction plays a leading role in the weakened API.Process analysis indicates that the increased O3 chemical production makes the greatest contribution to the weakened API effect in both summer and winter,while the enhanced vertical mixing is the key process for the weakened ARF effect.We also found that the enhancement of O3 concentration in eastern China is caused by the increased O3chemical production both in summer and winter.These result reveal that weakened aerosol-radiation interactions can increase O3 concentration,thus more coordinated air pollution control strategies are needed. |
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