Research On Effects Of Human Activities And Meteorological Conditions On Seasonal Air Pollution In China Over Recent Years

After the reform and opening up in China,along with the rapid development of industry and economy,living standards of urban residents have greatly improved.However,a large number of air pollutants emissions are concentrated in urban areas,exceeding the carrying capacity of the environment and resulting in a series of severe issues of urban air pollution.Haze phenomenon characterized by high concentration of fine particulate matter(PM_2.5,particulate matter with aerodynamic diameter less than 2.5μm)and photochemical smog characterized by surface ozone are two major air pollution events in cities.The occurrences of PM_2.5 pollution and ozone pollution exist significant seasonal differences,respectively in winter and summer.To deal with haze pollution in winter,China has implemented its toughest air pollution prevention and control action plan since 2013 on these aspects such as reducing pollutants emissions,reducing energy consumption,and carrying out cleaner production,etc.The implementation of these pollution prevention and control measures has resulted in a significant decline in the annual average mass concentration of PM_2.5 in China in recent years,but serious haze pollution events still occur frequently in winter.More unexpectedly,statistical data showed an insignificant decrease in PM_2.5 concentrations in central China,mainly focused on Henan province.It seems that anthropogenic emission controls have failed to improve air quality in central China during the winter,which caused a lot of confusion about the further air quality improvement strategy.On the other hand,China has undertaken a continuous ecological afforestation program in response to global climate change.Satellite data showed that China’s vegetation area has increased significantly since 2000,and vegetation will maintain this growth trend in the foreseeable future to achieve the strategic goal of carbon neutrality by 2060.Biogenic organic compounds（BVOCs）released by plants are important precursors of ozone formation,and vegetation growth has the highest abundance in summer,which is consistent with the high occurrence of ozone pollution in summer.However,limited work has examined the impact of greening pattern in China on summer surface ozone pollution.In view of the above problems,this paper firstly discussed the reasons for the frequent occurrence of haze pollution in winter in China,especially the insignificant decrease trend of PM_2.5 in central China,and then analyzed the influence of vegetation greening pattern in China on surface ozone in summer.The specific research contents and conclusions are as follows:（1）The inverse of atmospheric visibility,aerosol optical extinction coefficient can provide a proxy for assessing haze pollution levels,especially for submicron particles（i.e.,PM₁,aerodynamic diameter less than 1.0μm）because they are close to the wavelength of visible solar radiation and are very efficient in reducing atmospheric visibility.In past decades,many studies had indicated that optical extinction showed an opposite trend to fine aerosol particles(i.e.,PM_2.5,aerodynamic diameter less than 2.5μm).However,little work has examined the changing trend of optical extinction since the implementation of the toughest-ever clean air policy in 2013.In addition,it is still unclear whether the changing trend of extinction coefficient is consistent with that of PM_2.5 concentrations.For this,we established a random forest model based on environmental big data including air quality data,meteorological data,land cover data and other predictive variables to predict the extinction coefficient of eastern China during the winter of 2014-2019.The temporal and spatial variation characteristics of extinction coefficient were well captured by the model with a cross-validation R²=0.72 and RMSE=0.12.The simulation results showed that the high extinction coefficient is located in North China in winter,indicating that the region is still affected by haze pollution.From 2014 to 2018,the extinction coefficient in eastern China decreased at an annual rate of 3.6%,and the range of influence also shrank year by year.In addition,statistical analysis based on grid cells showed that the decreasing trend of extinction coefficient was slower than that of PM_2.5 mass concentrations,which may be related to the increased proportion of secondary aerosols in PM_2.5.It is worth noting that both the extinction coefficient and PM_2.5 concentration in the central region of China（mainly located in Henan province）showed insignificant downward trend,indicating that the current situation of air pollution in this region is still not optimistic.These results provide the latest evidence for the trend of aerosol extinction properties and emphasize the importance of reducing secondary aerosol formation to control haze pollution in eastern China.（2）In order to reveal the driving mechanism of the insignificant decrease trend of PM_2.5mass concentration in winter in central China（mainly located in Henan province）from 2014to 2018,we first analyzed the changes of anthropogenic source emissions and meteorological conditions in this region statistically,and compared them with the Beijing-Tianjin-Hebei（BTH）and Yangtze River Delta（YRD）megacity clusters.We found that in recent years,the anthropogenic pollutant emissions already had a big reduction.And compared with BTH and YRD regions,the central China had a larger reduction in emissions.On the contrary,in terms of meteorological conditions including relative humidity,precipitation,boundary layer height,wind speed,and surface temperature during the study,we calculated their changing trends.Among these meteorological factors,the change of relative humidity was the most significant（P<0.05）.Especially in central China,the relative humidity increases at a rate of 2.5%per year.The increase of humidity in the air may provide a good breeding ground for atmospheric physical and chemical reactions,thus exacerbating the formation of atmospheric particulate matter pollution in the central region.On this basis,we further quantified the contribution of anthropogenic emission reductions and meteorology changes to PM_2.5 and its components concentrations using the WRF-Chem regional air quality model.The simulation results showed that the emission reductions of anthropogenic primary pollutants can effectively reduce PM_2.5 concentrations in the central region by 41.8μg/m³,but the adverse meteorological conditions can significantly increase PM_2.5 concentrations by 27.0μg/m³.From the data of secondary components in simulated PM_2.5,nitrate is the main species with increased concentration in PM_2.5 components in the central region,and its contribution to PM_2.5 components increased from 22.4%in January 2015 to 39.7%in January 2019.This is mainly due to the insufficient reduction of nitrogen oxides compared with sulfur dioxide,which causes that more nitrogen oxides in the atmosphere can be converted to nitrate in the presence of ammonia.At the same time,adverse meteorological conditions,especially the increase in atmospheric humidity,further increased the formation of nitrate through heterogeneous reaction,resulting in an insignificant decreasing trend of PM_2.5 concentration in central China in winter.The above results indicate that it is necessary to strengthen the emission reduction of nitrogen oxides from anthropogenic sources,so as to reduce the production of nitrate in PM_2.5 components and improve the PM_2.5 pollution situation in the central region.（3）Vegetation greening process in China has experienced unprecedented changes,which may fundamentally increase the emissions of natural biogenic VOCs and thus have an impact on summer surface ozone pollution.In view of the current limited research on the effect of vegetation greening on summer surface ozone pollution,this study took vegetation leaf area（LAI）as an indicator of vegetation growth trend,and connected ozone concentrations with vegetation greening through regional model simulation.On the whole,vegetation change in China increased at an average rate of 0.016 m²/m² per year,and vegetation variation driven by human activities increased rapidly at a rate of 0.011 m²/m²,while vegetation growth rate was only 0.005 m²/m² under the influence of climate change.Using 2016 as the base year,we simulated the effects of vegetation changes on ozone concentrations in both historical（2000-2016）and future（2030-2050）periods using the WRF-Chem regional air quality models.During historical periods,we found a strong response of ozone concentrations to increasing LAI in China,especially in megacity clusters where ozone chemical is largely controlled by VOCs sensitivity or mixing mechanisms.Noticeably,in the BTH region,vegetation greening significantly increased ozone concentrations by as much as 4.1 ppb（4.7%）.In the future scenario of vegetation continuing to increase,we designed three anthropogenic VOCs reduction scenarios for offsetting the impact of vegetation greening.Overall,we estimate that a 10%reduction in anthropogenic VOCs emissions in 2030 will offset the impact of vegetation greening in megacity clusters of east China,and an about 15%reduction in 2050 is required to achieve the offset.Currently,China is sparing no effort to increase the vegetation cover,but the worry is that natural emissions over megacity clusters with highly dense vegetation may cause more surface ozone pollution,which will increase the pressure of anthropogenic VOCs reductions.This study emphasizes the need to consider the underappreciated effects of vegetation greening for future policies aimed at mitigating ozone pollution in China,especially in the megacity clusters.