The Study On Spatial-Temporal Variations And Formation Mechanisms Of Ozone And Secondary Inorganic Aerosols In North China Plain

The ambient air pollution in China has become increasingly serious and has become one of the most prominent environmental issues.The great mass of PM2.5 suspended in the air during haze can not only adversely affect public health,but also affect climate by scattering and absorbing solar radiation.Ozone,one of the most important reactive gases in the troposphere,being an atmospheric pollutant,tropospheric ozone could cause risks to human health,exert detrimental effects on vegetation and crops.It is also a greenhouse gas closely related to climate change and environmental issues.In order to solve the air quality problems,Chinese government has successively implemented a variety of regulation measures.Accordingly,the ambient concentrations of SO2 and NOx has decreased in different degrees.However,PM2 5 and ozone are still at relatively high concentrations.Severe massive haze episodes with particulate matter being the dominant pollutant bro out frequently in China,especially in the North China Plain in recent years,meanwhile,ozone pollution is also worsening,mainly due to a sharp increase in emissions of chemical precursors.Secondary inorganic aerosols(SIA,mainly sulfate,nitrate,and ammonium)could account for a major fraction of PM2 5,increasing with elevated pollution levels,and were suggested to be a principal driving force for the rapid increase of PM2.5 concentrations.To study the photochemical sensitivity of ozone,the main formation pathways of SO42-and NO3-during haze are the basis for the study of ozone and PM,crucial for controlling ozone and PM pollution.Meanwhile,the spatial-temporal distribution of ozone and PM are highly variable.To study the vertical structures of ozone and aerosols in low troposphere,especially in boundary layer closely related to human activities,is of great significance for a comprehensive understanding of mechanisms and processes of ozone and PM pollutions.Nevertheless,compared with PM2.5,the observation data of ozone and its chemical precursors including VOCs and NOx are relatively limited and the majority of them are confined to surface level.In this study,a comprehensive observation of ozone,its precursors including HCHO and NO2,as well as aerosols in the low troposphere in North China Plain was conducted to elucidate the O3 formation processes and sensitivities.The water-soluble ion composition of PM2.5 was also analyzed to calculate the aerosol pH value using ISORROPIA-II model.Via the analysis of SNA concentrations,aerosol acidity and the parameterized model calculations,the main pathways for the secondary formation of sulfate and nitrate aerosols in haze were interpreted.The relationship between ozone and fine PM was also discussed.The main contents and conclusions are as follows:(1)On haze days,O3 had a positive gradient in the low troposphere,and presented a highly stratified structure at night,with an O3 depletion in the nocturnal boundary layer and high concentrations of ozone preserved in the upper layer.Conversely,the aerosol extinction coefficients decreased with increasing altitude related to anthropogenic emissions.On clean days,both of them were completely uniformly distributed within the low troposphere.The polluted conditions were found to be characterized by higher levels of pollutants(PM,ozone,HCHO and NO2)and more dynamic variations in the boundary layer with respect to clean periods.In haze,ozone in lower to middle boundary layer and upper boundary layer exhibited similar single peak diurnal patterns.On clean days,ozone in boundary layer remained almost constant,with a weak dynamic diurnal cycle.(2)The ozone concentrations in boundary layer is affected by meteorological factors(as well as regional transport)significantly.During haze episode,the high concentrations of boundary layer O3 were attributed to the transport from industrial and urbanized regions in the south.On clean days O3 concentrations showed minor fluctuations,associated with northwestern air masses from background regions.(3)The HCHO/NO2 ratios were employed to indicate the photochemical sensitivity of ozone.The results showed that the sensitivity of O3 photochemical production was under a VOC-limited regime on haze days and turned into a mixed-VOC-NOx-limited regime on clean days.The overall variations of daily daytime O3 within the boundary layer were found to share a similar trend with HCHO VCDs,confirming that the variations of O3 concentrations were dominated by local photochemical production to a large extent.In this study,the reduction of VOCs would favor the mitigation of O3 pollution when the HCHO/NO2 ratios are below 1.5.The control of NO2 is trickier considering that it can exert adverse impacts on O3 formation under different regimes.(4)Sulfate,nitrate and ammonium were dominant ions in PM2 5 in both urban Beijing and rural site in Gucheng.The averaged SO42-,NO3-,and NH4+ concentration was 8.3,12.5 and 14.1 ?g m-3 at CMA site as well as 14.0,14.2 and 24.2 ?g m-3 at Gucheng site during the sample period.NO3-concentrations exceeded SO42-at both sites in contrast to previous studies,indicating the variations in SO2 and NOx emissions.The findings that NH4+ being the dominant component of SNA emphasized the significance of controlling on NH3 emissions in future.(5)The aerosol acidity was predicted using ISORROPIA-II model.Under haze conditions,the pH ranged from 4.9 to 5.2 with an average of 5.0 at CMA site,as well as 4.6 to 6.3 with a mean value of 5.3 at Gucheng site,both in a narrow range.The aerosol pH on clean days ranged from 6.0-7.7 with an average of 6.9.Aerosol particles were proved to be acidic in haze and also retained high neutralizing capacity associated with high NH3 emissions in North China Plain.The sensitivity analysis of aerosol pH during haze suggested a relatively weak sensitivity of pH to a wide range of SO42-,NO3-,and NH4+ levels.Also,the aerosol pH was more sensitive to SO42-and NH4+ with adverse response tendencies,compared with NO3-.(6)The secondary SO42-,NO3-,and NH4+ concentrations increased significantly under haze conditions.The contributions of gas-phase oxidation pathways,liquid-phase reactions occurring in clouds were found to be limited on the formation of SO42-and NO3-in haze.With the analysis of SOR,NOR,RH and the parameterized model calculation,heterogeneous reactions in the aerosol water were found to play an essential role in significant enhancement of sulfate and nitrate in haze pollution.In haze,according to the pH environment we predicted,oxidation by the dissolved NO2 and the hydrolysis of N2O5 on the acidic aerosol surface should be the major pathways for heterogeneous formation of sulfate and nitrate.(7)The surface O3 levels in both CMA and Gucheng site decreased with increasing PM2.5 concentrations during observation period.At UCAS site,O3 with higher concentrations occurred in PM pollution,and daytime O3 concentrations were found to correlate with AOD positively.Aerosols could cause either an increase or reduction in O3 concentrations via heterogeneous reactions on the surface of aerosols and changing photolysis rates of O3 in haze episode.Some common factors could be responsible for the enhancement in both O3 and aerosols,such as the increase of VOCs.