Tracing The Emission Sources And Formation Processes Of Airborne Secondary Inorganic Aerosols Using Multiple Stable Isotopes Techniques In Typical Cities Of China

Air pollution is a global environmental problem,which seriously affects human health,climate change and ecosystem.In recent years,secondary inorganic aerosols(NO₃^-,NH₄⁺and SO₄^2-)have become the key chemical components in China's haze events.At present,atmospheric chemical model and receptor model are widely used in the studies of the formation mechanisms and sources of secondary inorganic aerosols.These methods are difficult and uncertain.In this study,we selected typical polluted cities(Beijing,Gucheng and Xuzhou)and one clean air city(Guangzhou)to quantitatively analyze the formation mechanisms and sources of nitrogen and sulfur aerosols.The results showed that nitrogen-containing aerosols(TN)is the main chemical components in PM_2.5mass,and was mainly composed of inorganic nitrogen species(NH₄⁺-N and NO₃^--N),NO₃^-was the key component for the growth of PM_2.5concentration in haze days.The nitrogen isotope of TN(?¹⁵N-TN)in Xuzhou winter was 6.9±3.6‰.According to the optimized Bayesian model,our results demonstrated that NO_xand NH₃were important precursors for ammonium nitrate aerosols,controlling of combustion related NO_xand NH₃emissions might be an important way to reduce PM_2.5levels in this region.There is a lack of knowledge regarding the isotopic-determined contributions of different chemical processes to the vertical distributions of nitrate and sulfate aerosols within the boundary layer.Here,for the first time,we used the observations of?¹⁷O-NO₃^-at the three heights(8,120,and 260 m)to investigate the formation pathways of nitrate aerosols in urban Beijing during 2016 winter and 2017 summer.Our results showed significant vertical differences in oxidation pathways of nitrate in winter.The nitrate formation from OH adical oxidation and NO₃-related reactions(e.g.NO₃+HC)accounted for 42%and 29%at ground level,and 24%and 46%at 260 m,respectively.However,such vertical differences in nitrate formation were not found in the summer.This result may be related to the higher boundary layer height in summer,and the upper atmosphere in winter is more vulnerable to the influence of external polluted air masses.Stable sulfur isotopes(?³⁴S)of sulfate aerosols in PM_2.5were measured in Beijing,Gucheng and Guangzhou,in order to investigate the formation pathways of sulfate aerosols.The?³⁴S values of Beijing,Gucheng and Guangzhou were 4.4±1.4‰?4.5±1.2‰?2.3±0.6‰,respectively.Significant correlations were found between observed?³⁴S-SO₄^2-values and SO₂oxidation ratios,indicating the changes of sulfur isotopes were attributed to the SO₂oxidation processes.Combining sulfur isotopes and the Rayleigh fractionation law,we found TMI-catalyzed O₂and NO₂were the dominant pathways of sulfate formation in Beijing and Gucheng under haze pollution in the heating seasons.In addition,the results in Guangzhou showed that TMI pathway played an important role in the formation of sulfate in the atmosphere aloft(488 m above ground),which might be due to the transport of metal elements(e.g.Fe,Mn)to the upper atmosphere of Guangzhou by the air mass nearby.Based on the observation of stable isotopes,the formation mechanisms and sources of atmospheric nitrate and sulfate in heavy pollution periods and at different boundary layer heights in typical cities of China were quantitatively evaluated.The results provide valuable observation data for verifying the air quality model and solving the impact of regional pollution on haze formation.