Vertical Evolution Of The Concentrations And Sources Of Volatile Organic Compounds In Urban Beijing

Volatile organic compounds（VOCs）are not only important atmospheric pollutants,but also participate in atmospheric photochemical reactions to generate ozone（O₃）and secondary organic aerosols,exacerbating atmospheric pollution.In recent years,the concentrations of PM_2.5 and VOCs have significantly decreased,but O₃ remains high,indicating that the prevention and control strategies developed solely based on research in the horizontal direction near the ground are insufficient.Therefore,mastering the vertical evolution of VOCs is a necessary step to achieve precise prevention and control.In this study,two sets of vertical detection systems for pipelines and pods of the 325m meteorological tower of the Institute of Atmospheric Physics,Chinese Academy of Sciences in Beijing were constructed.In the summer of 2021,the summer of 2022 and the winter of 2023,three vertical detection experiments were carried out for VOCs in the low atmospheric boundary layer.The aim is to elucidate the vertical evolution characteristics of VOCs,discuss the differences in vertical distribution of VOCs in different atmospheric stability levels,analyze the sources of VOCs at different heights,and compare the vertical distribution structure and seasonal differences in sources of VOCs.The vertical VOCs concentration in summer was 22.0±9.6 ppbv.The concentration of VOCs was low in the afternoon and the vertical difference was not significant,while it decreased with increasing height in the morning,afternoon,and evening.Alkanes have the highest proportion at all heights（>44%）.To analyze the causes of vertical evolution differences,it was found that in convective and stable conditions,VOCs decreased with increasing height according to atmospheric stability classification.In convective conditions,the VOC concentrations decreased by 21%between 8-240 m;In stable conditions,it decreased by 33%.Background sources（23%）and fuel combustion（21%）were the two main sources of VOCs in the vertical direction.As atmospheric stability transitions from neutral to convective to stable,the contribution of background sources gradually decreased,while the contribution of combustion related sources gradually increased.The vertical VOCs concentration in winter was 41.5±20.8 ppbv.The vertical variation of VOCs concentration was not significant in the noon and afternoon of winter,but decreased with height in the morning and evening.Alkanes have the highest proportion at all heights（>48%）.According to the classification of atmospheric stability,it was found that in winter,in the three types of atmospheric stability,the VOC concentrations decreased with increasing height.Fuel combustion（37%）and gasoline vehicular emissions（24%）are the two main sources of vertical VOCs in Beijing during winter.The more stable the atmospheric stability,the lower the contribution of background sources,and the higher the contribution of fuel combustion and gasoline vehicular emissions.The concentration of VOCs in winter was 17.0 ppbv higher than that in summer.Except for halohydrocarbon,the concentration of other types of VOCs in winter was significantly higher than that in summer.The concentration of VOCs in winter was about1.8 times higher than that in summer at all heights.The majority of winter stable conditions result in a uniform distribution of VOCs above 50 m.The contribution of fuel combustion sources in winter was 16%higher than that in summer.With the control of air pollution,the potential for reducing volatile organic compounds emissions in Beijing has decreased.Regional joint prevention and control in summer and controlling the contribution of local combustion sources in winter are the main ways to reduce VOCs pollution in Beijing.Figure 35 Table 6 Reference 104...