Vertical evolution of the concentrations and sources of volatile organic compounds in the lower boundary layer in urban Beijing in summer.

Exploring the vertical variations in volatile organic compounds (VOCs) in the atmosphere and quantifying the sources of VOCs at different heights can help control atmospheric photochemical pollution in summer. Here, VOCs were vertically detected at three heights (47 m, 200 m and 320 m) along a 325 m tower of the Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, in the mornings (8:00) and afternoons (15:00) from May 19 to June 18, 2021. The VOC concentrations in Beijing in summer were 16.2 ± 5.6 ppbv, 14.7 ± 2.5 ppbv and 14.9 ± 3.8 ppbv at 47 m, 200 m and 320 m, respectively, and alkanes accounted for the largest proportion at all heights (>56%). The vertical gradients of the VOC concentrations and components did not significantly change, which was consistent with the summer observations of other stations in North China in recent years, but these results significantly differed from observations from more than a decade ago. To determine the reason for this, a classification based on atmospheric stability was performed, revealing that the vertical distribution of VOCs was uniform in convective and stable conditions and decreased with increasing height in neutral condition. With the transition of atmospheric stability from neutral to convective to stable, the contributions of fuel combustion sources and solvent use sources gradually increased, while those of biogenic sources and background sources gradually decreased. With increasing height, the contributions of background sources increased, those of biogenic sources, solvent use and gasoline vehicular emissions decreased, and those of fuel combustion and industrial emissions remained basically unchanged. The above results indicated that with air pollution treatment, the potential for reducing emissions of VOCs in Beijing has decreased. Therefore, regional joint prevention and control are the main ways to control VOC pollution in Beijing.