Vertical changes in volatile organic compounds (VOCs) and impacts on photochemical ozone formation

Abstract. Volatile organic compounds (VOCs) play crucial roles in regulating the formation of tropospheric ozone. However, limited knowledge on the interactions between vertical VOC variations and photochemical ozone formation has hindered effective ozone control strategies, especially in large cities. In this study, we investigated the vertical changes in concentrations, compositions, and key driving factors of a large suite of VOCs using online gradient measurements taken from a 325 m tall tower in urban Beijing. We also analyzed the impact of these vertical VOC variations on photochemical ozone formation using box model simulations. Our results indicate that the vertical variations of various VOC species are strictly regulated by the diurnal evolution of the planetary boundary layer. During daytime, reactive hydrocarbons are rapidly oxidized as they mix upwards, leading to the formation of OVOCs. This process plays a more significant role in regulating photochemical ozone formation with increasing height. In the lower layer, the photochemical formation of ozone responds positively to changes in both NOx and VOCs. As a result, the production rate of ozone decreases with height due to significant reductions in the concentrations of both NOx and VOCs, but remains high in the middle and upper layers. The strong production of ozone aloft is primarily driven by high concentrations of OVOCs and hydroxyl radicals, which can act as an important source of ozone at ground level. Therefore, careful consideration should be given to the vertical variations in both photochemical ozone production rates and formation regimes in the whole boundary layer when developing regional ozone control strategies.