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

Volatile organic compounds (VOCs) play crucial roles in regulating the photochemical formation of ozone. However, limited knowledge on the interactions between vertical VOCs change and ozone formation in the planetary boundary layer (PBL) has hindered effective ozone control strategies, particularly in large cities. To address concern, we investigated the vertical changes in concentrations, compositions, and key drivers of a large suite of VOCs using online gradient measurements taken from a 325 m tall tower in urban Beijing, China. The impacts of these vertical VOC variations on ozone formation were also analyzed using box model simulations. We find that VOCs exhibited differentiated vertical gradients due to their differences in both sources and chemical reactivities, along with the diurnal PBL evolution. In daytime, reactive VOCs (e.g., hydrocarbons) are rapidly oxidized and their concentrations generally decreased with height, accompanied by the formation and accumulation of oxygenated VOCs (OVOCs) in the middle and upper layers. We also find that the formation of ozone responds positively to changes in both NOx and VOCs. As a result, the production rate of ozone declines with height due to the simultaneous decreases in concentrations of reactive VOCs and NOx, but remains high in the middle and upper layers due to the presence of high OVOCs concentrations. Therefore, careful consideration should be given to the vertical variations in both ozone production rates and formation regimes in the whole PBL when developing regional ozone control strategies.