Effects of long-distance transport on O3 and secondary inorganic aerosols formation in Qingdao, China

Abstract

This work presents a three-year continuous observation of ozone (O3) and fine particle (PM2.5), as well as their precursors in Qingdao, China, from September 2018 to August 2021. The annual concentrations of O3 and PM2.5 were measured as 35.9–44.5 ppbv and 31.6–34.2 μg m−3, respectively. Analysis of the interannual variations of O3 and PM2.5 concentration indicated the effective control measures for PM2.5 and O3 pollution in Qingdao in recent years. Nevertheless, we still observed 85 O3 episodes and 80 PM2.5 episodes during the whole observation. And it was found that the fraction of secondary inorganic aerosols (SIA) in PM2.5 mass significantly increased during PM2.5 episodes. By employing the Lagrangian photochemical trajectory model (LPTM), we investigated the roles of local production and regional transport (i.e., short-distance transport and long-distance transport) that play in the O3 and SIA formation in these episodes. The contribution of local production, short-distance transport, and long-distance transport to O3 concentration was calculated as 36%, 25%, and 39%, respectively. Long-distance transport seemed to play a more significant role in the SIA formation, accounting for 56% of SIA concentration. Moreover, the calculation results of the relative incremental reactivity (RIR) showed that both O3 and SIA formation were mainly affected by NOx and VOCs emissions during the process of long-distance transport. The observed unexpectedly high contribution of long-distance transport to O3 and SIA formation suggests that more extensive regional joint prevention and control policies on NOx and VOCs emissions are warranted to mitigate secondary air pollution.