Impact of long-range atmospheric transport on volatile organic compounds and ozone photochemistry at a regional background site in central China

Abstract

Ozone (O3) photochemistry in remote areas can be altered by the injection of long-range transported air masses which contain volatile organic compounds (VOCs) and nitrogen oxides (NOx). In this study, VOCs and trace gases (i.e., SO2, NO, NO2, CO, and O3) were measured in summer (August) and autumn (October) in 2018 at Jinsha (JSH), a regional background station in central China. A unique pattern that most short-lived VOCs species presented higher mixing ratios in summer while long-lived VOCs showed higher mixing ratios in autumn was observed. Backward trajectory analysis revealed that higher mixing ratios of long-lived hydrocarbons were mostly associated with air masses originating from northern China, while lower concentrations related to air masses passed over southeastern China and the East China Sea. An observation-constrained photochemical box model coupled with Master Chemical Mechanism (PBM-MCM) was applied to simulate the net ozone production. It was found that the net ozone production rate in summer (average: 4.7 ± 0.8 ppbv/h) was higher than that in autumn (average: 2.5 ± 0.1 ppbv/h). O3 formation at JSH switched from transition regime in summer to VOC-constrained regime in autumn due to the high level of NOx in autumn. The relative incremental reactivity (RIR) analysis showed the isoprene was the most reactive VOC species to in-situ O3 production. These findings highlight the impact of long-range air pollution transport on background O3 photochemistry in central China.