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

Ozone (O-3) 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 O-3) 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). O-3 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 O-3 production. These findings highlight the impact of long-range air pollution transport on background O-3 photochemistry in central China.

Automated summary

The study in central China found that short-lived VOCs had higher mixing ratios in summer while long-lived VOCs had higher mixing ratios in autumn. Observations and modeling showed that the increase in NOx levels led to a transition from a transition regime to a VOC-constrained regime for O-3 formation in autumn.