The impacts of VOCs on PM2.5 increasing via their chemical losses estimates: A case study in a typical industrial city of China

Abstract

Volatile organic compounds (VOCs) have been explicitly required to be controlled by national or local governments to tackle heavy PM2.5 pollution in China. However, the role of VOCs chemistry in PM2.5 is still far away from being fully understood. In this study, we selected a typically industrialized city of China, conducted VOCs online measurements in July and December of 2020, estimated VOCs chemical losses, and explored their impacts on PM2.5 in the real atmosphere. Our measurements totally detected the mixing ratios of 53 VOCs species, sum of which reaching 20.8 ± 9.1 ppb and 45.5 ± 19.8 ppb in July and December respectively. Based on the change in the relative abundance of various VOCs species with different reactivity, daily OH exposure and daily O3 exposure (molecules·s·cm−3) were estimated, respectively with the mean value of 11.0 × 1010 and 5.2 × 1015 in July, 5.9 × 1010 and 3.1 × 1015 in December. Thus, there were about 6.7 ± 3.2 ppb and 11.1 ± 5.0 ppb VOCs chemically lost in July and December, with the average loss ratio of 29.7% and 19.3% in respective. In July, PM2.5 in the range of 20–60 μg m−3 positively responded to VOCs loss ratios, and high PM2.5 values were always accompanied by higher VOCs losses instead of high CO levels; but their relationship became negative in December when PM2.5 in the range of 40–150 μg m−3, and extremely high PM2.5 values happened in high CO conditions instead of high VOCs losses conditions. Moreover, PM2.5 measurements in this city also showed an apparent upward trend in OC/EC with PM2.5 in summer, but a slight downward trend in winter. These all indicated that in the conditions of stronger solar radiations (summer) and low-middle aerosol loads, VOCs chemistry played the important role in PM2.5 increasing through contributing SOA; but in the conditions of lower solar radiations (winter) and heavy aerosol loads, VOCs chemistry was greatly weakened and had the minor impact on PM2.5 increasing. The measure involving VOCs reduction to alleviate PM2.5 pollution in winter needs to be cautiously considered and examined.