Biogenic volatile organic compounds enhance ozone production and complicate control efforts: Insights from long-term observations in Hong Kong

Ground-level ozone (O3) pollution is a major air-quality issue in China. With decreasing emissions of anthropogenic precursors, natural precursors may become increasingly important in O3 pollution. However, understanding of the change in biogenic volatile organic compounds (BVOCs) and its environmental impact in China has been hindered by the lack of long-term measurements of VOCs. In this study, we analyze the continuous measurements of O3 and its precursors obtained in Hong Kong during 2013-2019 and demonstrate the increasingly important role of BVOCs in O3 production. Driven by the warming temperature, Biogenic isoprene at a suburban site (Tung Chung) increased by 0.05 & PLUSMN; 0.02 ppbv/yr (18%/yr), which is in sharp contrast to the decreases in anthropogenic precursors during the study period. Detailed chemical modeling shows that increased BVOCs enhanced local O3 production by 0.31 ppbv/h/yr (23%) annually, and the effect was the most obvious in summer (by 0.44 ppbv/h/yr or 27%). Increased BVOCs also affected the O3-precursor (anthropogenic) relationships by increasing the O3 sensitivity to nitrogen oxides (NOx) and decreasing the O3 sensitivity to anthropogenic VOCs (AVOCs). Despite changes in precursors (decreasing NOx, increasing BVOCs, and decreasing some AVOCs), the O3 formation remains in VOC-limited regimes at Tung Chung. Joint control of AVOCs and NOx (at a ratio greater than 1.2) would help avoid exacerbation of O3 pollution and reduce NO2 pollution. In the short term, an AVOCs-focused strategy would help reduce O3 to a moderate level; but because of the presence of the natural BVOCs, a drastic cut in NOx emissions would be needed to meet a more stringent O3 standard. Our findings also suggest the BVOCs increase likely occurred in the larger Pearl River Delta (PRD) region, and thus the results from our study in Hong Kong may have implications for developing AVOCs/NOx reduction measures in the PRD region and beyond.