Photochemistry of ozone pollution in autumn in Pearl River Estuary, South China

To explore the photochemical O₃ pollution over the Pearl River Estuary (PRE), intensive measurements of O₃ and its precursors, including trace gases and volatile organic compounds (VOCs), were simultaneously conducted at a suburban site on the east bank of PRE (Tung Chung, TC) in Hong Kong and a rural site on the west bank (Qi'ao, QA) in Zhuhai, Guangdong in autumn 2016. Throughout the sampling period, 3 days with high O₃ levels (maximum hourly O₃ > 100 ppbv) were captured at both sites (pattern 1) and 13 days with O₃ episodes occurred only at QA (pattern 2). It was found that O₃ formation at TC was VOC-limited in both patterns because of the large local NO_x emissions. However, the O₃ formation at QA was co-limited by VOCs and NO_x in pattern 1, but VOC-limited in pattern 2. In both patterns, isoprene, formaldehyde, xylenes and trimethylbenzenes were the top 4 VOCs that modulated local O₃ formation at QA, while they were isoprene, formaldehyde, xylenes and toluene at TC. In pattern 1, the net O₃ production rate at QA (13.1 ± 1.6 ppbv h⁻¹) was high, and comparable (p = 0.40) to that at TC (12.1 ± 1.5 ppbv h⁻¹), so was the hydroxyl radical (i.e., OH), implying high atmospheric oxidative capacity over PRE. In contrast, the net O₃ production rate was significantly higher (p < 0.05) at QA (16.3 ± 0.4 ppbv h⁻¹) than that at TC (4.7 ± 0.2 ppbv h⁻¹) in pattern 2, and the OH concentration and cycling rate were also higher, indicating much stronger photochemical reactions at QA. These findings enhanced our understanding of O₃ photochemistry in the Pearl River estuary, which could be extended to other estuaries.