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

To explore the photochemical O-3 pollution over the Pearl River Estuary (PRE), intensive measurements of O-3 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 O-3 levels (maximum hourly O-3 > 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 O-3 formation at TC was VOC-limited in both patterns because of the large local NOx emissions. However, the O-3 formation at QA was co-limited by VOCs and NOx 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 O-3 formation at QA, while they were isoprene, formaldehyde, xylenes and toluene at TC. In pattern 1, the net O-3 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 O-3 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 O-3 photochemistry in the Pearl River estuary, which could be extended to other estuaries. (C) 2020 Elsevier B.V. All rights reserved.