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 NOx emissions. However, the O3 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 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.