Photochemical ozone formation and radical chemistry in five Chinese megacities and Pearl Rriver Estuary : field observations and model simulations

Abstract

As a consequence of the rapid industrialization and urbanization in the past 30 years, ozone (O₃) pollution has become one of the major air pollution problems in China. O₃ is a big concern due to its adverse impact on air quality, human health, ecosystems and climate change. Many studies on O₃ photochemistry have been conducted in one or several megacity clusters in China, such as north China plain (NCP), Yangtze River Delta (YRD), Pearl River Delta (PRD) and Sichuan Basin (SCB) regions, and central and northwestern China. However, there is a dearth of nationwide studies on in-situ O₃ photochemistry in urban areas of major Chinese megacities. In addition, the O₃ photochemistry in small-scale environments such as Pearl River Estuary (PRE) area, including urban Hong Kong, remains unclear. This study aims to comprehensively investigate the O₃ photochemistry in: (1) urban areas of five major megacities in China in the summer of 2018; (2) two sites at the west and east banks of the PRE in the autumn of 2016; and (3) urban Hong Kong in the autumns of 2007, 2013 and 2016. To investigate the photochemical O3 pollution in urban areas in China, comprehensive measurements of O₃ and its precursors, including trace gases and volatile organic compounds (VOCs), were simultaneously conducted in urban areas of five major cities in China, i.e., Beijing, Shanghai, Wuhan, Chengdu and Lanzhou, in the summer of 2018. The results showed that Beijing had the highest O₃ mixing ratio (78.6±7.3 ppbv) followed by Lanzhou (67.2±7.8 ppbv) and Wuhan (63.9±6.8 ppbv), while Chengdu and Shanghai had relatively lower O3 (Chengdu: 52.5±7.5 ppbv; Shanghai: 20.7±1.9 ppbv) (p < 0.05), indicating the severe O₃ pollution in northern and central China in summer. Model simulations revealed that the net O₃ production rate in Lanzhou (8.9±1.7 ppbv h⁻¹) was the largest, followed by Beijing (6.4±1.3 ppbv h⁻¹), Wuhan (5.8±1.2 ppbv h⁻¹) and Chengdu (4.0±0.6 ppbv h⁻¹), while it was the lowest in Shanghai (2.8±0. 7 ppbv h-1) (p < 0.05). In addition, the simulated ROx (= OH+HO₂+RO₂) concentrations were comparable (p>0.1) in Lanzhou ((5.9±1.1) ×108 molecules cm⁻³ ), Beijing ((5.3±1.1) ×108 molecules cm-3) and Wuhan ((4.3±1.0) ×10⁸ molecules cm⁻³ ), which were significantly higher (p < 0.05) than those in Shanghai ((2.7±0.9) ×10⁸ molecules cm⁻³) and Chengdu ((1.8±0.7) ×10⁸ molecules cm⁻³ , indicating the stronger atmospheric oxidative capacity and severer O₃ pollution in northern and central China in summer. Furthermore, the Oformation was mainly controlled by VOCs in most urban areas but co-limited by both VOCs and nitrogen oxide (NOx) in Lanzhou, indicating that cutting NOx and VOCs would effectively lead to O₃ alleviation in Lanzhou. Moreover, the dominant VOC groups (species) contributing to O3 formation were oxygenated VOCs (OVOCs) (e.g., formaldehyde and acetaldehyde) in Beijing and Wuhan, alkenes (e.g., propene) in Lanzhou, and aromatics (e.g., xylenes and trimethylbenzenes) and OVOCs (e.g., formaldehyde and acetaldehyde) in Shanghai and Chengdu. Besides, 1-butene in Shanghai also needed stricter control given its high contribution to O₃ formation. Source apportionment simulations identified six VOC sources in the study cities, including liquefied petroleum gas (LPG) usage, diesel exhaust, gasoline exhaust, industrial emissions, solvent usage and biogenic emissions. VOCs from vehicular emissions including diesel and gasoline exhausts were more abundant in the central and northern cities (p < 0.05), while solvent usage contributed the most to ambient VOCs in Shanghai (p < 0.05), followed by Chengdu. Industry emission was the top source of VOCs in the northwestern city (i.e., Lanzhou) (p < 0.05). For the contributions of these VOC sources to O₃ formation in these cities, diesel exhaust and solvent usage were the largest contributors in Beijing (p < 0.05), while diesel exhaust, industrial emissions and solvent usage made comparable contributions in Lanzhou (p>0.1). The O₃ formation was solely dominated by solvent usage in the other three cities (p < 0.05). In addition to the five megacities in China, the Pearl River Estuary (PRE) including Hong Kong has also been suffering from severe photochemical O₃ pollution for many years. However, the in-situ O₃ photochemistry in this area is still not fully clear though many studies have been carried out. To explore the photochemical O₃ pollution over the PRE, intensive measurements 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, three O₃ episode days were captured at both sites (defined as pattern 1) and 13 days with O₃ episodes occurred only at QA (defined as pattern 2). The results indicated 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 rates were high at both QA and TC sites (p = 0.40), 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 than that at TC in pattern 2, and the OH concentration and cycling rate were also higher, indicating considerably stronger photochemical reactions at QA. Moreover, to discover the temporal variations of photochemical O₃ formation and radical chemistry during last decade in Hong Kong, comprehensive sampling campaigns were conducted in the autumns of 2007, 2013 and 2016. While the simulated locally-produced O₃ remained unchanged (p = 0.73) from 2007 to 2013, the observed O₃ increased (p < 0.05) at a rate of 1.78 ppbv yr⁻¹ driven by the rise in regionally transported O3 (1.77±0.04 ppbv yr⁻¹). Both the observed and locally produced O3 decreased (p < 0.05) from the VOC sampling days in 2013 to those in 2016 at a rate of -5.31±0.07 ppbv yr⁻¹ and -5.52±0.05 ppbv yr⁻¹, respectively. However, a leveling-off (p = 0.32) was simulated for the regionally transported O₃ from 2013 to 2016. The mitigation of autumn O₃ pollution in Hong Kong was further confirmed by the continuous monitoring data, which have never been reported. Benefiting from the air pollution control measures taken in Hong Kong, the local O₃ production rate decreased remarkably (p < 0.05) from 2007 to 2016, along with the lowering of the recycling rate of the OH. Specifically, VOCs emitted from the source of LPG usage and gasoline evaporation decreased in this decade at a rate of -2.61±0.03 ppbv yr⁻¹, leading to a reduction of the O₃ production rate from 0.51±0.11 ppbv h⁻¹ in 2007 to 0.10±0.02 ppbv h-1 in 2016. In addition, solvent usage made decreasing contributions to both VOCs (rate=-2.29±0.03 ppbv yr⁻¹) and local O₃ production rate (1.22±0.17 and 0.14±0.05 ppbv h⁻¹ in 2007 and 2016, respectively) in the same period. All the rates reported here were for the VOC sampling days in the three sampling campaigns. It is noteworthy that meteorological changes also play important roles in the inter-annual variations in the observed O₃ and the simulated O₃ production rates. Evaluations with more data in longer periods are therefore recommended. Overall, the findings of photochemical O₃ pollution in China's five megacities and PRE partially addressed the knowledge gaps in O₃ formation mechanism and radical chemistry in China, and provided a scientific basis for dealing with China's national O₃ pollution. In addition, this study analyzed the decadal changes of the local and regional photochemistry in Hong Kong, and the chemical characteristics of O₃ formation in PRE, which will help to understand the O₃ photochemistry in similar microenvironments. These findings are also useful for evaluating the effectiveness of existing and projected O₃ pollution control measures.