Observation-Based Summer O3 Control Effect Evaluation: A Case Study in Chengdu, a Megacity in Sichuan Basin, China

Qinwen Tan,Fumo Yang,Miao Feng,Yang Qiu,Li Zhou,Hefan Liu,Wenju Jiang,Fusheng Wei

doi:10.3390/atmos11121278

Abstract

Ground-level ozone (O3), which is mainly from the photochemical reactions of NOx and volatile organic compounds (VOCs), has become a crucial pollutant obstructing air quality improvement in China. Understanding the composition, temporal variability and source apportionment of VOCs is necessary for determining effective control measures to minimize VOCs and their related photochemical pollution. To provide a comprehensive analysis of VOC sources and their contributions to ozone formation in the city of Chengdu—a megacity with the highest rates of industrial and economic development in southwest China—we conducted a one-month monitoring project at three urban sites (Shuangliu, Xindu, Junpingjie; SL, XD and JPJ, respectively) along the main north–south meteorological transport channel before and during the implemented control measures. Alkanes were the dominant group at each site, contributing to around 50% of the observed total VOCs, followed by oxygen-containing VOCs (OVOCs), aromatics, halohydrocarbons and alkenes. During the control period, the mixing ratios of most measured VOC species decreased, and O3 concentrations were down by at least 20%. VOC species experiencing the most effect from control were aromatics and OVOCs, which had higher O3 formation reactivity. This indicated that the control policies had significant influence on reductions of reactive VOC species. We also identified VOC sources at SL and XD using positive matrix factorization (PMF) and assessed their contributions to photochemical O3 formation by calculating the O3 formation potential (OFP) based on mass concentrations and maximum incremental reactivity of related VOCs. Five dominant VOC sources were identified, with the highest contributions from vehicular exhaust and fuel evaporation before control, followed by solvent utilization, biogenic background and secondary formation, and industrial emissions. Contribution from vehicular exhaust was reduced the most at SL, while at XD, secondary formation VOCs decreased significantly. VOCs from vehicular and industrial emissions and solvent utilization were found to be the dominant precursors for OFPs, particularly the species of xylenes, toluene and propene. Our results therefore suggest that priority should be given to the alleviation of photochemical pollutants for effective control of O3 formation in Chengdu. The findings from this work have important implications for formulating effective emission control policies in Chengdu.

Highlights

Ozone, as a main product from photochemical reactions, is a widespread pollution problem in many of the world’s population centers, especially in China [1]
Heavy-polluting vehicles had been cleaned up off the road, in diesel vehicular emissions; and o-xylene;1,2,4-trimethylbenzene and ethene in industrial emissions the emission from industrial manufacturing was well controlled, and solvent utilization became were the dominant species from volatile organic compounds (VOCs) emissions contributing to photochemical O3 formation
Cutting were the dominant species from VOC emissions contributing to photochemical O3 formation

Summary

Introduction

As a main product from photochemical reactions, is a widespread pollution problem in many of the world’s population centers, especially in China [1]. Atmosphere 2020, 11, 1278 has been given to identifying and quantifying chemical processes leading to the generation of O3 and other secondary photochemical pollutants, and to putting forward their control countermeasures. Regional ozone pollution events occur frequently over China [2,3]. Sichuan Basin in southwest China has complex terrain, special meteorological conditions, and a fast urbanization rate [4,5]. Chengdu, located on the western edge of Sichuan Basin, is the capital of Sichuan province and a megacity with high anthropogenic emission; the city experiences stagnant meteorological conditions frequently. Chengdu has recorded high levels of PM2.5 and O3 pollution [6,7,8]

Methods

Results

Conclusion