Impacts of traffic emissions on atmospheric particulate nitrate and organics at a downwind site on the periphery of Guangzhou, China

Yi Ming Qin,Yong Jie Li,Fei Li,Hao Bo Tan,Chak K Chan,André S H Prévôt,Francesco Canonaco,Misha I Schurman

doi:10.5194/acp-17-10245-2017

Abstract

Abstract. Particulate matter (PM) pollution on the peripheries of Chinese megacities can be as serious as in cities themselves. Given the substantial vehicular emissions in inner-city areas, the direct transport of primary PM (e.g., black carbon and primary organics) and effective formation of secondary PM from precursors (e.g., NOx and volatile organic compounds) can contribute to PM pollution in buffer zones between cities. To investigate how traffic emissions in inner-city areas impact these adjacent buffer zones, a suite of real-time instruments were deployed in Panyu, downwind from central Guangzhou, from November to December 2014. Nitrate mass fraction was higher on high-PM days, with the average nitrate-to-sulfate ratio increasing from around 0.35 to 1.5 as the PM mass concentration increased from 10 to 160 µg m−3. Particulate nitrate was strongly correlated with excess ammonium (([NH4+] ∕ [SO42−] − 1.5) × [SO42−]), with higher concentrations in December than in November due to lower temperatures. The organic mass fraction was the highest across all PM1 levels throughout the campaign. While organic aerosols (OA) were dominated by secondary organic aerosols (SOA = semi-volatile oxygenated organic aerosols + low-volatility oxygenated organic aerosols) as a campaign average, freshly emitted hydrocarbon-like organic aerosols (HOA) contributed up to 40 % of OA during high-OA periods, which typically occurred at nighttime and contributed 23.8 to 28.4 % on average. This was due to daytime traffic restrictions on heavy-duty vehicles in Guangzhou, and HOA almost increased linearly with total OA concentration. SOA increased as odd oxygen (Ox = O3 + NO2) increased during the day due to photochemistry. A combination of nighttime traffic emissions and daytime photochemistry contributed to the buildup of PM in Panyu. The mitigation of PM pollution in inner-city areas by reducing vehicular traffic can potentially improve air quality in peripheral areas.

Highlights

Traffic emissions are one of the main contributors to air quality deterioration in rapidly expanding urban China (Kelly and Zhu, 2016; Zhang et al, 2017)
Pollutants emitted from vehicles – such as NOx, volatile organic compounds (VOCs), black carbon (BC) and other traffic-related particulate matter (PM) – have increased over the last decades (Wang et al, 2013; Zhang et al, 2012)
In the HR-ToF-AMS measurements (DeCarlo et al, 2006), ambient air was sampled through a PM2.5 cyclone on the rooftop with a flow rate of approximately 0.084 L min−1 drawn by the AMS and the remainder drawn by an auxiliary pump

Summary

Introduction

Traffic emissions are one of the main contributors to air quality deterioration in rapidly expanding urban China (Kelly and Zhu, 2016; Zhang et al, 2017). Pollutants emitted from vehicles – such as NOx, volatile organic compounds (VOCs), black carbon (BC) and other traffic-related particulate matter (PM) – have increased over the last decades (Wang et al, 2013; Zhang et al, 2012). Apart from the primary PM, the oxidation of traffic-related gaseous pollutants such as NOx, VOCs, semi-volatile VOCs (SVOCs) and lowvolatility VOCs (LVOCs) leads to the formation of particu-. Qin et al.: Impacts of traffic emissions on atmospheric particulate nitrate and organics late nitrate and secondary organic aerosols (SOA), exacerbating PM pollution. Multiple studies have shown that the concentrations and proportions of nitrate in PM have increased significantly in most Chinese megacities (Pan et al, 2016; Wen et al, 2015; Xue et al, 2014)

Methods

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Conclusion