Examining the sensitivity of in situ ozone production to its precursor chemical species in Beijing, China

In the past decade, the introduction of intensive clean air policies across China has led to a decline in particulate matter and NO_x concentrations, responsible for poor air quality detrimental to human health¹. Recent studies suggest that Beijing’s anthropogenic emissions of NO_x­ and PM_2.5 have reduced by 83.6% and 54.7% respectively, as a result of the Clean Air Action Plan, implemented in 2013.² However, despite much progress, concentrations of surface leave O₃, another harmful pollutant, have increased. In Beijing, O₃ increased at a rate of approximately 5% per year between 2013 and 2017_, with a mean increase of 19.32% per year observed at one monitoring site in the city.³ Due to the complex chemical processing leading to O₃ production, reductions in NO_x and PM may have inadvertently led to the increased secondary formation of O₃. To fully understand the chemical processes leading to surface-level O₃ production, a detailed analysis of its photochemical precursor species, volatile organic compounds (VOCs) and NO_x, and the role of aerosol-radical interactions, is required. This study utilises a detailed chemical box model to examine the propensity of observed VOCs at a site in Beijing to undergo oxidation, forming radical species, leading to in situ ozone production. The impact of the heterogenous uptake of the hydroxyl radical onto aerosol surfaces is also assessed.

During May/June 2017, concentrations of a large range of VOCs, NO_x, CO and O₃ were continuously measured at the Institute of Atmospheric Physics (IAP), an urban site in central Beijing. Measurements were taken as part of the Air Pollution and Human Health-Beijing (APHH) project. During the observation period, O₃ concentrations regularly breached recommended WHO 8-hour exposure limits of 50 ppb, with maximum concentrations exceeding 150 ppb. The sensitivity of in situ ozone production to changes in the observed reactive VOCs, NO_x, and aerosol surface area, are explored in detail using a chemical box model incorporating the Master Chemical Mechanism. The model is used to investigate the chemical regime of the measurement site in Beijing, and the key reactive species leading to in situ ozone production in the city are identified. This study aims to highlight the key species that could be targeted in future pollution reduction policies, to alleviate the continued increase in O₃ production rates in the city of Beijing.

1. Zhang, Q. et al.: Drivers of improved PM₅ air quality in China from 2013 to 2017, P. Natl. Acad. Sci. USA, 116, 24463–24469, 2019.

2. Cheng, J. et al.: Dominant role of emission reduction in PM_2.5 air quality improvement in Beijing during 2013–2017: a model-based decomposition analysis. Atmos. Chem. Phys., 19(9), 6125–6146, 2019.

3. Squires, F. A: Gas Phase Air Pollution in Remote and Urban Atmospheres: From then Azores to Beijing, PhD thesis, 2020.