Abstract
Abstract. We investigate the contributions of local and regional emission sources to air pollution in Beijing to inform the design of short-term emission control strategies for mitigating major pollution episodes. We use a well-evaluated version of the WRF-Chem model at 3 km horizontal resolution to determine the daily accumulation of pollution over Beijing from local and regional sources in October 2014 under a range of meteorological conditions. Considering feasible emission reductions across residential, transport, power, and industrial sectors, we find that 1 d controls on local emissions have an immediate effect on PM2.5 (particulate matter with diameter less than 2.5 µm) concentrations on the same day but can have lingering effects as much as 5 d later under stagnant conditions. One-day controls in surrounding provinces have the greatest effect in Beijing on the day following the controls but may have negligible effects under northwesterly winds when local emissions dominate. To explore the contribution of different emission sectors and regions, we perform simulations with each source removed in turn. We find that residential and industrial sectors from neighbouring provinces dominate PM2.5 levels in Beijing during major pollution episodes but that local residential emissions and industrial or residential emissions from more distant provinces can also contribute significantly during some episodes. We then perform a structured set of perturbed emission simulations to allow us to build statistical emulators that represent the relationships between emission sources and air pollution in Beijing over the period. We use these computationally fast emulators to determine the sensitivity of PM2.5 concentrations to different emission sources and the interactions between them, including for secondary PM, and to create pollutant response surfaces for daily average PM2.5 concentrations in Beijing. We use these surfaces to identify the short-term emission controls needed to meet the national air quality target of daily average PM2.5 less than 75 µg m−3 for pollution episodes of different intensities. We find that for heavily polluted days with daily mean PM2.5 higher than 225 µg m−3, even emission reductions of 90 % across all sectors over Beijing and surrounding provinces may be insufficient to meet the national air quality standards. These results highlight the regional nature of PM pollution and the challenges of tackling it during major pollution episodes.
Highlights
Beijing, located at the foot of the Yan Mountains on the northern edge of the heavily populated North China Plain, has consistently been named among the most polluted capital cities in the world (State of Global Air, 2019; Global burden of disease, 2016)
We find that the effects of local emission reductions in Beijing can be substantial but typically only contribute about 20 % of PM2.5 concentrations during pollution episodes, and the effects rarely extend beyond the day of emissions reduction
Controls on emissions in neighbouring regions over the North China Plain have a larger impact on PM2.5 in Beijing, typically about 35 % during pollution episodes, peaking the day following the application of controls, and they can persist for a number of days
Summary
Beijing, located at the foot of the Yan Mountains on the northern edge of the heavily populated North China Plain, has consistently been named among the most polluted capital cities in the world (State of Global Air, 2019; Global burden of disease, 2016). Additional emergency measures generally lasting 3–7 d are necessary to prevent these extreme pollution episodes, especially under stable meteorological conditions that are conducive to the formation and accumulation of very high levels of particulate matter. Such short-term emission controls have been tested, with some success, during special events such as the 2008 Beijing Olympics, the Asia–Pacific Economic Cooperation (APEC) summit in 2014, and the China Victory Day Parade in 2015 (Zhang et al, 2015; Xu et al, 2019, 2017). We generate pollutant response surfaces based on these different emission sectors and regions and demonstrate how they can be used to guide the development of future short-term emission control policies in the city
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