The impact of meteorological changes from 2013 to 2017 on PM2.5 mass reduction in key regions in China

Abstract

In 2013, China issued the “Action Plan for the Prevention and Control of Air Pollution” (“Ten Statements of Atmosphere”) and implemented a series of pollution reduction measures from 2013 to 2017. In key regions of China, the mass concentrations of particulate matter with aerodynamic equivalent diameters less than 2.5 μm (PM2.5) have dropped significantly. However, the contributions of meteorological changes to PM2.5 reduction are largely uncertain, which has attracted particular concern from the government and the public. Here, we investigated the impact of large-scale and boundary layer (BL) meteorological conditions on aerosol pollution and estimated the contributions of meteorological changes to PM2.5 reduction based on in-depth analysis and diagnosis of various observed meteorological elements and an integrated pollution-linked meteorological index (PLAM, which is approximately and linearly related to PM mass concentration). In this study, we found that the meteorological conditions worsened in 2014 and 2015 and improved in 2016 and 2017 relative to those in 2013 in key regions in China. In 2017 relative to 2013, only ~5% (approximately 13% of the total PM2.5 decline) of the 39.6% reduction in PM2.5 mass concentrations can be attributed to meteorological changes in the Beijing-Tianjin-Hebei (BTH) region, and only ~7% (approximately 20% of the total PM2.5 decline) of the 34.3% reduction can be attributable to meteorological changes in the Yangtze River Delta (YRD) region. Overall, the PM2.5 reduction due to meteorological improvement is much lower than the observed PM2.5 reduction in these areas, which indicates that emission reduction during the five-year implementation of the “Ten Statements of Atmosphere” is the dominant factor in the improvement in air quality. The changes in meteorology and climate are conducive to PM2.5 reduction but do not dominate the substantial improvement in air quality. Similar to the above regions, in the Pearl River Delta (PRD) region, the impact of meteorological changes on the annual averaged PM2.5 concentration from 2013 to 2017 was relatively weak, and the PM2.5 reduction was mainly due to emission reductions. During winter 2017 (January, February, and December of this year), the meteorological conditions improved ~20% in the BTH region (observed total PM2.5 reduction: 40.2%) and ~30% in the YRD region (observed total PM2.5 reduction: 38.2%) relative to those in 2013, showing the meteorological factors played more important role in the decrease of PM2.5 in winter of these years in the two regions, respectively. The meteorological conditions in winter 2016 were 14% better than those in winter 2017, but the PM2.5 reduction in winter 2016 was still less than that in winter 2017, reinforcing the significant contributions of the increasing efforts to reduce PM2.5 emissions in 2017. The substantial progress of strict emission measures was also confirmed by a comparison of several persistent heavy aerosol pollution episodes (HPEs) with similar meteorological conditions. It is found that the decrease of PM2.5 mass caused by emission reduction increases year by year, especially the decrease of PM2.5 concentration in 2016 and 2017. In China, HPEs mainly occur in winter, when meteorological conditions are approximately 40--100% worse than in other seasons. This worsening is partly due to the harbor effect of high topography, including downdrafts and the weak wind zone, and partly due to the increasingly stable regional BL structure caused by climate warming. For the formation of HPEs, it occurred under regional stagnant and stable conditions associated with upper-level circulation patterns, including the zonal westerly winds type and high-pressure ridges. After pollution formation, PM2.5 with mass accumulated to a certain degree can further worsen the BL meteorological conditions. The feedback effect associated with worsening conditions dominates PM2.5 mass explosive growth. In the context of high air pollutant emissions in China, unfavorable meteorological conditions are the necessary external conditions for the formation and accumulation of HPEs. Therefore, reducing aerosol pollution significantly during the earlier transport stage is critical in reducing persistent HPEs. Currently, even under favorable meteorological conditions, allowing emissions without restriction is also not advisable because aerosol pollution allowed to accumulate to a certain extent will significantly worsen the BL meteorological conditions and close the “meteorological channels” available for pollution dispersion.