Attribution of the worse aerosol pollution in March 2018 in Beijing to meteorological variability

Abstract

Fine particle matter (PM2.5) pollution frequently occurs in winter with increased consumption for heating and decreased radiation and boundary-layer height. Under strict emission controls since 2013, the mass concentrations of PM2.5 in Beijing decreased substantially in Winter 2017/2018. However, in March 2018, the mean PM2.5 concentration doubled from ~40 to 87 μg m−3, the reason for which is still unclear in the context of significant emission reductions. Here, using PM2.5 measurements, vertical observations, and reanalysis data, we found that this worsening was attributed to increasingly stable stratification characterized by temperature differences between 850 hPa and 1000 hPa, which reached the maximum value (−4 °C) for March from 1951 to 2018. Not limited to Beijing, positive anomalies in temperature differences occurred widely as a result of a westward transition in the center of the polar vortex. Away from the cold center and dominated by anomalous zonal westerlies, northern China was intruded by warm southerly winds. The warm regional advection increased positive anomalies in low-level temperature differences, and in turn, enhanced geopotential thickness throughout the troposphere. The adverse impact on pollution was also confirmed in comparison to the vortex pattern in January. The vortex changed from the elongated one with a split-flow pattern in January to the annular vortex with a major center over Novaya Zemlya in March. Correspondingly, Beijing was less affected by cold air masses and thereby under relatively stable stratification, which is unfavorable for pollution dispersion in March. Additionally, this aerosol pollution was further worsened by aerosol-induced deteriorating meteorological conditions.