Cross-Tropopause Transport of Surface Pollutants during the Beijing 21 July Deep Convection Event

Abstract

Abstract Air transport from the troposphere to the stratosphere plays an important role in altering the vertical distribution of pollutants in the upper troposphere and lower stratosphere (UTLS). On 21 July 2012, Beijing was hit by an unprecedented extreme rainfall event. In the present study, the Community Multiscale Air Quality Modeling System (CMAQ) is used to simulate the change in vertical profiles of pollutants during this event. The integrated process rate (IPR) method was applied to quantify the relative contributions from different atmospheric processes to the changes in the vertical profile of pollutants and to estimate the vertical transport flux across the tropopause. The results revealed that, in the tropopause layer, during the torrential rainfall event, the values of O3 decreased by 35% and that of CO increased by 98%, while those of SO2, NO2, and PM2.5 increased slightly. Atmospheric transport was the main cause for the change in O3 values, contributing 32% of the net increase and 99% of the net decrease of O3. The calculations showed that the transport masses of CO, O3, PM2.5, NO2, and SO2 to the stratosphere by this deep convection in 25 h were 6.0 × 107, 2.4 × 107, 7.9 × 105, 2.2 × 105, and 2.7 × 103 kg, respectively, within the ∼300 km × 300 km domain. In the midlatitudes of the Northern Hemisphere, penetrating deep convective activities can transport boundary layer pollutants into the UTLS layer, which will have a significant impact on the climate of this layer.