Reply on RC4

Abstract

Stratospheric ozone transported to the troposphere is estimated to account for 5–10 % of the tropospheric ozone sources. However, chances for intruded stratospheric ozone to reach the surface are low. Here, we report an event of strong surface ozone surge with stratospheric origins in the North China Plain (NCP, 34° N–40° N, 114° E–121° E) at night of 31 July 2021. The hourly measurements revealed that surface ozone concentrations were up to 80–90 ppbv at several cities over the NCP from 23:00 on 31 July 1 to 6:00 on 01 August, 2021, which was 40–50 ppbv higher than the corresponding monthly mean. A high-frequency surface measurement indicates that this ozone surge occurred abruptly and reached 40–50 ppbv within ~10 minutes. A concurrent decline in surface carbon monoxide (CO) concentrations suggests that this surface ozone surge resulted from downward transport of stratospheric ozone-rich and CO-poor airmass. This is further confirmed by the vertical evolutions of humidity and ozone profiles at night, based on radiosonde and satellite data, respectively. Such an event of stratospheric impact on surface ozone is rarely documented in terms of its magnitude, covering areas, abruptness, and duration. We find that this surface ozone surge was induced by a combined effect of a dying typhoon In-fa and shallow local mesoscale convective systems (MCS) that facilitated the transport of stratospheric ozone to the surface. This finding is based on analysis of meteorological reanalysis and radiosonde data, combining with high-resolution FLEXPART-WRF modeling. (WRF: Weather Research and Forecasting, FLEXPART: Flexible Lagrangian particle dispersion model). Although the synoptic-scale typhoon In-fa was in dissipation stage when it passed through the NCP, it could still bring down stratospheric dry and ozone-rich airmass. As a result, the stratospheric airmass descended to the middle-to-low troposphere over the NCP before the MCS formed. With the pre-existed stratospheric airmass, the convective downdrafts of the MCS facilitated the final descending of stratospheric airmass to the surface. Significant surface ozone enhancement occurred in the convective downdraft regions during the development and propagation of the MCS. This study underscores the non-negligible roles of dying typhoons and shallow convection in the transport of stratospheric ozone to the troposphere and even the surface, which have important implications for air quality, tropospheric ozone budget, and climate change.