Springtime variability of lower tropospheric ozone over Eastern Asia: contributions of cyclonic activity and pollution as observed from space with IASI

Abstract

We use satellite observations from IASI (Infrared Atmospheric Sounding Interferometer) on board the MetOp-A satellite to evaluate the springtime daily variability of lower tropospheric ozone at the scale of Eastern Asia. Lower tropospheric partial columns from surface to 6 km are retrieved from IASI with a maximum of sensitivity between 3 and 4 km. We focus our analysis on the month of May 2008 for which tropospheric ozone presents typically amongst the largest concentrations along the year. We combine IASI observations with meteorological reanalyses from ERA-Interim in order to investigate the processes that control the spatial and temporal distribution of lower tropospheric ozone, especially in case of ozone enhancement. The succession of low- and high-pressure systems drives the day-to-day variability of lower tropospheric ozone over North East Asia. The analysis of two episodes with ozone enhancement at the synoptic scale of East Asia shows that the reversible subsiding and ascending ozone transfers in the UTLS region occurring in the vicinity of low-pressure systems and related to tropopause height affect the upper and lower tropospheric ozone over large regions, especially north to 40° N and largely explain the ozone enhancement observed with IASI for these latitudes. Irreversible downward transport of ozone-rich air masses from the UTLS to the lower troposphere occurs more locally. Its contribution to the lower tropospheric ozone column is difficult to dissociate from the tropopause perturbations induced by the weather systems. For regions south to 40° N, a significant correlation between lower tropospheric ozone and carbon monoxide (CO) observations from IASI has been found, especially over North China Plain (NCP). Considering carbon monoxide observations as pollutant tracer, the O 3 -CO correlation indicates that the photochemical production of ozone from primary pollutants emitted over such large polluted regions significantly contributes to the ozone enhancements observed with IASI in the lower troposphere. When low-pressure systems circulate over NCP, stratospheric and pollution sources play a concomitant role in the ozone enhancements. Moreover, in that case, evidence of pollutant export from NCP towards the east is shown. Finally, we show that semi-independent columns of ozone from the surface up to 12 km associated with CO columns from IASI constitute a powerful observational dataset to investigate the processes controlling tropospheric enhancement of ozone at synoptic scales.