Influence of airmass transport events on the variability of surface ozone at Xianggelila Regional Atmosphere Background Station, Southwest China

Abstract

Abstract. In situ measurements of ozone (O3), carbon monoxide (CO) and meteorological parameters were made from December 2007 to November 2009 at the Xianggelila Regional Atmosphere Background Station (28.006° N, 99.726° E, 3580 m a.s.l.), Southwest China. It is found that both O3 and CO peaked in spring while the valleys of O3 and CO occurred in summer and winter, respectively. A normalized indicator (marked as "Y") of transport events on the basis of the monthly normalized O3, CO, and water vapor, is proposed to evaluate the occurrence of O3 transport events from the upper, O3-rich atmosphere. This composite indicator has the advantage of being less influenced by and seasonal or occasional variations of individual factors. It is shown that the most frequent transport events occurred in winter and they can make a significant contribution to surface O3 at Xianggelila. A case of strong O3 transport event under the synoptic condition of a~deep westerly trough is studied by the combination of the Y indicator, potential vorticity, total column ozone, and trajectory analysis. A 9.6 ppb increase (21.0%) of surface ozone is estimated based on the impacts of deep transport events in winter. Asian Monsoon plays an important role in suppressing O3 accumulation in summer and fall.