Enhanced Mid-Latitude Tropospheric Column Ozone over East Asia: Coupled Effects of Stratospheric Ozone Intrusion and Anthropogenic Sources

We analyze tropospheric column ozone (TCO) data observed by satellite instruments over East Asia for 15 years (from 1995 to 2009), and investigate the relationship between enhanced TCO (E-TCO) and ozone intrusion from the stratosphere near the subtropical jet (STJ). A belt of E-TCO is observed at mid-latitude over East Asia throughout the year; the belt is located at latitudes approximately equal to that of the STJ on seasonal, monthly, and daily timescales. The observed results are compared with a tagged tracer simulation by using a global chemical transport model. The simulation for East Asia indicates that the contribution from tropospheric origin to the enhancement of TCO is comparable to that from stratospheric origin at latitudes close to the STJ, resulting in the high correlation of the E-TCO belt and the STJ. The two origins of ozone cannot be differentiated in the tropospheric column ozone observed by a satellite, especially over East Asia where the anthropogenic source regions of ozone precursors are situated close to the latitudes of the STJ. Some occasional data, however, indicate split origins on a daily timescale, suggesting that the two origins really contribute to the enhancement of TCO. Our results strongly suggests an urgent need to develop a new satellite sensor and/or a new algorithm to distinguish boundary layer ozone from free tropospheric ozone in order to promote our understanding of atmospheric pollution over East Asia.