Distribution, variability and sources of tropospheric ozone over south China in spring: Intensive ozonesonde measurements at five locations and modeling analysis

We examine the characteristics of the spatial distribution and variability of tropospheric ozone (O 3 ) by analysis of 93 ozonesonde profiles obtained at five stations over south China (18–30°N) during a field campaign in April–May 2004. We use a global 3‐D chemical transport model (GEOS‐Chem) to interpret these characteristics and to quantify the sources of tropospheric O 3 over south China during this period. The observed tropospheric O 3 mixing ratios showed strong spatiotemporal variability due to a complex interplay of various dynamical and chemical processes. A prominent feature in the upper and middle troposphere (UT/MT) was the frequent occurrence of high O 3 mixing ratios shown as tongues extending down from the lower stratosphere or as isolated layers at all stations. The model largely captured the observed pattern of day‐to‐day variability in tropospheric O 3 mixing ratios at all stations, but often underestimated those tongues or isolated layers of O 3 enhancements observed in the UT/MT, especially at low‐latitude stations. We found that tropospheric O 3 along the southeast China coast was mainly produced within Asia. Lightning NO x emissions (over South Asia and equatorial Africa) and/or stratospheric influences were responsible for major events of high O 3 observed in the UT/MT at all stations. Underestimated contributions of these sources likely led to the model's underestimate in the low‐latitude UT/MT O 3 . This study emphasizes the need for improved understanding of lightning NO x emissions and stratospheric influences over the Eurasian and African continents and for better representation of these processes in current global models.