General characteristics of tropospheric trace constituent layers observed in the MOZAIC program

We present a statistical study on tropospheric layers as allowed by the mos t extensive ozone and water vapor database currently available. Considering O-3 and H2O deviations from an automatically calculated background, we def ine four types of layers. These tropospheric layers are a common feature, w ith the percentage of the troposphere occupied by such layers varying from 7% to 33% depending on the region and the season. Most of the layers are fo und between 4 and 8 km altitude, and the median thickness is about 500 m, A t northern midlatitudes we find 4 times more layers in summer than in winte r, while in tropical Asia we observe a spring maximum in the occurrence of the layers. The most abundant layer type everywhere is O-3 + H2O- and corre sponds to the signature of stratospheric intrusions or continental pollutio n. This suggests that stratosphere-troposphere exchanges or at least their influence are not negligible in summer at midlatitudes or in the tropics. A complete understanding of the layers could lead to a better empirical asse ssment of the different tropospheric ozone sources and to an assessment of the potential vorticity fluxes in the troposphere.