Analysis of high ozone events over Africa using aircraft data.

Abstract

Previous research has shown that there is considerable enhancement of tropospheric ozone over the African continent. The role of biomass burning as a source of ozone precursor gases and the atmospheric circulation, in particular the anticyclonic gyres over southern Africa, are known to be important in the accumulation of ozone over the subcontinent. In addition, the injection of ozone-rich air from the stratosphere is known to account for the occurrence of ozone peaks in the upper troposphere. This thesis is aimed at investigating the occurrence of high ozone events, defined as 2::100 ppbv, over Africa. Their variability in time (seasonal distribution) and space (latitudinal distribution) is quantified for the upper tropospheric region (~l 0 - 12 km) from 35°N to approximately 35°S . The data used are part of the MOZAIC (Measurement of OZone and wAter vapour by Airbus In-service airCraft) database. Ozone, water vapour. and temperature are measured at cruise altitude by these in-service aircraft, and this study explores data gathered on flights between Europe and Africa over the period January 1996 to December 1998. Complementary data from the European Centre for Medium Range Weather Forecasts (ECMWF) and satellite images are used to investigate case studies for selected flights in order to determine the origin of ozone episodes. Backward trajectory modeling is used to determine the source of air masses possibly related to ozone episodes. Correspondence with high potential vorticity and low water vapour content are used as stratospheric tracers and are indicative of stratospheric-tropospheric injection. Marked seasonal and latitudinal differences in the frequency and nature of ozone episodes are noted between the northern and southern latitudes. The seasonal variation of ozone episodes is more prominent in the extratropics than in the tropics. The highest frequencies are observed in the Northern Hemisphere to the north of 200N in spring and summer, and in the Southern Hemisphere to the south of 30°8 in winter and spring . Between these latitudes the distribution of peaks is sparse. An exception is observed during the austral spring when a relatively high number of peaks south of the equator is evident. The spring frequency peak is common to both hemispheres and is likely to result from a biomass burning influence, and stratospheric ozone injection into the troposphere during this season. Ozone peaks north of 15°N are greater in magnitude (up to 350 ppbv) than those in the tropics and in the Southern Hemisphere, where peaks do not exceed 200 ppbv. The horizontal extent of the ozone events ranged from a few tens of kilometers to over 1000 km in the boreal spring, although they seldom exceeded 100 km in the tropics. Two case studies were examined in detail. In the first case study, ozone peaks up to 180 ppbv were observed at approximately 12° S in the summer. They ranged between 3.5 and 21 km in horizontal extent and were accompanied by increased relative humidity and evidence of uplift. They were attributed to localized biomass burning and subsequent convective uplift to the upper troposphere. The possibility of enhancement due to convective uplift and lightning production was discussed. The second case study, in which an ozone peak of 287 ppbv was observed at approximately 31°S in springtime, had a horizontal extent of 840 km and was accompanied by low relative humidity values and high potential vorticity, indicative of air of stratospheric origin.