Ozone generation over the Indian Ocean during the South African biomass-burning period: case study of October 1992.

F G Taupin,P J Brémaud,M Beekmann,T Randriambelo

doi:10.5194/angeo-20-547-2002

Abstract

Abstract. In this study, we present an estimation of photochemical ozone production during free tropospheric transport between the African biomass burning area and Reunion Island (Indian Ocean) by means of trajectory-chemistry model calculations. Indeed, enhanced ozone concentrations (80–100 ppbv) between 5 and 8 km height over Reunion Island are encountered during September–October when African biomass burning is active. The measurements performed during flight 10 of the TRACE-A campaign (October 6, 1992) have been used to initialise the lagrangian trajectory-chemistry model and several chemical forward trajectories, which reach the area of Reunion Island some days later, are calculated. We show that the ozone burden already present in the middle and upper troposphere over Southern Africa, formed from biomass burning emissions, is further enhanced by photochemical production over the Indian Ocean at the rate of 2.5 - 3 ppbv/day. The paper presents sensitivity studies of how these photochemical ozone production rates depend on initial conditions. The rates are also compared to those obtained by other studies over the Atlantic Ocean. The importance of our results for the regional ozone budget over the Indian Ocean is briefly discussed.Key words. Atmospheric composition and structure (evolution of the atmosphere; troposphere – composition and chemistry); meterorology and atmospheric dynamics (tropical meteorology)

Highlights

Photochemical ozone production, from biomass burning emissions over Africa during the September–November period, affects large parts of the Southern tropical Indian Ocean as can be seen in TOMS/SBUV daily tropospheric ozone maps (Fishman et al, 1996a)
We show that the net upper tropospheric ozone production between the Southern African East coast and Reunion Island is about 2.5–3 ppbv/day for the three trajectories studied which corresponded to the most polluted upper tropospheric layers of TRACE-A flight 10
Our result falls in the range of values calculated by Jacob et al (1996) from all TRACE A data between 4 and 12 km, Jacob et al (1996) calculate net ozone production rates with fixed precursor concentrations whereas our study allows for decay of precursors

Summary

Introduction

Photochemical ozone production, from biomass burning emissions over Africa during the September–November period, affects large parts of the Southern tropical Indian Ocean as can be seen in TOMS/SBUV daily tropospheric ozone maps (Fishman et al, 1996a). Backtrajectory analyses from locations of enhanced ozone concentrations have shown an origin over southeastern Africa and Madagascar (Baldy et al, 1996; Diab et al, 1996). The forward trajectory analyses of Thompson et al (1996) from fire locations in southern Africa showed that Reunion Island was within the sphere of influence of southern African biomass burning products. The ozone contamination from biomass burning was shown to be maximum over Reunion Island during October. It is not clear (Baldy et al, 1996) whether the ozone build-up is confined to a region near emission sources, with subsequent transport to Reunion Island (Cook--Mix), or whether photochemical ozone production is still active over the Indian Ocean (Mix--Cook). In the present study, we wish to perform trajectory-chemistry modelling in order: 1. to explain the possible photochemical origin of ozone enriched layers observed at Reunion Island and

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Conclusion