Field observations of the vertical distribution of tropospheric ozone at the island of Reunion (southern tropics)

Abstract

An analysis of 1 year of tropospheric vertical soundings of ozone at the island of Reunion (21°S, 55°E) is presented. High values of ozone concentration are observed in the troposphere above the inversion level that caps the marine boundary layer (MBL) during the September to November period, concomitant with active biomass burning in the southeastern African continent and Madagascar. Tongues (prominences of higher values) of enhanced ozone are apparent on vertical profiles obtained during this period, and backward trajectories from tongue levels are generally traced back to these zones of intense burning. Profiles of high and almost constant ozone mixing ratio in the whole troposphere above the MBL are also obtained during this season and could not be traced back to a definite origin. Conversely, during January to March, MBL compounds and humidity are convected in the whole troposphere, and tropospheric ozone is very low. The seasonal variation of integrated tropospheric ozone is large, up to 30 Dobson units (DU) and the tropospheric contribution to total columnar ozone could be high, up to 50–55 DU. This seasonal variation is well correlated with total ozone mapping spectrometer (TOMS) data, anticorrelated with integrated water content, and approximately follows, with a small time lag, the seasonal fire activity in the southeastern Africa. Data from Reunion are in accord with equivalent results from Ascension Island (8°S, 15°W), Brazzaville (Congo; 4°S, 15°E) and Natal (Brazil; 6°S, 35°W), corroborate satellite observations, and suggest that the concentration of tropospheric ozone in the tropics is governed by the coupling of photochemical and dynamical processes.