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.

Tropospheric ozone climatology at two Southern Hemisphere tropical/subtropical sites, (Reunion Island and Irene, South Africa) from ozonesondes, LIDAR, and in situ aircraft measurements

“…A study of the effect of tropical convection on tropospheric ozone has also been reported (Leclair de Bellevue et al,2006) and showed some differences between the two sites, Reunion and Irene. In addition, photochemical sources have been identified: biomass burning activity in Southern Africa and Madagascar during austral spring associated with long range transport of air masses are a cause of tropospheric ozone increase over Reunion Island (Baldy et al, 1996;Taupin et al, 2002). At this site, the influence of biomass burning activity has been identified between 5 and 12 km on tropospheric ozone profiles (Randriambelo et al, 2000).…”

“…However, the increase in pollution and biomass burning activity since the 1990s needs to be confirmed by further investigation. On the contrary, lower layers over Reunion are less directly influenced by biomass burning because of the easterly trade wind regime (Baldy et al, 1996).…”

“…Meteorological analysis based on trajectory statistics showed that a large part of African emissions might be advected to the Indian Ocean Piketh et al, 2002). Mechanisms responsible for regional dispersion of biomass burning emissions have been documented (Baldy et al, 1996;Taupin et al, 1999Taupin et al, , 2002. A climatology by Randriambelo et al (1999) based on ozonesonde and satellite data focuses on the relative importance of the different mechanisms.…”

“…The reason for this positive trend for the upper layer is an interesting issue and we draw two hypotheses: first, the influence of the increase of biomass burning and pollution can play a role: the trend values in the upper troposphere at Reunion could be related to the complex dynamical mechanisms which are necessary for Reunion Island tropospheric ozone to be influenced by biomass burning: injection of pollutants in the free troposphere by convection, mix-thencook scheme and ozone production during westerly transport (Chatfield and Delany, 1990;Baldy et al, 1996). The second hypothesis is an increase in stratosphere-troposphere exchange which could be induced by climate change.…”

Distribution of ozone and its precursors over Bay of Bengal during winter 2009: role of meteorology

“…Tropospheric concentrations of ozone are also influenced by downward transport from the stratosphere (Sprenger et al, 2007;Ganguly and Tzanis, 2011), and deposition to surfaces (Graedel and Crutzen, 1992). Long range transport of ozone and other pollutants also modify the tropospheric level of ozone in different regions of the world (Baldy et al, 1996;Browell et al, 1996;Jacob et al, 1999). In the marine environment, all the dominant sources of ozone and precursor gases are absent/weak and their concentrations are governed primarily by transport and photochemistry.…”