Satellite estimate of tropospheric ozone columnar levels over the Tropical Atlantic Ocean show relatively low values (< 30 DU) during the Northern Hemisphere winter season (DJF), although there is biomass burning in West Africa (ozone paradox). Conversely, while relatively high tropospheric ozone columnar values (> 45 DU) are found during Northern Hemisphere autumn season (SON) when biomass burning occurs in South America and eastern Africa (tropospheric ozone maximum). We examine the transition period (March-August) and the factors (fires, aerosols, horizontal and vertical transport processes and lightning) that lead to elevated ozone by the month of September.

During March-April-May (MAM) tropospheric ozone column levels remain relatively low (< 36 DU) over much of the tropical Atlantic but begin to increase by May. We find during MAM that fires associated with biomass burning decline in West Africa, but by May fires begin to increase in Central Africa leading to tropospheric ozone columnar values of 39-42 DU off the coast of Central Africa (Equator-5S). At the same time, lightning, a potential source of ozone, migrates from Central Africa in March to areas north of the Equator (West Africa) by the month of May.

During June-July-August (JJA), tropospheric ozone columnar values (> 45 DU) over the Tropical Atlantic Ocean are significantly increase compared to MAM. High tropospheric columnar ozone values are also found land areas of West Africa during this period. In addition, an increase in aerosol indices most likely associated with biomass burning is found off the coast Central Africa between the Equator and 10S during this period. Tropospheric ozone mixing ratios at Ascension Island (7.98S, 14.42W) are increasing during this period. High tropospheric ozone columnar values over West Africa are puzzling given the lack of biomass burning north of the Equator. These high columnar values may be associated with middle/upper level ozone produced through lightning in West Africa or lower tropospheric horizontal transport from Central Africa.