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

Abstract. Measurements of ozone and NO₂ were carried out in the marine environment of the Bay of Bengal (BoB) during the winter months, December 2008–January 2009, as part of the second Integrated Campaign for Aerosols, gases and Radiation Budget conducted under the Geosphere Biosphere Programme of the Indian Space Research Organization. The ozone mixing ratio was found to be high in the head and the southeast BoB with a mean value of 61 ± 7 ppb and 53 ± 6 ppb, respectively. The mixing ratios of NO₂ and CO were also relatively high in these regions. The spatial patterns were examined in the light of airflow patterns, air mass back trajectories and other meteorological conditions and satellite retrieved maps of tropospheric ozone, NO₂, CO, and fire count in and around the region. The distribution of these gases was strongly associated with the transport from the adjoining land mass. The anthropogenic activities and forest fires/biomass burning over the Indo Gangetic Plains and other East Asian regions contribute to ozone and its precursors over the BoB. Similarity in the spatial pattern suggests that their source regions could be more or less the same. Most of the diurnal patterns showed decrease of the ozone mixing ratio during noon/afternoon followed by a nighttime increase and a morning high. Over this oceanic region, photochemical production of ozone involving NO₂ was not very active. Water vapour played a major role in controlling the variation of ozone. An attempt is made to simulate ozone level over the north and south BoB using the photochemical box model (NCAR-MM). The present observed features were compared with those measured during the earlier cruises conducted in different seasons.