Surface distributions of O3, CO and hydrocarbons over the Bay of Bengal and the Arabian Sea during pre-monsoon season

Abstract

Mixing ratios of ozone (O 3 ), carbon monoxide (CO), methane (CH 4 ) and few light non methane hydrocarbons (NMHCs) were measured on board the ocean research vessel Sagar Kanya over the Bay of Bengal and the Arabian Sea during the spring of 2006 as a part of an Integrated Campaign for Aerosol, gases and Radiation Budget (ICARB). North-westerly winds prevailing during this period transport large amount of anthropogenic pollutants from the Indo-Gangetic Plain (IGP) to the northern part of Bay of Bengal. The south-westerly and north-westerly winds carried cleaner marine air having lower abundance of pollutants over the southern Bay of Bengal and Arabian Sea. Ozone, CH 4 , CO, ethane and n-butane are found to be well correlated with each other over the northern Bay of Bengal indicating their common co-located sources. The latitudinal gradients of these species are found to be significant (O 3 ∼ 5.4 ppbv deg −1 , CH 4 ∼ 5.3 ppbv deg −1 , CO ∼ 10 ppbv deg −1 , ethane ∼ 93.2 pptv deg −1 and n-butane ∼ 59.7 pptv deg −1 ) over this region. Surprisingly, and in contrast to over the Bay of Bengal, the mixing ratios of these trace gases over the Arabian Sea are found comparatively higher over the southern region than over the northern region leading to negative latitudinal gradients. The short lived species with oceanic sources like ethene and propene show large variability and higher mixing ratios over southern parts of both the marine regions. These observations are compared with previous measurements made over these marine regions and the results obtained from the 3D MOZART chemistry transport model. The present study shows that the two marine regions adjacent to the Indian subcontinent are completely different from the perspective of surface level distributions of these species. ► First time near simultaneous measurements of trace gases over Bay of Bengal and Arabian Sea. ► Measurements of ozone, CO, CH 4 and various non-methane hydrocarbons during pre-monsoon season. ► Outflow of pollutants from Indo-Gangetic Plain over the Northern Bay of Bengal. ► Strong positive latitudinal gradients over Bay of Bengal and negative gradients over Arabian Sea. ► Comparison with a 3D chemistry transport model and CO obtained from MOPITT.