Atmospheric Environment 40 (2006) 4633–4645 Distributions of O 3 , CO and hydrocarbons over the Bay of Bengal: A study to assess the role of transport from southern India and marine regions during September–October 2002 L.K. Sahu à , S. Lal, S. Venkataramani Physical Research Laboratory (PRL), Navrangpura, Ahmedabad 38009, India Received 4 October 2005; received in revised form 10 February 2006; accepted 10 February 2006 Abstract Mixing ratios of O 3 and some related trace species like CO, CH 4 and C 2 –C 4 hydrocarbons were measured in the marine boundary layer (MBL) over the Bay of Bengal during September and October 2002. Air parcels having different and consequently variable compositions were encountered during the campaign. The strength of variability is reflected in their observed ranges; for O 3 (13–43 ppbv), CO (108–210 ppbv), CH 4 (1.67–1.86 ppmv), ethane (376–913 pptv) and acetylene (16–420 pptv). In general, continental influenced air masses dominated over the northern region while the cleaner marine air masses prevailed over the southern region of the Bay of Bengal. Ozone, CO, CH 4 and some anthropogenic NMHCs showed significant latitudinal gradients, while short-lived species like ethene, propene and butanes did not show any significant gradients. However, the latitudinal gradients in O 3 and CO are observed to be lower and much higher in CH 4 over the Bay of Bengal compared to those observed during the Indian Ocean Experiment (INDOEX) 1998 and 1999 campaigns. Study of wind fields, back-trajectories and C 2 H 2 /CO ratios support role of advection in the observed distribution of trace gases over the Bay of Bengal. r 2006 Elsevier Ltd. All rights reserved. Keywords: Bay of Bengal; Trace gases; NMHCs; Transport; India 1. Introduction The Asian region is developing very fast, leading to more energy consumption due to rapid indus- trialization and urbanization. Fossil fuel combus- tion is one of the most important sources of energy in automobiles and release many reactive O 3 precursors like non-methane hydrocarbons (NMHCs) and CO mainly due to incomplete burning of the fuel. Apart from fossil fuel combus- tion other human activities like use of biofuels and biomass burning are also important contributors of O 3 precursors in the tropics. In the entire Asia, about one quarter of the energy use depends on biofuels, whereas in India this fraction is close to 50%. Also, firewood contributes about two-thirds of biofuel consumption, whereas burning of dung and agricultural wastes contribute roughly equal to remaining one-third (Lelieveld et al., 2001). Sources and regional contribution of emissions of CH 4 , CO, ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.02.037 à Corresponding author. Tel.: +91 79 26314663; fax: +91 79 26314659. E-mail address: lokesh@prl.res.in (L.K. Sahu).