Distributions of O3, 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

Mixing ratios of O3 and some related trace species like CO, CH4 and C2–C4 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 O3 (13–43 ppbv), CO (108–210 ppbv), CH4 (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, CH4 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 O3 and CO are observed to be lower and much higher in CH4 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 C2H2/CO ratios support role of advection in the observed distribution of trace gases over the Bay of Bengal.