Seasonality in the latitudinal distributions of NMHCs over Bay of Bengal

Surface level measurements of light non-methane hydrocarbons (C2–C4 NMHCs) were conducted during summer and winter monsoon campaigns over Bay of Bengal (BOB). The mixing ratios of major NMHCs during winter monsoon were significantly higher compared to summer monsoon. During both seasons, the distributions of NMHCs show clear north–south decreasing gradients over open oceanic region of BOB. During the summer (winter) monsoons, the latitudinal gradients of ethane, propane and acetylene were 29 ± 3 (136 ± 4) pptv deg−1, 14 ± 2 (46 ± 8) pptv deg−1 and 14 ± 4 (148 ± 10) pptv deg−1, respectively over open oceanic region of BOB. For measurements during summer monsoon, the latitudinal gradients in the mixing ratios of NMHCs were caused by the transport of pollutants from peninsular India to the northern BOB and flow of cleaner marine air over southern BOB. On the other hand, the gradients in the mixing ratios of NMHCs during winter monsoon are due to the dilution and photochemical loss towards the southern BOB. In both seasons, no such clear latitudinal trends were observed along the east coast of India. The measurements along the coastal tracks were influenced by stagnant flow and photochemically aged air which also lacks clear north–south directionality. Alkenes show opposite seasonality to that of major NMHCs as the mixing ratios of both ethene and propene during summer monsoon were higher than the winter values. In addition, the distributions of alkenes also show significant heterogeneity, however, no clear latitudinal trends can be seen during both seasons. The investigations of wind and ocean color parameters suggest substantial emissions of alkenes from surface sea waters which also support the seasonality of these species over BOB.

► Seasonality in the latitudinal distribution of C2–C4 NMHCs over Bay of Bengal.
► Role of transport and photochemical oxidation in latitudinal gradients of NMHCs.
► Impact of oceanic emissions on the distributions of alkenes.