Airborne measurements of O3, CO, CH4 and NMHCs over the Bay of Bengal during winter

As part of the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (WICARB), airborne measurements of ozone (O3), carbon monoxide (CO), methane (CH4), ethane (C2H6), ethene (C2H4), acetylene (C2H2), propane (C3H8) and n-butane (n-C4H10) were conducted over the Bay of Bengal (BoB) at two altitude levels, 750 m and 1500 m, from Visakhapatnam, Chennai and Port Blair during January 2009. Airborne measurements, first of its kind over the BoB, revealed distinct chemical characteristics at these two altitudes over the eastern sector of Visakhapatnam away from the coastline. This feature is attributed to the presence of marine boundary layer inversion between the two altitudes that inhibited the vertical mixing. A comparative study of trace gas distributions during all air sorties showed enhanced mixing ratios of these gases over east of Visakhapatnam and south of Port Blair in comparison to the north/north–east and west of Port Blair and east of Chennai. The back-trajectory analysis showed the outflow from the highly polluted Indo-Gangetic Plain over eastern side of Visakhapatnam. The air masses showed lower photochemical processing (higher C2H2/CO ratio ∼ 4–4.5 pptv ppbv−1) with the age of ∼ 3–4 days over this location in comparison to other sorties. Chemical characteristics over south of Port Blair seem to be controlled by the outflow from Southeast Asian countries. The results presented in this work would serve as valuable inputs for the atmospheric chemistry transport models and will be helpful in reducing the uncertainty in the budget estimation of trace gases over tropical marine region.