O3 and CO in the South Asian outflow over the Bay of Bengal: Impact of monsoonal dynamics and chemistry

Abstract Monsoonal dynamics strongly impacts the composition over the South Asian region, however, in situ measurements remain limited mostly to the continental region. Whereas, remote oceanic regions of the Bay of Bengal (BoB) remain largely unexplored where shipping emissions can infuse into the South Asian outflow. In this direction, we performed ship-based measurements of ozone (O3) and carbon monoxide (CO) over the BoB during July‒August 2018 and corroborated the analyses with model results and satellite retrievals. Surface level mixing ratios of O3 and CO varied in the range of 14–45 and 50–164 ppbv, respectively during the expedition. Elevated O3 and CO levels were observed in coastal region downwind of a tropical megacity (Chennai), referred to as the Chennai-plume. Analysis of the Copernicus Atmosphere Monitoring Service (CAMS) model simulations suggests volatile organic compound (VOC)-limited O3 production in morning over most of the plume and east coast followed by gradual shift to NOx-limited regime over the BoB in afternoon. While the southern BoB (7–10° N) received generally pristine airmasses from the equatorial Indian Ocean, impacts of shipping emissions near 6° N are observed. Consequently, higher values of CO (as much as 120 ppbv) and net O3 production by 1.1–1.3 ppbv h−1 with a diurnal amplitude of 4–8 ppbv are observed over the southern BoB. Satellite-based observations from the Measurements of Pollution in the Troposphere (MOPITT) and results from CAMS model show decreasing trend (4.1–5.2 ± 1 ppbv decade−1) of surface CO, as also inferred from in situ measurements. Zonal fluxes of CO across the east coast show the outflow over the BoB with higher values (2–3 × 1019 molecules m−2 s−1) over the northern BoB. Positive vertical fluxes (~2 × 1016 molecules m−2 s−1) show the transport of CO from marine boundary layer to the free-troposphere. Due to strong updrafts, CO levels in upper and lower troposphere are seen to be comparable during monsoon. These findings in conjunction with higher methane and nitrogen dioxide in the upper-troposphere manifest the influences of monsoonal convection and trapping of trace gases in the anticyclone.