Variations in O 3 , CO, and CH 4 over the Bay of Bengal during the summer monsoon season: Ship-borne measurements and model simulations

15 We present ship-borne measurements of surface ozone, carbon monoxide and methane over the Bay of Bengal (BoB), the first time such measurements have been taken during the summer monsoon season, as a part of the Continental Tropical Convergence Zone (CTCZ) experiment during 2009. O 3 , CO, and CH 4 mixing ratios exhibited significant spatial and temporal variability in the ranges of 8–54 nmol mol -1 , 50–200 nmol mol -1 , and 1.57–2.15 µmol mol -1 , with means of 29.7±6.8 nmol mol -1 , 96±25 nmol mol -1 , and 1.83±0.14 µmol mol -1 , respectively. The 20 average mixing ratios of trace gases over northern BoB (O 3 : 30±7 nmol mol -1 , CO: 95±25 nmol mol -1 , CH 4 : 1.86±0.12 µmol mol -1 ), in airmasses from northern or central India, did not differ much from those over central BoB (O 3 : 27±5 nmol mol -1 , CO: 101±27 nmol mol -1 , CH 4 : 1.72±0.14 µmol mol -1 ), in airmasses from southern India. Spatial variability is observed to be most significant for CH 4 . The ship-based observations, in conjunction with backward air trajectories and ground-based measurements over the Indian region, are analyzed to estimate a net 25 ozone production of 1.5–4 nmol mol -1 day -1 in the outflow. Ozone mixing ratios over the BoB showed large reductions (by ~20 nmol mol -1 ) during four rainfall events. Temporal changes in the meteorological parameters, in conjunction with ozone vertical profiles, indicate that these low ozone events are associated with downdrafts of free-tropospheric ozone-poor airmasses. While the observed variations in O 3 and CO are successfully reproduced using the Weather Research and Forecasting model with Chemistry (WRF-Chem), this model overestimates mean 30 concentrations by about 20%, generally overestimating O 3 mixing ratios during the rainfall events. Analysis of the chemical tendencies from model simulations for a low-O 3 event on August 10, 2009, captured successfully by the model, shows the key role of horizontal advection in rapidly transporting ozone-rich airmasses across the BoB. Our study fills a gap in the availability of trace gas measurements over the BoB, and when combined with data from previous campaigns, reveals large seasonal amplitude (~39 and ~207 nmol mol -1 for O 3 and CO, respectively) over 35 the northern BoB. to be distributed over large region surrounding the measurement location. This large of elevated ozone mixing ratios is horizontally advected eastward 16:00 19:00 and then towards 22:00 UT As a result of this rapid advection, the high-ozone airmasses are transported from the coastal regions to deeper into the by the time they reached the location of observation, ozone mixing ratios are to be lower (25–35 nmol mol -1 during the time (22:00 UT). A was found to the east suggest played key role BoB diluted O 3 near coastal in southern India during the fourth event. Our study fills a gap of experimental data during the summer monsoon over the BoB, providing information on the extent of seasonal variability. We recommend supplementing these findings with ship-borne experiments featuring collocated vertical profile observations from balloon-borne and aircraft-based platforms over the oceanic regions surrounding India to better understand the role of both large-scale dynamics (e.g. Ojha et al., 2016) and of regional influences due to South Asian outflow (see Lawrence and Lelieveld, 2010, and references therein). Such a future study would also improve our understanding of the changes that take place in the atmospheric oxidation capacity during the summer monsoon season.