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We present shipborne measurements of surface ozone (O-3), carbon monoxide (CO), and methane (CH4) over the Bay of Bengal (BoB), the first time such measurements have been performed during the summer monsoon season, as a part of the Continental Tropical Convergence Zone (CTCZ) experiment during 2009. O-3, CO, and CH4 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 mu mol mol(-1), with means of 29.7 +/- 6.8 nmol mol(-1), 96 +/- 25 nmol mol(-1), and 1.83 +/- 0.14 mu mol mol(-1), respectively. The average mixing ratios of trace gases over BoB in air masses from central/northern India (O-3: 30 +/- 7 nmol mol(-1); CO: 95 +/- 25 nmol mol(-1); CH4: 1.86 +/- 0.12 mu mol mol(-1)) were not statistically different from those in air masses from southern India (O-3: 27 +/- 5 nmol mol(-1); CO: 101 +/- 27 nmol mol(-1); CH4: 1.72 +/- 0.14 mu mol mol(-1)) . Spatial variability is observed to be most significant for CH4 with higher mixing ratios in the air masses from central/northern India, where higher CH4 levels are seen in the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) data. O-3 mixing ratios over the BoB showed large reductions (by similar to 20 nmol mol 1) during four rainfall events. Temporal changes in the meteorological parameters, in conjunction with O3 vertical profile, indicate that these low-O-3 events are associated with downdrafts of free-tropospheric O-3-poor air masses. While the observed variations of O-3 and CO are successfully reproduced using the Weather Research and Forecasting model with Chemistry (WRF-Chem), this model overestimates mean concentrations by about 6 and 16% for O-3 and CO, respectively, generally overestimating O-3 mixing ratios during the rainfall events. An analysis of modelled O-3 along air mass trajectories show mean en route O-3 production rate of about 4.6 nmol mol 1 day 1 in the outflow towards the BoB. Analysis of the various tendencies from model simulations during an event on 10 August 2009, reproduced by the model, shows horizontal advection rapidly transporting O-3-rich air masses from near the coast across the BoB. This 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 (similar to 39 and similar to 207 nmol mol 1 for O-3 and CO, respectively) over the northern BoB.