Variations in O₃, CO, and CH₄ over the Bay of Bengal during the summer monsoon season: shipborne measurements and model simulations

Abstract. We present shipborne measurements of surface ozone (O₃), 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. O₃, 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 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 average mixing ratios of trace gases over BoB in air masses from central/northern India (O₃: 30 ± 7 nmol mol⁻¹; CO: 95 ± 25 nmol mol⁻¹; CH₄: 1.86 ± 0.12 µmol mol⁻¹) were not statistically different from those in air masses from southern India (O₃: 27 ± 5 nmol mol⁻¹; CO: 101 ± 27 nmol mol⁻¹; CH₄: 1.72 ± 0.14 µmol 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. O₃ 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 O₃ vertical profile, indicate that these low-O₃ events are associated with downdrafts of free-tropospheric O₃-poor air masses. While the observed variations of O₃ 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 O₃ and CO, respectively, generally overestimating O₃ mixing ratios during the rainfall events. An analysis of modelled O₃ along air mass trajectories show mean en route O₃ 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 O₃-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 ( ∼  39 and  ∼  207 nmol mol⁻¹ for O₃ and CO, respectively) over the northern BoB.