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We present shipborne measurements of surface ozone (O3), carbonmonoxide (CO), and methane (CH4) over the Bay of Bengal (BoB), the firsttime such measurements have been performed during the summer monsoon season,as a part of the Continental Tropical Convergence Zone (CTCZ) experimentduring 2009. O3, CO, and CH4 mixing ratios exhibited significantspatial 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 meansof 29.7 ± 6.8 nmol mol−1, 96 ± 25 nmol mol−1, and1.83 ± 0.14 µmol mol−1, respectively. The average mixingratios 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 statisticallydifferent 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 mostsignificant for CH4 with higher mixing ratios in the air masses fromcentral/northern India, where higher CH4 levels are seen in theSCIAMACHY (SCanning Imaging Absorption spectroMeter for AtmosphericCartograpHY) data. O3 mixing ratios over the BoB showed largereductions (by  ∼  20 nmol mol−1) during four rainfallevents. Temporal changes in the meteorological parameters, in conjunctionwith O3 vertical profile, indicate that these low-O3 events areassociated with downdrafts of free-tropospheric O3-poor air masses.While the observed variations of O3 and CO are successfully reproducedusing the Weather Research and Forecasting model with Chemistry (WRF-Chem),this model overestimates mean concentrations by about 6 and 16 % forO3 and CO, respectively, generally overestimating O3 mixing ratiosduring the rainfall events. An analysis of modelled O3 along air masstrajectories show mean en route O3 production rate of about 4.6 nmol mol−1 day−1 in the outflow towards the BoB. Analysis of thevarious tendencies from model simulations during an event on 10 August2009, reproduced by the model, shows horizontal advection rapidlytransporting O3-rich air masses from near the coast across the BoB. Thisstudy fills a gap in the availability of trace gas measurements over theBoB and, when combined with data from previous campaigns, reveals largeseasonal amplitude ( ∼  39 and  ∼  207 nmol mol−1 for O3 and CO, respectively) over the northern BoB

atmospheric chemistry and physics

Variations in O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, CO, and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; over the Bay of Bengal during the summer monsoon season: shipborne measurements and model simulations