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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. [ABSTRACT FROM AUTHOR]

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. 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 northern BoB (O3: 30 ± 7 nmol mol−1, CO: 95 ± 25 nmol mol−1, CH4: 1.86 ± 0.12 µmol mol−1), in airmasses from northern or central India, did not differ much from those over central BoB (O3: 27 ± 5 nmol mol−1, CO: 101 ± 27 nmol mol−1, CH4: 1.72 ± 0.14 µmol mol−1), in airmasses from southern India. Spatial variability is observed to be most significant for CH4. The ship-based observations, in conjunction with backward air trajectories and ground-based measurements over the Indian region, are analyzed to estimate a net 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 O3 and CO are successfully reproduced using the Weather Research and Forecasting model with Chemistry (WRF-Chem), this model overestimates mean concentrations by about 20 %, generally overestimating O3 mixing ratios during the rainfall events. Analysis of the chemical tendencies from model simulations for a low-O3 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 O3 and CO, respectively) over the northern BoB. [ABSTRACT FROM AUTHOR]

Abstract: Monthly mean tropospheric column O3 and NO2 retrieved from Ozone Monitoring Instrument on board Aura for the period December 2007 to November 2008 have been analysed to study the spatial and seasonal variations at selected locations over the Indian landmass and surrounding oceanic region. The annual change in tropospheric O3 and NO2 over the study region was ∼15–20 DU and ∼0.03–0.1 DU, respectively. Latitudinal gradient of −0.84 DU/° and 0.005 DU/°, respectively has been estimated for O3 and NO2 over this region. High levels of O3 were measured over parts of Arabian Sea and Bay of Bengal owing to transport from adjacent landmass. The dependence of O3 on NO2, solar radiation, cloud cover, rainfall, fire count, outgoing longwave radiation and boundary layer height (BLH) were investigated in detail along with the airflow pattern and airmass back trajectory analysis. There were regional variations in O3 seasonal patterns associated with differences in NO2, availability of solar radiation and prevailing airmass. BLH plays a role in the seasonal variation of tropospheric O3. The long-term trend in tropospheric O3 over a period of ten years has been examined for the different regions and both positive and negative trends were observed. [Copyright &y& Elsevier]