Variations of trace gases over the Bay of Bengal during the summer monsoon

Abstract

In situ measurements of near-surface ozone ( $$\hbox {O}_{3})$$ , carbon monoxide (CO), and methane ( $$\hbox {CH}_{4})$$ were carried out over the Bay of Bengal (BoB) as a part of the Continental Tropical Convergence Zone (CTCZ) campaign during the summer monsoon season of 2009. $$\hbox {O}_{3}$$ , CO and $$\hbox {CH}_{4}$$ mixing ratios varied in the ranges of 8–54 ppbv, 50–200 ppbv and 1.57–2.15 ppmv, respectively during 16 July–17 August 2009. The spatial distribution of mean tropospheric $$\hbox {O}_{3}$$ from satellite retrievals is found to be similar to that in surface $$\hbox {O}_{3}$$ observations, with higher levels over coastal and northern BoB as compared to central BoB. The comparison of in situ measurements with the Monitoring Atmospheric Composition & Climate (MACC) global reanalysis shows that MACC simulations reproduce the observations with small mean biases of 1.6 ppbv, –2.6 ppbv and 0.07 ppmv for $$\hbox {O}_{3}$$ , CO and $$\hbox {CH}_{4}$$ , respectively. The analysis of diurnal variation of $$\hbox {O}_{3}$$ based on observations and the simulations from Weather Research and Forecasting coupled with Chemistry (WRF-Chem) at a stationary point over the BoB did not show a net photochemical build up during daytime. Satellite retrievals show limitations in capturing $$\hbox {CH}_{4}$$ variations as measured by in situ sample analysis highlighting the need of more shipborne in situ measurements of trace gases over this region during monsoon.