Carbon monoxide over Indian region as observed by MOPITT

A comprehensive study has been carried out on tropospheric carbon monoxide (CO) over the Indian land mass and surrounding oceanic region using the CO retrievals from MOPITT (Measurements of Pollution in the Troposphere) for a period of ∼14 years (2000-2014). The lower-tropospheric CO maximises during winter and shows a broad minimum during summer-monsoon over most of the regions, but with regionally varying seasonal amplitudes. Tropospheric column CO also exhibits a seasonal pattern similar to lower-tropospheric CO. But the upper-tropospheric CO shows an opposite seasonal pattern which peaks during summer monsoon. Columnar CO showed strong positive correlation with fire counts over west, east and north-east India, indicating the dominant role of biomass burning in controlling the seasonal variation of CO. The lower-tropospheric and columnar CO showed decreasing trend of 2.0-3.4 ppb year−1 (1.1-2.0% year−1) and 6.0-13.6 × 1015 molecules cm−2 year−1 (0.3-0.6% year−1) respectively over most of the regions. However, over many land regions trend in columnar CO is not significant. Most strikingly, the upper tropospheric CO showed increasing trend of 1.4-2.4 ppb year−1 (1.8-3.2% year−1). The analysis of biases in the estimated trends due to temporal changes in the MOPITT averaging kernels shows that magnitude of the realistic trend may change depending upon the bias but the sign (positive or negative) of trend remains the same. The decreasing trend in lower tropospheric and columnar CO could be attributed partly to increase in lower tropospheric water vapour and/or tropospheric ozone. The strengthening of convective activity, uplifting the CO to higher altitudes, could be a reason for increasing trend in the upper-tropospheric CO.