Effects of convection and long-range transport on the distribution of carbon monoxide in the troposphere over India

Variability in the tropospheric distributions of carbon monoxide (CO) over five selected regions of India has been studied using the MOPITT data for the period of 2001–2014. The average seasonal profiles show highest mixing ratios at 900 hPa in the boreal winter and lowest in the Indian summer monsoon over all the study regions. We observe a slight increase in CO levels from 500 hPa to 200 hPa over all the locations. The CO mixing ratios are found to be higher by about 10–40% around 300–200 hPa as compared to 900 hPa over Ahmedabad, Hyderabad and Trivandrum during monsoon period. This could be due to lifting of polluted air by convection and getting trapped in the anticyclonic winds over the Indian region during the monsoon. Most of the 7 day back trajectories over these regions show transport of the polluted air mass from the major biomass burning regions of central Africa and SE Asia. The results show dominance of the seasonal amplitude at 900 hPa over all the regions, while inter-annual variability dominates mostly over Ahmedabad, Hyderabad and Trivandrum at 300 hPa. In order to check the ability of different models in capturing the observed variability, the results have been compared with simulations from two chemistry transport models (MOZART and EMAC). This comparison shows that both the models perform reasonably well in simulating the basic features such as annual variation as well as increase in CO around 300–200 hPa due to convection during the monsoon season.