Distribution of surface carbon monoxide over the Indian subcontinent: Investigation of source contributions using WRF-Chem

This study investigates fractional contribution of different carbon monoxide (CO) sources over the Indian subcontinent at the surface in 2015 using a tagged tracer approach in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). Model simulations are evaluated with respect to in-situ and satellite observations. The simulated CO levels reproduced in-situ observations in Pune, Anantapur, Udaipur, Ahmedabad, and Chennai reasonably well with mean bias ranging from −89.9 to 87.0 ppbv and RMSE ranging from 28 to 40% but a poor model performance was noticed in Hyderabad, Jabalpur, and Kanpur with larger mean bias (−259.0–190.9 ppbv) and RMSE (45–96%). Simulated CO concentrations are also compared with CO retrieved by MOPITT at 800 hPa and 200 hPa. At 800 hPa (200 hPa), the annual average WRF-Chem CO varies from 195 ± 69 to 224 ± 93 ppbv (102 ± 25 to 136 ± 21 ppbv) while MOPITT CO varies from 103 ± 26 to 114 ± 29 ppbv (103 ± 30 to 134 ± 33) over different Indian sub-regions. Over most of the regions, good correlation (coefficient of correlation > 0.7) is observed between simulated and satellite observed CO with mean bias of 92–118 ppbv at 800 hPa and −4 to 3 ppbv at 200 hPa. Over the total Indian region, we find the highest contribution from anthropogenic emissions (CO-ANT) and inflow into the model domain from domain boundaries (CO-BACK) with 45–46% contribution each and very small contribution (<5%) of biomass burning (CO-BIOM), biogenic (CO-BIOG) emissions, and photochemical production (CO-CHEM). Fractional contribution of these tracers are analysed over different Indian sub-regions. CO-ANT (21–69%) and CO-BACK (22–66%) contributions are found high throughout the year over all the Indian landmass regions. CO-BACK contribution is found to be maximum (70–95%) over the Bay of Bengal and Arabian Sea. During winter season, CO-ANT and CO-BACK contribute 32–69% and 27–56% respectively over landmass regions of India. During spring, highest CO-BACK (66%) over Western India as compared to other landmass regions is attributed to CO inflow from the Arabian Peninsula and Middle East region. CO-BIOM contributes more than 22% in NE&M (North-East India and Myanmar) during spring due to slash and burn agriculture practice. During summer, transport of cleaner marine air and stronger vertical mixing are major causes of lower CO concentration over all sub-regions. During autumn, CO-ANT contributes 33–70% over different landmass regions. Surface CO concentration is found to be mainly controlled by the direct anthropogenic emission in Indian megacities with a contribution exceeding 70% to CO in Delhi and Kolkata throughout the year.