Interstate transport of carbon monoxide and black carbon over India

The contribution of interstate pollution transport to surface carbon monoxide (CO) and black carbon (BC) levels in different Indian states and regions is studied by implementing a computationally efficient tagged tracer technique in the Weather Research and Forecasting model coupled with chemistry. Anthropogenic and fire emissions of CO and BC are tracked from each Indian state and the simulated CO and BC concentrations are used to estimate state-wise pairs of source and receptor states in 2014, the most recent year for which anthropogenic emissions are available. We observe strong relationship between direct emissions and concentrations of CO and BC with local emissions contributing 56-68% to the annual averaged CO and BC levels in Uttar Pradesh, Delhi, and West Bengal located in the Indo-Gangetic Plain. Interstate transport contributes 53 +/- 23% and 52 +/- 25% to annual averaged CO and BC, respectively, in different Indian states. Interstate transport contribution exceeds 60% in one third of the Indian states. Uttar Pradesh, West Bengal, Maharashtra, and Bihar are among the top five contributors to CO and BC levels in most of the Indian states. Seasonal changes in monsoonal circulation also led to a reversal of source-receptor relationship at the state level between winter and summer monsoon seasons. Our findings imply a need to extend India's air quality management framework to address interstate exchange of pollution.