Quantifying the impact of traffic emissions on PM2.5 over Delhi during the post-monsoon season

Every year during the post-monsoon season Delhi experiences levels of fine particulate matter (PM_2.5) pollution that far exceed the WHO air quality guidelines. This is due to unfavourable meteorological conditions and additional seasonal emissions. Much of the blame has been on emissions from seasonal burning of crop residue from upwind states of Punjab and Haryana, representing 20-40% of PM_2.5 pollution in Delhi during this season. However, other sources still represent the majority of surface PM_2.5 pollution. Local traffic emissions from the city of Delhi (National Capital Territory, NCT) are estimated to be one of the main contributors to PM_2.5 pollution in Delhi, but trial strategies to control emissions that involved limiting the volume of local passenger cars failed to address poor air quality over the city during winter and pre-monsoon seasons, reducing PM_2.5 maximum up to 10%. Previous studies have found that non-local anthropogenic sources from nearby states of the National Capital Region (NCR) also contribute substantially to air pollution over Delhi, emphasizing the need for the development of an integrated inter-sectoral and inter-state air pollution mitigation strategy. Here we use nested (4 km) WRF-Chem model simulations, driven by local inventories, to quantify the relative importance of the local (NCT) and non-local (NCR) transport sectors on PM_2.5 in Delhi during the post-monsoon season, and compare them against local and non-local anthropogenic sectors.