Temporal role of crop residue burning (CRB) in Delhi's air pollution

The National Capital Region of Delhi in India is one of the most polluted urban areas in the world, and its intense pollution episodes are attributed to crop residue burning (CRB). However, existing studies are limited in time and pollutant type, and do not often control for non-biophysical factors. We used ground-measured pollutant levels and aerosol optical depth (AOD) data to understand the role of CRB in Delhi's air pollution from 2015 to 2018. While the CRB peak in October-November is associated with pollution episodes, biophysical conditions in the April-May CRB period allow the pollutants to disperse. Elevation in SO2 and ozone is associated with CRB from more distant source areas than elevations in PM2.5 and PM10: gaseous molecules such as SO2 may travel further than particulate matter; and NO2 may convert to ozone during long-range transport. Pollution levels are very high in December-January despite low CRB in this period. Given the high contribution of biomass burning in this period in source apportionment studies, it is unclear whether the sharp elevation in pollutant levels with temperature drop in this period can be explained only with meteorological conditions, or whether there are unquantified sources contributing to pollutant load in December-January, such as small-scale CRB that is undetected by the MODIS active fire product or local burning for heat. There are limitations to the substitution of ground-measured pollutants with AOD in analyzing drivers of pollution since AOD, unlike ground-measured pollutants, is unable to identify fine-scale drivers such as construction. Further studies that use seasonal emissions inventories, particularly for waste and household burning for heat, are required to understand their contribution to pollution in December-January as they may have a disproportionate impact on pollution and human health.