Observational evidence of elevated smoke layers during crop residue burning season over Delhi: Potential implications on associated heterogeneous PM2.5 enhancements

Abstract

Post monsoonal agricultural Crop Residue Burning (CRB) over northwestern India is believed to severely affect the air quality of the megacity of Delhi. However, the mechanistic understanding remains elusive. Long-term satellite observations (2007–2020) of aerosol properties during CRB season (Oct 20th to Nov 20th) indicate a distinct airshed of CRB plume transport from Northwestern India (source region) to greater Delhi (downwind region). Theoretically, the smoke concentration should disperse downwind, and the CRB-associated PM2.5 enhancement over Northern India should be inversely proportional to the distance from the CRB source region. However, the in-situ PM2.5 observations illustrate that smoke-associated enhancement in PM2.5 over greater Delhi (downwind region) is disproportionately large compared to the source region. In this study, we examine satellite, radiosonde, and ground-based observations along with reanalysis data to provide robust evidence that aerosol- boundary layer - PM2.5 associations via semi-direct effect can explain the above heterogeneity. Vertically resolved satellite observations indicate that as the emitted CRB-smoke plumes travel downwind, a portion of the transported smoke plume injected relatively at a higher altitude leads to the formation of an elevated smoke layer over greater Delhi. These elevated smoke layers tend to suppress the mixing height via inducing strong temperature inversion, thereby severely constraining the daytime dilution effect of shallow boundary layers. Along with direct advection of smoke particles, enhanced accumulation of local urban emissions due to these semi-direct impacts can lead to the observed disproportionate PM2.5 enhancements over Delhi during CRB haze periods. Thus, control of the local anthropogenic emissions could bring relief during the extreme haze episodes over Delhi.