Robust observations of smoke - mixing layer height - air quality coupling during crop residue burning season: Implications of elevated smoke layers over Delhi

Agricultural Crop Residue Burning (CRB) over northwestern India during the post-monsoon period is considered to severely impact the air quality in the megacity Delhi; however, the mechanistic understanding remains largely unclear. Here, synergistic analysis using long-term high-resolution satellite observations, ground-based measurements, and reanalysis data; and illustrate a new mechanism through which CRB emissions affect the air quality of Delhi via aerosol semi-direct effect. Long-term satellite observations (2007-2020) of aerosol properties during the CRB season (Oct 20th to Nov 20th) show severe aerosol loading (AOD=1.14±0.15) and absorbing aerosol (AAOD = 0.11±0.02; UVAI=1.58±0.21) within a very distinct airshed of CRB plume transport from Northwestern India (source region) to greater Delhi (downwind region). It is expected that smoke concentrations should disperse downwind and that CRB-associated PM2.5 enhancements over Northern India will 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. Synergistic analysis of satellite, radiosonde, and ground-based observations along with reanalysis data provided robust evidence that aerosol- boundary layer - PM2.5 associations via semi-direct effect can explain the above heterogeneity. Further, vertically resolved satellite observations reveal that as the CRB-smoke plumes travel downwind, a portion of the transported smoke plume is injected relatively at a higher altitude resulting in an elevated smoke layer over greater Delhi. These elevated smoke layers suppress the mixing height via inducing strong temperature inversion, thereby severely constraining the daytime dilution effect of shallow boundary layers. In addition to the direct advection of smoke aerosols, enhanced accumulation of local urban emissions due to these semi-direct impacts can lead to 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.