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 PM_2.5 enhancement over Northern India should be inversely proportional to the distance from the CRB source region. However, the in-situ PM_2.5 observations illustrate that smoke-associated enhancement in PM_2.5 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 - PM_2.5 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 PM_2.5 enhancements over Delhi during CRB haze periods. Thus, control of the local anthropogenic emissions could bring relief during the extreme haze episodes over Delhi.

据信，印度西北部的季风后农业作物残渣燃烧 (CRB) 严重影响德里特大城市的空气质量。然而，对机械的理解仍然难以捉摸。CRB 季节（10 月 20 日至 11 月 20 日）气溶胶特性的长期卫星观测（2007-2020 年）表明，从印度西北部（源区）到大德里（顺风区）的 CRB 羽流传输存在明显的空气流域。理论上，烟雾浓度应顺风分散，印度北部与 CRB 相关的 PM _2.5增强应与距 CRB 源区的距离成反比。然而，现场 PM _2.5观察表明，PM 2.5 与烟雾相关的_增强与源区域相比，在大德里（顺风区域）上空大得不成比例。在这项研究中，我们检查了卫星、无线电探空仪和地面观测以及再分析数据，以提供强有力的证据表明气溶胶-边界层 - PM _2.5通过半直接效应的关联可以解释上述异质性。垂直分辨的卫星观测表明，随着排放的 CRB 烟雾羽流顺风行进，一部分被输送的烟雾羽流相对注入到较高的高度，导致在大德里上空形成升高的烟雾层。这些升高的烟层倾向于通过诱导强烈的逆温来抑制混合高度，从而严重限制了浅边界层的白天稀释效应。除了烟雾颗粒的直接平流外，由于这些半直接影响导致的局部城市排放物的积累会导致观察到的不成比例的 PM _2.5CRB 阴霾期间德里的增强。因此，控制当地人为排放可以在德里的极端雾霾事件中缓解压力。