Amplification of South Asian haze by water vapour-aerosol interactions

Abstract

<jats:p>Abstract. Air pollution and wintertime fog over South Asia is a major concern due to its significant implications on air quality, visibility and health. Using a coupled regional climate model with on-line chemistry, we assess the contribution of the hygroscopic growth of aerosols (wet-dry) to the total aerosol optical depth and demonstrate that the increased surface cooling due to the hygroscopic effects of aerosols further increases the humidity in the boundary layer and thus enhances the confinement of pollutants through aerosol-boundary layer interactions. This positive feedback mechanism plays an important role in the prevalence of wintertime fog and low air quality conditions over South Asia, where water vapor contributes more than half of the aerosol optical depth. The aerosol-boundary layer interactions lead to the moistening of the boundary layer and drying of the free-troposphere, which amplifies the long-term trend in relative humidity over the Indo-Gangetic Plain during winter. Hence, the aerosol-water vapor interaction plays a decisive role in the formation and maintenance of the wintertime thick fog conditions over South Asia, which needs to be considered for planning mitigation strategies. </jats:p>

Atmospheric rivers fueling the intensification of fog and haze over Indo-Gangetic Plains

“…Furthermore, hygroscopic experiments carried out over highly polluted locations of the IGP have also reported the hygroscopicity of aerosols and their ability to serve as cloud condensation nuclei (CCN) under both freshly emitted primary and secondary aerosols 44 – 46 . A recent observational study 47 has also reported an enhancement of aerosol size by around 1.34 times at 85% relative humidity and the aerosol hygroscopic growth reported to contribute up to 40% to the observed increase in the aerosol optical depth over IGP during the winter season 41 . Thus, the AR-induced enhancement in water vapor interacts with the prevailing atmospheric aerosols over the IGP region and triggers the aerosol hygroscopicity leading to the intensification of fog and haze.…”

“…S8 ). Thus, the AR-driven enhancement in water vapour over this highly polluted location caused aerosols to undergo aerosol-water vapor interactions 41 , 42 , leading to the intensification of fog and haze. In general, the detected AR events are observed to enhance the water vapour amount and AOD over IGP by 19 ± 5% and 29 ± 12% respectively (Supplementary Table S3 and S4 ).…”