Abstract
Abstract. Air pollution and wintertime fog over South Asia is a major concern due to its significant implications for air quality, visibility and health. Using a regional climate model coupled with chemistry, we assess the contribution of the hygroscopic growth of aerosols (ambient–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 poor air quality conditions over South Asia, where water vapour contributes more than half of the aerosol optical depth. The aerosol–boundary layer interactions lead to 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 vapour interaction plays a decisive role in the formation and maintenance of the wintertime fog conditions over South Asia, which needs to be considered for planning mitigation strategies.
Highlights
South Asia experiences severe air pollution events during the winter season
Except for dust and freshly emitted black carbon, most of the aerosol species have an affinity towards water vapour, with the hygroscopic growth of inorganic aerosols such as sulfate, ammonia, nitrate and sea salt being more pronounced compared to organic carbon (Solmon et al, 2006)
The regional climate model regional climate model version 4 (RegCM4) interactively coupled with atmospheric chemistry and aerosols is used to investigate the contribution of the hygroscopic growth of aerosols to the total aerosol optical depth and its meteorological feedback over the Indian subcontinent
Summary
South Asia experiences severe air pollution events during the winter season. Poor air quality and visibility persisting throughout the winter period has been a major concern for more than 900 million people living in the IGP (Gautam and Singh, 2018; Gurjar et al, 2008; Lelieveld et al, 2015). Various studies have shown that the winter concentrations of fine particles (PM2.5) and gaseous pollutants exceed national and international air quality standards over most of the IGP region and are considered extremely hazardous for human health (Ali et al, 2019; Bharali et al, 2019; Ghude et al, 2017; Gurjar et al, 2008; Kumar et al, 2015a; Nair et al, 2007; Rengarajan et al, 2011; Safai et al, 2008). During the last few decades, the intensity of fog and haze events has increased over the region by a factor of 3 (Ghude et al, 2017; Syed et al, 2012), which is in line with the observed increase in aerosol optical loading (2.3 % yr−1 from 1985 onwards) and surface dimming over South Asia reported from a network of radiometer observations (Babu et al, 2013)
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