Regional transport of aerosols from Northern India and its impact on boundary layer dynamics and air quality over Chennai, a coastal megacity in Southern India

Abstract. Westerly driven regional transport of aerosols from the heavily polluted North India towards south-eastern India is a prevalent phenomenon during the winter season. Here, the regional aerosol transport events on the boundary layer dynamics and air quality over Chennai, a tropical South Asian megacity, are investigated. The long-term satellite data enables us to depict such regional transport events prolonged for a few days, accounting for ~10–13 per cent of the winter season. The occurrence of these regional transport events is increasing over time in southeastern India which are associated with relatively calmer conditions under anticyclonic wind circulation over north India extending to south India. The transported aerosol layer is generally located around ~1–3 km across the entire southeastern India, capped by the strong atmospheric temperature inversion. The regional aerosol/ haze transport significantly reduces the boundary layer height (ABL-H) by ~38 % compared to clear sky conditions ( ~2–2.5 km). Consequently, an increase in PM_2.5 is observed to be ~30–35 % in association with the strong heating aloft ABL (~1.2–2.5 K), suppression of ABL-H and anticyclonic circulation over north India. This study provides robust observational evidence on the importance of regional transport of aerosols on air quality of downwind megacities and warrants more observational and modelling studies to constrain the inherent aerosol-induced effects on boundary layer dynamics.