Wintertime aerosol optical and radiative properties in the Kathmandu Valley during the SusKat-ABC field campaign

Abstract. Particulate air pollution in the Kathmandu Valley has reached severe levels that are mainly due to uncontrolled emissions and the location of the urban area in a bowl-shaped basin with associated local circulations. The AERONET measurements from December 2012 to August 2014 revealed a mean aerosol optical depth (AOD) of approximately 0.3 at 675 nm during winter, which is similar to that of the post-monsoon but half of that of the pre-monsoon AOD (0.63). The distinct seasonal variations are closely related to regional-scale monsoon circulations over South Asia and emissions in the Kathmandu Valley. During the SusKat-ABC campaign (December 2012–February 2013), a noticeable increase of both aerosol scattering (σs; 313 → 522 Mm−1 at 550 nm) and absorption (σa; 98 → 145 Mm−1 at 520 nm) coefficients occurred before and after January 4, 2013. This can be attributed to the increase of …