Free-tropospheric aerosols contribute to large aerosol optical depth at a remote, inland location with insignificant anthropogenic emissions

Abstract

In-situ measurements of atmospheric aerosols from remote environments marked with negligible human activities are required to reduce the uncertainties associated with natural aerosols in pre-industrial climate forcings (particularly the aerosol-cloud interactions). In this study, we present long-term continuous aerosol measurements, carried out using ground-based and satellite observations, both close to the surface and in the vertical column, over a remote, rural, inland location in southern peninsular India. The near-surface aerosol concentrations were low, in general conformity to the typical pattern expected for a remote location, with the temporal changes being primarily governed by the background meteorology. However, the columnar aerosol optical depth (AOD) was surprisingly high and resembled that of a typical semi-urban continental location, with an yearly mean AOD of 0.47 ± 0.06. Spaceborne lidar observations in conjunction with ground-based radiometer measurements revealed that around 60% of this AOD was contributed by free-tropospheric aerosol layers during summer and monsoon seasons. Airmass back-trajectory analysis along with particle depolarisation ratio profiles indicate that these aerosol layers are mostly associated with long-range transported dust aerosols. Our findings indicate that benign surface concentration of aerosols at a location need not necessarily be sufficient to consider it as an optimal choice for studying background aerosols.