Impact of Relative Humidity on the vertical distribution of aerosols over India

Water uptake by aerosol particles significantly changes its light-scattering characteristics, alters the physical and optical properties of aerosols. In this article, we present the effect of ambient relative humidity (RH) on the optical properties of surface and columnar aerosols under different RH conditions during different seasons over India. In particular, relation between surface RH and surface aerosol properties, surface RH and aerosol optical depth (AOD) and profile of RH on the AOD is presented. An increase in the surface aerosol scattering coefficient (sigma scat) and total aerosol number concentration (N(d)) is observed under high humidity (50-100%) condition than low humidity (0-30%). This is more prominent in the winter and pre-monsoon seasons. An increase in the AOD is observed as surface RH increase in all the seasons except in monsoon season where it showed an inverse relation. The profile measurements (Lidar and radiosonde) also showed a consistent increase in AOD (0-6 km) from low to high RH (0-6 km) conditions. This relationship is significant during winter followed by the post-monsoon, pre -monsoon, and monsoon seasons. An increase in AOD with RH is observed regardless of altitude in all seasons, except during the monsoon at higher altitudes (2-6 km). Our findings clearly demonstrated that during the winter and monsoon (pre-and post-monsoon) seasons, aerosols within (above) the boundary layer are more sensitive to the RH. This relationship has been tested in different surface conditions across India using CALIPSO and IMD radiosonde measurements. These measurements also confirmed an increase in AOD with increasing RH in all the regions except East Coast (EC) region. Irrespective of the region, this feature is more prominent at lower altitudes (0-1.5 km) during winter and higher altitudes (1.5-6 km) during other seasons. At higher altitudes, only a few regions (Central India, Indo-Gangetic Plain, and West Coast) showed an inverse relationship between AOD and RH during the monsoon and post-monsoon seasons. Further, our results illustrated the contribution of hygro-scopic particles to the RH increases the AOD in a given season. This finding implies that hygroscopic growth caused by water-soluble particles in the lower troposphere can significantly alter the magnitude of aerosol radiative forcing both at the surface and at the top of the atmosphere.