Diurnal Characteristics of column-integrated Aerosol Optical and Radiative Properties over Kanpur, Indo-Gangetic Plains

Aerosol optical and radiative properties are the key parameters to assess associated impacts on climate forcing. Over southern Asia, several studies in the past decade have indicated significant perturbations in the regional radiation budget due to enhanced aerosol loading, especially over the densely-populated Indo-Gangetic Plains (IGP). While the seasonal and inter-annual variations of aerosols and radiative forcing over the IGP have been studied extensively, diurnal characteristics of aerosols are not well understood. Here, we present results pertaining to daytime variability of aerosol optical and radiative properties over Kanpur (central IGP) during 2001-2010. Key aerosol properties, such as Aerosol Optical Depth (AOD), Angstrom Exponent, Size Distribution (ASD), Single Scattering Albedo (SSA), are analyzed from column-integrated measurements/retrievals obtained from the ground-based Aerosol Robotic Network (AERONET). Our analysis focuses on the two contrasting seasons over the IGP, i.e. dust-dominated pre-monsoon (March - May) and winter-haze (December - February) months. Generally, higher AOD departures were observed in early morning hours during the pre-monsoon season, relative to lower aerosol loading conditions during afternoon/evening (14:30 - 18:30 LT). Water vapor appears to be well-correlated with AOD variations and also exhibits higher morning departures associated with changes in boundary layer dynamics. Variations of Angstrom Exponent suggest greater abundance of coarse-aerosol fraction during afternoon, likely associated with enhanced dust loading and a well-mixed turbulent boundary layer. Case studies of dust outbreaks are also analyzed in terms of diurnal characteristics of AOD, SSA and ASD. In contrast, for winter season, relatively low departures were found during early morning and evening hours, obtained during early afternoon (13:00-14:00 LT). This strong diurnal variation appears to be impacted by the persistent fog/low-cloud cover during morning hours in winter months possibly leading to elevated aerosol loading after episodes of fog burnoff. Variability of meteorological parameters, such as wind field, RH, temperature, is also studied alongside - as likely factors influencing diurnal characteristics of aerosols in this region.