Aerosol radiative forcing deduced from observations and models over an urban location and sensitivity to single scattering albedo

Aerosol radiative forcing at the Earth’s surface is estimated by simultaneous measurements of broad-band global fluxes and aerosol optical depths (AODs) over an urban location in western India during 2008. AODs at 0.5 μm show large seasonal variability with higher values (0.52) during monsoon. Higher AOD during monsoon is mainly due to increase in relative humidity which overwhelms the effects of wet removal of aerosols and addition of sea salt. Forcing efficiency for monsoon season is found to be lower as compared to other seasons. Surface aerosol radiative forcing has the highest value of −44 Wm−2 during monsoon. The forcing values are similar for model independent and semi model dependent methods. Single scattering albedo (SSA) is higher in monsoon followed by pre-monsoon, winter and lowest in post-monsoon. SSA derived from ground-based measurements (aethalometer and nephelometer) is lower than columnar SSA estimated from Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and derived from Ozone Monitoring Instrument (OMI). Lower radiative forcing at the surface is attributed to higher SSA during pre-monsoon. Model estimated surface forcing using ground-based SSA is about two times higher than observed forcing for different seasons except for monsoon. However, model estimated forcing using columnar SSA agrees well with observations except in monsoon. The differences during monsoon are probably caused by overestimation of SSA from GOCART and OMI. The study reveals that a small change in SSA can lead to significant change in aerosol forcing.