Influence of aerosol properties and surface albedo on radiative forcing efficiency of key aerosol types using global AERONET data

According to the particle size and absorptivity as determined by the fine mode fraction and the single-scattering albedo (SSA) retrievals from AErosol RObotic NETwork (AERONET) sites around the world, aerosols are classified into four key categories: coarse and absorptive aerosol (Type I), mixed aerosol (Type II), fine and absorptive aerosol (Type III), fine and non-absorptive aerosol (Type IV). Seasonal variations of aerosol types with their corresponding direct radiative forcing efficiency (RFE) are observed on different continents. The RFE at the surface (RFEsfc) and top of the atmosphere (RFEtoa) reach their maximum (minimum) values over Asia and North America (Europe, Oceania, and South America) from June to August. The effects of solar zenith angle (SZA), surface albedo (SA), and SSA on RFEsfc and RFEtoa are investigated. The absolute values of RFE at TOA of all types of aerosols are largest at cos(SZA) =0.3 to 0.4. The increased SA reduces the absolute value of RFE both at SFC and TOA for all types of aerosols, and when SA reaches a specific threshold, depending on the type of aerosol, the RFEtoa turns positive. RFEtoa increases while RFEsfc decreases with decreasing SSA. The RFEsfc of the four categories of aerosol varies slightly in the same SZA, SSA and SA conditions, while RFEtoa is aerosol type dependent. It is found that larger particles reflect more solar energy into space per optical depth, resulting in an enhanced cooling effect under similar SZA, SSA, and SA conditions.