Aerosol radiative forcing for Asian continental outflow

Asian aerosols in elevated layers over the Pacific Ocean were sampled with NASA wire-impactors and a FSSP optical particle spectrometer-probe aboard t he NASA DC-8 aircraft in early March 1994. Strong variations in aerosol pro perties, primarily aerosol concentration, lead to derived mid-visible extin ctions between 0.003 and 0.5/km. FSSP data usually identified two size-mode s. The larger 'coarse mode' (radii of 1-3 mum) was assumed to be dust. The composition of the smaller 'accumulation mode' (radii of 0.1-0.3 mum) was b ased on the analysis of the wire-impactor samples, as significant amounts o f soot reduce mid-visible single scattering albedos to the 0.87-0.92 range. Radiative forcing simulations investigated the impact of Asian outflow aero sol on atmospheric radiative fluxes and heating rates. Only events with lar ger optical depths were important. In those events the solar attenuation of the smaller size mode dominated the net-flux losses at the surface, with v alues similar those of urban-polluted and/or biomass burning aerosol types (as observed during the TARFOX and INDOEX field experiments). In contrast, changes to net-fluxes at the top of the atmosphere (ToA) for outflow cases are less negative-primarily due to the added greenhouse effect of the dust component. For the climate of the Earth-Atmosphere-System, ToA net-flux los ses are considered a cooling, ToA net-flux gains are associated with warmin g. Weak cooling is determined for the Asian outflow cases under cloud-free conditions. The addition of a reported 50% cloud cover below the aerosol la yer causes a switch to slight warming. (C) 2001 Elsevier Science Ltd. All r ights reserved.