Surface dimming and cloud response due to black carbon and absorbing organic carbon aerosols

Description

Surface measurements have shown significant decreases in solar radiation (dimming) in many regions since the 1960s. At least half of the observed dimming could be linked to the direct radiative effect of anthropogenic aerosols, especially absorbing aerosols like black carbon (BC) due to their strong atmospheric absorption. However, model-data comparisons indicate that absorption by aerosols is largely underestimated in current GCM simulations by several factors over some regions. Using a global chemical transport model coupled with a radiative transfer model, we include a treatment for absorbing organic carbons (OC) from bio-fuel and open biomass burnings in optical calculations and estimate aerosol radiative forcings for two anthropogenic aerosol emission scenarios representative of the 1970s and circa 2000. Assumptions about aerosol mixing and the OC absorption spectrum are examined by comparing simulated aerosol absorption and atmospheric heating against surface and aircraft measurements. We will also examine regional variations in cloud liquid content and thickness in the presence of absorbing aerosols (BC, OC, and dust), with a focus on the ARM fixed sites and AMF deployments. The calculated aerosol single-scattering albedo (0.93+/-0.044) is generally consistent with the AERONET data (0.93+/-0.030) for the year 2001. On a global scale, inclusion of absorbing OC enhances the absorption in the atmosphere by 11% for July, globally. The estimated present-day direct radiative forcing by anthropogenic aerosols (-0.24 W/m2) is similar to the TOA average value by the AeroCom models based on the same 2000 emissions, but significantly enhanced negatively at surface by about 53% (-1.56 W/m2) and increased in the atmosphere by +61% (+1.32 W/m2). About 87% of the atmosphere absorption and 42% of the surface dimming is contributed by BC. Between 1975 and 2000, the calculated all-sky flux at surface has decreased about 6 W/m2 over east China (equivalent to about 5 W/m2 per decade in clear sky), but increased about 1 W/m2 in Europe. The non-uniformly regional trends in surface solar radiation correspond to changes in BC and sulfur emissions from fossil fuel and bio-fuel combustion sources. By considering OC absorption and internal mixing of aerosols, we improved the simulated dimming trend in regions like Asia where the observed clear-sky reduction is as much as 9 W/m2 per decade due to rapidly increasing anthropogenic emissions, which have been linked to the recent regional climate anomalies.