Modeling Aerosol Layers Observed during the Two Column Aerosol Project (TCAP)

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Description

Recent studies have shown that some of the differences in radiative forcing among global climate models can be attributed to differences in simulated black carbon profiles. The U.S. Department of Energy’s Two Column Aerosol Project (TCAP) conducted during July 2012 was designed to address this issue by providing key measurements of aerosol composition, size, and optical properties needed to evaluate both detailed process models and 3-D atmospheric models. In situ measurements on the G-1 aircraft and the new NASA Langley airborne High Resolution Spectral Lidar (HRSL-2) indicated a range of aerosol layers in the vicinity of Cape Cod, including large variability of aerosol extinction within residual layers transported from the continent over the ocean and thinner aerosol layers at various altitudes in the free troposphere. In this study, we use the TCAP data to quantify how well a regional model represents the vertical structure of aerosol layers, the concentration of various aerosol species within those layers, and optical properties in the vicinity of Cape Cod. The model domain encompasses most of North America to represent the wide range of contributions from anthropogenic, biomass burning, biogenic and other natural sources that can be transported over the TCAP sampling region. In addition to a detailed treatment of aerosols and cloud-aerosol interactions for resolved clouds, the model also uses a new treatment that accounts for vertical transport, wet scavenging, and aqueous chemistry in sub-grid scale shallow and deep convective clouds that likely influence the height, depth, concentration, and lifetime of aerosol layers. Sensitivity studies are performed to quantify the relative contribution of anthropogenic, biomass burning, and natural sources of aerosols on the total aerosol optical depth as well as the layering structure. The model reproduces the general structure and composition associated with some of the aerosol layers observed by the G-1 and HSRL. For example, large fires in central and northwest Canada, far upwind of the TCAP region, contributed to a distinct aerosol layer above another layer dominated by anthropogenic sources on one day. However, small errors in aerosol mass and composition (that also affect aerosol water uptake) contribute to differences between observed and simulated optical properties, such as extinction and single scattering albedo.