Direct radiative forcing and atmospheric absorption by boundary layer aerosols in the southeastern US: model estimates on the basis of new observations

In an effort to reduce uncertainties in the quantification of aerosol direc t radiative forcing (ADRF) in the southeastern United States (US), a field column experiment was conducted to measure aerosol radiative properties and effects at Mt. Mitchell, North Carolina, and at an adjacent valley site. T he experimental period was from June 1995 to mid-December 1995. The aerosol optical properties (single scattering albedo and asymmetry factor) needed to compute ADRF were obtained on the basis of a procedure involving a Mie c ode and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse-to-direct solar irra diance ratio. The regional values of ADRF at the surface and top of atmosph ere (TOA), and atmospheric aerosol absorption are derived using the obtaine d aerosol optical properties as inputs to the column radiation model (CRM) of the community climate model (CCM3). The cloud-free instantaneous TOA ADR Fs for highly polluted (HP), marine (M) and continental (C) air masses rang e from 20.3 to -24.8, 1.3 to -10.4, and 1.9 to -13.4 W m(-2), respectively. The mean cloud-free 24-h ADRFs at the TOA (at the surface) for HP, M, and C air masses are estimated to be -8 +/-4 (-33 +/- 16), -7 +/- 4 (-13 +/- 8) , and -0.14 +/- 0.05 (-8 +/- 3) W m(-2), respectively. On the assumption th at the fractional coverage of clouds is 0.61, the annual mean ADRFs at the TOA and the surface are -2 +/- 1, and -7 +/- 2 W m(-2), respectively. This also implies that aerosols currently heat the atmosphere over the southeast ern US by 5 +/- 3 W m(-2) on annual timescales due to the aerosol absorptio n in the troposphere. (C) 2001 Elsevier Science Ltd. All rights reserved.