Temporal trends of black carbon concentrations and regional climate forcing in the southeastern United States

The effect of black carbon (BC) on climate forcing is potentially important , but its estimates have large uncertainties due to a lack of sufficient ob servational data. The BC mass concentration in the southeastern US was meas ured at a regionally representative site, Mount Gibbes (35.78 degreesN, 82. 29 degreesW, 2006 m MSL). The air mass origin was determined using 48-h bac k trajectories obtained from the hybrid single-particle Lagrangian integrat ed trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the wint er, the overall average BC value was 74.1 ng m(-3), whereas the overall sum mer mean BC value is higher by a factor of 3. The main reason for the seaso nal difference may be enhanced thermal convection during summer, which incr eases transport of air pollutants From the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998. abnormall y high BC concentrations from the continental sector were measured. These c oncentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth probe total ozone mapping s pectrometer. The BC average concentrations of air masses transported from t he polluted continental sector during summer are low on Sunday to Tuesday w ith a minimum value of 256 ng m(-3) occurring on Monday, and high on Wednes day to Friday with a maximum value of 379 ng m(-3) occurring on Friday. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL is calculated to be - 1.38 x 10(-3) W m(-3) for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering is reduced by 15 +/- 7% due to the BC absorption. (C) 2001 Elsevier Science Ltd. All rights reserved.