Wednesday, 16 January 2002: 1:30 PM
Direct radiative forcing due to absorbing aerosols
The effect of black carbon (BC) on climate forcing is potentially important, but its estimates have large uncertainties due to a lack of sufficient observational data. The BC mass concentration in the southeastern US was measured at a regionally representative site near Mount Mitchell, NC (35.73 N, 82.29 W, 2038 m MSL), the highest peak in eastern North America. The air mass origin was determined using 48-hr back trajectories obtained from the hybrid single-particle Lagrangian integrated trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the winter the overall average BC value was 74.1 ng m^(-3), whereas the overall summer mean BC value is higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth Probe Total Ozone Mapping Spectrometer. The BC average concentrations of air masses transported from the polluted continental sector during summer are low on Sunday through Tuesday with a minimum value of 256 ng m^(-3) on Monday, and high on Wednesday through Friday with a maximum value of 379 ng m^(-3) on Friday. The daily mean absorption and scattering coefficients are well correlated (R=0.86), which suggests the aerosols originated from the same emission sources. 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.