| Abstract: The direct radiative forcing (DRF) of sulfate and
black carbon (BC) aerosols is investigated using a new multispectral radiation
code within the R30 Geophysical Fluid Dynamics Laboratory general circulation
model (GCM). Two independent sulfate climatologies from chemical transport
models are applied to the GCM; each climatology has a different atmospheric
burden, vertical profile, and seasonal cycle. The DRF is calculated to
be approximately -0.6 and -0.8 W m-2 for
the different sulfate climatologies. Additional sensitivity studies show
that the vertical profile of the sulfate aerosol is important in determining
the DRF; sulfate residing near the surface gives the strongest DRF due
to the effects of relative humidity. Calculations of the DRF due to BC
reveal that the DRF remains uncertain to approximately a factor of 3 due
to uncertainties in the total atmospheric burden, the vertical profile
of the BC, and the assumed size distribution. Because of the uncertainties
in the total global mass of BC, the normalized DRF (the DRF per unit column
mass of aerosol in watts per milligram (W mg-1))
due to BC is estimated; the range is +1.1 to +1.9 W mg-1
due to uncertainties in the vertical profile. These values correspond to
a DRF of approximately +0.4 W m-2 with
a factor of 3 uncertainty when the uncertainty in the total global mass
of BC is included. In contrast to sulfate aerosol, the contribution to
the global DRF from cloudy regions is very significant, being estimated
as approximately 60%. The vertical profile of the BC is, once again, important
in determining the DRF, but the sensitivity is reversed from that of sulfate;
BC near the surface gives the weakest DRF due to the shielding effects
of overlying clouds. Although the uncertainty in the estimates of the DRF
due to BC remains high, these results indicate that the DRF due to absorption
by BC aerosol may contribute a significant positive radiative forcing and
may consequently be important in determining climatic changes in the Earth-atmosphere
system. |