C. Pilinis et al., SENSITIVITY OF DIRECT CLIMATE FORCING BY ATMOSPHERIC AEROSOLS TO AEROSOL-SIZE AND COMPOSITION, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 100(D9), 1995, pp. 18739-18754
We evaluate, using a box model, the sensitivity of direct climate forc
ing by atmospheric aerosols for a ''global mean'' aerosol that consist
s of fine and coarse modes to aerosol composition, aerosol size distri
bution, relative humidity (RH), aerosol mixing state (internal versus
external mixture), deliquescence/crystallization hysteresis, and solar
zenith angle. We also examine the dependence of aerosol upscatter fra
ction on aerosol size, solar zenith angle, and wavelength and the depe
ndence of single scatter albedo on wavelength and aerosol composition.
The single most important parameter in determining direct aerosol for
cing is relative humidity, and the most important process is the incre
ase of the aerosol mass as a result of water uptake. An increase of th
e relative humidity from 40 to 80% is estimated for the global mean ae
rosol considered to result in an increase of the radiative forcing by
a factor of 2.1. Forcing is relatively insensitive to the fine mode di
ameter increase due to hygroscopic growth, as long as this mode remain
s inside the efficient scattering size region. The hysteresis/deliques
cence region introduces additional uncertainty but, in general, errors
less than 20% result by the use of the average of the two curves to p
redict forcing. For fine aerosol mode mean diameters in the 0.2-0.5 mu
m range direct aerosol forcing is relatively insensitive (errors less
than 20%) to variations of the mean diameter. Estimation of the coars
e mode diameter within a factor of 2 is generally Sufficient for the e
stimation of the total aerosol radiative forcing within 20%. Moreover,
the coarse mode, which represents the nonanthropogenic fraction of th
e aerosol, is estimated to contribute less than 10% of the total radia
tive forcing for all RHs of interest. Aerosol chemical composition is
important to direct radiative forcing as it determines (1) water uptak
e with RH, and (2) optical properties. The effect of absorption by aer
osol components on forcing is found to be significant even for single
Scatter albedo values of omega=0.93-0.97. The absorbing aerosol compon
ent reduces the aerosol forcing from that in its absence by roughly 30
% at 60% RH and 20% at 90% RH. The mixing state of the aerosol (intern
al versus external) for the particular aerosol considered here is foun
d to be of secondary importance, While sulfate mass scattering efficie
ncy (m(2) (g SO42-)(-1)) and the normalized sulfate forcing(W (g SO42-
)(-1))increase strongly with RH, total mass scattering efficiency (m(2
) g(-1))and normalized forcing (W g(-1)) are relatively insensitive to
RH, wherein the mass of all species, including water, are accounted f
or. Following S. Nemesure et al. (Direct shortwave forcing of climate
by;anthropogenic sulfate aerosol: sensitivity to particle size, compos
ition, and relative humidity, submitted to Journal of Geophysical Rese
arch, 1995), we find that aerosol forcing achieves a maximum at a part
icular solar zenith angle, reflecting a balance between increasing ups
catter fraction with increasing solar zenith angle and decreasing sola
r flux (from Rayleigh scattering) with increasing solar zenith angle.