of Commerce / NOAA / PMEL
/ Atmospheric Chemistry
Aerosol Climate Forcing
Patricia Quinn & Timothy Bates
Derek Coffman, Drew Hamilton,
Jim Johnson, & Theresa Miller
provide information about tropospheric aerosol particles in order to improve
estimates of aerosol radiative forcing which will, in turn, improve predictions
of future climatechanges due to human-induced changes in the compositionof
PMEL’s Research Strategy:
Aerosol particles exert both a direct
effect on climate by reflecting and absorbing shortwave solar radiation
and an indirect effect by influencing the optical properties and lifetimes
The uncertainty in aerosol radiative
forcing is considerably larger than that due to greenhouse gases (IPCC,
1996). This limits the ability to predict future surface temperature
changes and to unambiguously detect a greenhouse warming signal.
PMEL’s research strategy is to provide
data to serve as both input to and validation of models employed in the
estimation of aerosol radiative forcing.
The geographical distribution of
aerosol forcing is very inhomogeneous. Therefore, PMEL’s measurement
program covers a wide range of oceanic and land-based regions in order
to characterize a variety of aerosol types (clean marine, perturbed marine,
clean continental, and polluted continental).
Improvement of the IPCC(95)
Parameterization of Aerosol Chemical Composition
The IPCC(95) estimate of aerosol radiative
forcing focused on a highly simplified sulfate only aerosol even though
there are other aerosol components which have very different optical properties.
Thus, improving this estimate relies on accurate measurements of the aerosol
chemical composition over wide geographical regions.
The PMEL data base indicates that only
20 to 60% of the aerosol mass is sulfate such that, in all geographical
regions, there is a significant fraction of the aerosol mass that may have
very different optical properties than those included in the IPCC(95) estimate.
Presumably this mass is composed of mineral dust or carbonaceous material.
These measurements verify the need
to improve the IPCC(95) depiction of the aerosol in order to improve estimates
of aerosol radiative forcing.
Assessment and Interpretation of
Satellite-Derived Aerosol Properties
NOAA’s advanced very high resolution
radiometer (AVHRR) onboard a polar orbiting satellite provides global distributions
of aerosol optical thickness (AOT) which are crucial in estimating the
direct radiative forcing by aerosols.
Seasonally composited images reveal
an enhanced AOT over the Pacific Ocean just north of the Intertropical
Convergence Zone during March, April, and May. The satellite images
do not indicate, however, if the enhanced AOT is a result of a radiative
forcing or is a natural aerosol feature
PMEL has conducted four cruises in
this region during different seasons. The resulting in situ observations
of the aerosol chemical composition indicate that the enhanced AOT
is not a result of anthropogenic aerosol but rather natural sea salt aerosols.
The seasonal increase in AOT corresponds to the time of year when the ITCZ
has migrated to its southern-most location and the wind speed and resulting
sea salt concentrations are high.
These measurements have revealed the
source of the aerosol yielding an enhanced AOT in the north equatorial
region of the Pacific thereby allowing for an accurate interpretation of
the satellite observations.
|Improvement of Model Estimates
of Aerosol Radiative Forcing
Recent model estimates of the
aerosol optical thickness (AOT) due to a variety of aerosol types (sulfate,
dust, carbonaceous, sea salt) indicate that, on a relative basis, the extinction
due to sea salt is small on both global and regional scales (Tegen et al.,
Geophys. Res., 102, 23,895, 1997).
These model estimates assume that sea
salt exists only in those particles greater than 4 microns in diameter.
This is stark contrast to PMEL’s in situ data which show a significant
amount of sea salt mass in smaller particles. This sea salt mass
is comparable to or larger than the observed sulfate mass over much of
the Pacific Ocean.
Based on the in situ data, for the
20(S to 20(N latitude band of the Pacific, the sea salt contribution to
AOT relative to sulfate is 91% while the model estimates a sea salt contribution
of only 28%. Hence, PMEL’s measurements reveal that the model significantly
underpredicts the contribution of the natural background aerosol to the
radiative properties of the aerosol in this region.
Quantification, identification, and
determination of the radiative properties of the non-sulfate aerosol mass.
How does this mass affect estimates of aerosol radiative forcing?
Participation in large multi-platform
experiments which are focused on anthropogenic aerosol and are designed
to improve estimates of aerosol radiative forcing (INDOEX and ACE ASIA).
Further integration of measurement
results with models
Funding support provided by the
NOAA Office of Global Programs.
Atmospheric Chemistry Group, http://saga.pmel.noaa.gov