U.S.Dept of Commerce / NOAA / PMEL / Atmospheric Chemistry

Aerosol Climate Forcing

Patricia Quinn & Timothy Bates
Derek Coffman, Drew Hamilton,
Jim Johnson, & Theresa Miller

 

Goal:  To 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 the atmosphere.


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 of clouds.
  • 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).


 
 
Accomplishments: 

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., J. 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. 
 



 
Future directions: 
 
  • 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