In order to address questions concerning climate forcing by anthropogenic aerosol particles (Charlson et al., 1992; Penner et al., 1994), ESRL/GMD is continually making measurements at a variety of regional aerosol monitoring stations. Two of the stations are in rural continental locations that are frequently perturbed by anthropogenic aerosols. A third station is in a marine location which may not only be perturbed by anthropogenic emissions from the US, but may also be impacted by dust and pollutants transported across the Pacific Ocean. The above links describe the site characteristics, and their current status. Each station operates in close collaboration with a local university or government agency, which provides on-site support for the measurements.
The scientific questions that define the context of the measurements at these sites include
- What are the sign, mechanism, magnitude, uncertainty, and spatial distribution of the climate forcing by anthropogenic aerosol particles?
- What are the physical and chemical processes, including their rates and spatial distributions, leading to formation and removal of the particles responsible for the forcing, and how do these processes determine the size and chemical-composition distributions of the particles?
- What is the sensitivity of the forcing and its spatial distribution to changes in these parameters?
- How has the forcing changed in the past, and how will it change in the future?
Clearly, ground-based measurements at a few sites will provide answers to only a few of these questions. Recognizing this, the strategy of the ESRL/GMD regional aerosol measurement program is
- to determine means, variability, and possible trends of key optical, chemical and microphysical properties for a number of important aerosol types.
The measurements will provide ground-truth for satellite measurements and global models, as well as key aerosol parameters for global-scale models (e.g., scattering efficiency of sulfate particles, hemispheric backscattering fraction). An important aspect of this strategy is that the chemical measurements are linked to the physical measurements through simultaneous, size-selective sampling and thermal analysis, which allows the observed aerosol properties to be connected to the atmospheric cycles of specific chemical species.
When the sites are fully operational, continuous measurements will include the total particle number concentration (Ntot), and components of the aerosol extinction coefficient at one or more wavelengths (total scattering (ssp), backwards hemispheric scattering (sbsp), and absorption (sap)). Size-resolved impactor and filter samples (submicrometer and supermicrometer size fractions) will be obtained for gravimetric and chemical (ion chromatographic) analyses. All size-selective sampling, as well as the measurements of the components of the aerosol extinction coefficient, will be performed at a low, controlled relative humidity (<40%) to eliminate confounding effects due to changes in ambient relative humidity.
Summary of Measurements at Active Regional Stations
|Category||Perturbed Continental||Perturbed Continental||Perturbed Marine|
|Location||Bondville, Illinois, USA||Lamont, Oklahoma, USA||Trinidad Head, California, USA|
|Collaborating Institute||University of Illinois,
Illinois State Water Survey
|Status||Operational July, 1994||Operational July, 1996||Operational April, 2002|
|Sample Size Fractions||D<1 µm, 10<D<1 µm||D<1 µm, 10<D<1 µm||D<1 µm, 10<D<1 µm|
|Optical measurements||ssp(3l), sap(1l), sbsp(3l)||ssp(3l), sap(1l), sbsp(3l),f(RH)||ssp(3l), sap(1l), sbsp(3l),f(RH)|
|Chemical measurements)||major ions, mass||major ions, mass||major ions, mass|
- Charlson, R.J., S.E. Schwartz, J.M. Hales, R.D. Cess, J.A. Coakley, Jr., J.E. Hansen, and D.J. Hofmann, Climate forcing by anthropogenic aerosols, Science 255, 423-430, 1992.
- Penner, J.E., R.J. Charlson, J.M. Hales, N. Laulainen, R. Leifer, T. Novakov, J. Ogren, L.F. Radke, S.E. Schwartz, and L. Travis, Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols, Bull. Amer. Meteor. Soc. 75, 375-400.