Regional Aerosol Observations

**This is a section of NOAA's Climate Monitoring and Diagnostics Laboratory Summary Report No. 22 from 1993

In order to address questions concerning climate forcing by anthropogenic aerosol particles (Charlson et al., 1992; Penner et al., 1994), CMDL is establishing a network of regional aerosol monitoring stations. Two of the stations are located in marine locations and three in continental locations; for each category, one site in relatively free of anthropogenic influences and the others are frequently perturbed by anthropogenic aerosols. Table 1 lists the sites, their 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.

Table 1: Regional Aerosol Monitoring Sites

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 CMDL 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 (N_tot), cloud condensation nucleus number concentration (N_ccn), aerosol optical depth (d), and components of the aerosol extinction coefficient at one or more wavelengths (total scattering s_sp, backwards hemispheric scattering s_bsp), and absorption s_ap). Size-resolved impactor and filter samples (submicrometer and supermicrometer size fractions) will be obtained for gravimetric and chemical (ion chromatographic, organic/elemental carbon) 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.

A limited sampling program was conducted from April 1993 to March 1994, to evaluate the suitability for long-term, ground-based, aerosol measurements of the proposed monitoring site outside of Laramie, WY. This site was chosen because vertical profiles of aerosol size distribution have regularly been obtained by the University of Wyoming for the last 20 years at this site. However, our twelve months of surface measurements of s_sp and N_tot revealed that the site is very frequently contaminated by local sources in the Laramie Valley, making it unrepresentative as a clean continental monitoring site. Consequently, measurements of s_sp and N_tot were initiated in November, 1993 to evaluate the feasibility of using the University of Colorado's facility on Niwot Ridge as a clean continental site. Initial results from this site are promising, although transport of polluted air from metropolitan Denver occurs regularly during afternoon, upslope flow conditions in the summer.

The Cheeka Peak site is operated independently by the University of Washington, with major funding from the NOAA Climate and Global Change Program. Although the sampling protocols are essentially identical to those used at the other CMDL sites, all data acquisition and processing is the responsibility of the University of Washington. Eventually, data from this site will be included in the CMDL archive and tabulated in a future CMDL Annual Report.

Summary results for Sable Island for 1992 and 1993 are shown in Figures 1 and 2 (daily geometric means) and Tables 2 and 3 (monthly geometric means). Table 4 contains statistics on the measured parameters at Sable Island (August 1992-June 1994) for three categories: all cases, and those cases when both Ntot and s_sp (550 nm, D< 1 µm) are below and above, respectively, one standard deviation of their mean value ("clean" and "dirty", respectively). These cases correspond to Ntot < 398 cm^-3 and s_sp < 5.0 Mm^-1 (clean) and Ntot > 2089 cm^-3 and s_sp > 22.9 Mm^-1 (dirty). The Sable Island site was chosen to provide information on the properties of pollution aerosols subsequent to long-range transport from the continent, as well as information on aerosols in the clean marine boundary layer. The initial results reflect the different transport patterns influencing the site, with periods of low aerosol number concentration and light scattering coefficient interspersed with periods of high aerosol loadings. Trajectory analyses of a limited number of cases reveal that transport from the south and southeast (clean marine) is associated with low aerosol loadings, and that trajectories from the west and southwest (polluted continental) are associated with high aerosol loadings. Monthly mean aerosol loadings are considerably higher than the levels encountered at the CMDL baseline stations, due to the proximity of Sable Island to continental pollution sources.

Table 2: Sable Island Monthly means for 1992

Table 3: Sable Island Monthly Means for 1993

Table 4: Mean Observed Values at Sable Island (August 1992-June 1994)

Notes: (1) The Ångström exponent is denoted å to avoid confusion in the future with the mass scattering efficiency a. (2) The entries for "s_sp fine/total" are the fraction of the total aerosol light scattering coefficient attributable to submicrometer particles. (3) Geometric means are reported for Ntot and s_sp, while arithmetic means are reported for å and s_sp fine/total. (4) Clean and dirty cases are those hours when both Ntot and s_sp are below and above, respectively, one standard deviation of the mean value.

Inclusion of size- and wavelength-dependence of s_sp in the sampling protocol allows identification of systematic shifts in the aerosol size distribution under different conditions (Table 4). Fine particles (diameter < 1 µm) are responsible for less than half of the aerosol light scattering (550 nm wavelength), and this fraction systematically increases from clean to dirty conditions. Similarly, the Ångström exponent å systematically increases from clean to dirty conditions, for both fine and total size fractions. Together, these two findings indicate that fine particles are systematically smaller, and more abundant, under dirty conditions. Linear regression analysis demonstrates that 75% of the variance in the Ångström exponent (total size fraction) can be explained by variance in the fraction of light scattering caused by fine particles (550 nm wavelength), i.e., the major factor controlling å is the relative abundance of fine and coarse particles (diameter > 1 µm).

Information and data from the aerosol group at CMDL is now available on the Internet via FTP and World Wide Web servers. Recently processed data, file format specifications, documents summarizing data processing and flow, and clean processed data presented in hourly averaged files for all years of station operation are available via anonymous FTP to ftp.cmdl.noaa.gov, directory "aerosol".

References

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.