Variability of Aerosol Properties as Determined by Long-term Surface Observations

Determination of aerosol radiative forcing from satellites requires assumptions about the chemical, microphysical, and optical properties of the aerosols because satellites do not measure several important aerosol properties. Therefore, uncertainties in satellite-retrieved parameters can result from variability in aerosol properties that are assumed to be constant in retrieval algorithms. Documenting the magnitude of the variation in aerosol properties is the first step in determining its contribution to the uncertainty of a satellite-retrieved parameter. NOAA begin monitoring aerosols in the mid-1970s with baseline stations at Barrow (Alaska), Mauna Loa (Hawaii), American Samoa, and the South Pole. Since 1992, NOAA has been expanding its aerosol measurement program to include monitoring stations at Sable Island (Nova Scotia, Canada), Bondville (Illinois), and Lamont (Oklahoma) that are designed to obtain representative regional measurements of aerosol properties. The aerosol parameters measured at each station consist of some or all of the following: condensation nuclei concentration, light absorption coefficient, total light scattering coefficient, and hemispheric back scattering coefficient. From these measurements, it is possible to determine the aerosol single-scattering albedo, hemispheric backscatter fraction, and Ångström exponent.

In general, changes in long-range transport patterns dominate the annual cycles at baseline stations. At Barrow, the highest aerosol concentrations are observed during the spring. The aerosol single-scattering albedo is close to one with little annual variability, which is indicative of highly scattering sulfate and sea salt aerosol. At Mauna Loa, the highest scattering and absorption values occur in the springtime and result from the long-range transport of pollution and mineral dust from Asia; however, little seasonality is seen in CN concentrations. Both the aerosol scattering coefficient and the Ångström exponent display seasonal cycles at the South Pole with a scattering coefficient maximum and an Ångström exponent minimum in winter associated with the transport of coarse mode sea salt from the Antarctic coast to the interior of the continent. Monthly median scattering coefficients are nearly two orders of magnitude higher for regional sites than the baseline stations. The Bondville and Lamont sites display autumn highs in absorption coefficient and a low in aerosol single-scattering albedo. Little seasonal variability is observed in aerosol properties at Sable Island, although the Ångström exponent tends to be higher in the summer. Aerosol properties are not observed to have weekly cycles at any of the monitoring stations. Daily cycles in CN concentrations are observed at the Bondville and Lamont sites, with low CN concentrations near sunrise and high CN concentrations in mid-afternoon. Daily cycles in aerosol scattering and absorption are observed at Bondville and to a lesser extent at Lamont but not at the Sable Island and Barrow sites. Daily cycles in relative humidity should not be the cause for the daily aerosol cycles since the aerosol sample is heated to maintain the relative humidity below 40%.