Measured and Derived Aerosol Properties

In situ measurements of a variety of aerosol optical properties are being made at ESRL/GML stations. The measurement suite enables calculation of direct aerosol climate forcing. The measured values relevant for climate forcing calculations are: light absorption (σap), total scattering (σsp) and hemispheric backscattering (σbsp). Additionally, most stations in the cooperative aerosol network measure total aerosol number concentration (N). The optical measurements (typically made at several wavelengths) are used to derive parameters required in the radiative forcing calculation as well as to calculate parameters which help further characterize the aerosol (e.g., by size or type) (Table 1).

Table 1: Aerosol radiative properties that can be derived from measurements made by the ESRL/GML cooperative aerosol network.

åsp (or SAE) The scattering Ångström exponent, defined by the power-law σsp(λ), describes the wavelength-dependence of scattered light. Situations where the scattering is dominated by submicrometer particles typically have values around 2, while values close to 0 occur when the scattering is dominated by particles larger than a few micrometers in diameter.
åap (or AAE) The absorption Ångström exponent, defined by the power-law σap(λ), describes the wavelength-dependence of light absorption. The AAE can provide indications of aerosol composition, for example, black carbon (BC), has a theoretical AAE value of 1, while dust aerosol typically has AAE values greater than 2.
ωo (or SSA) The aerosol single-scattering albedo, defined as σsp/(σapsp), describes the relative contributions of scattering and absorption to the total light extinction. Purely scattering aerosols (e.g., sulfuric acid) have values of 1, while very strong absorbers (e.g., elemental carbon) have values around 0.3.
g, b, β Radiative transfer models commonly require an integral property of the angular distribution of scattered light (phase function): the asymmetry factor g, the hemispheric backscatter fraction b, or the upscatter fraction β. The asymmetry factor is the cosine-weighted average of the phase function, ranging from a value of -1 for entirely backscattered light to +1 for entirely forward-scattered light. The hemispheric backscatter fraction b is σbspsp. The upscatter fraction depends on the solar zenith angle, and is equal to b when the sun is directly overhead
FMF The fine mode fraction (FMF) of aerosol light scattering (and absorption) is the ratio of sub1-μm to sub10-μm (PM1 and PM10) scattering (or absorption). Because many natural aerosols (e.g., sea salt and dust) tend to have sizes larger than 1 μm while anthropogenically-generated particles typically are submicron in diameter, the FMF can indicate the relative contribution of anthropogenic sources in an air mass.