Urban aerosol radiative properties: Measurements during the 1999 Atlanta Supersite Experiment

As part of the Atlanta Supersite 1999 study, aerosol radiative and related physical and chemical properties are examined on the basis of measurements of PM 2.5 (aerosol particles with aerodynamic diameters, D p , less than 2.5 μm) in urban Atlanta. In addition to potential compliance issues with proposed regulatory standards, PM 2.5 concentrations in Atlanta and the surrounding region are large enough to have an important impact on atmospheric radiative transfer and hence visibility and potentially regional climate. Arithmetic means and standard deviations of the light scattering by PM 2.5 (σ sp at 530 nm) and absorption coefficients (σ ap at 550 nm) measured at a controlled relative humidity of 49 ± 5% are 121 ± 48 and 16 ± 12 Mm −1 , respectively. Though the mean light extinction coefficient (σ ep ) in Atlanta is much larger than background sites, it is comparable to nonurban areas in the interior southeast United States highlighting the contribution of a regional haze here. The single scattering albedo (ω o ) in Atlanta is 0.87 ± 0.08 and is ∼10% lower than reported in nonurban polluted sites, likely a result of the emission of elemental carbon (EC) from mobile sources. A pronounced diel pattern in aerosol properties is observed with clear influences from mobile sources (morning rush hour maxima in concentrations, particularly soot‐related indicators) and atmospheric mixing (afternoon minima). A strong linear relationship between σ sp and PM 2.5 is observed, and using several techniques, gives a range of mean mass scattering efficiencies (E sp ) from = 3.5 to 4.4 m 2 g −1 . EC and σ ap are observed to have a relationship though less strongly correlated than σ sp and PM 2.5 . Four methods of determining the mass absorption efficiency of EC give E ap ranging from 5.3 to 18.3 m 2 g −1 . This wide range of values is a result of the variability in aerosol properties, uncertainties in the light absorption method, and in particular, differences in the EC measurement techniques. Best agreement was found using measured EC mass distributions using a multistage impactor in comparison to σ ap calculated with a Mie code yielding E ap = 9.5 ± 1.5 m 2 g −1 , while EC mass summed from the impactor stages in comparison to measured σ ap gives E ap = 9.3 ± 3.2 m 2 g −1 . Mie light‐scattering calculations using inputs of measured mass and EC size distributions give geometric mean light scattering and absorption D p = 0.54 and 0.13 μm, respectively, and show the dominance of the submicrometer diameter particles to light extinction in the urban environment. Based on the measured aerosol optical depth in Atlanta, δ a (500 nm) = 0.44 ± 0.22, and other radiative measurements, a best estimate of the average direct aerosol radiative forcing at the top of the atmosphere (a measure of the climate significance) is ΔF = −11 ± 6 W m −2 in Atlanta. This value is an order of magnitude greater than global mean estimates for aerosols underscoring the influence of aerosol particles on radiative transfer in the urban environment.