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

Abstract

[1] 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 mm) 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 (s sp at 530 nm) and absorption coefficients (s ap at 550 nm) measured at a controlled relative humidity of 49 ± 5% are 121 ± 48 and 16 ± 12 Mm À1 , respectively. Though the mean light extinction coefficient (s 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 (w 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 s 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 À1 . EC and s ap are observed to have a relationship though less strongly correlated than s 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 À1 . 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 s ap calculated with a Mie code yielding E ap = 9.5 ± 1.5 m 2 g À1 , while EC mass summed from the impactor stages in comparison to measured s ap gives E ap = 9.3 ± 3.2 m 2 g À1 . 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 mm, 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, d 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 ÁF = À11 ± 6 W m À2 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.

Physical and optical properties of aerosols over an urban location in Spain: seasonal and diurnal variability

“…However, our σ sp average value is relatively larger than the average values measured by Pereira et al (2008), at Evora, Portugal (46 Mm −1 ), and by Vrekoussis et al (2005), in remote areas of Greece and Turkey (50 and 45 Mm −1 ). Nevertheless, the measured σ sp values at Granada are lower than those obtained in highly polluted areas in US (Greenwald et al, 2007;Carrico et al, 2003) and are much smaller than those observed in large urban areas in Asia (Bergin et al, 2001;Andreae et al, 2008;Garland et al, 2008;Ganguly et al, 2006). These results indicate that Granada aerosol scattering coefficient values are typical of moderately polluted urban areas.…”

“…The single scattering albedo values obtained in Granada are similar to the mean value of 0.68±0.07 obtained by Eidels-Dubovoi (2002) at Pedregal site in Mexico City. Nevertheless, the single scattering albedo measured in Granada is lower than those obtained in Marseille, France, (0.85±0.05), Toulon, France, (0.73±0.07 to 0.79±0.07), Atlanta (0.87±0.08), Beijing, China, (0.81±0.08) and Guangzhou, China, (0,82±0.05) by Mallet et al (2003), Saha et al (2008), Carrico et al (2003), Bergin et al (2001) and Andreae et al (2008), respectively. The lower values of single scattering albedo in an urban area are generally related to black carbon, although dust can also contribute to aerosol light absorption (Bohren and Hoffman, 1983;Horvath, 1998).…”

Changes in visibility with PM2.5 composition and relative humidity at a background site in the Pearl River Delta region

“…However, b ext in this study was 1.2-3.7 times that reported in summer in Guangzhou, China (419/Mm), in Nanjing, China (249/Mm), and in Atlanta, United States (137/Mm) (Carrico et al, 2003;Jung et al, 2009;Shen et al, 2014). As discussed in Section 2.1, the annual median visibility seemed to improve less with the significant drop of PM 2.5 mass concentrations in recent years.…”

Effects of Continentally Polluted Air Mass on Aerosol Optical Properties over the East China Sea

“…For aerosol samples collected in APEX-E2, sample filters were heated in an electric furnace at 300 C in ambient air for 30 minutes to remove OC and then used to determine the elemental carbon content (EC) (Ohta and Okita 1984). Though such a combustion method is famous to estimate mass concentration of EC, several studies Schmid et al 2001;Clarke et al 2002;Carrico et al 2003) suggested that this method can overestimate the mass concentration of EC if OC chars at low temperature and produces EC. Such an artifact was likely to be a possible reason for overestimating mass concentrations of EC.…”

“…Analysis of EC is one of the most difficult challenges facing atmospheric chemists (Hering et al 1990;Huebert and Charlson 2000). Mass concentration of EC depends strongly on the analytical method (e.g., Schmid et al 2001;Carrico et al 2003). For aerosol samples collected in APEX-E2, sample filters were heated in an electric furnace at 300 C in ambient air for 30 minutes to remove OC and then used to determine the elemental carbon content (EC) (Ohta and Okita 1984).…”