Abstract. Black carbon, brown carbon, and mineral dust are three of the most important light absorbing aerosols. Their optical properties differ greatly and are distinctive functions of the wavelength of light. Most optical instruments that quantify light absorption, however, are unable to distinguish one type of absorbing aerosol from another. It is thus instructive to separate total absorption from these different light absorbers to gain a better understanding of the optical characteristics of each aerosol type. During the EAST-AIRE (East Asian Study of Tropospheric Aerosols: an International Regional Experiment) campaign near Beijing, we measured light scattering using a nephelometer, and light absorption using an aethalometer and a particulate soot absorption photometer. We also measured the total mass concentrations of carbonaceous (elemental and organic carbon) and inorganic particulates, as well as aerosol number and mass distributions. We were able to identify periods during the campaign that were dominated by dust, biomass burning, fresh (industrial) chimney plumes, other coal burning pollution, and relatively clean (background) air for Northern China. Each of these air masses possessed distinct intensive optical properties, including the single scatter albedo andÅngstrom exponents. Based on the wavelength-dependence and particle size distribution, we apportioned total light absorption to black carbon, brown carbon, and dust; their mass absorption efficiencies at 550 nm were estimated to be 9.5, 0.5 (a lower limit value), and 0.03 m 2 /g, respectively. While agreeing with the common consensus that black carbon is the most important light absorber in the mid-visible, we demonstrated that brown carbon and dust could also cause significant absorption, especially at shorter wavelengths.

Abstract. Black carbon, brown carbon, and mineral dust are three of the most important light absorbing aerosols. Their optical properties differ greatly and are distinctive functions of the wavelength of light. Most optical instruments that quantify light absorption, however, are unable to distinguish one type of absorbing aerosol from another. It is thus instructive to separate total absorption from these different light absorbers to gain a better understanding of the optical characteristics of each aerosol type. During the EAST-AIRE (East Asian Study of Tropospheric Aerosols: an International Regional Experiment) campaign near Beijing, we measured light scattering using a nephelometer, and light absorption using an aethalometer and a particulate soot absorption photometer. We also measured the total mass concentrations of carbonaceous (elemental and organic carbon) and inorganic particulates, as well as aerosol number and mass distributions. We were able to identify periods during the campaign that were dominated by dust, biomass burning, fresh (industrial) chimney plumes, other coal burning pollution, and relatively clean (background) air for Northern China. Each of these air masses possessed distinct intensive optical properties, including the single scatter albedo and Ångstrom exponents. Based on the wavelength-dependence and particle size distribution, we apportioned total light absorption to black carbon, brown carbon, and dust; their mass absorption efficiencies at 550 nm were estimated to be 9.5, 0.5, and 0.03 m2/g, respectively. While agreeing with the common consensus that BC is the most important light absorber in the mid-visible, we demonstrated that brown carbon and dust could also cause significant absorption, especially at shorter wavelengths.

[1] During the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) intensive experiment in the spring of 2001 we used a total aerosol sampler (TAS) and a micro-orifice impactor (MOI) to collect dust and pollution aerosols for ion chromatographic analysis. An aerodynamic particle sizer (APS) was used to estimate the total coarse-mode volume. We conducted postexperiment passing efficiency measurements on the APS, the MOI, and their delivery tubing to constrain the inevitable (and sometimes large) artifacts associated with sampling supermicron particles from an aircraft. We have combined TAS and corrected MOI data to estimate ambient coarse and fine sulfate, ammonium, nitrate, calcium, sodium, chloride, potassium, magnesium, and oxalate. We found significant differences between aerosol composition in the free troposphere (FT) and boundary layer (BL). The molar ratio of nitrate to soluble calcium averaged 1.8 in the BL, but only 0.2 in the FT. Nitrate and calcium frequently had identical coarse size distributions, while sulfate and ammonium often had identical fine distributions. Dust clearly directs NO y toward coarse-mode nitrate. Sulfate in the FT was closest to ammonium bisulfate (half neutralized), while non-sea-salt sulfate (NSS) in the BL was usually completely neutralized to ammonium sulfate. In the presence of dust, up to half the NSS was found in the coarse mode, probably the result of SO 2 uptake by CaCO 3 in the dust. Soluble calcium averaged 5-8% of the coarse dust mass inferred from the APS. BL aerosol chemistry was seldom a good indicator of ionic composition in the FT.

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