Comparisons of the Measured and Calculated Specific Absorption Coefficients for Urban Aerosol Samples in Vienna

Abstract

Simultaneous measurements of the aerosol absorption coefficient σ_a, using an integrating plate photometer, and of the mass size distribution of the aerosol between 0.06 and 16 μm in aerodynamic equivalent diameter, with a low-pressure Berner impactor, were conducted during a fairly stable period in April 1991. The size distributions were analyzed for total carbon (TC; i.e., the sum of organic and black carbon) by a combustion method; for SO₄ ²⁻, NO₃ ⁻, Cl⁻, NH₄ ⁺, Na⁺, and K⁺ by ion chromatography; and Ca and Mg by atomic absorption spectroscopy. This chemical analysis yielded nearly identical size distributions for SO₄ ²⁻, NO₃ ⁻, and NH⁺ ₄, with mean aerodynamic equivalent diameters typically ∼ 0,7 μm. The ionic balance showed that the aerosol bad been chemically neutral all the time. More than 70% of the accumulation mode mass was due to TC, SO₄ ²⁻, NO₃ ⁻, and NH₄ ⁺. The relative amount of internally and externally mixed carbon (with respect to the other accumulation mode species) was estimated. The externally mixed part of TC contributes up to 80% of TC mass for particles < 0.5 μm. The measured values of the specific absorption coefficient B_a are compared to calculations of B_a from chemical size distributions as functions of the black to total carbon ratios for three different mixing models. The measured values are always higher than the calculated ones, an effect that can be quantitatively explained by the calibration factor for the integrating plate photometer developed in Hitzenberger (1993). For the aerosol during this study, the overestimation of B _a due to the integrating plate method lies in the range of 0.2–0.3 m²/g, which corresponds to ∼ 20–30% of the true B _a. For less absorbing aerosols, the integrating plate photometer would give overestimations by 50% and for remote aerosols by 100%.