Variation of Absorption Ångström Exponent in Aerosols From Different Emission Sources

Abstract

<p>The absorption Ångström exponent (AAE) describes the spectral dependence of light absorption by aerosols. AAE is typically used to differentiate between different aerosol types for example., black carbon, brown carbon, and dust particles. In this study, the variation of AAE was investigated mainly in fresh aerosol emissions from different fuel and combustion types, including emissions from ships, buses, coal-fired power plants, and residential wood burning. The results were assembled to provide a compendium of AAE values from different emission sources. A dual-spot aethalometer (AE33) was used in all measurements to obtain the light absorption coefficients at seven wavelengths (370–950 nm). AAE<sub>470/950</sub> varied greatly between the different emission sources, ranging from −0.2 ± 0.7 to 3.0 ± 0.8. The correlation between the AAE<sub>470/950</sub> and AAE<sub>370-950</sub> results was good (R<sup>2</sup> = 0.95) and the mean bias error between these was 0.02. In the ship engine exhaust emissions, the highest AAE<sub>470/950</sub> values (up to 2.0 ± 0.1) were observed when high sulfur content heavy fuel oil was used, whereas low sulfur content fuels had the lowest AAE<sub>470/950</sub> (0.9–1.1). In the diesel bus exhaust emissions, AAE<sub>470/950</sub> increased in the order of acceleration (0.8 ± 0.1), deceleration (1.1 ± 0.1), and steady driving (1.2 ± 0.1). In the coal-fired power plant emissions, the variation of AAE<sub>470/950</sub> was substantial (from −0.1 ± 2.1 to 0.9 ± 1.6) due to the differences in the fuels and flue gas cleaning conditions. Fresh wood-burning derived aerosols had AAE<sub>470/950</sub> from 1.1 ± 0.1 (modern masonry heater) to 1.4 ± 0.1 (pellet boiler), lower than typically associated with wood burning, while the burn cycle phase affected AAE variation.</p>