Variation of Absorption Ångström Exponent in Aerosols From Different Emission Sources
Aku Helin, Aki Virkkula, John Backman, Liisa Pirjola, Olli Sippula, Päivi Aakko-Saksa, Sampsa Väätäinen, Fanni Mylläri, Anssi Järvinen, Matthew Bloss, Minna Aurela, Gert Jakobi, Panu Karjalainen, Ralf Zimmermann, Jorma Jokiniemi, Sanna Saarikoski, Jarkko Tissari, Topi Rönkkö, Jarkko V. Niemi, Hilkka Timonen
Abstract
Abstract 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 470/950 varied greatly between the different emission sources, ranging from −0.2 ± 0.7 to 3.0 ± 0.8. The correlation between the AAE 470/950 and AAE 370‐950 results was good ( R 2 = 0.95) and the mean bias error between these was 0.02. In the ship engine exhaust emissions, the highest AAE 470/950 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 470/950 (0.9–1.1). In the diesel bus exhaust emissions, AAE 470/950 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 470/950 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 470/950 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.