Variation of Absorption Ångström Exponent in Aerosols From Different Emission Sources

• Published in: Journal of geophysical research. Atmospheres Vol. 126; no. 10
• Main Authors: Helin, A., Virkkula, A., Backman, J., Pirjola, L., Sippula, O., Aakko‐Saksa, P., Väätäinen, S., Mylläri, F., Järvinen, A., Bloss, M., Aurela, M., Jakobi, G., Karjalainen, P., Zimmermann, R., Jokiniemi, J., Saarikoski, S., Tissari, J., Rönkkö, T., Niemi, J. V., Timonen, H.
• Format: Journal Article
• Language: English
• Published: 27.05.2021
• Subjects: absorption Ångström exponent; aethalometer, black carbon, emissions; source apportionment
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2020JD034094

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). AAE470/950 varied greatly between the different emission sources, ranging from −0.2 ± 0.7 to 3.0 ± 0.8. The correlation between the AAE470/950 and AAE370‐950 results was good (R2 = 0.95) and the mean bias error between these was 0.02. In the ship engine exhaust emissions, the highest AAE470/950 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 AAE470/950 (0.9–1.1). In the diesel bus exhaust emissions, AAE470/950 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 AAE470/950 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 AAE470/950 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. Key Points Absorption Ångström exponent (AAE) values were on average the highest in shipping and small‐scale wood combustion emissions AAE values were the lowest in aerosols originating from ethanol‐fueled bus and coal‐fired power plant emissions The results raise the awareness of the possible shortcoming in using AAE for aerosol characterization particularly in fresh emissions