Variation of Absorption Ångström Exponent in Aerosols From Different Emission Sources

Aku Helin ,Jarkko Tissari ,Panu Karjalainen ,Sanna Saarikoski ,Olli Sippula ,Jarkko V Niemi ,Ralf Zimmermann ,Päivi Aakko-Saksa ,Liisa Pirjola ,Gert Jakobi ,Jorma Jokiniemi ,Aki Virkkula ,Matthew Bloss ,Anssi Järvinen ,Topi Rönkkö ,Sampsa Väätäinen ,Fanni Mylläri ,John Backman ,Minna Aurela ,Hilkka Timonen

doi:10.1029/2020jd034094

Abstract

AbstractThe 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.

Highlights

The absorption Ångström exponent (AAE) describes the spectral dependence of light absorption by aerosols
The bus chasing experiments conducted at the bus depot allowed for the investigation of the effect of different driving conditions on the Abstract The absorption Ångström exponent (AAE) values derived from bus exhaust emissions
The total particle number (PN) concentration showed a similar trend: the highest PN concentrations were observed during the acceleration phase and more so with older type buses, whereas the lowest PN concentrations were observed during steady driving and with alternative fuel buses (Pirjola et al, 2015)

Summary

Introduction

The absorption Ångström exponent (AAE) describes the spectral dependence of light absorption by aerosols. The light-absorbing part of organic aerosols is commonly referred to as brown carbon (BrC); a heterogeneous group of compounds from both primary and secondary sources (Andreae & Gelencsér, 2006; Kirchstetter et al, 2004; Laskin et al, 2015). Both BrC and mineral dust particles absorb light relatively stronger at the near-ultraviolet (UV) wavelengths than at the near-infrared (IR) wavelengths (Bond et al, 2013; Moosmüller et al, 2009), HELIN ET AL

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Results

Conclusion