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 (R² = 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.

Aerosols emitted from various anthropogenic and natural sources undergo constant physico-chemical transformations in the atmosphere, altering their impacts on health and climate. This paper presents the design and characteristics of a novel Photochemical Emission Aging Flow Tube Reactor (PEAR). The PEAR was designed to provide sufficient aerosol mass and flow for simultaneous measurement of the physico-chemical properties of aged aerosols and emission exposure studies (in-vivo and in-vitro). The performance of the PEAR was evaluated by using common precursors of secondary aerosols as well as combustion emissions from a wood stove and a gasoline engine. The PEAR was found to provide a near laminar flow profile, negligible particle losses for particle sizes above 40 nm, and a narrow residence time distribution. These characteristics enable resolution of temporal emission patterns from dynamic emission sources such as small-scale wood combustion. The formation of secondary organic aerosols (SOA) in the PEAR was found to be similar to SOA formation in a smog chamber when toluene and logwood combustion emissions were used as aerosol sources. The aerosol mass spectra obtained from the PEAR and smog-chamber were highly similar when wood combustion was used as the emission source. In conclusion, the PEAR was found to plausibly simulate the photochemical aging of organic aerosols with high flow rates, needed for studies to investigate the effects of aged aerosols on human health. The method also enables to study the aging of different emission phases in high time resolution, and with different OH-radical exposures up to conditions representing long-range transported aerosols.