Advanced Characterization of Aerosol Optical Properties, Sources and BrC Ageing With the New 9λ Aethalometer Model AE36s

ASTA GREGORIČ, Bálint Alföldy, Gašper Lavrič, Matic Ivančič, Irena Ježek Brecelj, Amalia Muñoz, Mila Ródenas, Ruben Soler, Esther Borrás, Teresa Vera, Eduardo Yubero, Rok Podlipec, Martin Rigler, Aerosol d.o.o.

     Abstract Number: 302
     Working Group: Carbonaceous Aerosol

Abstract
Aerosol light absorption measurements are important for assessing the effects of aerosols and their mixing state on the radiative balance of the atmosphere. The absorption spectral dependence can be expressed by the absorption Ångström exponent (AAE), which assumes a power law relationship and is used to identify the types and sources of light absorbing aerosols, such as black carbon (BC) and brown carbon (BrC).

AAE is usually calculated by fitting a linear function to the logarithm of the absorption coefficients at two or more wavelengths. However, the absorption spectra show different features across the UV and visible wavelength range, due to the specific absorption properties of different BrC compounds. To better characterize BrC absorption, two additional wavelengths were added in the new 9λ Aethalometer model AE36s (Aerosol Magee Scientific): (a) 400 nm, which fills the gap between 370 nm and 470 nm, and (b) 340 nm, which extends the measurement range to the UV spectrum.

Long-term measurements with the AE36s were performed at an urban background site in Ljubljana (Slovenia) in winter – spring period in 2021/22 and 2023, and compared with results obtained in the EUPHORE simulation chamber for different fuel types, burning conditions and aging states. AAE was analyzed for the long (AAE_590-950; 590-660-880-950 nm) and short spectral range (AAE_400-520; 400-470-520 nm) separately, which allowed us to distinguish between different absorption patterns.

Results from the Ljubljana campaign presented as a dual AAE plot revealed several specific events, which could be differentiated from the urban aerosol absorption pattern: e.g. industrial fire emission, open fire burning and fresh BrC emission.