Absorption instruments inter-comparison campaign at the Arctic Pallas station

• Published in: Atmospheric Measurement Techniques
• Main Authors: E. Asmi, J. Backman, H. Servomaa, A. Virkkula, M. I. Gini, K. Eleftheriadis, T. Müller, S. Ohata, Y. Kondo, A. Hyvärinen
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2021
• Subjects: Environmental engineering; TA170-171; Earthwork. Foundations; TA715-787; Arctic; black carbon; Northern Finland
• Online Access: https://doi.org/10.5194/amt-14-5397-2021; https://doaj.org/article/e87ee00b4aac44f0b0ea90561d047508

Aerosol light absorption was measured during a 1-month field campaign in June–July 2019 at the Pallas Global Atmospheric Watch (GAW) station in northern Finland. Very low aerosol concentrations prevailed during the campaign, which posed a challenge for the instruments' detection capabilities. The campaign provided a real-world test for different absorption measurement techniques supporting the goals of the European Metrology Programme for Innovation and Research (EMPIR) Black Carbon (BC) project in developing aerosol absorption standard and reference methods. In this study we compare the results from five filter-based absorption techniques – aethalometer models AE31 and AE33, a particle soot absorption photometer (PSAP), a multi-angle absorption photometer (MAAP), and a continuous soot monitoring system (COSMOS) – and from one indirect technique called extinction minus scattering (EMS). The ability of the filter-based techniques was shown to be adequate to measure aerosol light absorption coefficients down to around 0.01 Mm −1 levels when data were averaged to 1–2 h. The hourly averaged atmospheric absorption measured by the reference MAAP was 0.09 Mm −1 (at a wavelength of 637 nm). When data were averaged for >1 h, the filter-based methods agreed to around 40 %. COSMOS systematically measured the lowest absorption coefficient values, which was expected due to the sample pre-treatment in the COSMOS inlet. PSAP showed the best linear correlation with MAAP ( slope=0.95 , R 2 =0.78 ), followed by AE31 ( slope=0.93 ). A scattering correction applied to PSAP data improved the data accuracy despite the added noise. However, at very high scattering values the correction led to an underestimation of the absorption. The AE31 data had the highest noise and the correlation with MAAP was the lowest ( R 2 =0.65 ). Statistically the best linear correlations with MAAP were obtained for AE33 and COSMOS ( R 2 close to 1), but the biases at around the zero values led to slopes clearly below 1. The sample pre-treatment in the ...