Abstract
Abstract. 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, R2=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 (R2=0.65). Statistically the best linear correlations with MAAP were obtained for AE33 and COSMOS (R2 close to 1), but the biases at around the zero values led to slopes clearly below 1. The sample pre-treatment in the COSMOS instrument resulted in the lowest fitted slope. In contrast to the filter-based techniques, the indirect EMS method was not adequate to measure the low absorption values found at the Pallas site. The lowest absorption at which the EMS signal could be distinguished from the noise was >0.1 Mm−1 at 1–2 h averaging times. The mass absorption cross section (MAC) value measured at a range 0–0.3 Mm−1 was calculated using the MAAP and a single particle soot photometer (SP2), resulting in a MAC value of 16.0±5.7 m2 g−1. Overall, our results demonstrate the challenges encountered in the aerosol absorption measurements in pristine environments and provide some useful guidelines for instrument selection and measurement practices. We highlight the need for a calibrated transfer standard for better inter-comparability of the absorption results.
Highlights
The development of a filter-based aerosol absorption measurement method began with an experiment by Rosen et al (1978)
The measured aerosol light absorption is frequently reported as equivalent black carbon mass, which relies on a specific wavelength-dependent mass absorption cross section (MAC) coefficient (Bond and Bergstrom, 2006; Petzold et al, 2013a)
Subsets in such a way that if N is the number of subsets of yj measurements, each averaged over the measurement interval t, the Allan variance for a measurement time period is defined as σy2 (t )
Summary
The development of a filter-based aerosol absorption measurement method began with an experiment by Rosen et al (1978). The Raman spectral measurements confirmed that the light attenuation is proportional to the graphitic soot content on a filter. After this discovery the development was continued by Hansen et al (1982, 1984), and today, the various filter-based techniques are commonly used in aerosol absorption measurements (Tørseth et al, 2019). It has become evident that the filter-based methods are prone to several filter artifacts These include the dependence of light attenuation on the filter tape mass loading and the interference by aerosol light scattering with the absorption measurement (Müller et al, 2011a). The measured aerosol light absorption is frequently reported as equivalent black carbon (eBC) mass (in units of ng m−3), which relies on a specific wavelength-dependent mass absorption cross section (MAC) coefficient (Bond and Bergstrom, 2006; Petzold et al, 2013a)
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