Mass absorption cross section of black carbon for Aethalometer in the Arctic

Long-term measurements of the mass concentration of black carbon (BC) in the atmosphere (M_BC) with well-constrained accuracy are indispensable to quantify its emission, transport, and deposition. The aerosol light absorption coefficient (b_abs), usually measured by a filter-based absorption photometer, including an Aethalometer (AE), is often used to estimate M_BC. The measured b_abs is converted to M_BC by assuming a value for the mass absorption cross section (MAC). Previously, we derived the MAC for AE (MAC (AE)) from measured b_abs and independently measured M_BC values at two sites in the Arctic. M_BC was measured with a filter-based absorption photometer with a heated inlet (COSMOS). The accuracy of the COSMOS-derived M_BC (M_BC (COSMOS)) was within about 15%. Here, we obtained additional MAC (AE) measurements to improve understanding of its variability and uncertainty. We measured b_abs (AE) and M_BC (COSMOS) at Alert (2018–2020), Barrow (2012–2022), Ny-Ålesund (2012–2019), and Pallas (2019–2022). At Pallas, we also obtained four-wavelength photoacoustic aerosol absorption spectrometer (PAAS-4λ) measurements of b_abs. b_abs (AE) and M_BC (COSMOS) were tightly correlated; the average MAC (AE) at the four sites was 11.4 ± 1.2 m² g⁻¹ (mean ± 1σ) at 590 nm and 7.76 ± 0.73 m² g⁻¹ at 880 nm. The spatial variability of MAC (AE) was about 11% (1σ), and its year-to-year variability was about 18%. We compared MAC (AE) in the Arctic with values at mid-latitudes, measured by previous studies, and with values obtained by using other types of filter-based absorption photometer, and PAAS-4λ.