Estimates of mass absorption cross sections of black carbon for filter-based absorption photometers in the Arctic

• Main Authors: Ohata, Sho, Mori, Tatsuhiro, Kondo, Yutaka, Sharma, Sangeeta, Hyvärinen, Antti, Andrews, Elisabeth, Tunved, Peter, Asmi, Eija, Backman, John, Servomaa, Henri, Veber, Daniel, Koike, Makoto, Kanaya, Yugo, Yoshida, Atsushi, Moteki, Nobuhiro, Zhao, Yongjing, Matsushita, Junji, Oshima, Naga
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Canada; Ny-Ålesund; Pallastunturi; Svalbard; Barrow; black carbon; Ny Ålesund; Alaska
• Online Access: https://doi.org/10.5194/acp-2020-1190; https://acp.copernicus.org/preprints/acp-2020-1190/

Long-term measurements of black carbon (BC) are warranted for investigating changes in its emission, transport, and deposition. However, depending on instrumentation, parameters related to BC such as aerosol absorption coefficient ( b abs ) have been measured instead. Most ground-based measurements of b abs in the Arctic have been made by filter-based absorption photometers, including multi-angle absorption photometers (MAAP), particle soot absorption photometers (PSAP), continuous light absorption photometer (CLAP), and Aethalometers. The measured b abs can be converted to atmospheric mass concentrations of BC ( M BC ) by assuming the value of the mass absorption cross section (MAC = b abs / M BC ). However, the accuracy of conversion of b abs to M BC has not been adequately assessed. Here, we introduce a systematic method for deriving MAC values from b abs measured by these instruments and independently measured M BC . In this method, M BC was measured with a filter-based absorption photometer with a heated inlet (COSMOS). COSMOS-derived M BC ( M BC (COSMOS)) is traceable to a rigorously calibrated single particle soot photometer (SP2) and the absolute accuracy of M BC (COSMOS) has been demonstrated previously to be about 15 % in Asia and the Arctic. The necessary conditions for application of this method are a high correlation of the measured b abs with independently measured M BC , and long-term stability of the correlation slope, which represents the MAC. In general, b abs – M BC (COSMOS) correlations were high ( r 2 = 0.84–0.96 for hourly data) at Fukue in Japan, Barrow in Alaska, Ny-Ålesund in Svalbard, Pallastunturi in Finland, and Alert in Canada, and stable up to for 10 years. We successfully estimated MAC values (11.0–15.2 m 2 g −1 at a wavelength of 550 nm) for these instruments and these MAC values can be used to obtain error-constrained estimates of M BC from b abs measured at these sites even in the past, when COSMOS measurements were not made. Because the absolute values of M BC in these Arctic sites estimated by this method are consistent with each other, they are applicable to study spatial and temporal variation of M BC and to evaluate performance of numerical model calculations.