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

Long-term measurements of atmospheric mass concentrations of black carbon (BC) are needed to investigate 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 particle soot absorption photometers (PSAP), continuous light absorption photometer (CLAP), Aethalometers, and multi-angle absorption photometers (MAAP). The measured babs can be converted to mass concentrations of BC (M_BC) by assuming the value of the mass absorption cross section (MAC; M_BC = b_abs/MAC). However, the accuracy of conversion of b_abs to MBC 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 MBC. In this method, MBC 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 MBC, and long-term stability of the regression slope, which is denoted as MAC_cor (MAC derived from the correlation). In general, b_abs-M_BC (COSMOS) correlations were high (r² = 0.76-0.95 for hourly data) at Alert in Canada, Ny-Ålesund in Svalbard, Barrow in Alaska, Pallastunturi in Finland, and Fukue in Japan, and stable for up to 10 years. We successfully estimated MAC_cor values (10.6-15.2 m² g^-1 at a wavelength of 550 nm) for these instruments and these MAC_cor values can be used to obtain error-constrained estimates of MBC 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 the study of spatial and temporal variation of M_BC in the Arctic and to evaluation of the performance of numerical model calculations. [ABSTRACT FROM AUTHOR]