Comparison of different aethalometer correction schemes and a reference multi-wavelength absorption technique for ambient aerosol data

Deriving absorption coefficients from Aethalometer attenuation data requires different corrections to compensate for artifacts related to filter-loading effects, scattering by filter fibers, and scattering by aerosol particles. In this study, two different correction schemes were applied to 7-wavelength Aethalometer data, using Multi-Angle Absorption Photometer (MAAP) data as a reference absorption measurement at 637 nm. The compensation algorithms were compared to 5-wavelength offline absorption measurements obtained with a Multi-Wavelength Absorbance Analyzer (MWAA), which serves as a multiple-wavelength reference measurement. The online measurements took place in the Amazon rainforest, from the wet-to-dry transition season to the dry season (June–September 2014). The mean absorption coefficient (at 637 nm) during this period was 1.8 ± 2.1 Mm −1 , with a maximum of 15.9 Mm −1 . Under these conditions, the filter-loading compensation was negligible. One of the correction schemes was found to artificially increase the short-wavelength absorption coefficients. It was found that accounting for the aerosol optical properties in the scattering compensation significantly affects the absorption Angstrom exponent (AAE) retrievals. Proper Aethalometer data compensation schemes are crucial to retrieve the correct AAE, which is commonly implemented in brown carbon contribution calculations. We found that a "hybrid" algorithm was more appropriate to achieve optimal correlations with the MAAP absorption coefficients and with the AAE retrieved from offline MWAA measurements.