Improving measurements of aerosol light absorption coefficients from DualSpot aethalometers using an internal consistency diagnostic and an alternative loading compensation method
| Authors: | Ryan P. Poland, Zachary C. McQueen, Chase K. Glenn, Omar El Hajj, Mac A. Callaham, Joseph J. O'Brien, Rawad Saleh, Geoffrey D. Smith |
| Year: | 2025 |
| Type: | Scientific Journal |
| Station: | Southern Research Station |
| DOI: | https://doi.org/10.1080/02786826.2025.2587817 |
| Source: | Aerosol Science and Technology |
Abstract
Aethalometers are widely used to measure aerosol light absorption and to infer concentrations of black carbon (BC) in aerosols. These instruments measure the transmission of light through a particle-laden filter but are known to suffer from decreased sensitivity as the filter is loaded. This so-called “filter loading effect” can be quantified and corrected for by comparing attenuation measured on two filter spots through which the same aerosol has been sampled but at different flow rates. This approach, termed “DualSpot,” is commonly employed in modern aethalometers. Here, we test the default method for deriving the loading compensation parameter in the AE33 DualSpot Aethalometer by comparing to a reference photoacoustic instrument with aerosols sampled from simulated wildland fires and ambient air. We demonstrate that this compensation method suffers from three sources of error: 1) the use of a weighted average of the compensation parameter, 2) the use of cumulative volume of air sampled (as opposed to instantaneous volumetric flow rate), and 3) an error-prone flow correction. By comparing compensated absorption coefficients on both sample spots, we establish an internal consistency check that helps identify when the compensation algorithm is in error, a diagnostic we recommend to users of DualSpot aethalometers. Finally, we demonstrate and recommend the use of an alternative compensation parameter, derived by equating the compensated absorption coefficients on both spots, which improves measurements of aerosol absorption and its wavelength dependence.