Bias in filter-based aerosol light absorption measurements due to organic aerosol loading: Evidence from ambient measurements

During the 2006 Texas Air Quality Study/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006) a filter-based (Particle Soot Absorption Photometer, or PSAP) and a photoacoustic-based aerosol light absorption technique were deployed and here the data are compared. The level of agreement between the two techniques with ambient aerosol depended on the abundance of organic aerosol (OA), with the ratio of OA to light absorbing carbon (LAC) mass (ROA-LAC) of particular importance. When OA mass concentration was low the agreement between the methods was within instrumental uncertainties (PSAP measuring 12% higher), however at high (ROA-LAC) (similar to 15-20) the difference in agreement was between 50 and 80%. This difference is similar to the bias observed in the laboratory studies of a companion paper using non-absorbing OA and LAC (Cappa et al. 2008a). It was found that most of the OA was oxidized and non-absorbing in nature. We postulate that the observed differences results from a bias in the filter-based measurements due to (a) the redistribution of liquid-like organic particulate matter (PM) around the fiber filters thereby modifying the filter surface and subsequent light scattering, and (b) the possible coating and absorption enhancement of pre-existing absorbing PM (i.e., soot) as OA deposition and redistribution occurs. We stop short or recommending a universal correction using these findings due to the magnitude of the bias showing some dependence on air mass type. Any use of this data for a correction must consider the uncertainties in measuring OA mass concentrations, LAC mass concentrations and type of OA present. The observed PSAP bias may have a significant impact on the accuracy of aerosol single scatter albedo (SSA) and LAC mass concentrations derived from filter-based aerosol absorption methods from regions impacted by large amounts of OA. Although this study was performed using the PSAP we caution users of other filter-based instrumentation to the possibility of a similar bias in those instruments.