Light Absorption by Soil Dust in Regional Haze over the United States

Abstract:

The IMPROVE network (Interagency Monitoring of Protected Visual Environments) monitors regional haze at about 170 sites throughout the U.S. About 20,000 samples of fine particulate matter (D_p < 2.5 um) are collected each year on PTFE filters that are weighed for total mass and analyzed by XRF for elemental concentrations. Light absorption by these samples is measured at 633 nm with the U.C. Davis Hybrid Integrating Plate/Sphere (HIPS) system. Absorption by haze depends in general on the composition, morphology, and microstructure of the individual suspended particles. The ambient particle mix varies widely in the real atmosphere, and is not easily reproduced in controlled laboratory settings. Absorption by a collected sample depends additionally on the filter, sample loading, and heterogeneity of the particle population. Absorption by a filter deposit is thus likely to differ quantitatively from that of the suspended particles in the air column from which it was sampled. This is a recognized limitation of any filter-based absorption measurement, but is widely tolerated in return for the huge increase that filtration provides in sample concentration. Tailoring the absorption measurement to the samples routinely analyzed by XRF does allow us to observe aspects of the absorption-composition relationship in the real world. Newly calibrated measurements from the HIPS system yield a consistent record of light absorption throughout the IMPROVE monitoring network since 2003. Comparisons with IMPROVE chemical data identify both carbon soot and iron-bearing dust as significant contributors to light absorption by particulate matter.