A critical review of filter transmittance measurements for aerosol light absorption, and de novo calibration for a decade of monitoring on PTFE membranes

摘要

The IMPROVE (Interagency Monitoring of PROtected Visual Environments) network monitors the attenuation of light by PM(2.)5 samples (fine particulate matter, D-aero <= 2.5 mu m) routinely collected on polytetrafluoroethylene (PTFE) filters throughout the United States. The results of this measurement have long been reported as an indicator of absorption, with no rigorous calibration as such. Filterbased absorption measurements more conventionally employ optically thick quartz- or glass-fiber collection media, for which a substantial calibration literature offers algorithms to correct for particle scattering and filter loading effects. PTFE membranes are optically thinner and less homogeneous than the fiber media, but they avoid interference from adsorbed organic gases that is associated with quartz and glass fiber media. IMPROVE's measurement system is a hybrid of integrating sphere and integrating plate that records the light backscattered as well as transmitted by each filter. This article introduces and validates a theory-based model for calibration and data reduction that accounts for particle scattering effects as well as variations in filter optics. Tests based on historical analyses of field blanks and recent reanalyses of archived samples establish that the current system has operated with a stable calibration since 2003.