Absorption photometry of patterned deposits on IMPROVE PTFE filters.

Abstract

The IMPROVE program (Interagency Monitoring of PROtected Visual Environments) tracks long-term trends in the composition and optics of regional haze aerosols in the United States. The absorptance of red (633-nm) light is monitored by filter photometry of 24 h-integrated samples of fine particulate matter (PM 2.5). These are collected every third day at about 150 rural and often remote locations. Systematic reanalyses of archived samples have established the reproducibility of these optical absorption measurements across decades and instrument systems, with a consistent calibration that is traceable back to 2003.IMPROVE samples for nondestructive analyses by photometry, gravimetry, and X-ray fluorescence are all collected on ring-mounted membranes of expanded PTFE (polytetrafluoroethylene). Although attractively inert, low in blank mass, and optically thin, these media yield visibly nonuniform deposits that do not admit direct interpretation according to a naïve "Beer-Lambert" formulation of optical absorption. Most IMPROVE PTFE deposits exhibit a fine-scale "pixelation" shaped by the perforated screen that supports the membrane during sample collection.This paper extends the traditional Beer-Lambert interpretive model to accommodate the patterned deposits generated by IMPROVE and similarly aggressive sampling protocols on PTFE filters. The extended model is then used to assess the bias and epistemic uncertainty in the aerosol absorption coefficient that IMPROVE has historically reported.ImplicationsAmbient fine aerosol particles are widely collected at high sample face velocities on PTFE (Teflon®) membrane filters backed by perforated support screens. The screens' localized blockage of airflow can channel the resulting PM2.5 sample deposits into a pattern of discrete heavily loaded "pixels", where the screen perforations allow flow through the membrane, separated by a lightly loaded background that reflects blockage by the screen. Such non-uniform deposits, collected throughout the United States on FRM filters to monitor NAAQS compliance and on IMPROVE filters to support the Regional Haze Rule, do not meet the requirements of the Beer-Lambert model for optical absorption measurements. This paper extends Beer-Lambert theory to accommodate patterned deposits, and assesses the bias and epistemic uncertainties introduced by historical (mis)reporting based on "Beer's Law". For samples of moderate (and lesser) absorptance, including the bulk of those collected at rural IMPROVE sites to track regional haze, usefully tight bounds are developed for this pixelation bias.