Assessing the Suitability of Historical PM_2.5 Element Measurements for Trend Analysis

The IMPROVE (Interagency Monitoring of Protected Visual Environments) network has characterized fine particulate matter composition at locations throughout the United States since 1988. A main objective of the network is to evaluate long-term trends in aerosol concentrations. Measurements inevitably advance over time, but changes in measurement technique have the potential to confound the interpretation of long-term trends. Problems of interpretation typically arise from changing biases, and changes in bias can be difficult to identify without comparison data that are consistent throughout the measurement series, which rarely exist. We created a consistent measurement series for exactly this purpose by reanalyzing the 15-year archives (1995–2009) of aerosol samples from three sites – Great Smoky Mountains National Park, Mount Rainier National Park, and Point Reyes National Seashore–as single batches using consistent analytical methods. In most cases, trend estimates based on the original and reanalysis measurements are statistically different for elements that were not measured above the detection limit consistently over the years (e.g., Na, Cl, Si, Ti, V, Mn). The original trends are more reliable for elements consistently measured above the detection limit. All but one of the 23 site-element series with detection rates >80% had statistically indistinguishable original and reanalysis trends (overlapping 95% confidence intervals)

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

<p>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 μm) 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. Filter-based 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.</p> <p>The newly calibrated IMPROVE absorption values correlate strongly with the refractory carbon fraction reported by thermal-optical analysis as “elemental” (EC). EC is sometimes treated as the only significant light absorber in PM_2.5, but the general decline observed between 2005 and 2014 in IMPROVE EC was not accompanied by a comparable decline in IMPROVE absorption. Absorption also exhibits a distinct association with Fe concentrations, which at IMPROVE sites are attributable mainly to mineral dusts and have generally held steady or risen since 2003. An increased relative contribution by mineral dusts can explain some, but not all, of the observed difference between recent absorption and EC trends.</p

Evaluating IMPROVE PM_2.5 element measurements

The Interagency Monitoring of PROtected Visual Environments (IMPROVE) network has collected airborne particulate matter (PM) samples at locations throughout the United States since 1988 and provided chemical speciation measurements on the samples using several techniques including X-ray fluorescence (XRF). New XRF instruments for measuring PM elemental content of IMPROVE samples were introduced in 2011. To evaluate the performance of these new instruments relative to the old instruments, archived sample from three IMPROVE monitoring sites were retrieved and analyzed on the new instruments. The agreement between the two instruments varied by element. Comparisons of the results were very good (slopes within 10% of unity) for most elements regularly measured well above the detection limits (sulfur, chlorine, potassium, titanium, vanadium, manganese, iron, copper, zinc, selenium, lead). Different particle compositions at the three sites highlighted different measurement interferences. High sea salt concentrations at the coastal site emphasized corrections applied in the old systems to light elements – sodium and magnesium – and resulted in poor agreement for these elements. Comparisons of the XRF measurements with collocated sulfate measurements by ion chromatography suggest that sulfur measurements from the new instruments are more precise but slight underestimates. Comparing elemental ratios to expected ratios for soil-derived PM demonstrate the new instruments are better at resolving the aluminum and silicon peaks.</p