Nicole P. Hyslop

Estimating Precision Using Duplicate Measurements

Abstract Precision is a concept for which there is no universally accepted metric. Reports of precision vary depending on the formula and inclusion criteria used to calculate them. To properly interpret and utilize reported precisions, the user must understand exactly what the precision represents. This paper uses duplicate Interagency Monitoring of Protected Visual Environments (IMPROVE) measurements to illustrate distinctions among different approaches to reporting precision. Three different metrics are used to estimate the precision from the relative differences between the duplicate measurements: the root mean square (RMS), the mean absolute value, and a percentile spread. Precisions calculated using the RMS relative difference yield wide distributions that tend to overestimate most of the observed differences. Precisions calculated using percentiles of the relative differences yield narrower distributions that tend to fit the bulk of the observed differences very well. Precisions calculated using the mean absolute relative difference lie between the other two precision estimates. All three approaches underestimate the observed differences for a small percentage of outliers.

Earlier onset of the spring fine dust season in the southwestern United States

Particulate matter (PM)2.5 dust concentrations (mineral particles with aerodynamic diameters less than 2.5 µm) typically peak in spring and early summer at rural and remote sites across the southwestern United States. Trend analyses indicate that springtime regional mean PM2.5 dust concentrations have increased from 1995 to 2014, especially in March (5.4% yr−1, p < 0.01). This increase reflects an earlier onset of the spring dust season across the Southwest by 1 to 2 weeks over the 20 year time period. March dust concentrations were strongly correlated with the Pacific Decadal Oscillation index (r = −0.65, p < 0.01), which was mostly in its negative phase from 2007 to 2014, during which the region was drier, windier, and less vegetated. The positive spring trend and its association with large-scale climate variability have several important implications for visibility, particulate matter, health effects, and the hydrologic cycle in the region.

Wintertime Vertical Variations in Particulate Matter (PM) and Precursor Concentrations in the San Joaquin Valley during the California Regional Coarse PM/Fine PM Air Quality Study

Abstract Air quality monitoring was conducted at a rural site with a tower in the middle of California’s San Joaquin Valley (SJV) and at elevated sites in the foothills and mountains surrounding the SJV for the California Regional PM10/M2.5 Air Quality Study. Measurements at the surface and on a tower at 90 m were collected in Angiola, CA, from ecember 2000 through February 2001 and included hourly black carbon (BC), particle counts from optical particle counters, nitric oxide, ozone, temperature, relative humidity, wind speed, and direction. Boundary site measurements were made primarily using 24-hr integrated particulate matter (PM) samples. These measurements were used to understand the vertical variations of PM and PM precursors, the effect of stratification in the winter on concentrations and chemistry aloft and at the surface, and the impact of aloft-versus-surface transport on PM concentrations. Vertical variations of concentrations differed among individual species. The stratification may be important to atmospheric chemistry processes, particularly nighttime nitrate formation aloft, because NO2 appeared to be oxidized by ozone in the stratified aloft layer. Additionally, increases in accumulation-mode particle concentrations in the aloft layer during a fine PM (PM2.5) episode corresponded with increases in aloft nitrate, demonstrating the likelihood of an aloft nighttime nitrate formation mechanism. Evidence of local transport at the surface and regional transport aloft was found; transport processes also varied among the species. The distribution of BC appeared to be regional, and BC was often uniformly mixed vertically. Overall, the combination of time-resolved tower and surface measurements provided important insight into PM stratification, formation, and transport.

Assessing the Suitability of Historical PM2.5 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).

Reanalysis of archived IMPROVE PM2.5 samples previously analyzed over a 15-year period.

The IMPROVE (Interagency Monitoring of Protected Visual Environments) network has collected airborne particulate matter (PM) samples at locations throughout the United States since 1988. These samples have been analyzed for elemental content using analytical methods that evolved over the years. Changes in analytical methods sometimes introduced shifts in reported concentrations that are evident in the historical record. We sought to illuminate the effects of methodological changes by reanalyzing archived samples with current methods. To test the feasibility of this approach, the 15-year archive of PM samples from Great Smoky Mountains National Park was selected for reanalysis as a single analytical batch using a common protocol and calibration. Comparisons of the reanalyses and original analyses indicate that concentrations of all but one measured element, Br, remained stable on the filters over years of storage. The agreement between the two analyses varied with element and original measurement method. For elements measured well above their contemporary detection limits - S, K, Ca, Fe, and Zn - the reanalysis established that method changes had limited impacts on reported concentrations, generally <10%. For elements originally measured near their detection limits, reanalysis confirmed the presence of discontinuities in the data record, many of which were previously recognized and documented as method-related.

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

ABSTRACT The IMPROVE (Interagency Monitoring of PROtected Visual Environments) network monitors the attenuation of light by PM2.5 samples (fine particulate matter, Daero = 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. IMPROVEs 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. 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 PM2.5, 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.

A decreasing vanadium footprint of bunker fuel emissions

The Interagency Monitoring of Protected Visual Environments (IMPROVE) network measures the chemical composition of atmospheric particulate matter at over 160 locations throughout the United States. As part of the routine quality control process, we noted decreases in the network-wide vanadium (V) and nickel (Ni) concentrations in 2015 relative to the previous years. Enriched V and Ni with respect to soil are indicative of heavy fuel oil burning and are often used as tracers for emissions from marine vessels. Multiple regulations on the fuel used by marine vessels were implemented in North America since 2010, and the most sweeping regulation was implemented at the start of 2015. The 2015 regulations reduced the allowable fuel oil sulfur concentrations within the North America Emissions Control Area from 1.0% to 0.1% to reduce the environmental and human health impacts of sulfates. As a side effect, these requirements economically favored fuels with lower V and Ni concentrations. The atmospheric concentrations of V and Ni decreased markedly at many IMPROVE monitoring sites, particularly sites near major ports. Between 2011 and 2015, annual mean V concentrations measured on IMPROVE samples collected near the ports of Seattle, Washington and New Orleans, Louisiana decreased by 35% and 85%, respectively. These decreases have brought the coastal V and Ni concentrations much closer to those measured far inland.