Elemental Characterization of Ambient Particulate Matter for a Globally Distributed Monitoring Network: Methodology and Implications.

Global ground-level measurements of elements in ambient particulate matter (PM) can provide valuable information to understand the distribution of dust and trace elements, assess health impacts, and investigate emission sources. We use X-ray fluorescence spectroscopy to characterize the elemental composition of PM samples collected from 27 globally distributed sites in the Surface PARTiculate mAtter Network (SPARTAN) over 2019-2023. Consistent protocols are applied to collect all samples and analyze them at one central laboratory, which facilitates comparison across different sites. Multiple quality assurance measures are performed, including applying reference materials that resemble typical PM samples, acceptance testing, and routine quality control. Method detection limits and uncertainties are estimated. Concentrations of dust and trace element oxides (TEO) are determined from the elemental dataset. In addition to sites in arid regions, a moderately high mean dust concentration (6 µg/m3) in PM2.5 is also found in Dhaka (Bangladesh) along with a high average TEO level (6 µg/m3). High carcinogenic risk (>1 cancer case per 100000 adults) from airborne arsenic is observed in Dhaka (Bangladesh), Kanpur (India), and Hanoi (Vietnam). Industries of informal lead-acid battery and e-waste recycling as well as coal-fired brick kilns likely contribute to the elevated trace element concentrations found in Dhaka.

Evaluating IMPROVE PM2.5 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.Implications: The presented work represents a comprehensive analysis of the method change enacted within the Interagency Monitoring of PROtected Visual Environments (IMPROVE) air monitoring network. This work describes the implications of the last change in elemental quantification methodology. The most important point for data users performing longitudinal analyses is that light elements (e.g., sodium - sulfur) were affected; the old instrumentation overestimated these elements while the current measurements are slightly underestimated. The authors recommend these results to be taken into consideration when interpreting sea salt and crustal sources of atmospheric dust.

Evaluating PM2.5 element concentration measurements for a nationwide monitoring network.

Particulate matter (PM) concentrations have decreased dramatically over the past 20 years, thus lower method detection limits (MDL) are required for these measurements. Energy-dispersive X-ray fluorescence (XRF) spectroscopy is used to quantify multiple elements simultaneously in the U.S. Environmental Protection Agency (EPA) Chemical Speciation Network (CSN). Inductively-coupled plasma mass spectrometry (ICP-MS) is an alternative analysis with lower MDL for elements. Here, we present a side-by-side comparison of XRF and ICP-MS for elements in PM2.5 samples collected via the EPA's CSN. For ICP-MS, a simple extraction and ICP-MS analysis technique was applied to a wide variety of samples to minimize effort and cost and serve as a feasibility test for a large monitoring network. Filter samples (N = 549) from various urban locations across the US were analyzed first analyzed via XRF at UC Davis and then ICP-MS at RTI International. Both methods measured 29 of the same elements out of the 33 usually reported to CSN. Of these 29, 14 elements (Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb) were found to be frequently detected (i.e. had more than 10% of values above both XRF and ICP-MS MDL). ICP-MS was found to have lower MDL for 26 out of 29 elements, namely Na, Mg, Al, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, As, Se, Rb, Sr, Zr, Ag, Cd, In, Sn, Sb, Cs, Ba, Ce, Pb; conversely, XRF had lower MDL for 3 elements, namely, P, K, Zn. Intra-method quality checks using (1) inter-elemental inspection of scatter plots using a priori knowledge of element sources and (2) scatter plots of routine versus collocated measurements reveal that ICP-MS exhibits better measurement precision. Lower detection limits for element measurements in nationwide PM monitoring networks would benefit human-health and source apportionment research.Implications: We demonstrate that ICP-MS with adilute-acid digestion method would significantly improve the element detection rates and thus be avaluable addition to the current analysis techniques for airborne PM samples in anationwide monitoring network. In this paper, we show that a hybrid method of elemental analysis for airborne particulate matter (PM) would significantly improve the detection rates for elements in PM. This would be a valuable addition to the current analysis techniques for airborne PM samples in nationwide and other large-scale monitoring networks, such as the EPA's Chemical Speciation Network (CSN). The techniques explored in this study (i.e., X-ray Fluorescence Spectroscopy or XRF and Inductively Coupled Plasma-Mass Spectrometry or ICP-MS) are relevant to the PM monitoring and regulatory community audience of JAWMA, especially agencies and states that are already involved in CSN. In addition, our results outline considerations that give insight on factors to consider for other large-scale and long-term ambient air monitoring efforts.

Wintertime haze and ozone at Dinosaur National Monument.

Dinosaur National Monument (DINO) is located near the northeastern edge of the Uinta Basin and often experiences elevated levels of wintertime ground-level ozone. Previous studies have shown that high ozone mixing ratios in the Uinta Basin are driven by elevated levels of volatile organic compounds (VOCs) and nitrogen oxides (NOx) from regional oil and gas development coupled with temperature inversions and enhanced photochemistry from persistent snow cover. Here, we show that persistent snow cover and temperature inversions, along with abundant ammonia, also lead to wintertime haze in this region. A study was conducted at DINO from November 2018 through May 2020 where ozone, speciated fine and coarse aerosols, inorganic gases, and VOCs were measured. Three National Ambient Air Quality Standards (NAAQS) ozone exceedances were observed in the first winter, and no exceedances were observed in the second winter. In contrast, elevated levels of particulate matter were observed both winters, with 24-h averaged particle light extinction exceeding 100 Mm-1. These haze events were dominated by ammonium nitrate, and particulate organics were highly correlated with ammonium nitrate. Ammonium nitrate formation was limited by nitric acid in winter. As such, reductions in regional NOx emissions should reduce haze levels and improve visibility at DINO in winter. Long-term measurements of particulate matter from nearby Vernal, Utah, suggest that visibility impairment is a persistent issue in the Uinta Basin in winter. From April through October 2019, relatively clean conditions occurred, with average particle extinction of ~10 Mm-1. During this period, ammonium nitrate concentrations were lower by more than an order of magnitude, and contributions from coarse mass and soil to haze levels increased. VOC markers indicated that the high levels of observed pollutants in winter were likely from local sources related to oil and gas extraction activities.Implications: Elevated ground-level ozone and haze levels were observed at Dinosaur National Monument in winter. Haze episodes were dominated by ammonium nitrate, with 24-h averaged particle light extinction exceeding 100 Mm-1, reducing visual range near the surface to ~35 km. Despite elevated ammonium nitrate concentrations, additional gas-phase ammonia was available, such that any increase in NOx emissions in the region is likely to lead to even greater haze levels.

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.

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).

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.

Identifying sources of uncertainty from the inter-species covariance of measurement errors.

A standard metric of measurement precision in environmental monitoring is the variance of differences between duplicate (collocated) samples. With duplicate measurements of multiple species, we can extend this variance analysis to include the interspecies covariance of differences between duplicate samples; these covariances can provide clues about the sources of error. We illustrate the potential of such an analysis with atmospheric aerosol measurements from two national air quality monitoring networks: Interagency Monitoring of Protected Visual Environments (IMPROVE) and Speciation Trends Network (STN). These aerosol "speciation" networks provide the multivariate data sets needed to characterize error covariance by operating duplicate samplers at several of their monitoring locations and analyzing both the collected aerosol samples for multiple species. We observe covariance among the measurement differences for multiple species in both networks. The covariance among measurement differences for soil-derived elements suggests an error associated with the particle size discrimination step in sampling, which is not currently included in either network's uncertainty estimates. The multivariate statistical analyses of aerosol speciation data performed by standard source apportionment models assume that measurement errors in different species are independent of each other; the present analysis invalidates this assumption for several species measured by IMPROVE and STN.

An empirical approach to estimating detection limits using collocated data.

Measurements of trace species generally become less certain as concentrations decrease. Data analysts need guidance on the ranges in which particular measurements are meaningful. This guidance is normally stated in the form of detection limits. The International Union for Pure and Applied Chemistry (IUPAC) has defined several parameters to characterize measurement detection limits (Currie, L. A. Pure Appl. Chem. 1995, 67, 1699). The published guidelines envision an ability to prepare reference materials with concentrations close to the detection limits using the same methods as for normal samples. For multianalyte methods such as X-ray fluorescence (XRF), multiple reference materials may be required for each analyte to characterize the effects of interferences. The creation and characterization of such complex reference materials atthe detection limits of modern XRF systems represents a considerable technical challenge. This paper describes an observational approach to estimating the detection limits defined by IUPAC. Our empirical approach takes advantage of collocated (duplicate) measurements that are routinely collected by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and Speciation Trends Network (STN). The analysis is successfully demonstrated by deriving detection limits at the measurement system level for six elements measured on PM2.5 samples by XRF in both networks. The two networks' detection limits are found to be similar in terms of loading (areal density, ng cm(-2)) on the filters as measured by the XRF instruments despite many differences in sample collection, handling, and analysis. IMPROVE detection limits are an order of magnitude lowerthan STN's in terms of atmospheric concentrations (ng m(-3)), because IMPROVE uses smaller fitters and higher flow rates which lead to more concentrated sample deposits.

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.

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/ PM2.5 Air Quality Study. Measurements at the surface and n a tower at 90 m were collected in Angiola, CA, from December 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.