Advances in Integrated and Continuous Measurements for Particle Mass and Chemical Composition

Abstract

Recent improvements in integrated and continuous PM 2.5 mass and chemical measurements from the Supersite program and related studies in the past decade are summarized. Analytical capabilities of the measurement methods, including accuracy, precision, interferences, minimum detectable levels, comparability, and data completeness are documented. Upstream denuders followed by filter packs in integrated samplers allow an estimation of sampling artifacts. Efforts are needed to: (1) address positive and negative artifacts for organic carbon (OC), and (2) develop carbon standards to better separate organic versus elemental carbon (EC) under different temperature settings and analysis atmospheres. Advances in thermal desorption followed by gas chromatography/ mass spectrometry (GC/MS) provide organic speciation of approximately 130 nonpolar compounds (e.g., n-alkanes, alkenes, hopanes, steranes, and polycyclic aromatic hydrocarbons [PAHs]) using small portions of filters from existing integrated samples. Speciation of water-soluble OC (WSOC) using ion chromatography (IC)-based instruments can replace labor-intensive solvent extraction for many compounds used as source markers. Thermal gasbased continuous nitrate and sulfate measurements underestimate filter ions by 10 -50% and require calibration against on-site filter-based measurements. IC-based instruments provide multiple ions and report comparable (Ϯ10%) results to filter-based measurements. Maintaining a greater than 80% data capture rate in continuous instruments is labor intensive and requires experienced operators. Several instruments quantify black carbon (BC) by optical or photoacoustic methods, or EC by thermal methods. A few instruments provide real-time OC, EC, and organic speciation. BC and EC concentrations from continuous instruments are highly correlated but the concentrations differ by a factor of two or more. Site-and season-specific mass absorption efficiencies are needed to convert light absorption to BC. Particle mass spectrometers, although semiquantitative, provide much information on particle size and composition related to formation, growth, and characteristics over short averaging times. Efforts are made to quantify mass by collocating with other particle sizing instruments. Common parameters should be identified and consistent approaches are needed to establish comparability among measurements.

U.S. National PM2.5Chemical Speciation Monitoring Networks—CSN and IMPROVE: Description of networks

“…The variation between the two approaches for OC measured on typical urban and rural samples ranges from about 10 to 30%; for EC the difference can be up to a factor of 2 or more for the different analytical protocols used in the comparison studies (Solomon et al, 2000;Chow et al, 2001;Schmid et al, 2001;Solomon et al, 2003;Chow et al, 2004;Watson et al, 2005;Chow et al, 2008;Cheng et al, 2011). Total carbon (TC), equal to the sum of OC and EC, usually agrees to within 10-15%.…”

“…All channels employ mass flow control and all use 47-mm diameter filters. Sampling (Chow et al, 2007); OC and EC fractions reported in 8 temperature steps Chow et al, 2008), analysis by DRI; Details in Table 4 Sunset Thermal Optical Transmittance (TOT), modified NIOSH method 5040 (RTI 2009e); OC reported in five temperature steps; Details in (Hering and Cass, 1999;Solomon et al, 2000). Therefore, for this sampler only, nitrate is the sum of nitrate measured on the Teflon front and nylon backup filter.…”

“…For the collection of nitrate, both networks use denuders and reactive nylon filters to minimize sampling artifacts caused by nitrate volatilization (Hering et al, 1988;Hering and Cass, 1999). Carbon artifacts are accounted for by using various blank correction approaches (Kim et al, 2005;Chow et al, 2008;Watson et al, 2009;Chow et al, 2010;Maimone et al, 2011), as discussed later. Therefore, these samplers are designed to minimize sampling artifacts for nitrate while allowing flexibility for estimating organic carbon artifacts.…”

“…Studies examined uncertainty issues in only one network or the other, whereas other studies examined it between networks. These studies include the initial CSN intercomparisons (Solomon et al, 2000;Coutant and Stetzer, 2001;Solomon et al, 2003), estimating the overall measurement uncertainty for mass or one or more chemical components for CSN and IMPROVE (Solomon et al, 2000;Chow et al, 2001;Solomon et al, 2003;Ashbaugh et al, 2004aAshbaugh et al, , 2004bChow et al, 2004;Watson et al, 2005;White et al, 2005;Yu et al, 2006;Smiley, 2013;Hyslop and White, 2008b), and estimating different uncertainty components (blanks, MDL, sampling artifacts) for OC, mass, ions, and elements by EDXRF (Iyer et al, 2000;Gutknecht et al, 2001;Kim et al, 2001;Lewtas et al, 2001;Fehsenfeld et al, 2004;Subramanian et al, 2004;Gego et al, 2005;Kim et al, 2005;Gutknecht et al, 2006;Zhao et al, 2006;Chow et al, 2008;Hyslop and White, 2008a;Watson et al, 2009;Chow et al, 2010;Gutknecht et al, 2010;Cheng et al, 2011;Maimone et al, 2011;Malm et al, 2011). Several special EPA Office of Air Quality Planning and Standards (OAQPS) studies also were conducted to examine sampling artifacts associated with QFF medium caused by a specific filter cassette (Clark, 2002b); the effect of QFF filter type (manufacturer) for collection and analysis of OC and EC (Peterson et al, 2007); the extraction efficien...…”

Source Apportionment: Findings from the U.S. Supersites Program

“…It was intended that these studies would take advantage of the comprehensive Supersite measurements at various time resolutions 88,89 and attempt to use appropriate modeling techniques. SCEs were made for several PM size fractions and for VOCs.…”