Methods to Assess Carbonaceous Aerosol Sampling Artifacts for IMPROVE and Other Long-Term Networks

Abstract

Volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs) adsorb to quartz fiber filters during fine and coarse particulate matter (PM 2.5 and PM 10 , respectively) sampling for thermal/optical carbon analysis that measures organic carbon (OC) and elemental carbon (EC). Particulate SVOCs can evaporate after collection, with a small portion adsorbed within the filter. Adsorbed organic gases are measured as particulate OC, so passive field blanks, backup filters, prefilter organic denuders, and regression methods have been applied to compensate for positive OC artifacts in several long-term chemical speciation networks. Average backup filter OC levels from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network were approximately 19% higher than field blank values. This difference is within the standard deviation of the average and likely results from low SVOC concentrations in the rural to remote environments of most IMPROVE sites. Backup filters from an urban (Fort Meade, MD) site showed twice the OC levels of field blanks. Sectioning backup filters from top to bottom showed nonuniform OC densities within the filter, contrary to the assumption that VOCs and SVOCs on a backup filter equal those on the front filter. This nonuniformity may be partially explained by evaporation and readsorption of vapors in different parts of the front and backup quartz fiber filter owing to temperature, relative humidity, and ambient concentration changes throughout a 24-hr sample duration. OC-PM 2.5 regression analysis and organic denuder approaches demonstrate negative sampling artifact from both Teflon membrane and quartz fiber filters.

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

“…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...…”

“…When sampling atmospheric particles, artifacts are typically due to collection of gases on a sampling substrate or volatilization of sample already collected. Determining the impact of an artifact on the measurement may be complicated (e.g., Chow, 1995;Hering and Cass, 1999;Solomon et al, 2000;Turpin et al, 2000;Kim et al, 2001;Pang et al, 2002;Ashbaugh et al, 2004aAshbaugh et al, , 2004bSubramanian et al, 2004;Chow et al, 2005aChow et al, , 2005bKim et al, 2005;White et al, 2005;Yu et al, 2006;Watson et al, 2009;Chow et al, 2010;Maimone et al, 2011). Sampling artifacts are primarily associated with semivolatile components in organic carbon and ammonium nitrate.…”

“…Examples of these include use of a single QFF backup filter behind the primary QFF, such as IMPROVE uses, or use of a QFF behind a Teflon filter; use of a denuder (e.g., activated-carbon-impregnated strips or XAD-coated glass annular denuder) in front of a reactive backup filter (e.g., carbonor XAD-impregnated glass-fiber filter) behind the primary QFF, and application of a linear regression method where OC (y-axis) is plotted versus PM 2.5 mass as obtained on a Teflon filter (Peters et al, 2000;Turpin et al, 2000;Solomon et al, 2000;Lewtas et al, 2001;Tolocka et al, 2001;Subramanian et al, 2004;Kim et al, 2005;Watson et al, 2009;Chow et al, 2010;Maimone et al, 2011). Other statistical methods also have been investigated for CSN (Kim et al, 2001).…”

“…The new approach as described in the following employed a QFF filter behind the front QFF filter with the assumption that the same artifact would be observed on both filters (Subramanian et al, 2004;Watson et al, 2009;Maimone et al, 2011). CSN results in AQS are reported without the dynamic OC blank correction, although the dynamic blank results are posted in AQS and can be used to make the correction.…”