Equivalence of Elemental Carbon by Thermal/Optical Reflectance and Transmittance with Different Temperature Protocols

Abstract

Charring of organic carbon (OC) during thermal/optical analysis is monitored by the change in a laser signal either reflected from or transmitted through a filter punch. Elemental carbon (EC) in suspended particulate matter collected on quartz-fiber filters is defined as the carbon that evolves after the detected optical signal attains the value it had prior to commencement of heating, with the rest of the carbon classified as organic carbon (OC). Heretofore, operational definitions of EC were believed to be caused by different temperature protocols rather than by the method of monitoring charring. This work demonstrates that thermal/ optical reflectance (TOR) corrections yield equivalent OC/ EC splits for widely divergent temperature protocols. EC results determined by simultaneous thermal/optical transmittance (TOT) corrections are 30% lower than TOR for the same temperature protocol and 70-80% lower than TOR for a protocol with higher heating temperatures and shorter residence times. This is true for 58 urban samples from Fresno, CA, as well as for 30 samples from the nonurban IMPROVE network that are individually dominated by wildfire, vehicle exhaust, secondary organic aerosol, and calcium carbonate contributions. Visual examination of filter darkening at different temperature stages shows that substantial charring takes place within the filter, possibly due to adsorbed organic gases or diffusion of vaporized particles. The filter transmittance is more influenced by the within-filter char, whereas the filter reflectance is dominated by charring of the near-surface deposit that appears to evolve first when oxygen is added to helium in the analysis atmosphere for these samples. The amounts of charred OC (POC) and EC are also estimated from incremental absorbance. Small amounts of POC are found to dominate the incremental absorbance. EC estimated from absorbance are found to agree better with EC from the reflectance charring correction than with EC from the transmittance charring correction.

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

“…The different temperature protocols are shown in Table 4. The different optical approaches occur during the He temperature steps (Chow et al, 1993;Chow et al, 2001;Chow et al, 2004;Chow et al, 2005a;Chow et al, 2007;Peterson and Richards, 2002;Watson et al, 2005;RTI, 2009e). Beginning in 2008 and finishing in 2009, CSN transitioned to the IMPROVE_A analysis protocol to obtain consistency between the networks.…”

“…Beginning with samples collected in January 2005, IMPROVE provides both TOR and TOT results (DRI, 2005;Chow et al, 2007). Several studies have been conducted comparing the IMPROVE, CSN, and in some cases other protocols for OC and EC Schmid et al, 2001;Conny et al, 2003;Chow et al, 2004;Watson et al, 2005;Cheng et al, 2011).…”

“…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%.…”

“…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...…”

Application of 14C analyses to source apportionment of carbonaceous PM2.5 in the UK

“…This is the only PM material for which there are 31 The assignment of the split between OC and EC for particulate matter carbon remains a 5 methodologically-defined quantity (Chow et al, 2004a). The extent to which this is so is 6 illustrated by the greater than 6-fold range in EC/TC mass ratios (0.075 to 0.52) from 19 7 different methods or research groups which have been collated for SRM-1649a (Currie et al, 8…”