Effect of Peak Inert-Mode Temperature on Elemental Carbon Measured Using Thermal-Optical Analysis

Abstract

Thermal-optical analysis is a conventional method for classifying carbonaceous aerosols as organic carbon (OC) and elemental carbon (EC). This article examines the effects of three different temperature protocols on the measured EC. For analyses of parallel punches from the same ambient sample, the protocol with the highest peak helium-mode temperature (870• C) gives the smallest amount of EC, while the protocol with the lowest peak helium-mode temperature (550 • C) gives the largest amount of EC. These differences are observed when either sample transmission or reflectance is used to define the OC/EC split. An important issue is the effect of the peak helium-mode temperature on the relative rate at which different types of carbon with different optical properties evolve from the filter. Analyses of solvent-extracted samples are used to demonstrate that high temperatures (870 • C) lead to premature EC evolution in the helium-mode. For samples collected in Pittsburgh, this causes the measured EC to be biased low because the attenuation coefficient of pyrolyzed carbon is consistently higher than that of EC. While this problem can be avoided by lowering the peak helium-mode temperature, analyses of wood smoke dominated am- • C) allow non-light absorbing carbon to slip into the oxidizing mode of the analysis. If this carbon evolves after the OC/EC split, it biases the EC measurements high. Given the complexity of ambient aerosols, there is unlikely to be a single peak helium-mode temperature at which both of these biases can be avoided.

Harmonizing Aerosol Carbon Measurements between Two Conventional Thermal/Optical Analysis Methods

“…14,15,17,22,27 PC generally has a higher light-absorbing efficiency than native EC, which mostly demands that, prior to the split point in the He/O 2 mode, more light-absorbing carbon (LAC, including native EC and PC) be burned to compensate for the charring-caused laser signal decreases, resulting in a lower EC in the NIOSH protocol. 15 The third factor is the laser pattern for the charring correction (reflection or transmission). Studies have revealed that PC is always distributed throughout the filter section rather than only on or close to the filter's outer surface.…”

“…First, apart from OC abundance, there are also other possible causes for the discrepancy between NIOSH and IMPROVE protocols. For example, brown carbon can badly influence the OC/EC split, 30 metals in the sample can start to remove EC already in the inert gas mode, 18 heavily loaded filter can change laser attenuation coefficient and charring rate, 15,27 and differing abundances of SOA may have inconsistent influence on laser correction. 46 More importantly, the equation was deduced from coal-burning samples (Group I) and was directly verified by only 13 source samples and 20 urban samples (Group II), and more comprehensive and systematic verifications should be carried out for ambient samples with various regional features, difficult as this may be.…”

“…As a result, there exist several NIOSH-derived Environmental Science & Technology ARTICLE programs that are usually referred to as a NIOSH protocol, for example, EPA-NIOSH (STN), 7,8 EMEP/NIOSH, 9 ACE-Asia, [10][11][12] and Birch/NIOSH. 13 Previous pillar papers have demonstrated that the NIOSH protocol always reports a lower EC value than the IMPROVE protocol, 12,[14][15][16][17][18][19] but there remains a major problem that the EC difference between NIOSH and IMPROVE measurements is not fixed, but is variable at even more than an order of magnitude from sample to sample, depending on the physical, morphological, and chemical properties of the samples originating from various fuels, sites, or processes. 12,18 This leads to an ambiguous understanding of ambient measurements and inventory estimates of aerosol carbon, and it calls for a novel way to determine the relation between the two TOA results.…”

Modal Characteristics of Elemental and Organic Carbon in an Urban Location in Guangzhou, China

“…• C) by minerals in the aerosol particles or collection substrates (Chow et al 2001(Chow et al , 2004Subramanian et al 2006). We propose here to use positive matrix factorization (PMF) to apportion the evolved C peaks (OC 1 -OC 4 and EC 1 -EC 5 ) to OC and EC.…”

Estimating ambient particulate organic carbon concentrations and partitioning using thermal optical measurements and the volatility basis set

“…It has also been shown that PC defined by different temperature protocols can differ significantly (Subramanian et al 2006). Therefore, another issue associated with PC involves the choice of the charring correction method-transmittance or reflectance.…”