Charring Characteristics of Atmospheric Organic Particulate Matter in Thermal Analysis

Abstract

The charring of organic materials during carbon analysis bythermal methods makes it difficult to differentiate elemental carbon (EC) from organic carbon (OC). Failure to correct for charring results in the overestimation of EC and the underestimation of OC. The charring characteristics andthermal behaviors of aerosol OC are studied by subjecting hexane and water extracts of ambient aerosols to various analysis conditions. The complete evolution of water-soluble organic carbon (WSOC) aerosol materials is found to require a temperature as high as 850 degrees C and the presence of oxygen. EC would be oxidized under these thermal conditions as well. As a result, thermal methods relying only on temperature for the differentiation of EC and OC would give unreliable OC and EC concentrations. Our investigation also reveals that WSOC accounts for a large fraction (13-66%) of charring, while hexane extractable organic compounds produce little charring. The extent of charring from WSOC, defined as the ratio between pyrolytically generated EC to the total WSOC, is found to increase with the WSOC loading in each analysis when the loadings are below a certain value. This ratio remains constant when the loadings are above this value. This may account for the high variability in the extent of charring among aerosol samples from different locations as well as among samples from a single location collected at different times. Charring is reduced if the residence time at each temperature step in a helium atmosphere is sufficiently long to allow for maximum C evolution at each step. Charring is also influenced by the presence of inorganic constituents such as ammonium bisulfate. For the few tested organic materials, it is observed that ammonium bisulfate enhances the charring of starch and cellulose but reduces the charring of levoglucosan.

Water-soluble organic carbon (WSOC) and its temperature-resolved carbon fractions in atmospheric aerosols in Beijing

“…We found that the extent of charring (defined as the ratio of WSOP/WSOC) positively correlates (r = 0.6268, p b 0.05) with WSOC loadings. This means that higher WSOC concentrations caused more charring during the carbon evolution process, which is consistent with the study of Yu et al (2002). One reason for the charring of organic carbon was suggested to be associated with catalytic oxides such as Fe and Mn oxides (Wang et al, 2010).…”

“…One reason may be the uncertainty of OC/EC split, since water soluble inorganic constitutes (such as ammonium bisulfate) which may influence OC charring behavior were also washed away by water (Yu et al, 2002). However, Yang et al (2003) showed that the method based on TOC analyzer and OC/EC carbon analyzer gave equivalent measurement results when determining water extracts of WSOC.…”

“…It was suggested that the evolution features of carbon, such as the number and position of the peaks, are greatly influenced by the chemical composition of the aerosols (Yu et al, 2002). Investigation of thermograms of single standards of water-soluble organic species found that organics evolved at lower temperatures (such as oxalic acid, adipit acid and glucose) are generally associated with low molecular weight polar organic and organics evolved at higher temperatures (such as humic acid) might be associated with high molecular weight polar compounds (Matsumoto et al, 2014;Miyazaki et al, 2007;Yu et al, 2004).…”

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

“…Birch and Cary (1996) note that with an "optimized" setting they find only 0.86% EC a in cigarette smoke, while with a "less than optimal" setting 20-30% of the carbon in cigarette smoke was designated elemental. Furthermore, thermal analysis of starch and cellulose in the presence of NH 4 HSO 4 produced 2-3 times the amount of char than was formed in the absence of the inorganic compound, while the charring of levoglucosan decreased in the presence of NH 4 HSO 4 (Yu et al, 2002). …”

“…One could argue that the brown carbon from biomass burning is not very susceptible to charring, given that it has been volatilized in a fire. This is, however, contradicted by the findings that water-soluble organic carbon (WSOC), which is a major component of pyrogenic aerosols and contains a significant amount of C brown , has been shown to be especially prone to charring (Yu et al, 2002;Yu et al, 2004). This also applies to the most abundant organic compound in pyrogenic aerosols, levoglucosan.…”