Different characteristics of char and soot in the atmosphere and their ratio as an indicator for source identification in Xi'an, China

Abstract

Abstract. Numerous definitions and analytical techniques for elemental (or black) carbon (EC) have been published in the scientific literature, but still no generally accepted interdisciplinary definition exists. EC is not a single chemical compound, but is mainly composed of two parts of carbon contents: combustion residues from pyrolysis and combustion emissions formed via gas-to-particle conversion. Accordingly EC is subdivided into two classes: char and soot. Char is defined as carbonaceous materials obtained by heating organic substances and formed directly from pyrolysis, or as an impure form of graphitic carbon obtained as a residue when carbonaceous material is partially burned or heated with limited access of air. Soot is defined as only those carbon particles that form at high temperature via gasphase processes. Since the different classes of EC have different chemical and physical properties, their optical lightabsorbing properties differ, so that it is essential to differentiate them in the environment. The thermal optical reflectance (TOR) method was used to differentiate between char-EC and soot-EC according to its stepwise thermal evolutional oxidation of different carbon fractions under different temperatures and atmosphere. Char-EC and soot-EC are operationally defined as EC1-OP and EC2+EC3 (EC1, EC2 and EC3 corresponding to carbon fractions evolved at 550, 700 and 800 • C in a 98% He/2% O 2 atmosphere, respectively), respectively. One year of observations of the daily and seasonal variations of carbonaceous particles were conducted in Xi'an, China in 2004 to demonstrate the different characteristics of char and soot in the atmosphere. Total carbon (TC), organic carbon (OC), EC and char-EC showed Correspondence to: Y. M. Han (yongming@ieecas.cn) similar seasonal trends, with high concentrations in winter and low concentrations in summer, while soot-EC revealed relatively small seasonal variations, with maximum concentration (1.85±0.72 µg m −3 ) in spring and minimum concentration (1.15±0.47 µg m −3 ) in summer. The strong correlation between EC and char-EC (R 2 = 0.99) and poor correlation between EC and soot-EC (R 2 = 0.31) indicate that previously reported total EC in the literature reflected the distribution characteristics of char only, while overlooking that of soot. However, soot exhibits stronger light-absorbing characteristics than char, and merits greater focus in climate research. The small seasonal variation of soot-EC indicates that soot may be the background fraction in total EC, and is likely to have an even longer lifetime in the atmosphere than previously estimated for total EC, which suggests that soot may has a greater contribution to global warming. While both char-EC/soot-EC and primary OC/EC ratios vary with emission sources, only OC/EC ratio is affected by SOA. Thus char-EC/soot-EC may be a more effective indicator than OC/EC in source identification of carbonaceous aerosol. Comparison of seasonal variations of OC/EC and char-EC/soot-EC ratios in Xi'an confirms this point. ...

Ionic and carbonaceous compositions of PM<sub>10</sub>, PM<sub>2.5</sub> and PM<sub>1.0</sub> at Gosan ABC Superstation and their ratios as source signature

“…EC was well correlated with sulfate, resulting in good relationships of sulfate with both aerosol scattering coefficient measured by Nephelometer and BC concentration. Our measurements of EC confirmed the definition of EC1 as char-EC emitted from smoldering combustion and EC2+3 as soot-EC generated from higher-temperature combustion such as motor vehicle exhaust and coal combustion (Han et al, 2010). In particular, EC1 was strongly correlated with potassium, a traditional biomass burning indicator, except during the summer, when the ratio of EC1 to EC2+3 was the lowest.…”

“…Especially for EC, there are various definitions and analytical methods to quantify its atmospheric concentration Han et al, 2010). EC is usually referred to a near-elemental soot-carbon-like composition and to the fraction of carbon that is oxidized in combustion analysis above a certain temperature threshold.…”

PM<sub>1</sub> geochemical and mineralogical characterization using SEM-EDX to identify particle origin – Agri Valley pilot area (Basilicata, southern Italy)

“…Soot results from incomplete hydrocarbons combustion processes and forms at high temperatures by gasto-particle conversion (e.g. Wolff, 1981;Han et al, 2010). .…”

Absorption and scattering properties of organic carbon versus sulfate dominant aerosols at Gosan climate observatory in Northeast Asia

“…Their study pointed out biofuel from residential sectors as a predominant source of OC, especially in winter. In our study, for the 370 and 660 nm groups, the concentrations of PM 1.0 EC1, referred to as char EC (Han et al, 2010;Lim et al, 2012) were well correlated with the sum of OC3, OC4, and ) is a potential emission sensitivity distribution of 40 000 particles released in a particular grid cell at the measurement location during the measurement interval and followed backward in time. This is considered proportional to the particle residence time in that cell.…”

“…The char EC is known to be produced from incomplete combustion at lower temperatures; e.g., biofuel combustion (Han et al, 2010). This implies that the major source of PM 1.0 OC for the 370 and 660 nm groups were biofuel combustion.…”