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

Elemental carbon (EC) is a collective term encompassing all thermally altered carbonaceous materials and can be subdivided into two classes: char and soot. Since the different classes of EC have different chemical and physical properties, their optical light-absorbing properties differ, so that it is essential to differentiate them in the environment. 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 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 μg m 3) in spring and minimum concentration (1.15 μg m 3) in summer. The strong correlation between EC and char-EC (R2=0.99) indicates that previously reported total EC reflected the characteristics of char only, while overlooking that of soot. However, soot exhibits stronger light-absorbing characteristics than char, and merits greater focus. 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 has a greater contribution to global warming. Although char-EC/soot-EC ratio is similar to primary OC/EC ratio as both vary with emission sources, OC/EC ratio is affected by the secondary organic aerosol (SOA) formation. 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. However, wet scavenging by snow and rain was more effective for char than for soot and influenced the char-EC/soot-EC ratio, and this factor should be considered in source identification as well.