Thermal behaviour of carbonaceous aerosol from a continental background site

In this paper thermal profiling (programmed heating in oxygen and simultaneous recording of evolved carbon dioxide) is used in combination with sample pre-treatment (heating and aqueous extraction) for the characterisation of aerosol particles from a continental background site. It is found that the thermal profile of fine aerosol (<1 μm) can be characterised with a well-defined and fairly reproducible double peak. On average, volatile organic carbon and less volatile/more refractory organic and elemental carbon (may be termed as “air polymers”) represent 27 and 73% of total carbon, respectively. As a comparison thermal profile of a humic acid standard is also recorded, producing a single peak approximately in the same position as the second peak of fine aerosol. Thermal pre-treatment of the fine mode samples (heating in air) removes a considerable fraction of the total carbon at 250°C (65%), leaving little behind at 340°C (1.8%). The correlation between the amount of carbon remained after heating at 340°C and black carbon determined by an independent optical method is found to be significant at the 0.001 probability level. The ratio of optically determined black carbon to organic carbon is consistently much lower than those reported in the literature. Aqueous extraction of the samples removes nearly half of the total carbon from the filter (48%). The two thermal profiles of the larger size fractions of the same samples (1–10 and >10 μm) look almost identical, but are markedly different from that of the fine aerosol, implying different origin for carbonaceous particles in the fine and coarse mode. The distribution of carbon is dominated by fine particles, on average 87% of the total carbon are in this size range. The results are interpreted as an implication that the bulk of organic compounds in fine mode aerosol may be air polymers having a significant coverage of polar, hydrophilic functional groups.