Abstract
Abstract. Sources of particulate organic carbon (OC) with different volatility have rarely been investigated, despite the significant importance for better understanding of the atmospheric processes of organic aerosols. In this study we develop a radiocarbon-based (14C) approach for source apportionment of more volatile OC (mvOC) and apply to ambient aerosol samples collected in winter in six Chinese megacities. mvOC is isolated by desorbing organic carbon from the filter samples in helium (He) at 200 ∘C in a custom-made aerosol combustion system for 14C analysis. Evaluation of this new isolation method shows that the isolated mvOC amount agrees very well with the OC1 fraction (also desorbed at 200 ∘C in He) measured by a thermal–optical analyzer using the EUSAAR_2 protocol. The mvOC, OC and elemental carbon (EC) of 13 combined PM2.5 samples in six Chinese cities are analyzed for 14C to investigate their sources and formation mechanisms. The relative contribution of fossil sources to mvOC is 59±11 %, consistently larger than the contribution to OC (48±16 %) and smaller than that to EC (73±9 %), despite large differences in fossil contributions in different cities. The average difference in the fossil fractions between mvOC and OC is 13 % (range of 7 %–25 %), similar to that between mvOC and EC (13 %, with a range 4 %–25 %). Secondary OC (SOC) concentrations and sources are modeled based on the 14C-apportioned OC and EC and compared with concentrations and sources of mvOC. SOC concentrations (15.4±9.0 µg m−3) are consistently higher than those of mvOC (3.3±2.2 µg m−3), indicating that only a fraction of SOC is accounted for by the more volatile carbon fraction desorbed at 200 ∘C. The fossil fraction in SOC is 43 % (10 %–70 %), lower than that in mvOC (59 %, with a range of 45 %–78 %). Correlation between mvOC and SOC from nonfossil sources (mvOCnf vs. SOCnf) and from fossil sources (mvOCfossil vs. SOCfossil) is examined to further explore sources and formation processes of mvOC and SOC.
Highlights
Aerosol particles are of importance for atmospheric chemistry and physics and exert crucial influence on the climate system, air quality and human health (Fuzzi et al, 2015)
The reproducibility of the extracted more volatile OC (mvOC) amount was tested for two independent test filters with mvOC loadings of 6 and 18 μg cm−2, respectively (Fig. S5)
When filter pieces are large and placed outside the centered 3.5 cm, the desorption temperature for part of the filter pieces will lower than 200 ◦C, leading to lower desorbed mvOC amount
Summary
Aerosol particles are of importance for atmospheric chemistry and physics and exert crucial influence on the climate system, air quality and human health (Fuzzi et al, 2015). Carbonaceous aerosols are often operationally subdivided into organic carbon (OC) and elemental carbon (EC). OC consists of a large variety of organic species that cover a great range of volatilities and are not or only weakly light absorbing, while EC is nonvolatile, resistant to chemical transformation and strongly light absorbing (Pöschl, 2005). It should be noted that since no clear distinction between OC and EC exists, OC and EC are operationally defined based on measurement techniques (Petzold et al, 2013). When a thermal–optical method is used to separate and determine OC and EC, EC is described as a thermally refractory carbon continuum and OC is weakly refractory (Pöschl, 2003, 2005; Petzold et al, 2013)
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