Abstract
Abstract. Organic aerosols (OA) can be separated with factor analysis of aerosol mass spectrometer (AMS) data into hydrocarbon-like OA (HOA) and oxygenated OA (OOA). We develop a new method to parameterize H:C of OOA in terms of f43 (ratio of m/z 43, mostly C2H3O+, to total signal in the component mass spectrum). Such parameterization allows for the transformation of large database of ambient OOA components from the f44 (mostly CO2+, likely from acid groups) vs. f43 space ("triangle plot") (Ng et al., 2010) into the Van Krevelen diagram (H:C vs. O:C) (Van Krevelen, 1950). Heald et al. (2010) examined the evolution of total OA in the Van Krevelen diagram. In this work total OA is deconvolved into components that correspond to primary (HOA and others) and secondary (OOA) organic aerosols. By deconvolving total OA into different components, we remove physical mixing effects between secondary and primary aerosols which allows for examination of the evolution of OOA components alone in the Van Krevelen space. This provides a unique means of following ambient secondary OA evolution that is analogous to and can be compared with trends observed in chamber studies of secondary organic aerosol formation. The triangle plot in Ng et al. (2010) indicates that f44 of OOA components increases with photochemical age, suggesting the importance of acid formation in OOA evolution. Once they are transformed with the new parameterization, the triangle plot of the OOA components from all sites occupy an area in Van Krevelen space which follows a ΔH:C/ΔO:C slope of ~ −0.5. This slope suggests that ambient OOA aging results in net changes in chemical composition that are equivalent to the addition of both acid and alcohol/peroxide functional groups without fragmentation (i.e. C-C bond breakage), and/or the addition of acid groups with fragmentation. These results provide a framework for linking the bulk aerosol chemical composition evolution to molecular-level studies.
Highlights
The study of organic aerosols (OA) in the atmosphere is challenging due to the large number of molecular species involved and the continuous evolution of OA concentration, composition, and properties (Jimenez et al, 2009)
SV-oxygenated OA (OOA) components have higher H:C and lower oxidation states than low volatility OOA (LV-OOA) components for almost all sites, with the separation being clearer for the oxidation state
The variation inf43 of the semi-volatile OOA (SV-OOA) components in the triangle plot is still preserved in the VK-triangle diagram, with SV-OOA components spanning a range of H:C ratios
Summary
The study of organic aerosols (OA) in the atmosphere is challenging due to the large number of molecular species involved and the continuous evolution of OA concentration, composition, and properties (Jimenez et al, 2009). Total OA composition is driven by several effects, including physical mixing, condensation/evaporation effects, and photochemical transformation In this manuscript we deconvolve total OA into distinct primary (HOA and others) and secondary (total OOA, SV-OOA, LV-OOA) components and explicitly examine ambient OOA evolution alone in a manner that is analogous to and can be compared with chamber SOA experiments. Each component provides a “snapshot” of the OOA as it is formed and aged in the atmosphere, but together the components from multiple worldwide sites define a continuum of OOA properties (Ng et al, 2010; Morgan et al, 2010) We use this integrated analysis of OOA components to examine the trends in Van Krevelen space and the implied changes in chemical composition of ambient OOA evolution. We have not derived H:C values for these primary components from f43
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