Abstract
Abstract. Scanning transmission X-ray microscopy coupled with near-edge X-ray absorption and fine structure (STXM-NEXAFS) spectroscopy can be used to characterize the morphology and composition of aerosol particles. Here, two inorganic ∕ organic systems are used to validate the calculation of organic volume fraction (OVF) and determine the level of associated error by using carbon K-edge STXM data at 278, 285.4, 288.6, and 320 eV. Using the mixture of sodium chloride and sucrose as one system and ammonium sulfate and sucrose as another, three solutions were made with 10:1, 1:1, and 1:10 mass ratios (inorganic to organic). The OVFs of the organic-rich aerosols of both systems deviated from the bulk OVF by less than 1%, while the inorganic-rich aerosols deviated by approximately 1 %. Aerosols from the equal mass mixture deviated more (about 4 %) due to thick inorganic regions exceeding the linear range of Beer's law. These calculations were performed after checking the data for poor image alignment, defocusing issues, and particles too thick to be analyzed. The potential for systematic error in the OVF calculation was also tested by assuming the incorrect composition. There is a small (about 0.5 %) OVF difference if the organic is erroneously assumed to be adipic acid rather than the known organic, sucrose. A much larger difference (up to 25 %) is seen if sodium chloride is assumed instead of ammonium sulfate. These results show that the OVF calculations are fairly insensitive to the organic while being much more sensitive to the choice of inorganic.
Highlights
Atmospheric aerosols are airborne mixtures of solid and liquid phase components such as soot, inorganic salts, trace metals, and organics (Seinfeld and Pandis, 2006)
Particle mixing state and morphology can potentially impact their effectiveness as either ice nuclei or cloud condensation nuclei (CCN) (Baustian et al, 2012, 2013; Pöschl et al, 2010; DeMott et al, 2003)
Because each of the solutes are quite soluble in water (Haynes, 2014) and because the nebulization process ensures a well-mixed solution, the resulting droplets are expected to be similar in composition to the homogenous bulk solution and little particle-to-particle variability is anticipated
Summary
Atmospheric aerosols are airborne mixtures of solid and liquid phase components such as soot, inorganic salts, trace metals, and organics (Seinfeld and Pandis, 2006). These aerosols have been shown to cause detrimental health effects upon inhalation and can negatively impact visibility, especially around large cities (Villeneuve et al, 2002). Aerosols currently represent the largest source of uncertainty in radiative forcing from anthropogenic sources according to the 2013 Intergovernmental Panel on Climate Change (IPCC) report (Stocker et al, 2013). Two of the main ways in which aerosols can affect radiative forcing are through aerosol–radiation interactions ( known as the “direct effect”) and aerosol–cloud interactions ( known as the “indirect effect”). Because the complex and varied compositions of aerosols are linked to their impacts on health and the environment, quantitative characterizations of detailed aerosol chemical and physical properties are necessary
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