Quantitative capabilities of STXM to measure spatially resolved organic volume fractions of mixed organic ∕ inorganic particles
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.