Distinguishing Gas-Phase and Nanoparticle Contributions to Small-Angle X-ray Scattering in Reacting Aerosol Flows

Abstract

We have developed a strategy for distinguishing between small-angle X-ray scattering (SAXS) from gas-phase species and newly formed nanoparticles in mixed gas- and particle-phase reacting flows. This methodology explicitly accounts for temperature-dependent scattering from gases. We measured SAXS in situ in a sooting linear laminar partially premixed co-flow ethylene/air diffusion flame. The scattering signal demonstrates a downward curvature as a function of the momentum transfer (q) at q values of 0.2–0.57 Å–1. The q-dependent curvature is consistent with the Debye equation and the independent-atom model for gas-phase scattering. This behavior can also be modeled using the Guinier approximation and could be characterized as a Guinier knee for gas-phase scattering. The Guinier functional form can be fit to the scattering signal in this q range without a priori knowledge of the gas-phase composition, enabling estimation of the gas-phase contribution to the scattering signal while accounting for changes in the gas-phase composition and temperature. We coupled the SAXS measurements with in situ temperature measurements using coherent anti-Stokes Raman spectroscopy. This approach to characterizing the gas-phase SAXS signal provides a physical basis for distinguishing among the contributions to the scattering signal from the instrument function, flame gases, and nanoparticles. The results are particularly important for the analysis of the SAXS signal in the q range associated with particles in the size range of 1–6 nm.

Soot-particle core-shell and fractal structures from small-angle X-ray scattering measurements in a flame

“…At all HABs the value of C 0 is negligible as a result of the background-subtraction process applied to the raw data [56]. The primary-particle mode of the bimodal unified model is well constrained by the data.…”

“…3.1 Background Subtraction. The processing of the raw data and background subtraction approach are described in detail by Michelsen et al [56]. The scattering signal recorded on the detector was azimuthally averaged using the Nika software (Release 1.74) [63], which yielded 1D profiles of scattering signal as a function of q.…”

“…We isolated and subtracted these contributions from the normalized signal. The SAXS data presented in this paper were processed used the scattering signal recorded at an HAB of 2 mm for the differenced signal, i.e., HAB0 = 2 mm (see [56] for details). These data are supplied as tabulated text and Excel files in the supplementary material.…”

“…A paper by Hexemer et al [55] provides details of the beamline setup. A detailed description of the experimental flame setup is provided by Michelsen et al [56]. Only a brief description is provided here.…”

“…Because the flame length changes with height above the burner, however, this smearing effect is not accounted for in the data modeling. The change in flame length with flame height is discussed in Michelsen et al [56].…”