Infrared extinction spectroscopy and micro‐Raman spectroscopy of select components of mineral dust mixed with organic compounds

Abstract

[1] Radiative transfer calculations as well as satellite and ground-based retrieval algorithms often use Mie theory to account for atmospheric mineral dust. However, the approximations used in Mie theory are often not appropriate for mineral dust and can lead to inaccuracies in modeling optical properties. Analytic models that are based on Rayleigh theory and account for particle shapes can offer significant advantages when used to model the IR extinction of mineral dust in the accumulation size mode. Here we extend our investigations of the IR optical properties of mineral dust to include samples that have been processed with organic acids. In particular, we aerosolize several individual components of mineral dust with organic compounds that are common in the atmosphere. Through online and offline analysis of the resulting aerosol particles combining Fourier transform infrared (FTIR) extinction spectroscopy, micro-Raman spectroscopy, and scanning electron microscopy, we have identified three distinct outcomes of the interactions, which depend on the nature of the mineral and the organic acid: reactions with segregation of the products within the particle, formation of a uniform coating on the particle, or a formation of external mixture when there is no significant chemical interaction. Analysis of FTIR extinction spectra of the different dust components that have undergone processing shows red shifts of the prominent IR resonance peaks. The extent of the red shift, which varies from 2 to 10 cm À1 , depends on the mineral and the nature of the interaction. Spectral simulations showed that the deviation from Mie theory becomes even more pronounced for these processed mineral dust aerosol components.

Infrared Optical Constants of Organic Aerosols: Organic Acids and Model Humic-Like Substances (HULIS)

“…In order to evaluate the optical properties of multicomponent aerosols, a variety of mixing methods can be used to produce accurate multicomponent optical constants, but these methods require high resolution optical constants of the components. The presence of organic coatings on mineral dust can alter position of IR resonance peaks (Laskina et al 2013). The shifts of the resonance peaks can be predicted if accurate optical constants for the coating material are available.…”

Characterization of individual ice residual particles by the single droplet freezing method: a case study in the Asian dust outflow region

“…The peaks were assigned to specific compounds and minerals by comparison with reported values from the literature (Tang and Fung, 1989;Ivleva et al, 2007;Freeman et al, 2008;Baustian et al, 2012;Laskina et al, 2013) and Raman databases. The peak positions were also verified by measuring standard pure chemical compounds using the current Raman system.…”

“…The Raman-shift peak positions used for the identification of compounds and minerals are summarized in Table 1. The identification of organic components was defined by detection of the C-H vibrational mode that appears as peaks or a broad peak in the range 2800 to 3100 cm −1 Ault et al, 2013;Laskina et al, 2013). The strong broad peak obtained from atmospheric particles in the range 1200 to 1700 cm −1 , such as that shown in Fig.…”

Hygroscopic behavior and chemical composition evolution of internally mixed aerosols composed of oxalic acid and ammonium sulfate

“…Raman spectroscopy is a powerful technique to characterize aerosol compositions, water contents, molecular interactions and particle phases especially for the efflorescence process (Ma and He, 2012;Laskina et al, 2013;Zhou et al, 2014;Wang et al, 2015). In this study, the phase transformations and hygroscopic properties of OA and mixed OA / AS droplets with varying OA content were studied by confocal Raman spectroscopy in conjunction with optical microscopy.…”