Applications of Raman Microspectroscopy to the Study of Atmospheric Aerosol Particles

[摘要] Atmospheric aerosol particles impact climate by scattering or absorbing solar radiation and acting as cloud condensation and ice nuclei, but there is high uncertainty regarding the magnitude of these climate effects. The physicochemical properties of aerosol particles dictate their climate impacts, yet are challenging to accurately and quantitatively measure, as aerosols are highly complex in terms of chemical composition, size, phase, and morphology (i.e. mixing state). Methods enabling more detailed and quantitative investigations of aerosol particle properties are necessary in order to improve mechanistic understanding of multiphase aerosol processes occurring in the atmosphere. In this dissertation, novel Raman microspectroscopic techniques for improved analysis of the chemical composition of both laboratory-generated and ambient aerosol particles were developed to further understanding of aerosol mixing state, which will enable better predictions of the climate impacts associated with aerosols. Computer-controlled Raman mapping (CC-Raman) and surface enhanced Raman microspectroscoy (SERS) were applied to the study of aerosol particles to improve characterization of chemical composition. CC-Raman used automated mapping to increase throughput and particle statistics by analyzing up to 100 particles per hour, in comparison to much slower manual characterization. CC-Raman analysis provides detailed information regarding functional groups present, size, morphology, and the mixing of secondary chemical species with primary components.SERS enables detection of trace organic and/or inorganic species present within particles, observation of complex inter- and intraparticle variability, and characterization of smaller, more atmospherically-relevant sized aerosol particles with diameters smaller than the diffraction limit (as small as 150 nm). In comparison to traditional vibrational spectroscopy techniques, these advances greatly increase the potential for characterization of aerosol particle physicochemical properties with Raman analysis.A Raman microspectroscopic method for measuring single particle pH was developed. Aerosol pH plays an important role in many multiphase processes, such as secondary organic aerosol (SOA) formation, but is difficult to measure due to the minute volumes of aerosol particles and the non-conservative nature of the hydronium ion. Traditional indirect measurement methods and predictions of aerosol pH via thermodynamic modeling often disagree and are associated with limitations regarding measurement inputs and equilibrium assumptions. In contrast, this method coupling Raman microspectroscopy with extended Debye-Hückel activity calculations allows for direct determination of acidity of individual particles based on measurements of acid and conjugate base vibrational modes. Several atmospherically relevant inorganic and organic acid-base equilibria systems are compatible with this method, including HNO3/NO3-, HSO4-/SO42-, HC2O4-/C2O42-/ CH3COOH/CH3COO-/ and HCO3-/CO32-, covering a broad pH range (-1 to 10). A second complementary method for direct measurement of size-resolved bulk aerosol pH using quantitative colorimetric image processing of aerosol particles collected on pH indicator paper was also developed. In addition to aerosol pH measurements, these methods were used to investigate the effect of RH on particle acidity, gas-particle partitioning of acidic chemical species, the relationship between ionic strength and H+ activity coefficients, and other aspects of ion behavior under non-ideal conditions. Direct measurement of aerosol pH through these methods will improve fundamental mechanistic understanding of critical pH-dependent aerosol processes.The methods developed in this dissertation and their application to the study of atmospheric aerosol particles will yield more detailed measurements of particle physicochemical properties, providing new insights into the mechanisms of multiphase atmospheric processes and improving understanding of the impact of aerosols on climate.

[发布机构] University of Michigan

[效力级别] Atmospheric Aerosol Particles

[关键词] Raman Microspectroscopy;Atmospheric Aerosol Particles;Chemistry;Science;Chemistry