Raman Microspectroscopy

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO42–) and νs(HSO4–), the acid dissociation constant, and activity coefficients from extended Debye–Hückel calculations. Shifts in pH and peak positions of νs(SO42–) and νs(HSO4–) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.

Figure caption: Raman spectra of νs(SO4 2−) and νs(HSO4−) for initial aerosol particles generated from each seed aerosol bulk solution (a) and average aerosol pH as a function of [SO4 2−]/[HSO4 −] (b). Error bars are based on the standard deviation for multiple trials.  Corresponding spectra (c and d) for ν(SO4 2−) at ~985 cm−1 and ν(HSO4 −) at ~1050 cm−1.

Rindelaub, J. D.†; Craig, R. L.†,G; Nandy, L.; Bondy, A. L.G; Dutcher, C. S.; Shepson, P. B. Ault, A. P.* Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity. Journal of Physical Chemistry - A, 2016, 120(6), 911-917 

Craig, R. L.G; Nandy, L.; Axson, J. L.P; Dutcher, C. S.; Ault, A. P.*; Spectroscopic Determination of Aerosol pH from Acid-Base Equilibria in Inorganic, Organic,and Mixed Systems, Journal of Physical Chemistry – A, (Veronica Vaida Festschrift), 2017, 121(30), 5690-5699. 

The Ault Lab Raman Microspectrometer can also perform fluorescence spectroscopy and has been used to determine the amount of algae in freshwater samples collected from the Great Lakes. 

Figure caption: Optical images (left) and fluorescence maps with fluorescence intensity scales in counts, (right), of particles generated from the Lake Michigan – Michigan City (top), Lake Erie – Catawba (middle), and Lake Erie – Maumee (bottom) freshwater samples collected in 2014, as a function of freshwater blue green algae concentrations measured at the time of collection. All scale bars are 5 µm.

May, N. W.; Olson, N. E.G; Panas, M.; Axson, J. L.P; Tirella, P. S.U; Kirpes, R. M.; Craig, R. L.G; Gunsch, M. J.; China, S.; Laskin, A.; Ault, A. P.*; Pratt, K. A.*Aerosol Emissions from Great Lakes Harmful Algal Blooms, Environmental Science & Technology, 2018, 52 (2), 397-405. 

DOI: 10.1021/acs.est.7b03609