An investigation of the factors influencing the detection sensitivity of cavity enhanced Raman scattering for probing aqueous binary aerosol droplets

Abstract

Stimulated Raman scattering (SRS) from single aerosol droplets can be observed at extremely low laser threshold intensities at wavelengths commensurate with whispering gallery modes. Although droplet size can routinely be determined from the ensuing cavity enhanced Raman scattering (CERS) fingerprint, determining droplet composition is a considerably more challenging measurement. We present here an examination of the factors that influence and limit the detection sensitivity of CERS in quantifying the concentrations of sulfate and nitrate in water droplets, 20-50 microm in radius. In particular, we consider the variation in nitrate and sulfate SRS signal with variation in species concentration, probe laser intensity and droplet size. We illustrate that the band contour of the OH stretching band can be used as a relative measure of the internal light intensity circulating within the droplet and experimentally investigate how the threshold condition for SRS is achieved.

Determining the composition of aqueous microdroplets with broad-band cavity enhanced Raman scattering

“…This is in marked contrast to the measurement of a strong dependence on droplet size for the nitrate-water system, with the sensitivity to nitrate increasing as the droplet size is decreased. 19 We showed in this previous publication that this enhancement in sensitivity to nitrate is consistent with the increased internal intensity that arises with decrease in droplet size. As a further demonstration of the insensitivity of the alcohol-water calibration ratio to the droplet size, and consequently internal intensity, we have constructed a broad-band calibration curve for droplets of a second size range (33-35 mm), shown in Fig.…”

“…Of these parameters, we have already examined the effect of the pulse energy on quantifying the alcohol component in an alcohol-water aerosol and the inorganic ion in nitrate-water and sulfatewater aerosols. 18,19 The effect of pulse duration has not been fully elucidated, although studies have indicated a change in the mechanism of the build-up of the spontaneous scatter. 20,21 We have also compared the effect of using two different pump wavelengths (532 nm and 355 nm) of comparable linewidth, pulse energy and temporal duration on the threshold sensitivity.…”

Spectroscopic characterization of aqueous microdroplets containing inorganic salts

“…) are relatively narrow compared with the bandwidth of the OH vibration, and we do not always observe MDRs at these Raman wavelengths, dependent on the size of the droplet. 14 We use the computer program called SCATA (Size and Composition Analysis of Tweezed Aerosol) 15 to analyse Raman spectra of aqueous microdroplets. The software identifies the wavelengths of MDRs from recorded-data files and then estimates the size of the microdroplet.…”

Heterogeneous oxidation of nitrite anion by gas-phase ozone in an aqueous droplet levitated by laser tweezers (optical trap): is there any evidence for enhanced surface reaction?

“…The nitrite peak occurred at B1049 rel cm À1 , which concurs with literature, 21 and the nitrate peak occurred at B1333 rel cm À1 , which concurs with literature. [22][23][24][25][26]…”