Faculty of Graduate Studies
Graduate Programme in Chemistry
ORAL EXAMINATION PROSPECTUS
A Candidate for the Degreee of
Master of Science.
Title of Thesis: Further development of resonant optical cavities for the measurement of atmospheric trace gases: ambient measurements of atmospheric nitrate (NO3) radical and dinitrogen pentoxide (N2O5)
December 21, 2012 @ 1:00 pm – 1:45 pm
317 Petrie Science and Engineering Bldg.
York University
Abstract: Cavity ring-down is a highly versatile spectroscopic technique that can be used to investigate the ambient concentrations of important atmospheric species of trace amounts. The design of a CRD-based detector for the measurement of highly reactive radicals such as NO3, presents a significant challenge.
We have developed a compact, single channel instrument for the sensitive measurement of ambient NO3 and N2O5 offering moderately high temporal resolution and low detection limits. Light from a tunable diode laser emitting in the visible spectrum (~662 nm) was coupled off-axis to achieve an effective absorption path of ~20 km. Light transmitted through the cavity was detected by a newly implemented photomultiplier sensor. Using this system we have performed laboratory calibrations and demonstrated sensitivities as low as αmin = 4.55 × 10-10 cm-1 in 30 s signal integration. The equivalent detection limits for NO3 were 1.4 ppt (1σ) and 2.8 ppt (2σ) in 30 s integration. In ambient settings the best measured sensitivity was 7.18 × 10-10 cm-1 and the 30 s detection limits were 1.7 ppt and 3.4 ppt for the 1σ and 2σ intervals, respectively. In addition the sample flow rates through the sampling cell were improved 4-fold which resulted in reduced residence times for the reactive radical. Specifically, it was verified that the system exhibits stable signal transmission over weeks of laboratory or ambient sampling.
The instrument was used to perform ambient measurements at York University over 5 nights during September, 2012. The data from two nights of continuous NO3 sampling are presented and discussed along with data from a single night of a N2O5 measurement.