Investigating reactive tropospheric nitrogen oxides by thermal-dissociation cavity ring-down spectroscopy
Date
2021-01-18
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Abstract
The nitrogen oxides are important trace constituents of the troposphere that originate mainly from anthropogenic sources. Accurate quantification techniques are needed to characterize their abundance and to assess their impact on air quality. This thesis describes a compact and portable source that delivers nitrous acid (HONO) in high purity (>95%). HONO is produced dynamically by reacting a gas stream containing < 1 part per million (by volume) HCl gas at relative humidity of 30% - 40% with solid sodium nitrite. The production of HONO and absence of impurities such as nitric oxide (NO), nitrogen dioxide (NO2) and nitrosyl chloride (ClNO) were verified by Fourier transform infrared (FTIR) and thermal dissociation cavity ring-down spectroscopy (TD CRDS). The interference from HONO in the measurement of NO2 by photolytic conversion with chemiluminescence detection (P-CL) was investigated in two prototype converters. The first used radiation centred around 395 nm, common in P-CL. The second utilized 415 nm light, where the overlap with the HONO absorption spectrum and expected HONO interference are lower. Mixing ratios of NO2, NOx and HONO entering and exiting the converters were quantified by TD-CRDS. Both converters exhibited high NO2 conversion efficiency (CFNO2 >90%). Plots of CF against flow rate delivered photolysis frequencies of 4.2 s-1 and 2.9 s-1 for NO2 and 0.25 s-1 and 0.10 s-1 for HONO at 395 nm and 415 nm, respectively. The CFHONO was larger than predicted implying that measurements of NO2 by P-CL overestimate NO2 concentrations. Mixing ratios of nitrogen oxides were quantified during the SNOWDOGS campaign in Fort MacKay, AB, in January 2020. The group's TD-CRDS was modified to simultaneously quantify mixing ratios of NO2, HNO3, gas phase NOy, and total NOy (including particulate nitrate). High nitrogen oxide concentrations at night and faster than expected daytime conversion of NO2 to nitric acid (HNO3) and particulate nitrate were observed, indicating that the nitrogen oxides are processed primarily by photochemical reactions in polluted regions at high latitude during winter.
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Chemistry, nitrogen oxides, nitrous acid, cavity ring-down spectroscopy, photolytic conversion, Athabasca oil sands
Citation
Gingerysty, N. J. L. (2021). Investigating reactive tropospheric nitrogen oxides by thermal-dissociation cavity ring-down spectroscopy (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.