Department of Chemistry

Departmental Speaker

Professor Hans Osthoff, Department of Chemistry, University of Calgary.

will present a seminar titled:

“New insights into the atmospheric chemistry of nitrogen oxide reservoir species”

Abstract:

The oxides of nitrogen (NO_x = NO + NO₂) are important trace gas constituents of the troposphere. During daytime, NO_x catalyzes the photochemical production of ozone (O₃), which is of interest to regional air quality and human health. The lifetime of NO_x is on the order of days; its main sink is conversion to nitric acid (HNO₃), which deposits. NO_x can be stored in so-called reservoir species, whose lifetimes are longer than that of NO_x and whose decomposition, usually after long-range transport to remote regions, regenerates NO_x. Peroxycarboxylic nitric anhydrides (PANs, RC(O)O₂NO₂) and alkyl nitrates (ANs, RONO₂) are important NO_x reservoir species formed during daytime.  At night, a significant fraction of NO_x can be converted to the nocturnal reservoir species dinitrogen pentoxide (N₂O₅) and nitryl chloride (ClNO₂). While the atmospheric chemistry of PANs and N₂O₅ in ambient air is relatively well established, considerable uncertainty remains about the atmospheric chemistry of ANs and ClNO₂, mainly due to a lack of analytical techniques to measure these species in ambient abundance.

 In this presentation, two novel analytical methods to quantify the mixing ratios of NO_x reservoir species with high sensitivity and accuracy are presented and their advantages and limitations discussed. In the first method, thermal dissociation cavity ring-down spectroscopy (TD-CRDS) is used to quantify the mixing ratios of SPAN, SAN, ClNO₂, or N₂O₅ in laboratory-generated samples with detection limits on the order of tens of parts-per-trillion by volume (pptv). In the second method, chemical ionization mass spectrometry (CIMS), calibrated against TD-CRDS, is used to quantify the mixing ratios of ClNO₂, the major PAN species, nitric, and nitrous acid. The new methods were applied in laboratory experiments and field measurements. In the lab, the gaseous nitrogen oxides generated during the photolysis of nitrate anion in ice were characterized. The photochemical generation of NO_x from nitrate photolysis was observed to be dependent on acidity. Using CIMS, two new species, peroxynitric (HO₂NO₂) and peroxynitrous (HO₂NO) acid, were observed as photochemical products in the gas phase, which suggests that these molecules may be important but overlooked intermediates. Preliminary results from field measurements of ClNO₂ in Calgary, AB, and Pasadena, CA, are presented.

Friday, January 27, 2012 at 3:00 p.m. in ICT 121

Department Contact: Dr. Kevin Thurbide.

thurbide@ucalgary.ca