Development of a Near-IR Cavity Enhanced Absorption Spectrometer for the detection of atmospheric oxidation products and amines Nathan C. Eddingsaas Breanna Jewell and Emily Thurnherr Rochester Institute of Technology 69 th International Symposium on Molecular Spectroscopy June 17, 2014

Atmospheric oxidation Atmospheric oxidation is complex leading to many different products. Important to isolate a single reaction pathway to understand the chemistry. To date there are many unknowns about gas phase oxidation and particle formation and composition.

Studying atmospheric oxidation in the lab Want to promote relevant reactions and minimize competing reaction Ex. RO 2 + HO 2 not RO 2 + RO 2 Need to take kinetics and thermodynamics into account R-OO bond has been shown to be labile. Peroxy radicals have lifetimes of up to 10s of seconds in the atmosphere. Want to be able to detect a wide range of oxidation products Many methods to detect alcohols, carbonyls, and carboxylic acids More difficult to detect hydroperoxides and amines. Want a system with high sensitivity that can detect many oxidation products continuously and in real time. Our plan is to use IR absorption spectroscopy.

Vibrational spectra of hydroxyethyl hydroperoxide OO-H O-H OO-H O-H Fry, J. L.; et al. JPC A, Fundamental bandsFirst overtones

O-H stretch mid- vs near-IR Fundamental bandsFirst overtones Butanol Butyric acid Hydrogen peroxide PNNL IR database Better spectral separation of functional groups in the near-IR. Clean window for amines: nm, terminal epoxides: ~1600 nm. Loss an order of magnitude of sensitivity using the first overtone (σ: – cm 2 molecules -1 ). Need a highly-sensitive technique

IBBCEAS Incoherent Broadband Cavity Enhanced Absorption Spectroscopy Direct absorption technique. Highly reflective mirrors: 99.9 – % reflective giving path length in the 10s of km. Has been implemented in the visible region with sensitivity of sub ppt. IBBCEAS only recently been implemented in the near-IR, only with FT detection. Simple to operate, robust, sensitive and selective.

CEAS setup

Xe arc lamp Optical cell HeNe laser To spec. via fiber optic Atmospheric chamber Pt catalyst Air or N 2 Purge gas To pump MirrorValve Lens ¼” tubing Light path Gas path Filter

Broadband dielectric mirrors

Raw CEAS data Dry air Diethylamine

CEAS detection of diethylamine

Testing the sensitivity and detection limit Have detected diisopropyl amine down to 50 ppb. Still working on improving the limits of detection of CEAS instrument.

Inference from water vapor CEAS spectrum of 2% Relative Humidity (~525 ppm water vapor) Linear response tested up to 30 % RH

Accounting for water vapor Spectrum of 8 ppm ethanol and 8 ppm acetic acid from 500 L teflon bag. Same sample passed through an Pt catalyst at 350° C.

CEAS using Pt catalyst as reference

CEAS using C-trap reference Pt catalyst converts organic compounds into water vapor and carbon dioxide. As long as the water vapor does not result in nonlinear absorption, the absorption from water can be accounted for. 8 ppm ethanol, 7 ppm acetic acid

Summary Near-IR CEAS can be used to qualitatively and quantitatively detect the oxidized species of atmospheric oxidation. Water vapor can be accounted for using a carbon trap. Amines can be monitored in real time using near-IR CEAS. Working on improving limits of detection. At this time we are studying the overtone spectra of hydroperoxides. Next will look at compounds with multiple functional groups. Future plans include studying gas phase oxidation and determining the composition of semi-volatile fraction of aerosols using thermal desorption.

Acknowledgments Undergraduates at RIT Breanna Jewell Emily Thurnherr Funding: RIT School of Chemistry and Material Science RIT College of Science RIT Grant writers bootcamp