Spectroscopic Studies of N₂0 and HNO₄: A Window into the Global Biogeochemistry of Nitrogen

Citation

Zhang, Hui (2001) Spectroscopic Studies of N₂0 and HNO₄: A Window into the Global Biogeochemistry of Nitrogen. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2ts6-9d87. https://resolver.caltech.edu/CaltechTHESIS:01302013-092114374

Abstract

Nitrogen is an essential nutrient for all living organisms. This thesis focuses on the spectroscopic studies of two species that participate in the global biogeochemical cycle of nitrogen: N_2O and HNO_4. Both play important roles in the radiative and chemical processes in the terrestrial atmosphere.

In terms of experimental instrumentation, this thesis takes great advantage of the recent advances in both Optical Parametric Oscillator (OPO) and high power, narrow-linewidth pulsed laser technology. Chapter 1 describes a β-BaB_2O_4 (BBO) OPO pumped by a high repetition rate Nd:YAG laser (Coherent Infinity^(TM). This combination provides a unique light source with wide tunability and high average output power, making it ideally suited for the photochemical and spectroscopic studies carried out in this thesis.

N_2O is a prominent greenhouse gas and the major natural source of NO that initiates the catalytic NO_x ozone destruction cycles in the stratosphere. It has been suggested (Yung and Miller 1997, Science 78, 1778, referred to as YM97 hereafter) that N_2O should be isotopically fractionated as a result of photolysis in the upper atmosphere, which represents the primary sink of N_2O. Chapter 2 studies the photolytic fractionation of N_2O in an attempt to test the YM97 model. These measurements have consistently shown large heavy enrichment of the residual N_2O isotopomers. The magnitude of the observed fractionation, however, is significantly larger than predicted but in accord with the sizable fractionation observed in the stratosphere. An attempt to reconcile the differences is given which notes the existence of vibrationally "hot" N_2O molecules at room temperature and the possible involvement of more than two electronic states in the photolysis. A fully quantitative test of YM97 theory will require accurate wavelength and temperature dependent differential cross sections for each of the N20 isotopomers that are not yet available.

HNO_4 is an important reservoir species coupling the HO_x and NO_x families in the upper troposphere and lower stratosphere. Chapter 3 investigates the cleavage of the HOO-NO_2 bond in HNO_4 via absorption of red/near infrared (NIR) solar radiation. Experiments are designed to determine the cross sections and quantum yields for gas phase HN04 photodissociation. HN04 is found to dissociate at wavelengths as long as 1600 nm. It is argued that molecular internal energy available for thermal excitation in addition to the photon energy can explain the observed dissociation of HN04 beyond its thermodynamic dissociation threshold. Accordingly, a temperature-dependent quantum yield is predicted. The 1st OH stretching overtone is found to be partially dissociative. Because it is significantly brighter than the 2nd overtone, it contributes significantly to the photodissociation of HNO_4. Based on these experimental results, the strength of the HOO-NO_2 bond is constrained and compared to literature values. The atmospheric significance of the NIR photodissociation of HNO_4 is then discussed.

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