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 recentadvances in both Optical Parametric Oscillator (OPO) and high power, narrow-linewidthpulsed laser technology. Chapter 1 describes a β-BaB_2O_4 (BBO) OPO pumped by a highrepetition rate Nd:YAG laser (Coherent Infinity^(TM). This combination provides a uniquelight source with wide tunability and high average output power, making it ideally suitedfor 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 thecatalytic NO_x ozone destruction cycles in the stratosphere. It has been suggested (Yungand Miller 1997, Science 78, 1778, referred to as YM97 hereafter) that N_2O should beisotopically fractionated as a result of photolysis in the upper atmosphere, which representsthe primary sink of N_2O. Chapter 2 studies the photolytic fractionation of N_2O in anattempt to test the YM97 model. These measurements have consistently shown large heavyenrichment of the residual N_2O isotopomers. The magnitude of the observed fractionation,however, is significantly larger than predicted but in accord with the sizable fractionationobserved in the stratosphere. An attempt to reconcile the differences is given which notesthe existence of vibrationally "hot" N_2O molecules at room temperature and the possibleinvolvement of more than two electronic states in the photolysis. A fully quantitative testof YM97 theory will require accurate wavelength and temperature dependent differentialcross 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 uppertroposphere and lower stratosphere. Chapter 3 investigates the cleavage of the HOO-NO_2bond in HNO_4 via absorption of red/near infrared (NIR) solar radiation. Experiments aredesigned 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 arguedthat molecular internal energy available for thermal excitation in addition to the photon energycan explain the observed dissociation of HN04 beyond its thermodynamic dissociationthreshold. Accordingly, a temperature-dependent quantum yield is predicted. The 1st OHstretching overtone is found to be partially dissociative. Because it is significantly brighterthan the 2nd overtone, it contributes significantly to the photodissociation of HNO_4. Basedon these experimental results, the strength of the HOO-NO_2 bond is constrained and comparedto literature values. The atmospheric significance of the NIR photodissociation ofHNO_4 is then discussed.
