Desorption Kinetics of Model Polar Stratospheric Cloud Films Measured Using Films Measured Using Fourier Transform Infrared Spectroscopy and Temperature-Programmed Desorption

摘要

This study combines Fourier transform infrared (Fourier transform infrared (FTIR) spectroscopy and temperature-pogrammed desorption to examine the evaporation kinetics of thin films of crystalline nitric acid hydrates, solid ambrphoO+-120/f-lNO3 mlXtutes, H20-ice% Ice coi:ltedwlthHCr;and solidHNO3. lR spectrdscopy measured the thickness of each fi.(masit evaporated, either at <:onstant temperature or during a Imeartemperature rampjtempetature-programmed infrared, TPIR). Simultaneously.. amass spectrometer measured the rate of evaporation directly by monitoring the evolution of the molecules into the gas phase (temperature-programmedde- sorption, TPD) ,.Both TPIRand TPD data provide a measurement of.the desorption rate and yi~ld the activatiQnenergy and preexponential factor .for desorption, TPDmeasurements have theadyantage 9f produdng many datapqi~s but are subj.ect to interference from experimental gifficl!ties such ~sun~vepheatif)gfromcthe edge of a sample and sample-support as well as pumping-speeQlimitation,5.. TPIRexperi~ents giv~ c.lean but fewer data points. Evaporation occurred between 170 and 215 K for the varIous films; Ice evaporates wIth an actlvatlo~ energy , Cc ,of 12. 9 +-I kcal/moT and apreexponehtial factor of f X [032" 15 moledcm2 S, in good agreement with the literature. The beta form dfniitic;gcid trihydrate, .8-NAT:has'an Edes of 15.6 +-2 kcal/ mot with tog A = 34.3 +- 2.3; the alpha form ofnitric acid trihydrate, a-NAT;is around 17.7+-3 kcal/molwith tog A = 31.2+-4. For nitric acid dihydrate, NAD, Edes is 17.3 +-2 kcalf mol with logA =35.9+-2.6;..for nitric acid monohydrate, NAM, Edes is 13+-3kcal/mol with log A = 31.4+-3. The a-NAT converts to .8- NATduringevaporation, and the amorphous solid H2O/HNO3miXtures crystallize during evaporation. The barrier to evaporation for pure .nitric acid is 14.6+-3 kcaI/mol with log A = 34.4+-3