TEMPERATURE-DEPENDENT ABSORPTION CROSS-SECTIONS AND ATMOSPHERIC PHOTOLYSIS RATES OF NITRIC-ACID

Absorption cross-sections for HNO3 have been measured in the wavelength region 220-340 nm, using a dual-beam diode array spectrometer, with a spectral resolution of 0.3 nm, The results at room temperature were in good agreement with earlier measurements. Absorption over most of the wavelength range showed a distinct temperature dependence, with a decline in cross-section with decreasing temperature in the range 295-239 K. The results lead to quite large effects on the calculated photodissociation rate of HNO3 in the lower stratosphere, especially at the low temperatures and high solar zenith angles characteristic of the polar winter and spring.