The photochemistry of the nitrate radical and the kinetics of the nitrogen pentoxide-ozone system

Abstract

The kinetics of the nitrogen pentoxide catalyzed decomposition of ozone were studied in the dark and with photolytic light absorbed by the NO/sub 3/ radical. Ultraviolet, visible, or infrared absorption cross sections were measured for N/sub 2/O/sub 5/, HNO/sub 3/, NO/sub 2/ and NO/sub 3/. The equilibrium constant for N/sub 2/O/sub 5/ reversible NO/sub 2/ + NO/sub 3/ was found to be (molecules cm/sup -3/, 298--329 K) K = (8.4 +- 1.8) x 10/sup 26/ exp(-(11180 +- 100)/T). The rate constants for several reactions were measured: (cm/sup 3/ molecule/sup -1/s/sup -1/) 2NO/sub 3/ ..-->.. 2NO/sub 2/ + O/sub 2/, g = (8.5 +- 2.8) x 10/sup -13/ exp(-(2450 +- 100)/T); O + N/sub 2/O/sub 5/ ..-->.. product, m less than or equal to 2 x 10/sup -14/; O + NO/sub 3/ ..-->.. O/sub 2/ + NO/sub 2/, n = (1.0 +- 0.4) x 10/sup -11/. By combining the equilibrium constant K with literature values of Ke and Kf, rate constants for reactions e and f were evaluated: NO/sub 2/ + NO/sub 3/ ..-->.. NO + O/sub 2/ + NO/sub 2/, e = (2.5 +- 0.5) x 10/sup -14/ exp(-(1230 +- 100)/T); NO + NO/sub 3/ ..-->.. 2NO/sub 2/, f =more » (1.9 +- 0.4) x 10/sup -11/ at 297 K. At 1 atm total pressure, the photolysis of NO/sub 3/ occurs with a primary quantum yield less than one in the red region of the spectrum. The solar photolysis constants at 298 K for the two paths of dissociation are consistent with the following values: NO/sub 3/ + h..nu.. ..-->.. NO + O/sub 2/, j/sub 1/ = 0.040 +- 0.02 s/sup -1/; NO/sub 3/ + h..nu.. ..-->.. NO/sub 2/ + O, j/sub 2/ = 0.099 +- 0.02 s/sup -1/. The average quantum yield for process two was approximately 0.77 for light with wavelengths between 470 and 610 nm, and that for process one was about 0.23 between 470 and 610 nm but only 0.07 in the strong absorption region between 610 and 670 nm. The photolysis constants may be larger at lower total pressures. Infrared absorption ascribed to NO/sub 3/ is due to N/sub 2/O/sub 5/; previously reported visible cross sections for NO/sub 3/ were found to be about a factor of 4 too low and are corrected here; previously reported ultraviolet cross sections for N/sub 2/O/sub 5/ are reversed slightly.« less