On the Photodecomposition of ClONO₂ in the Middle Ultraviolet

Abstract

Herrn Professor G. O. Schenck zu seinem 70. Geburtstag gewidmetPhotodecomposition of gaseous C10N0 2 was investigated in a static system at wavelengths 265 nm and 313 nm. From the quantitative determination of the products 0 2 , N 2 0 5 , and C10 l5 N0-> formed, and from <£(-C10N0 2 ) it is concluded that the predominant primary step is C10NÖ 2 + h v CI + N0 3 (3).Chlorine nitrate, C10N0 2 , is considered to be formed in the stratosphere via the pressure dependent recombination reaction C10 + N0 2 + N 2^ C10N0 2 + N 2 .(1)As its major removal process the photodissociation of atmospheric C10N0 2 in the ultraviolet is discussed. It has repeatedly been pointed out that significance should be attributed to the pathways of chlorine nitrate formation and its destruction since this compound might be involved in the ozone depletion mechanism through both the C10 x and NO x atmospheric cycles.This communication summarizes our results on the photodecomposition of gaseous chlorine nitrate in the middle ultraviolet using a static experimental set-up. Its publication is prompted by a report [1] containing in essence conclusions which are in agreement with an earlier letter by Smith et al. [2] but being at variance with our results [3,4] and those of others [5].All experiments were carried out in thermostated all-quartz cells. One set of cells was equipped with an additional set of sealed-on Si windows which are transparent in the ir region of interest. Illumination occurred with a Hanovia medium pressure Hglamp. The Schott filters UV-250 and UV-340 were used either separately or in combination with cutoff filters. The effective wavelength in the experiments was either / = 265 nm with a relative total intensity < 1% for / < 263 nm or / = 313 nm with no measurable contributions at /, < 300 nm. Irradiations were performed in the temperature range +20 to -30 °C. The C10N0 2 pressure was adjusted be-* Present address: GKSS Forschungszentrum Geesthacht GmbH. tween 3 and 50 mbar depending on the wavelength. Conversion was always ^ 5%. At 265 nm, HI was used for actinometry. At 313 nm, C1NO or N0 2 were applied for this purpose. Product analysis was carried out mass spectrometrically and/or by uv and ir absorption measurements.Three major routes are energetically available for the photodecomposition of C10N0 2 at wavelengths /. ^ 265 nm. They are listed in the table below.The energy requirements for simple one center fragmentations are also given using recommended values [6] and more recent data [7] The molecular elimination processes C10N0 2 -» C1NO + 0 2 (5) and C10N0 2^ C10 2 + N0 (6) are not included, although energetically feasible, because of their inherent low probability. Also, the spin-allowed dissociationis not considered. The energy required for this process to occur is given as 460 kJ/mole equivalent to a < 260 nm. ResultsTo obtain information about the relative importance of the pathways (2)-(4) in the photodeUnauthenticated Download Date | 5/12/18 8:17 AM

Photodissociation of ClONO2at 235 nm: Final Product Yields and Energy Partitioning

“…Although the ClO−NO 2 bond is the weakest in ClONO 2 , initial experimental studies reported that the ClO + NO 2 channel was only of minor importance. Knauth and Schindler reported that channel (3) was the dominant dissociation channel at 265 and 313 nm in a static cell . Margitan observed that Cl and O atoms were the two major products with quantum yields of 0.9 and 0.1 respectively at 266 and 355 nm using the atomic resonance fluorescence detection method .…”

Direct Observation of ClO from Chlorine Nitrate Photolysis

“…The direct study by Marindli and Johnston (8) found that the NO, yield, corrected for a revised calibration factor (1 I), is less than 40%, and NO, and secondary N20, products have been detected in steady state (11) and static cell (5,9) experiments. 26.…”