Pressure-Dependent Yields and Product Branching Ratios in the Broadband Photolysis of Chlorine Nitrate

Abstract

The photolysis of chlorine nitrate was studied using broadband flash photolysis coupled with long-path ultraviolet-visible absorption spectroscopy. Branching ratios for the Cl + NO 3 and ClO + NO 2 product channels were determined from time-dependent measurements of ClO and NO 3 concentrations. Yields of the ClO and NO 3 products displayed a dependence on the bath gas density and the spectral distribution of the photolysis pulse. Product yields decreased with increasing bath gas density regardless of the spectral distribution of the photolysis pulse; however, the decrease in product yield was much more pronounced when photolysis was limited to longer wavelengths. For photolysis in a quartz cell (λ > 200 nm) the yield decreased by a factor of 2 over the pressure 10-100 Torr. In a Pyrex cell (λ > 300 nm), the yield decreased by a factor of 50 over the same pressure range. When photolysis was limited to λ > 350 nm, the yield decreased by a factor of 250. Branching ratios for the photolysis channels [ClONO 2 + hν f ClO + NO 2 (1a) and ClONO 2 + hν f Cl + NO 3 (1b)] were determined from the relative ClO and NO 3 product yields at various pressures. Although the absolute product yield displayed a pressure dependence, the branching between the two channels was independent of pressure. The relative branching ratios (assuming negligible contributions from other channels) are 0.61 ( 0.20 for channel 1a and 0.39 ( 0.20 for channel 1b for photolysis with λ > 200 nm and 0.44 ( 0.08 for channel 1a and 0.56 ( 0.08 for channel 1b for photolysis with λ > 300 nm. The implications of these results for the chemistry of the lower stratosphere are discussed.

The coupling of ClONO2, ClO, and NO2 in the lower stratosphere from in situ observations using the NASA ER‐2 aircraft

“…Several suggestions have been forwarded to explain the discrepancy. A laboratory measurement of the C1ONO2 absorption cross section led to speculation that the photolysis rate is overestimated due to an unusual pressure-dependent quantum yield for photodissociation [Nickolaisen et al, 1996]. A decrease in Jc•oNo2 would dramatically increase estimated C1ONO2, but would decrease estimated C10 as well, in significant disagreement with in situ measurements of C10.…”

Midlatitude ClO during the maximum atmospheric chlorine burden: in situ balloon measurements and model simulations

“…The quantum yields of the two ClONO 2 photolysis channels (ClONO 2 +hν→ClO+NO 2 and ClONO 2 +hν→Cl+NO 3 ) are taken from standard recommendations (Sander et al, 2002). Nickolaisen et al (1996) observed Cl pressure dependence of the quantum yield for the ClONO 2 photolysis beyond 300 nm. If model simulations do not consider the effect of pressure on the ClONO 2 photolysis, models would tend to over-predict HCl and under-predict ClONO 2 and thus ClO mixing ratios in the lower stratosphere.…”

“…In a model study based on balloon-borne in situ measurements of ClO (Avallone et al, 1993), observed ClO was greater by as much as a factor of four below 20 km altitude. Studies on the broadband photolysis of ClONO 2 led to the speculation that there is a pressure dependence of the ClONO 2 quantum yield beyond 300 nm (Nickolaisen et al, 1996), but first in situ measurements of ClONO 2 on board the ER-2 aircraft during the northern high-latitude summer found no evidence in support of a pressure-dependent quantum yield for photodissociation of ClONO 2 . Further, no evidence was found from these ER-2 measurements in support of missing inorganic chlorine species that would constitute a significant fraction of Cl y (Bonne et al, 2000).…”

Chlorine nitrate in the atmosphere

“…Since solar absorption spectrometry is not possible during polar night, the moon was identified as an alternative source of background radiation. Lunar absorption spectrometry enabled ClONO 2 measurements during the entire winter and was applied in Ny-Ålesund, Spitsbergen (Notholt et al, 1993;Notholt et al, 1995).…”