Ionization of Nitric Acid on Ice

Abstract

The ionization of nitric acid on the surface of crystalline ice was examined from 130 to 150 K using FTIR transmission spectroscopy. A spectral feature of the hydronium ion, H 3 O + , was monitored as a function of time. The results are best understood when they are separated into (a) low and (b) high nitric acid exposure, depending upon the amount of nitric acid adsorbed on the ice surface. (a) For low nitric acid exposure ((∼2-20) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a single exponential (i.e., first-order expression). The resulting rate constant, k ) (4.9 ( 0.7) × 10 -3 s -1 , is attributed to the dissolution of the ions in the ice surface layer (i.e., a reactive layer on the ice surface). The thickness of this ice surface layer is estimated to be 10 nm. (b) For high nitric acid exposure ((∼20-200) × 10 15 molecules/cm 2 ), the absorbance of H 3 O + can be fit to a double exponential expression that is composed of the first-order rate constant above, along with another first-order rate constant, k′ ) (5.2 ( 0.7) × 10 -4 s -1 . This rate constant is attributed to the dissolution of ions in an acid-rich ice surface layer. Both rate constants are independent of temperature, indicating a small activation energy (E a ) 0 ( 2 kcal/mol).

Photolytic control of the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

“…However, some observational evidence suggests that the physically driven loss of molecular HNO 3 from the snow matrix is a slow process. Dissolution of HNO 3 and therefore also release across the air-QLL interface was observed to occur within minutes (Pursell et al, 2002). But on the snow grain scale, HNO 3 diffusion measured in ice is too slow to explain rapid loss of NO (Thibert and Domine, 1998) and yields for typical snow grain diameters d of 0.05-0.1 cm an average diffusion time τ=d 2 (2 D 0 ) −1 of 1.5-3 yr.…”

“…Regarding the chemical form we know that in present-day Antarctica and past interglacials NO − 3 occurs predominantly as HNO 3 as opposed to the salt Ca(NO 3 ) 2 during glacials (Legrand et al, 1999). It has been suggested that HNO 3 newly adsorbed on ice ionizes in a 2-step process, HNO (Pursell et al, 2002). Molecular dynamics simulations provide support for the notion that nitrate at the air-water interface remains molecular HNO 3 , whereas it dissociates when embedded at various depths within the aqueous layer (e.g.…”

Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling

“…This explained that evaporative losses were lowest for highly dissociated acids such as hydrochloric acid (HCl) and not detectable for HNO 3 during 96 h long experiments (Sato et al, 2008). However, in another study dissolution of HNO 3 and therefore also release across the air-QLL interface was observed to occur within minutes (Pursell et al, 2002). Thus, from a thermodynamic point of view it is conceivable that temperature-dependent adsorption and desorption contributes to NO − 3 loss from snow at DC.…”

“…where subscripts g, s, ad and aq denote gas, surface, adsorbed and solvated (aqueous) species, respectively (Pursell et al, 2002). Evaporation requires then just reversing Reaction (R6).…”

Release of Oxygen Atoms and Nitric Oxide Molecules from the Ultraviolet Photodissociation of Nitrate Adsorbed on Water Ice Films at 100 K

“…No REMPI signals were detected just after deposition of HNO 3 since the dissolution and ionization of HNO 3 into H + and NO 3 -on the PCI film surface is a slow process, on the order of 10 min. 18 The measured evolution curve in Figure 2 can be fitted to a single-exponential curve with a rate constant of (5.3 ( 0.2) × 10 -3 s -1 . During the intercept of this measurement, the photodissociation laser pulse at 305 nm was blocked.…”