Interaction of Nitrous Acid with Polycrystalline Ice: Adsorption on the Surface and Diffusion into the Bulk

Abstract

The thermodynamic and kinetic constants needed to model the interactions of nitrous acid (HONO) with ice were measured. The experiments were done in a packed ice bed flow tube at temperatures between 213 and 253 K and at HONO gas phase concentrations between 0.4 and 137 ppbv. The experiments were performed using radioactively labeled HO13NO molecules. The use of the short-lived isotope 13N (t 1/2 ≈ 10 min) enabled in situ monitoring of the migration of HO13NO in the flow tube. The measurements showed that the interactions do not only occur through adsorption but also via diffusion into polycrystalline ice. A method is suggested to disentangle the bulk and the surface processes. Using this method, the temperature dependence of the linear partition coefficient was found to be K lin,C,HONO = 7.4 × 10−11 exp(5.4 × 103/T) in units of m. The enthalpy and entropy of adsorption of HONO on ice were ΔH ads,HONO = −45(±20) kJ mol−1 and ΔS ads,HONO 0 = −17(±89) J mol−1 K−1, respectively for the standard conditions A std/V std = 1.7 × 109 m−1. The time dependent uptake into the bulk was expressed using H cc,HONO *(D b,HONO)1/2, the product of the dimensionless effective Henry’s law constant and the square root of the effective diffusion constant in the bulk of the polycrystalline ice matrix. H cc,HONO *(D b,HONO)1/2 was evaluated to 1.6(±69%) × 10−2 m s−1/2. The results are discussed in the context of HONO partitioning in snowpacks.

A novel synthesis of the N-13 labeled atmospheric trace gas peroxynitric acid

“…For this isothermal chromatographic methods are well suited [7,29]. The advantage of isothermal methods is that the highly temperature-depended partitioning coefficient is the observable of such experiments, and can be directly measured at the temperatures of interest.…”

“…The partitioning of NO 2 and HNO 2 to ice is much weaker compared to HNO 3 : Significant adsorption of NO 2 to ice has not been observed at temperatures above 195 K [26,27]. HNO 2 partitioning to ice surfaces occurs at higher temperature (above 170 K) [7,28,29]. Consequently, we can assign HNO 2 deposition to the second zone at 184 K and NO 2 to the third zone at 126 K. Again, the current IUPAC recommendations for HNO 2 support this conclusion with 1 × 10 4 molecules cm −2 HNO 2 on the surface per molecule cm −3 in the gas-phase at 184 K. This assessment of HNO 3 , HNO 2 , and NO 2 is supported by our earlier experiments where we have used selective traps to identify the deposition zones of nitrogen oxides along a negative temperature gradient in a packed bed flow tube [14].…”

“…For each experiment, a new packed bed flow tube was prepared as described earlier [7,14]: Spraying of ultra pure water into liquid nitrogen rapidly froze small ice droplets. After a minimum of 2 d of annealing at 253 K, the ice spheres were sieved with calibrated sieves (Retsch, Germany) to grain sizes between 400 and 600 μm, filled into a PFA tube (8 mm inner diameter, 360 mm length), and stored again for at least 12 h at −20…”

“…Similarly, on the ground, trace gas uptake by snow may significantly alter the concentration both in the air above snow-covers and in the snow itself [4,5]. Nitrous acid (HNO 2 ) partitions less to ice or snow, but its interactions with the ice are still strong enough to significantly slow down its diffusion through surface snow [6,7]. This longer residence time in the snow pack, as compared to non-interacting species such as NO 2 , makes its photolytic dissociation and thus its role as source of the strong oxidant OH more probable.…”

The adsorption of peroxynitric acid on ice between 230 K and 253 K

“…The non-acidic H 2 O 2 shows no long term uptake (Clegg and Abbatt, 2001;Pouvesle et al, 2010). For the weak acids HONO and SO 2 a long term uptake has been observed in packed bed flow tubes that have a much larger ice volume and surface area and are thus much more sensitive to slow bulk and surface effects (Huthwelker et al, 2001;Kerbrat et al, 2010b). Molecular solubility seems not to be a strong driver for long-term uptake (Table 1).…”