No abstract

The reaction of gaseous N 2 O 5 with sea salt and its components is a potential source of halogen atoms in the marine boundary layer. There are two possible reaction paths when water is present on the salt surface. Reaction with the chloride ion forms nitryl chloride (ClNO 2 ), a photolyzable compound: N 2 O 5 + NaCl ! ClNO 2 + NaNO 3 , while hydrolysis of N 2 O 5 generates HNO 3 that can react further with NaCl to form gaseous HCl: N 2 O 5 + H 2 O (on NaCl) ! 2 HNO 3 , HNO 3 + NaCl ! HCl + NaNO 3 . We report here Knudsen cell studies at 23 C of the reaction of N 2 O 5 with NaCl, using less than one layer of salt particles. A model, which takes into account the effective salt surface area exposed to the gas, was applied, allowing for the determination of uptake coefficients without introducing uncertainties associated with diffusion into multiple layers of salt particles. The net uptake coefficient for the sum of both channels for the N 2 O 5 reaction was measured to be g N 2 O 5 ¼ (2.9 AE 1.7) Â 10 À3, where the error cited is the 2s statistical error. The cumulative error is estimated to be better than a factor of three. Both ClNO 2 and HCl were observed as gaseous products from the N 2 O 5 -salt reaction and the branching ratio for ClNO 2 was 0.73 AE 0.28 (2s). A limited number of experiments were carried out for the reaction with synthetic sea salt, resulting in an uptake coefficient of about an order of magnitude larger than for NaCl, and a ClNO 2 yield of 100%. We propose a mechanism for this reaction in which surfaceadsorbed water plays a key role in the competition between hydrolysis of N 2 O 5 to generate HNO 3 and the reaction with NaCl to generate ClNO 2 . Reaction with NaCl is shown to be a potentially important source of ClNO 2 , and thus, of highly reactive chlorine atoms in urban marine regions at dawn. Application of our model to previous data from this laboratory for the reaction of chlorine nitrate (ClONO 2 ) with fractional layers of NaCl gives a corrected uptake coefficient of g ClONO 2 ¼ (2.4 AE 1.2) Â 10 À2 (2s), which suggests that the ClONO 2 -NaCl reaction may contribute significantly to the observed concentrations of Cl 2 in the marine boundary layer.

The uptake and reaction of HNO3 with NaCl was studied at 298 K using a Knudsen cell coupled to a quadrupole electron impact mass spectrometer. Experiments were conducted using less than one layer of particles to clearly define the available reactive surface area. The uptake of HNO3 was observed to be faster initially than at longer reaction times. A new model is proposed that incorporates reactions at two different types of sites:  (1) steps and edges on the NaCl surface holding surface adsorbed water (SAW) and (2) dry terrace sites. The initial, more rapid uptake is attributed to reaction with both types of sites on a fresh NaCl surface, while the slower uptake at longer reaction times is due only to reaction at the steps and edges where new reaction sites are generated by the SAW-assisted recrystallization of the NaNO3 product. The initial uptake coefficient based on data at short reaction times is γ0 = (2.3 ± 1.9) × 10-3, in agreement within the experimental errors with a value of γ0 = (1.1 ± 0.4) × 10-3 (2s) derived from the dependence of the uptake coefficient on the initial HNO3 concentration. The fraction of the surface area covered by sites holding SAW was estimated to be approximately 50% based on the uptake coefficient, averaged over three cycles of exposure to HNO3, γ = (1.0 ± 0.8) × 10-3 (2s). The data suggest that the reaction of HNO3 with effluoresced sea salt particles is less important than previously thought relative to reactions with N2O5 and ClONO2, which generate chlorine atom precursors.

Gaseous nitric acid removal by surfaces in experimental systems and in the atmospheric boundary layer is rapid. However, neither the form of HNO 3 on surfaces nor its impact on the properties of the thin surface film are known. We report here studies of surfaces that have been exposed at room temperature (295 AE 2 K) to gaseous mixtures of water vapor with HNO 3 at concentrations from 46 ppb to 4 Â 10 3 ppm. The surfaces were probed using a combination of Fourier transform infrared spectrometry (FTIR), non-contact atomic force microscopy (AFM), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and X-ray photoelectron spectroscopy (XPS). Exposure of borosilicate glass, quartz, and thin Teflon films to mixtures of gaseous HNO 3 and water vapor leads to the subsequent uptake of much larger amounts of water than occurs on the corresponding unexposed surfaces. Infrared spectra show evidence for the formation of nitric acid-water complexes on the surface that leads to this enhanced water uptake. On borosilicate glass, exposure to the nitric acid-water vapor mixture results in surface segregation of the trace metal oxides and their nitrates formed from reaction with HNO 3 . The majority of these oxides can be removed by rinsing with water; however, smaller, segregated regions of ZnO remain on the surface. The implications for heterogeneous reactions in thin films on surfaces in laboratory systems and in the atmosphere are discussed.