The reaction probability of N₂O₅ with sulfuric acid aerosols at stratospheric temperatures and compositions

Abstract

We have measured the rate of reaction of N2O5 with H2O on monodisperse, submicrometer H2SO4 particles in a low‐temperature flow reactor. Measurements were carried out at temperatures between 225 K and 293 K on aerosol particles with sizes and compositions comparable to those found in the stratosphere. At 273 K, the reaction probability was found to be 0.103±0.006, independent of H2SO4 composition from 64 to 81 wt %. At 230 K, the reaction probability increased from 0.077 for compositions near 60% H2S04 to 0.146 for compositions near 70% H2SO4. Intermediate conditions gave intermediate results except for low reaction probabilities of about 0.045 at 260 K on aerosols with about 78% H2SO4. The reaction probability did not depend on particle size. These results imply that the reaction occurs essentially at the surface of the particle. A simple model for this type of reaction that reproduces the general trends observed is presented. The presence of formaldehyde did not affect the reaction rate.

Heterogeneous uptake of ClONO2 and N2O5 by sulfuric acid solutions

“…At this time, however, there are insufficient experimental data to derive such a surface reaction model as in the halogen uptake case. Fried et al [1994] have proposed a surface reaction model for N20 s on H2SO 4 that predicts an increase in 3'0 with increasing concentration at cold temperatures (T < 240 K) and which contradicts the data for concentrations above 70 wt % (Figures 5c and 5d). In this context we note that the present model predicts that 3'0 decreases at higher H2SO 4 concentrations (Figures 5d and 10); this agrees well with the very low uptake rate (3'0 "• 10-4) reported by Baldwin and Golden [1979] for N2Os on 96 wt % H2SO 4 at room temperature.…”

“…Our model expands upon previous work by which modeled the uptake of C1ONO2 on H2SO 4 in terms of bulk liquid phase solubility and reactivity parameters. The present model differs from one developed by Fried et al [1994] which describes N20 5 uptake in terms of processes occurring at the liquid surface. The implications of our model for the mechanism of XNO 3 hydrolysis in concentrated acid solutions and for lower stratospheric chemistry are also addressed.…”

“…For example, the present ARI/BC droplet data are somewhat higher than the National Oceanic and Atmospheric Administration (NOAA) flow tube data [Hanson and Ravishankara, 1991]. Between the two aerosol data sets, the National Center for Atmospheric Research (NCAR) data [Fried et al, 1994] are consistently higher than those from NOAA [Hanson and Lovejoy, 1994]. The 70 wt % NOAA flow tube data are low, even relative to the aerosol data (see below).…”

“…However, if a reactive process that is confined to the liquid surface were involved in N20 5 uptake at higher acidity, then krx n could be lower at all concentrations, leading to better agreement between the model and the data for dilute solutions. Such liquid surface reactions have been observed in the case of uptake of halogen molecules into halide solutions and have been proposed for N20 s interacting with H2SO 4 Fried et al, 1994]. The contribution of surface reactions to N2Os uptake would be most significant at high acid concentrations where the hydrolysis rate constant (krxn) is largest and the liquid phase diffusion coefficient (D1) is smallest ( Figures 10 and 11) [Hu et al, 1995].…”

Reactive uptake coefficients for heterogeneous reaction of N&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; with submicron aerosols of NaCl and natural sea salt

“…This equation takes into account the gas phase diffusion limitation for large particles in the term containing the Knudsen number (Kni=3D g /ωr i ), as discussed by Fuchs and Sutugin (1970) and Fried et al (1994). It also specifies a size dependence uptake coefficient γ (r i ), arising for example from slow chemical processing in small particles, as discussed by Hanson et al (1994).…”