A molecular diffusion tube study of N₂O₅ and HONO₂ interacting with NaCl and KBr at ambient temperature

Abstract

We have measured the reaction probabilities γ and surface residence times τ_surf of N₂O₅ and HONO₂ on sea-salt aerosol surrogates such as NaCl and KBr using a new experimental technique operating under molecular flow conditions which we have called molecular diffusion tube (MDT) technique. N₂O₅ showed a relatively weak interaction with both NaCl and KBr in good agreement with previous work using a Knudsen flow reactor. Upper limits for τ_surf were of the order of 0.1 ms and reaction probabilities were measured as γ=(3±1)×10^-4 and (2.5±1)×10^-3 for the interaction of N₂O₅ with NaCl and KBr, respectively. HONO₂ showed much stronger adsorption on NaCl and KBr surfaces with reaction probabilities γ=0.04±0.01 and 0.02±0.01, respectively. Residence times ranged from 1 ms on NaCl to 15 ms on KBr making the adsorption process prone to surface saturation under the present experimental conditions. Reference experiments showed that HONO₂ also has a strong affinity for "‘non-reactive’' surfaces such as PTFE Teflon^TM, Pyrex glass and stainless steel. These results and others obtained from molecular diffusion tube experiments so far suggest a correlation between measured surface residence times τ_surf and reaction probabilities γ based on a precursor-mediated mechanism of heterogeneous reactivity. A linear free energy relationship for the molecular adsorbate is suggested involving surface reaction and desorption back into the gas phase.

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