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

We have measured the reaction probabilities γ and surface residence times τsurf 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 τsurf 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 TeflonTM, 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 τsurf 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.