Production and decay of ClNO2, from the reaction of gaseous N2O5 with NaCl solution: Bulk and aerosol experiments

The chemistry of N2O5 on liquid NaCl aerosols or bulk NaCl solutions was studied at 291 K by aerosol smog chamber and wetted-wall flow tube experiments. The uptake of N2O5 on deliquescent aerosol was obtained to be (3.2 +/- 0.2) x 10(-2) (1 sigma error) from the aerosol experiments. In the wetted-wall flow tube we observed that nitryl chloride (ClNO2) is the main product of the reaction at NaCl concentrations larger than approximately 0.5 M and almost the only product at concentrations larger than 1 M. The ClNO2 yield does not depend linearly on the NaCl concentration, especially at small sodium chloride concentrations (i.e., smaller than 1 M). It appeared that a simple mechanism where N2O5 undergoes two reaction channels (hydrolysis and reaction with Cl-) is unable to explain the observed concentration dependence of the product yield. We propose that N2O5 dissociates to NO2+ and NO3- (rate constant k(1) > 10(4) s(-1)) mainly. The directly hydrolysis of N2O5 (k(3)[H2O]) is less than 20% of the total reaction. NO2+ reacts with water to form 2H(+) and NO3- (k(5)) or with Cl- to form ClNO2 (k(4)). Neglecting the influence of ionic strength we evaluate k(4)/k(5) to be 836 +/- 32 (1 sigma error). Using the wetted-wall flow tube technique, we studied the uptake of nitryl chloride by aqueous solutions containing NaCl. We observed that the uptake coefficient gamma decreased from (4.84 +/- 0.13) x 10(-6) on pure water to (0.27 +/- 0.02) x 10(-6) on a 4.6 M NaCl solution. The sharp decrease of gamma with increasing salt concentrations is evidence of reversible hydrolysis. ClNO2 dissociates to Cl- + NO2+ (k(6)). In the absence of Cl- we evaluate H . k(6)(1/2) to be 0.44 +/- 0.01 mol L(-1) atm(-1) s(-1/2). Finally, we discuss that atomic Cl from photolysis of ClNO2 may play a role in the marine boundary layer at high northern latitudes.