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

Abstract

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)×10−2 (1σ 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 kl>104 s−1) mainly. The directly hydrolysis of N2O5 (k3[H2O]) is less than 20% of the total reaction. NO2+ reacts with water to form 2H+ and NO3− (k5) or with Cl− to form ClNO2 (k4). Neglecting the influence of ionic strength we evaluate k4/k5 to be 836±32 (1σ 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 γ decreased from (4.84±0.13)×10−6 on pure water to (0.27±0.02)×10−6 on a 4.6 M NaCl solution. The sharp decrease of γ with increasing salt concentrations is evidence of reversible hydrolysis. ClNO2 dissociates to Cl− + NO2+(k6). In the absence of Cl− we evaluate H ⋅ k61/2 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.