Production and decay of ClNO2 from the reaction of gaseous N2O5 with NaCl solution: Bulk and aerosol experiments
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Abstract
The chemistry of N 2 O 5 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 N 2 O 5 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 (ClNO 2 ) 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 ClNO 2 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 N 2 O 5 undergoes two reaction channels (hydrolysis and reaction with Cl − ) is unable to explain the observed concentration dependence of the product yield. We propose that N 2 O 5 dissociates to NO 2 + and NO 3 − (rate constant k l >10 4 s −1 ) mainly. The directly hydrolysis of N 2 O 5 ( k 3 [H 2 O]) is less than 20% of the total reaction. NO 2 + reacts with water to form 2H + and NO 3 − ( k 5 ) or with Cl − to form ClNO 2 ( k 4 ). Neglecting the influence of ionic strength we evaluate k 4 / k 5 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. ClNO 2 dissociates to Cl − + NO 2 + ( 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 ClNO 2 may play a role in the marine boundary layer at high northern latitudes.
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