Chemistry of NOx and HNO3 Molecules with Gas-Phase Hydrated O.- and OH- Ions.

The gas-phase reactions of O. - (H2 O)n and OH- (H2 O)n , n=20-38, with nitrogen-containing atmospherically relevant molecules, namely NOx and HNO3 , are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and theoretically with the use of DFT calculations. Hydrated O. - anions oxidize NO. and NO2 . to NO2 - and NO3 - through a strongly exothermic reaction with enthalpy of -263±47 kJ mol-1 and -286±42 kJ mol-1 , indicating a covalent bond formation. Comparison of the rate coefficients with collision models shows that the reactions are kinetically slow with 3.3 and 6.5 % collision efficiency. Reactions between hydrated OH- anions and nitric oxides were not observed in the present experiment and are most likely thermodynamically hindered. In contrast, both hydrated anions are reactive toward HNO3 through proton transfer from nitric acid, yielding hydrated NO3 - . Although HNO3 is efficiently picked-up by the water clusters, forming (HNO3 )0-2 (H2 O)m NO3 - clusters, the overall kinetics of nitrate formation are slow and correspond to an efficiency below 10 %. Combination of the measured reaction thermochemistry with literature values in thermochemical cycles yields ΔHf (O- (aq.))=48±42 kJ mol-1 and ΔHf (NO2 - (aq.))=-125±63 kJ mol-1 .