On the occurrence of nitrite in urban fogwater

Nitrite was observed in fogwater samples 3 – 52 μmol L−1, in a polluted source region in Germany. The fogwater contained high concentrations of various pollutants but only little or no acidity. The concentrations of oxidizable S(IV) were in the range 6 – 290 μmol L−1. A two-phase chemical box model is used to interprete the chemistry. Fogwater N(III) is found to be sensitive to gas-phase HNO2 and also to HCHO (which were both not measured). From the distribution of S(IV) between oxidizable and non-oxidizable species (the latter is suspected to be dominated by the complex formed from S(IV) and dissolved HCHO), it is infered that HCHO most likely was low. The reaction between dissolved NO2 and S(IV) was an important N(III) source and could even explain the observed fogwater nitrite concentrations, if no gas-phase HNO2 was fed into the fog. The comparison between observed and model predicted suggests that interphase mass transfer of NO2 is characterized by a low mass accommodation coefficient.