Two field experiments to investigate the formation of nitrate as an airstream passes through a hill cap cloud have been performed at the UMIST field station on Great Dun Fell. Techniques chosen for the measurement of various nitrogen species are described. The results of the second field experiment are discussed and compared with those of the first. Evidence is found in support of the hypothesis that under the range of conditions studied, the dominant pathway for nitrate production is the solution of N2O5 formed from the reaction of NOx with O3 upwind. The effectiveness of this pathway by night and by day is observed to be a function of the NOx mixing ratio. A surface reaction rate constant of around 300 cm3 cm−2 s−1 for the hydration of N2O5 is inferred from the observations. These results are shown to be consistent with recent laboratory measurements of the rates of reaction of nitrogen species. It is suggested that pathways other than via N2O5 may be significant sources of nitrate under certain conditions that merit further investigation.\ud \u
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