OBSERVATION ON GREAT DUN FELL OF THE PATHWAYS BY WHICH OXIDES OF NITROGEN ARE CONVERTED TO NITRATE

Two field experiments to investigate the formation of nitrate as an ai rstream passes through a hill cap cloud have been performed at the UMI ST field station on Great Dun Fell. Techniques chosen for the measurem ent of various nitrogen species are described. The results of the seco nd 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 NO(x) with O3 upwind. The effectiveness of this pathway by night and by day is observed to be a function of the NO(x) mixing ratio. A surface reaction rate const ant of around 300 cm3 cm-2 s-1 for the hydration of N2O5 is inferred f rom the observations. These results are shown to be consistent with re cent laboratory measurements of the rates of reaction of nitrogen spec ies. It is suggested that pathways other than via N2O5 may be signific ant sources of nitrate under certain conditions that merit further inv estigation.