Influence of airmass back trajectory upon nitrogen compound composition

The speciation of nitrogen-containing compounds occurring in airmasses arriving at a site on the north Norfolk coast (UK) during 1993–1995 is examined. The data are disaggregated according to four types of back-trajectory, broadly categorised as characterising: (1) land (southern England) (2) land followed by a short stretch of shallow sea (3) the ocean (but encountering land during the last 100 km or so) and (4) the Atlantic Ocean or Arctic via the North Sea. We find clear differences in NOy, NOx/NOy, Fuchs surface area and total inorganic nitrate between the airmass types, although oxidised nitrogen speciation, i.e. nitrogen dioxide NO2, HONO, HNO3 and PAN remains broadly similar. For trajectories arriving off the land, summer samples reveal a greater degree of oxidation than those in winter. The percentage of total inorganic nitrate is enhanced and the NOx/NOy ratio appears to be suppressed in the more aged airmasses arriving over the sea. When data are disaggregated according to trajectory type, total NOy can be seen to relate to airmass age and clear compositional differences relating to airmass age and history appear, which are not seen in the analysis of the dataset as a whole. This suggests that the inference in some earlier published work that NOy composition is largely insensitive to airmass age may be incorrect if it is based on inadequate markers of airmass age. Concentrations of ammonia and ammonium are especially sensitive to back-trajectory, reflecting a strong source over the land and depletion in maritime air.