An analysis of the data obtained during AASE II was made to characterize small- (Δx<24 km) and large-scale (30<Δx<260 km) increases of reactive nitrogen species. By using the NOx/NOy ratio, the increases were classified into fresh emissions and aged air parcels. The sources of the NOy increases were then assessed by the freshness of the increases and by examining the correlation (or absence thereof) between NOy and other trace species including tracers of various NOx sources: for example, aircraft emissions, lightning, the stratospheric source, and sources in the boundary layer. We found that the majority (43%) of NOy increases in the upper troposphere and lower stratosphere were due to displacement of constant mixing ratio surfaces relative to isobaric flight paths. Aircraft emissions were the second most abundant (17%). The next significant contributor was surface emissions, including fossil fuel combustion and biomass burning. Only two lightning spikes were found, apparently due to the winter season and emphasis on high latitudes in the AASE II experiment. The relative contribution from each source to the increases (or positive variability) represents a measure of the relative source strength. However, it is important to note that our findings on the relative contribution of each source to the NOy increases should not be applied directly to estimating the relative source strength of the ambient or background NOy abundance. Knowledge of the detailed temporal and spatial distribution of the relative contribution of each source is needed. This requires a substantially higher precision for the instruments of tracer gases than those used in this study. ¿ American Geophysical Union 1996