Measurements of ambient levels of PAN (peroxyacetyl nitrate or, more properly, peroxyacetic nitric anhydride), C2--C5 alkyl nitrates, NOx (NO+NO2), HNO3, and NO-3 particulates were made concurrently with a measurement of total reactive nitrogen species (NOy) at Scotia, Pennsylvania, from July 16 to August 31, 1988. Each of the organic nitrates exhibited a marked diurnal variation. The concentration of PAN varied from 0.05 to 3 ppbv (parts per billion vy volume). The sum of the alkyl nitrate concentrations varied from less than 2 to 200 pptv (parts per trillion by volume). The NOy levels varied from 1 to 30 ppbv. During periods of high photochemical activity the contributions of NOy were 30--45% from HNO3, 25--30% from PAN, 15--25% from NOx, 5% from NO-3, and an additional 1.5% from the C2--C5 alkyl nitrates, on average. During these periods, about 15% of the measured NOy could not be accounted for by the sum of the individually measured species. The unidentified species that constitute this 15% exhibit behavior similar to that of the measured organic nitrates. ¿ American Geophysical Union 1990