The reactive odd nitrogen chemistry is examined for enviornments both typical of the urban troposphere and in which biogenic hydrocarbon emissions are appreciable. Results are presented in terms of a nitrogen shortfall. X/NOy, the fraction of reactive nitrogen presents as species other than NO, NO2, HNO3, NO-3, and peroxyacetyl nitrate. In urban environoments, the calculated nitrogen shortfall is 0.02--0.33. Both higher initial hydrocarbon concentrations and higher initial hydrocarbon-to-NOx ratios lead to greater shortfalls, with the latter effect being more important. Products of biogenic hydrocarbon emissions greatly increase the nitrogen shortfall in urban air that has aged for 48 hours. The nitrogen shortfall increases from 0.25 for a simulation with background isoprene and pinene emissions to 0.45 for a scenario with high pinene emissions. In addition, two sensitivity studies which examine the stability of nitrate species formed from the biogenic hydrocarons are also presented. These show that for our scenarios, the stability of ISON, a species formed during isoprene oxidation can change the calculated nitrogen shortfall by 0.15. However, the stability of nitrates from the reactions of isoprene and pinene with NO3 changes the calculated nitrogen shortfall by only 0.05. ¿American Geophysical Union 1990