The photochemical equations describing O3 formation in the lower troposphere contain 2 major sink terms for free radicals; combination reactions and reactions with NOx. Knowing the fraction of radicals removed by reactions with NOx, termed LN/Q, allows one to predict the sensitivity of O3 production to NO and VOCs. We derive an analytic formula that gives LN/Q in terms of readily measured O3 precursors and test this formula using constrained steady state calculations based on field observations gathered in Phoenix, Arizona. The formula quantifies well-known results regarding the effects of dilution, oxidation, and the production of oxidants on the transition from VOC to NOx sensitive behavior as an air parcel is advected away from an urban source. ¿ 2001 American Geophysical Union