We report measurements of N2O5, CO, and O3 at a subarctic site near Fairbanks, Alaska (65¿N), during two periods of approximately 2 weeks each, one in December 2002 and the other in February 2004. N2O5 mixing ratios range from below the instrument detection limit of 2 parts per trillion by volume (pptv) up to approximately 250 pptv. Ozone and carbon monoxide measurements help distinguish several different air mass types: "clean" (near background CO and ozone), "titrated" (ozone titrated to near zero and elevated CO), and "mixed" (intermediate ozone and some CO elevation). Observed N2O5 mixing ratios are near zero for air masses that are either clean or titrated. Times with measurable N2O5 are almost always associated with mixed air masses. Measurements of CO and the CO:NOx emissions ratio in Fairbanks are used to estimate NO2 mixing ratios and calculate steady state lifetimes for N2O5. The lifetime estimates vary from subminute to 4 hours, with a median for the February study of 10 min and a median for the December study of 4 min. These lifetimes are quite short compared to the lifetime of NO2 with respect to reaction with ozone and formation of NO3, suggesting that the rate limiting step of NOx oxidation in the nighttime atmosphere is the formation of NO3, not the losses of NO3 and N2O5. These data show sinks of N2O5 (presumably mostly heterogeneous hydrolysis) are an efficient mechanism of NOx removal near the surface in the dark, high-latitude winter.