Spectroscopic measurements from Mauna Loa, Hawaii, yield mixing ratios for NO3 and NO2 at night of ~0.3 and ~30 ppt, respecitvely, at 3 km altitude in the free troposphere. The mixing ratio of NO2 in November is close to that measured earlier in June, and no significant seasonal change is apparent. The mixing ratio of NO3 is far below that expected, implying the existence of a scanvenger for NO3; the scavening rate (~10-3 s-1) is close to that found earlier in the Colorado mountains. While the scavenger remains unidentified, it appears to be a well-mixed and long-lived atmospheric component. It is not known whether the scavenger acts as a sink for NOx or whether it merely sequesters it until the following day.