Variations in NO y composition at Idaho Hill, Colorado

Measurements of NO y and the principal constituent NO y species (NO, NO 2 , PAN, HNO 3 , PPN, NO 3 -) were taken during the fall of 1993 at a high-elevation site in the Colorado mountains west of Boulder. The meteorology provided two principal flow regimes: strong westerly flow, bringing drier and cleaner air over the Continental Divide from the west where emission sources are sparse, and weaker easterly upslope flow which brought moist and more polluted air from the Denver-Boulder urban corridor and perhaps from smaller more local sources such as the mountain town of Nederland. The upslope flow data generally indicate a balance between measured NO y and the separately measured NO y species with NO x , PAN, and HNO 3 accounting for over 90%. Under these conditions, NO x was the dominant fraction of NO y , and occasionally fresh emissions were indicated as shown by NO x /NO y close to unity. For downslope flow there was generally a shortfall in the NO y balance where measured NO y was higher than the constituent species by about 20%, on average. Known potential interferences in the measurements do not appear to account for the NO y deficit. Missing NO y correlated directly with NO x /NO y , O 3 , and aerosols and inversely with temperature. These relationships suggest the presence of one or more NO y species that were not measured by the individual techniques.