Measurements of oxides of nitrogen and nitric acid in clean air

We have measured oxides of nitrogen, nitric acid, and ozone in air at a remote site in the Rocky Mountains west of Boulder, Colorado. The site is infrequently polluted by upslope air movements from the Denver‐Boulder metropolitan area and most often experiences flows of clean air from the west and north. We find NO x (NO + NO 2 ) levels commonly ≈0.2 ppb, with considerable diurnal variation around this mean value, and occasional excursions well below it. Nitric acid (HNO 3 ) levels were <0.03 to 0.1 ppb, and were always smaller than the NO x level. We find the photostationary state equation does not hold in clean air, and we suggest that the deviation from theory may arise from the presence of peroxy radicals.