Observed and model-calculated photostationary state at Mauna Loa observatory during MLOPEX 2

During the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), simultaneous clear sky measurements of [NO2], [NO], [O-3], and NO2 photodissociation rate were collected during autumn 1991, winter, spring, and summer 1992 intensive measurement periods at Mauna Loa Observatory (MLO), Hawaii. These measurements were used to assess the O-3-NO-NO2 photostationary state, and to infer peroxy radical concentration, ozone photochemical production and destruction rates, as well as the relationship between the deviation of the photostationary state parameter, phi = j(2)[NO2]/k(1)[O-3][NO], from unity and environmental variables. In addition, a photochemical box model was used for the purpose of comparison with observations and interpretation of the results. A systematic deviation of phi from unity was determined from measurements, this parameter reached values of 1.5-2 for high-sun conditions depending on the season. The inferred total peroxy radical concentrations were generally larger (25-70 pptv for high-sun conditions depending on the season) than those directly measured (20-25 pptv during noontime). Model results seem generally to underestimate the value of phi and to overpredict the hydroxyl and total peroxy radical concentrations, suggesting loss processes not accounted for in the model for these species. Analysis of the contributions to the net ozone photochemical production indicates total production and destruction terms reaching 10-15 ppbv/day which tend to nearly cancel on a daily average basis. High-sun values of phi - 1 were shown to be independent of NOx (= NO + NO2) for NOx concentrations above about 50 pptv. However, below this level, observations show that phi - 1 generally decreases with increasing nitrogen oxide concentrations. Furthermore, the measured phi - 1 correlation with root H2O]/[O-3] suggests the existence of NO to NO2 oxidants not accounted for in a simplified HOx chemistry.