Photostationary state analysis of the NO₂‐NO system based on airborne observations from the western and central North Pacific

On the basis of measurements taken during the NASA Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐West A (PEM‐West A), photostationary state model calculations were carried out for approximately 1300 three‐minute sample runs. The objective of this study was to look at a subset of this processed data to assess the level of agreement between observed ratios of NO 2 to NO and those estimated using current photochemical theory. This filtered data subset consisted of 562 NO 2 ‐NO data pairs. The comparison between observations and predictions was based on the use of the photochemical test ratio (NO 2 ) expt /(NO 2 ) calc , designated here as R e / R c . Although the expected median value for this test ratio was unity, for the PEM‐West A data set it was found to be 3.36. The value of the ratio R e /R c showed a general trend of increasing magnitude with increasing altitude and decreasing latitude. Attempts to understand the sizable discrepancy between observation and prediction (especially for the high‐altitude and low‐latitude data) were explored in the context of two hypotheses: (1) incomplete model chemistry and (2) interferences in the measurement of NO 2 . Efforts to quantify the levels of HO 2 , CH 3 O 2 , RO 2 , and/or ClO x needed to correct the R e /R c discrepancy led to major inconsistencies in the predicted levels of other chemical species. Bromine and iodine chemistries were also investigated with results requiring Br x and I x radical levels well in excess of what would seem reasonable given our current understanding of the source strengths for these elements. This suggests that incompleteness in the model's chemistry was unlikely the major cause of the discrepancy. The second hypothesis, involving interference in the measurement of NO 2 , now appears to be the most likely explanation for the largest component of the deviation in R e /R c from unity. For example, the disagreement between (NO 2 ) expt and (NO 2 ) calc was found to be a strong function of the NO x /NO y ratio. Also, the magnitude of the discrepancy between (NO 2 ) expt and (NO 2 ) calc fell within the possible limits defined by other reactive nitrogen species (e.g., ΔNO y ) available to generate the interference. These results suggest that the further development of a new direct measurement technique for NO 2 , involving a wall collision‐free inlet system, should be considered a high priority. We should also continue, however, to examine the chemical basis of current photochemical models to assess whether yet untested mechanisms might not provide an explanation for these observations.