A photostationary state analysis of the NO₂‐NO system based on airborne observations from the subtropical/tropical North and South Atlantic

The Chemical Instrumentation Test and Evaluation 3 (CITE 3) NO‐NO 2 database has provided a unique opportunity to examine important aspects of tropospheric photochemistry as related to the rapid cycling between NO and NO 2 . Our results suggest that when quantitative testing of this photochemical system is based on airborne field data, extra precautions may need to be taken in the analysis. This was particularly true in the CITE 3 data analysis where different regional environments produced quite different results when evaluating the photochemical test ratio (NO 2 ) Expt /(NO 2 ) Calc , designated here as R E /R C . The quantity (NO 2 ) Calc was evaluated using the following photostationary state expression: [NO 2 ] Calc = (k 1 [O 3 ] + k 4 [HO 2 ] + k 5 [CH 3 O 2 ] + k 6 [RO 2 ]) [NO] Expt /J 2 . The four most prominent regional environmental data sets identified in this analysis were those labeled here as free‐tropospheric northern hemisphere (FTNH), free‐tropospheric tropical northern hemisphere (FTTNH), free‐tropospheric southern hemisphere (FTSH), and tropical‐marine boundary layer (plume) (TMBL(P)). The respective R E /R C mean and median values for these four data subsets were 1.74, 1.69; 3.00, 2.79; 1.01, 0.97; and 0.99, 0.94. Of the four data subsets listed, the two that were statistically the most robust were FTNH and FTSH; for these the respective R E /R C mean and standard deviation of the mean values were 1.74 ± 0.07 and 1.01, ± 0.04. The FTSH observations were in good agreement with theory, whereas those from the FTNH data set were in significant disagreement. An examination of the critical photochemical parameters O 3 , UV(zenith), NO, NO 2 , and non‐methane hydrocarbons (NMHCs) for these two databases indicated that the most likely source of the R E /R C bias in the FTNH results was the presence of a systematic error in the observational data rather than a shortcoming in our understanding of fundamental photochemical processes. Although neither a chemical nor meteorological analyses of these data identified this error with complete certainty, they did point to the three most likely possibilities: (1) an NO 2 interference from a yet unidentified NO y species; (2) the presence of unmeasured hydrocarbons, the integrated reactivity of which would be equivalent to ∼2.7 parts per billion by volume (ppbv) of toluene; or (3) some combination of points (1) and (2). Details concerning hypotheses (1) and (2) as well as possible ways to minimize these problems in future airborne missions are discussed.