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-NO2 dat abase has provided a unique opportunity to examine important aspects o f tropospheric photochemistry as related to the rapid cycling between NO and NO2. Our results suggest that when quantitative testing of this photochemical system is based on airborne field data, extra precautio ns may need to be taken in the analysis. This was particularly true in the CITE 3 data analysis where different regional environments produc ed quite different results when evaluating the photochemical test rati o (NO2)Expt/(NO2)Calc, designated here as R(E)/R(C). The quantity (NO2 )Calc was evaluated using the following photostationary state expressi on: [NO2]Calc = (k1[O3] + k4[HO2] + k5[CH3O2] + k6[RO2])[NO]Expt/J2. T he four most prominent regional environmental data sets identified in this analysis were those labeled here as free-tropospheric northern he misphere (FTNH), free-tropospheric tropical northern hemisphere (FTTNH ), free-tropospheric southern hemisphere (FTSH), and tropical-marine b oundary layer (plume) (TMBL(P)). The respective R(E)/R(C) mean and med ian 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 tha t 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 w ere 1.74 +/- 0.07 and 1.01, +/- 0.04. The FTSH observations were in go od agreement with theory, whereas those from the FTNH data set were in significant disagreement. An examination of the critical photochemica l parameters O3, UV(zenith), NO, NO2, and non-methane hydrocarbons (NM HCs) 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 systemati c error in the observational data rather than a shortcoming in our und erstanding of fundamental photochemical processes. Although neither a chemical nor meteorological analyses of these data identified this err or with complete certainty, they did point to the three most likely po ssibilities: (1) an NO2 interference from a yet unidentified NO(y) spe cies; (2) the presence of unmeasured hydrocarbons, the integrated reac tivity of which would be equivalent to approximately 2.7 parts per bil lion by volume (ppbv) of toluene; or (3) some combination of points (1 ) and (2). Details concerning hypotheses (1) and (2) as well as possib le ways to minimize these problems in future airborne missions are dis cussed.