The Tropospheric Oxidation Capacity Research Group

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The first direct in situ measurements of HO(2)NO(2) in the upper troposphere were performed from the NASA DC-8 during the Intercontinental Chemical Transport Experiment-North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HO(x) chemistry in the free troposphere and complement direct observations of HOx, because of the dual dependency of HO(2)NO(2) on HO(x) and NO(x). On average, the highest HO(2)NO(2) mixing ratio of 76 pptv ( median = 77 pptv, sigma = 39 pptv) was observed at altitudes of 8-9 km. Simple steady state calculations of HO(2)NO(2), constrained by measurements of HO(x), NO(x), and J values, are in good agreement (slope = 0.90, R(2) = 0.60, and z = 5.5-7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO(2)NO(2) is in poor agreement with observed values (R(2) = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HOx show reasonable agreement with the observed HO(2)NO(2) in both the midtroposphere ( slope = 0.96, intercept = 7.0, and R(2) = 0.63) and upper troposphere ( slope = 0.80, intercept = 32.2, and R(2) = 0.58). These results indicate that observed HO(2) and HO(2)NO(2) are in poor agreement in the upper troposphere but that HO(2)NO(2) levels are consistent with current photochemical theory.

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