The first direct in situ measurements of HO2NO2 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 HOx chemistry in the free troposphere and complement direct observations of HOx, because of the dual dependency of HO2NO2 on HOx and NOx. On average, the highest HO2NO2 mixing ratio of 76 pptv (median = 77 pptv, σ, = 39 pptv) was observed at altitudes of 8-9 km. Simple steady state calculations of HO2NO2, constrained by measurements of HOx, NOx, and J values, are in good agreement (slope = 0.90, R² = 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 HO2NO2 is in poor agreement with observed values (R² = 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 HO2NO2 in both the midtroposphere (slope = 0.96, intercept = 7.0, and R² = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R² = 0.58). These results indicate that observed HO2 and HO2NO2 are in poor agreement in the upper troposphere but that HO2NO2 levels are consistent with current photochemical theory. Copyright 2007 by the American Geophysical Union.

The first direct in situ measurements of HO₂NO₂ 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 HO_x, because of the dual dependency of HO₂NO₂ on HO_x and NO_x. On average, the highest HO₂NO₂ mixing ratio of 76 pptv (median = 77 pptv,σ = 39 pptv) was observed at altitudes of 8–9 km. Simple steady state calculations of HO₂NO₂, constrained by measurements of HO_x, NO_x, and J values, are in good agreement (slope = 0.90, R² = 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 HO₂NO₂ is in poor agreement with observed values (R² = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HO_x show reasonable agreement with the observed HO₂NO₂ in both the midtroposphere (slope = 0.96, intercept = 7.0, and R² = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R² = 0.58). These results indicate that observed HO₂ and HO₂NO₂ are in poor agreement in the upper troposphere but that HO₂NO₂ levels are consistent with current photochemical theory.