Measurement of HO₂NO₂ in the free troposphere during the Intercontinental Chemical Transport Experiment - North America 2004

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.