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

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.