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

Abstract

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 HO x , 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, σ = 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 HO x 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.