Validation of POAM III NO2 Measurements

The Polar Ozone and Aerosol Measurement (POAM) III instrument is a nine-channel (354 to 1018 nm) solar occultation instrument designed to measure stratospheric profiles of ozone, nitrogen dioxide and water vapor densities, aerosol extinction at five wavelengths, and temperature. It was launched onboard the Satellite Pour l'Observation de la Terre (SPOT) 4 in March of 1998 into a sun synchronous orbit, and is currently operational. In this paper we describe POAM III NO2 measurements and their associated errors, and show comparisons to correlative data. POAM III NO₂ is derived through differential measurements at 439.6 nm (NO₂-on channel) and 442.2 nm (NO₂-off channel), between the altitudes of 20 and 40 km. The random error of the NO₂ retrievals is less than 5% between about 21 and 38 km, and increases to about 6% at 40 km and 8% at 20 km. The vertical resolution of the retrieved NO₂ is about 1.5 km or better between altitudes of 25 and 35 km, increasing to nearly 3 km at altitudes of 20 and 40 km. We show comparisons between POAM NO₂ and NO₂ derived from the satellite-based Halogen Occultation Experiment (HALOE), as well as other airborne and ground-based instruments. These comparisons demonstrate that POAM measurements of NO₂ are reasonable in terms of their magnitude, profile structure, and temporal variations. A statistical analysis of the POAM-HALOE differences shows that, on average, these differences are less than 0.2 ppbv below 34 km, increasing to about 0.7 ppbv by 40 km (POAM higher than HALOE). This corresponds to a relative difference at 40 km of about 15%, which is within the range quoted for previous comparisons between HALOE and other correlative data. We conclude that the POAM III NO₂ data are valid for scientific use.