Validation of NO₂ and HNO₃ measurements from the Improved Limb Atmospheric Spectrometer (ILAS) with the version 5.20 retrieval algorithm

Abstract

The Improved Limb Atmospheric Spectrometer (ILAS) on board the Advanced Earth Observing Satellite (ADEOS) measured nitrogen dioxide (NO 2 ) and nitric acid (HNO 3 ) profiles from November 1996 to June 1997 at high latitudes in both hemispheres. The ILAS NO 2 profiles (version 5.20) are compared with those obtained by balloon‐borne and satellite measurements to validate ILAS NO 2 data. Comparisons with balloon‐borne measurements indicate that ILAS NO 2 at 25–30 km has a positive bias of 0.3–0.4 ppbv (6–11%). The random difference in NO 2 at 25–30 km is 0.2–0.3 ppbv (3–9%). The random error in the ILAS NO 2 measurements is larger than 100% below 20 km and above 45 km, where the NO 2 mixing ratios were less than 1.0 ppbv. It is possible that ILAS NO 2 values were lowered by optically thick aerosols with aerosol extinction coefficients at 780 nm of greater than 0.001 km −1 . The lack of diurnal correction along the line of sight contributes to the positive bias in the ILAS NO 2 values below 25 km. Agreement of the ILAS NO 2 values with those by the Polar Ozone and Aerosol Measurement (POAM) II instrument is within 10–30% at 25–35 km. The agreement with the Halogen Occultation Experiment (HALOE) is as good as ±10% at 25–40 km. ILAS HNO 3 (version 5.20) agrees with balloon‐borne HNO 3 to within 0.1 ppbv (0–1%), and the random difference is within 10% at 25–30 km.