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 (NO2) and nitric acid (HNO3) profiles from November 1996 to June 1997 at high latitudes in both hemispheres. The ILAS NO2 profiles (version 5.20) are compared with those obtained by balloon‐borne and satellite measurements to validate ILAS NO2 data. Comparisons with balloon‐borne measurements indicate that ILAS NO2 at 25–30 km has a positive bias of 0.3–0.4 ppbv (6–11%). The random difference in NO2 at 25–30 km is 0.2–0.3 ppbv (3–9%). The random error in the ILAS NO2 measurements is larger than 100% below 20 km and above 45 km, where the NO2 mixing ratios were less than 1.0 ppbv. It is possible that ILAS NO2 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 NO2 values below 25 km. Agreement of the ILAS NO2 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 HNO3 (version 5.20) agrees with balloon‐borne HNO3 to within 0.1 ppbv (0–1%), and the random difference is within 10% at 25–30 km.