Abstract
Abstract. The increasing amounts of reactive nitrogen in the stratosphere necessitate accurate global measurements of stratospheric nitrogen dioxide (NO2). Over the past decade, the SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) instrument on ENVISAT (European Environmental Satellite) has been providing global coverage of stratospheric NO2 every 6 days. In this study, the vertical distributions of NO2 retrieved from SCIAMACHY limb measurements of the scattered solar light are validated by comparison with NO2 products from three different satellite instruments (SAGE II, HALOE and ACE-FTS). The retrieval algorithm based on the information operator approach is discussed, and the sensitivity of the SCIAMACHY NO2 limb retrievals is investigated. The photochemical corrections needed to make this validation feasible, and the chosen collocation criteria are described. For each instrument, a time period of two years is analyzed with several hundreds of collocation pairs for each year. As NO2 is highly variable, the comparisons are performed for five latitudinal bins and four seasons. In the 20 to 40 km altitude range, mean relative differences between SCIAMACHY and other instruments are found to be typically within 20 to 30%. The mean partial NO2 columns in this altitude range agree typically within 15% (both global monthly and zonal annual means). Larger differences are seen for SAGE II comparisons, which is consistent with the results presented by other authors. For SAGE II and ACE-FTS, the observed differences can be partially attributed to the diurnal effect error.
Highlights
As a minor constituent of the atmosphere, NO2 is known for its influence on ozone concentrations
This work gives an overview of the performance and sensitivity of SCIAMACHY NO2 limb retrieval relying mainly on a range of occultation instruments (SAGE II, HALOE, ACEFTS)
Thousands of profile pairs are investigated after applying collocation criteria, using data from each instrument obtained during two years over the whole globe
Summary
As a minor constituent of the atmosphere, NO2 is known for its influence on ozone concentrations. The SCIAMACHY results discussed here are retrieved from measurements of the scattered solar light in limb viewing geometry. The first part of this work gives a description of the SCIAMACHY limb NO2 retrieval, its sensitivity and error sources (including pointing, aerosols, clouds and diurnal effect error), and explains the photochemical model correction method used to match the measurements at different local times. The absolute radiometric calibration and polarization correction do not affect the retrieval results significantly because of the normalization by a limb measurement at an upper tangent height and a usage of the differential spectral structure, respectively. For this reason, these calibration steps were not applied
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