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A 13-year analysis (2000–2012) of the NO2 vertical columndensities derived from ground-based (GB) instruments and satelliteshas been carried out over the Izaña NDACC (Network for the Detection ofthe Atmospheric Composition Change) subtropical site. Ground-based DOAS(differential optical absorption spectroscopy) and FTIR (Fourier transforminfrared spectroscopy) instruments are intercompared to test mutualconsistency and then used for validation of stratospheric NO2 from OMI(Ozone Monitoring Instrument) and SCIAMACHY (SCanning Imaging AbsorptionspectroMeter for Atmospheric CHartographY). The intercomparison has beencarried out taking into account the various differences existing ininstruments, namely temporal coincidence, collocation, sensitivity, field ofview, etc. The paper highlights the importance of considering an “effectivesolar zenith angle” instead of the actual one when comparing direct-suninstruments with zenith sky ones for a proper photochemical correction.Results show that NO2 vertical column densities mean relativedifference between FTIR and DOAS instruments is 2.8 ± 10.7 % fora.m. data. Both instruments properly reproduce the NO2 seasonal and theinterannual variation. Mean relative difference of the stratosphericNO2 derived from OMI and DOAS is −0.2 ± 8.7 % and from OMI andFTIR is −1.6 ± 6.7 %. SCIAMACHY mean relative difference is of 3.7 ± 11.7and −5.7 ± 11.0 % for DOAS and FTIR,respectively. Note that the days used for the intercomparison are not thesame for all the pairs of instruments since it depends on the availabilityof data. The discrepancies are found to be seasonally dependent with largestdifferences in winter and excellent agreement in the spring months (AMJ). Apreliminary analysis of NO2 trends has been carried out with theavailable data series. Results show increases in stratospheric NO2columns in all instruments but larger values in those that are GB than thatexpected by nitrous oxide oxidation. The possible reasons for the discrepancybetween instruments and the positive trends are discussed in the text

atmospheric measurement techniques

Intercomparison of stratospheric nitrogen dioxide columns retrieved from ground-based DOAS and FTIR and satellite DOAS instruments over the subtropical Izana station