Systeme d'Analyse Par Observation Zenithale Research Articles

Accuracy of satellite total column ozone measurements in polar vortex conditions: Comparison with ground-based observations in 1979–2013

This study presents the first detailed long-term analysis of satellite and ground-based Dobson, Brewer, SAOZ (Système D'Analyse par Observations Zénithales) and DOAS (Differential Optical Absorption Spectrometer) measurements inside the Antarctic vortex for the 1979–2013 period. In general, the satellite measurements show a good agreement with ground-based measurements at all stations (correlation coefficient > 0.95). The average relative difference between ground-based and satellite measurements is about ±1–3% and the mean bias error (MBE) is within ±4 DU, depending on the satellite instrument and ground-based station. The satellite measurements show good stability over their operational period, for which the estimated drifts are about ±0.1–5%/decade and are statistically insignificant for most instruments. The Nimbus TOMS (Total Ozone Monitoring Spectrometer) and OMI (Ozone Monitoring Instrument) measurements are stable over the period, but the Earth Probe-TOMS measurements show deterioration after 2000. Also, most satellite instruments show no dependency on satellite solar zenith angle (SZA), but reveal significant dependency on stratospheric temperature. In addition, the mean relative difference shows larger variation with lower total column ozone (TCO). This long-term and multi-instrument comparison exercise would help both ground-based and satellite measurement community to better analyse accuracy of the TCO measurements from the instruments and examine the stability of their long-term measurements. In addition, the SZA and temperature dependency assessment would also help the ozone observation community to use better absorption cross-sections for TCO retrievals.

Evaluating a new homogeneous total ozone climate data record from GOME/ERS‐2, SCIAMACHY/Envisat, and GOME‐2/MetOp‐A

AbstractThe European Space Agency's Ozone Climate Change Initiative (O3‐CCI) project aims at producing and validating a number of high‐quality ozone data products generated from different satellite sensors. For total ozone, the O3‐CCI approach consists of minimizing sources of bias and systematic uncertainties by applying a common retrieval algorithm to all level 1 data sets, in order to enhance the consistency between the level 2 data sets from individual sensors. Here we present the evaluation of the total ozone products from the European sensors Global Ozone Monitoring Experiment (GOME)/ERS‐2, SCIAMACHY/Envisat, and GOME‐2/MetOp‐A produced with the GOME‐type Direct FITting (GODFIT) algorithm v3. Measurements from the three sensors span more than 16 years, from 1996 to 2012. In this work, we present the latest O3‐CCI total ozone validation results using as reference ground‐based measurements from Brewer and Dobson spectrophotometers archived at the World Ozone and UV Data Centre of the World Meteorological Organization as well as from UV‐visible differential optical absorption spectroscopy (DOAS)/Système D′Analyse par Observations Zénithales (SAOZ) instruments from the Network for the Detection of Atmospheric Composition Change. In particular, we investigate possible dependencies in these new GODFIT v3 total ozone data sets with respect to latitude, season, solar zenith angle, and different cloud parameters, using the most adequate type of ground‐based instrument. We show that these three O3‐CCI total ozone data products behave very similarly and are less sensitive to instrumental degradation, mainly as a result of the new reflectance soft‐calibration scheme. The mean bias to the ground‐based observations is found to be within the 1 ± 1% level for all three sensors while the near‐zero decadal stability of the total ozone columns (TOCs) provided by the three European instruments falls well within the 1–3% requirement of the European Space Agency's Ozone Climate Change Initiative project.

Tropospheric nitrogen dioxide column retrieval from ground-based zenith–sky DOAS observations

Abstract. We present an algorithm for retrieving tropospheric nitrogen dioxide (NO2) vertical column densities (VCDs) from ground-based zenith–sky (ZS) measurements of scattered sunlight. The method is based on a four-step approach consisting of (1) the differential optical absorption spectroscopy (DOAS) analysis of ZS radiance spectra using a fixed reference spectrum corresponding to low NO2 absorption, (2) the determination of the residual amount in the reference spectrum using a Langley-plot-type method, (3) the removal of the stratospheric content from the daytime total measured slant column based on stratospheric VCDs measured at sunrise and sunset, and simulation of the rapid NO2 diurnal variation, (4) the retrieval of tropospheric VCDs by dividing the resulting tropospheric slant columns by appropriate air mass factors (AMFs). These steps are fully characterized and recommendations are given for each of them. The retrieval algorithm is applied on a ZS data set acquired with a multi-axis (MAX-) DOAS instrument during the Cabauw (51.97° N, 4.93° E, sea level) Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI) held from 10 June to 21 July 2009 in the Netherlands. A median value of 7.9 × 1015 molec cm−2 is found for the retrieved tropospheric NO2 VCDs, with maxima up to 6.0 × 1016 molec cm−2. The error budget assessment indicates that the overall error σTVCD on the column values is less than 28%. In the case of low tropospheric contribution, σTVCD is estimated to be around 39% and is dominated by uncertainties in the determination of the residual amount in the reference spectrum. For strong tropospheric pollution events, σTVCD drops to approximately 22% with the largest uncertainties on the determination of the stratospheric NO2 abundance and tropospheric AMFs. The tropospheric VCD amounts derived from ZS observations are compared to VCDs retrieved from off-axis and direct-sun measurements of the same MAX-DOAS instrument as well as to data from a co-located Système d'Analyse par Observations Zénithales (SAOZ) spectrometer. The retrieved tropospheric VCDs are in good agreement with the different data sets with correlation coefficients and slopes close to or larger than 0.9. The potential of the presented ZS retrieval algorithm is further demonstrated by its successful application on a 2-year data set, acquired at the NDACC (Network for the Detection of Atmospheric Composition Change) station Observatoire de Haute Provence (OHP; Southern France).

Pollution events over the East Mediterranean: Synergistic use of GOME, ground-based and sonde observations and models

The behaviour of ozone (O 3) and two important precursors, nitrogen dioxide (NO 2) and formaldehyde (HCHO), over the East Mediterranean in spring from 1996 to 2002 is studied in order to characterise the buildup of tropospheric O 3. The vertical distribution of O 3 observed over Crete during the Photochemical Activity and Solar Ultraviolet Radiation (PAUR II) campaign in May 1999 has been used for validation of satellite-derived data. Retrievals of O 3 columns from measurements of backscattered radiation by Global Ozone Monitoring Experiment (GOME) are compared with Total Ozone Mapping Spectrometer (TOMS), balloon, Systeme d’Analyse par Observation Zenithale (SAOZ) and LIDAR observations. The total O 3 vertical columns vary between 270 and 402 DU and correlate well with changes in air circulation patterns. The total observed variability in tropospheric O 3 is about 25 DU. Chemical box model calculations associate the GOME-observed NO 2 and HCHO tropospheric columns with a potential of daily photochemical enhancement in the tropospheric O 3 columns of about 0.8–1 DU over Crete and estimate the daily potential of regional photochemical buildup within upwind polluted air masses at about 2–8 DU. A Langrangian analysis attributes at most 10–20 DU of tropospheric O 3 to stratosphere–troposphere exchange (STE). The remainder is attributed to long-range transport of O 3 from industrial regions in Central Europe. From 1996 to 2002, in May no significant inter-annual variation in the tropospheric NO 2 and HCHO columns over Crete has been observed by GOME suggesting no detectable increase in regionally produced tropospheric O 3.

Lightning-produced NO<sub>2</sub> observed by two ground-based UV-visible spectrometers at Vanscoy, Saskatchewan in August 2004

Abstract. Ground-based measurements of ozone and NO2 differential slant columns by the SAOZ (Système d'Analyse par Observations Zénithales) and UT-GBS (University of Toronto Ground-Based Spectrometer) instruments during the MANTRA 2004 field campaign are presented herein. During the afternoon of 28 August, a thunderstorm passed over the instruments, which were installed at Vanscoy, Saskatchewan (52° N, 107° W). Enhanced differential slant columns of ozone and NO2 were observed by both instruments during the storm, with maximum values of two and 25 times the expected clear sky columns, respectively. The enhanced ozone differential slant columns are primarily due to the longer path traversed by the solar radiation caused by multiple scattering inside the thick cloud layer associated with the thunderstorm. The enhanced NO2 columns are partly attributed to NOx production by lightning. Two new methods are used to separate the NO2 enhancements into contributions from the longer path length and production by lightning. Combining the observed excess NO2 with lightning flash data from the Canadian Lightning Detection Network and Environment Canada Doppler radar measurements, the production of NO2 molecules per lightning flash is determined. Using these two methods, the best estimate of the production rate is found to be (7.88±2.52)×1026 molecules NO2/flash from the UT-GBS and (6.81±2.17)×1026 molecules NO2/flash from SAOZ. These results are consistent with the range of previous estimates reported in the literature.

Balloon-borne stratospheric BrO measurements: comparison with Envisat/SCIAMACHY BrO limb profiles

Abstract. For the first time, results of four stratospheric BrO profiling instruments, are presented and compared with reference to the SLIMCAT 3-dimensional chemical transport model (3-D CTM). Model calculations are used to infer a BrO profile validation set, measured by 3 different balloon sensors, for the new Envisat/SCIAMACHY (ENVIronment SATellite/SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) satellite instrument. The balloon observations include (a) balloon-borne in situ resonance fluorescence detection of BrO (Triple), (b) balloon-borne solar occultation DOAS measurements (Differential Optical Absorption Spectroscopy) of BrO in the UV, and (c) BrO profiling from the solar occultation SAOZ (Systeme d'Analyse par Observation Zenithale) balloon instrument. Since stratospheric BrO is subject to considerable diurnal variation and none of the measurements are performed close enough in time and space for a direct comparison, all balloon observations are considered with reference to outputs from the 3-D CTM. The referencing is performed by forward and backward air mass trajectory calculations to match the balloon with the satellite observations. The diurnal variation of BrO is considered by 1-D photochemical model calculation along the trajectories. The 1-D photochemical model is initialised with output data of the 3-D model with additional constraints on the vertical transport, the total amount and photochemistry of stratospheric bromine as given by the various balloon observations. Total [Bry]=(20.1±2.5) pptv obtained from DOAS BrO observations at mid-latitudes in 2003, serves as an upper limit of the comparison. Most of the balloon observations agree with the photochemical model predictions within their given error estimates. First retrieval exercises of BrO limb profiling from the SCIAMACHY satellite instrument on average agree to around 20% with the photochemically-corrected balloon observations of the remote sensing instruments (SAOZ and DOAS). An exception is the in situ Triple profile, in which the balloon and satellite data mostly does not agree within the given errors. In general, the satellite measurements show systematically higher values below 25 km than the balloon data and a change in profile shape above about 25 km.

Retrieval of nitrogen dioxide stratospheric profiles from ground-based zenith-sky UV-visible observations: validation of the technique through correlative comparisons

Abstract. A retrieval algorithm based on the Optimal Estimation Method (OEM) has been developed in order to provide vertical distributions of NO2 in the stratosphere from ground-based (GB) zenith-sky UV-visible observations. It has been applied to observational data sets from the NDSC (Network for Detection of Stratospheric Change) stations of Harestua (60° N, 10° E) and Andøya (69° N, 16° E) in Norway. The information content and retrieval errors have been analyzed following a formalism used for characterizing ozone profiles retrieved from solar infrared absorption spectra. In order to validate the technique, the retrieved NO2 vertical profiles and columns have been compared to correlative balloon and satellite observations. Such extensive validation of the profile and column retrievals was not reported in previously published work on the profiling from GB UV-visible measurements. A good agreement - generally better than 25% - has been found with the SAOZ (Système d'Analyse par Observations Zénithales) and DOAS (Differential Optical Absorption Spectroscopy) balloons. A similar agreement has been reached with correlative satellite data from the HALogen Occultation Experiment (HALOE) and Polar Ozone and Aerosol Measurement (POAM) III instruments above 25km of altitude. Below 25km, a systematic underestimation - by up to 40% in some cases - of both HALOE and POAM III profiles by our GB profile retrievals has been observed, pointing out more likely a limitation of both satellite instruments at these altitudes. We have concluded that our study strengthens our confidence in the reliability of the retrieval of vertical distribution information from GB UV-visible observations and offers new perspectives in the use of GB UV-visible network data for validation purposes.