From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI’s Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Its main objectives were to determine the accuracy of state-ofthe-art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers and lidar instruments simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the CESAR site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO 2, O4 and HCHO with MAX-DOAS agree within 5 to 15 %, vertical profiles of NO2 derived from several independent instruments agree within 25 % of one another, and MAX-DOAS aerosol optical thickness agrees within 20–30 % with AERONET data. For the in-situ NO2 instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters.

This article focuses on the validation of the total ozone column (TOC) data set acquired by the Global Ozone Monitoring Experiment (GOME) and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite remote sensing instruments using the Total Ozone Retrieval Scheme for the GOME Instrument Based on the Ozone Monitoring Instrument (TOGOMI) and Total Ozone Retrieval Scheme for the SCIAMACHY Instrument Based on the Ozone Monitoring Instrument (TOSOMI) retrieval algorithms developed by the Royal Netherlands Meteorological Institute. In this analysis, spatially colocated, daily averaged ground-based observations performed by five well-calibrated Brewer spectrophotometers at the Iberian Peninsula are used. The period of study runs from January 2004 to December 2009. The agreement between satellite and ground-based TOC data is excellent (R-2 higher than 0.94). Nevertheless, the TOC data derived from both satellite instruments underestimate the ground-based data. On average, this underestimation is 1.1% for GOME and 1.3% for SCIAMACHY. The SCIAMACHY-Brewer TOC differences show a significant solar zenith angle (SZA) dependence which causes a systematic seasonal dependence. By contrast, GOME-Brewer TOC differences show no significant SZA dependence and hence no seasonality although processed with exactly the same algorithm. The satellite-Brewer TOC differences for the two satellite instruments show a clear and similar dependence on the viewing zenith angle under cloudy conditions. In addition, both the GOME-Brewer and SCIAMACHY-Brewer TOC differences reveal a very similar behavior with respect to the satellite cloud properties, being cloud fraction and cloud top pressure, which originate from the same cloud algorithm (Fast Retrieval Scheme for Clouds from the Oxygen A-Band (FRESCO+)) in both the TOSOMI and TOGOMI retrieval algorithms.

In June 2009, 22 spectrometers from 14 institutes measured tropospheric and stratospheric NO2 from the ground for more than 11 days during the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI), at Cabauw, NL (51.97° N, 4.93° E). All visible instruments used a common wavelength range and set of cross sections for the spectral analysis. Most of the instruments were of the multi-axis design with analysis by differential spectroscopy software (MAX-DOAS), whose non-zenith slant columns were compared by examining slopes of their least-squares straight line fits to mean values of a selection of instruments, after taking 30-min averages. Zenith slant columns near twilight were compared by fits to interpolated values of a reference instrument, then normalised by the mean of the slopes of the best instruments. For visible MAX-DOAS instruments, the means of the fitted slopes for NO2 and O4 of all except one instrument were within 10% of unity at almost all non-zenith elevations, and most were within 5%. Values for UV MAX-DOAS instruments were almost as good, being 12% and 7%, respectively. For visible instruments at zenith near twilight, the means of the fitted slopes of all instruments were within 5% of unity. This level of agreement is as good as that of previous intercomparisons, despite the site not being ideal for zenith twilight measurements. It bodes well for the future of measurements of tropospheric NO2, as previous intercomparisons were only for zenith instruments focussing on stratospheric NO2, with their longer heritage.

The study of total ozone columns above Kiev and variations of ozone concentrations in the troposphere at different altitudes above Kiev was carried out using ground-based Fourier Transform InfraRed (FTIR) spectrometric observations that are taken on a routine basis at the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (MAO NASU). This study was performed within the framework of the international ESA-NIVR-KNMI OMI-AO project no.2907 entitled OMI validation by ground-based remote sensing: ozone columns and atmospheric profiles during the time frame 2005-2008. The infrared FTIR spectral observations of direct solar radiation in the wavelength range of 2-12 micron as transmitted through the Earth's atmosphere were performed during the months of April-October of each year. The aim of the project was the validation of total ozone columns and vertical ozone profiles as obtained by the Ozone Monitoring Instrument (OMI)) onboard of the NASA EOS-Aura scientific satellite platform. The modeling of the ozone spectral band shape near 9.6 microns was performing with the MODTRAN code and a molecular band model based on the HITRAN-2004 molecular database. The a-priori information for the spectral modeling consisted of water vapor and temperature profiles from the NASA EOS-Aqua-AIRS satellite instrument, stratospheric ozone profiles from the NASA EOS-Aura-MLS satellite instrument, TEMIS-KNMI climatological ozone profiles and surface ozone concentration measurements performed at the specific times of infrared spectra observations. The troposphere ozone variability was analyzed for two typical episodes: the spring episode of enhanced total ozone columns and the summer episode of enhanced surface ozone concentrations.

This article focuses on the global‐scale validation of the empirically corrected Version 8 total ozone column data set acquired by the NASA Total Ozone Mapping Spectrometer (TOMS) during the period 1996–2004 when this instrument was flying aboard the Earth Probe (EP) satellite platform. This analysis is based on the use of spatially co‐located, ground‐based measurements from Dobson and Brewer spectrophotometers. The original EP‐TOMS V8 total ozone column data set was also validated with these ground‐based measurements to quantify the improvements made by the empirical correction that was necessary as a result of instrumental degradation issues occurring from the year 2000 onward that were uncorrectable by normal calibration techniques. EP‐TOMS V8‐corrected total ozone data present a remarkable improvement concerning the significant negative bias of around ∼3% detected in the original EP‐TOMS V8 observations after the year 2000. Neither the original nor the corrected EP‐TOMS satellite total ozone data sets show a significant dependence on latitude. In addition, both EP‐TOMS satellite data sets overestimate the Brewer measurements for small solar zenith angles (SZA) and underestimate for large SZA, explaining a significant seasonality (∼1.5%) for cloud‐free and cloudy conditions. Conversely, relative differences between EP‐TOMS and Dobson present almost no dependence on SZA for cloud‐free conditions and a strong dependence for cloudy conditions (from +2% for small SZA to −1% for high SZA). The dependence of the satellite ground‐based relative differences on total ozone shows good agreement for column values above 250 Dobson units. Our main conclusion is that the upgrade to TOMS V8‐corrected total ozone data presents a remarkable improvement. Nevertheless, despite its quality, the EP‐TOMS data for the period 2000–2004 should not be used as a source for trend analysis since EP‐TOMS ozone trends are empirically corrected using NOAA‐16 and NOAA‐17 solar backscatter ultraviolet/2 data as external references, and therefore, they are no longer considered as independent observations.

This paper focuses on the validation of the empirically corrected total ozone column (TOC) data provided by the Earth Probe Total Ozone Mapping Spectrometer (EP-TOMS) using ground-based measurements recorded by a well-calibrated Brewer spectroradiometer located at El Arenosillo (Spain). In addition, satellite TOC observations derived from the Ozone Monitoring Instrument (OMI) with the TOMS algorithm are also used in this paper. The agreement between EP-TOMS TOC data and Brewer measurements is excellent (R 2 ~ 0.92) even for the period 2000-2005 when a higher EP-TOMS instrument degradation occurred. Despite its low magnitude, the EP-TOMS-Brewer relative differences depend on the solar zenith angle (SZA), showing a clear seasonal cycle with amplitude between ±2% and ±4%. Conversely, OMI-Brewer relative differences show a constant negative value around -1% with no significant dependence on SZA. No significant dependence on the ground-based to satellite-based differences with respect to the EP-TOMS scene or to the OMI crosstrack position is observed for either satellite retrieval algorithm. Finally, TOC, estimated by the two satellite instruments, have also been compared, showing a good agreement (R 2 ~ 0.88). Overall, we conclude that the empirical correction of the EP-TOMS data record provides a reprocessed set of high quality. However, EP-TOMS data after year 2000 should not be used in calculations of global-ozone trending due to remaining errors in the data set and because it is no longer an independent data set.

In this paper we present the validation results of the operational vertical ozone profiles retrieved from the nadir observations by the Ozone Monitoring Instrument (OMI) aboard the NASA Earth Observing System (EOS) Aura platform. The operational ozone profile retrieval algorithm was developed at the Royal Netherlands Meteorological Institute and the OMI mission data has been processed and made publicly available. Advantages of these nadir sounded ozone profiles are the excellent spatial resolution at nadir and daily global coverage while the vertical resolution is limited to 6-7 km. Comparisons with well-validated ozone profile recordings by the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES), both aboard the NASA EOS-Aura platform, provide an excellent opportunity for validation because of the large amount of collocations with OMI due to the instruments significant geographical overlap. In addition, comparisons with collocated ozone profiles from the Stratospheric Aerosol and Gas Experiment (SAGE-II), the Halogen Occultation Experiment (HALOE), the Global Ozone Monitoring by the Occultation of Stars (GOMOS) and the Optical Spectrograph and Infrared Imager System (OSIRIS) satellite instruments and balloon-borne electrochemical concentration cell (ECC) ozonesondes are presented. OMI stratospheric ozone profiles are found to agree within 20% with global correlative data except for both the polar regions during local spring. For ozone in the troposphere OMI shows a systematic positive bias versus the correlative data sets of order 60% in the tropics and 30% at midlatitude regions. The largest source of error in the tropospheric ozone profile is the fit to spectral stray light in the operational algorithm.

Daily total ozone column (TOC) measurements from the Brewer spectroradiometer located at El Arenosillo (Spain) and the Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) located at Evora (Portugal) are analyzed for the period 2006-2008. The main goal of this study is to compare TOC estimates retrieved with the passive differential optical absorption spectroscopy (DOAS) methodology from zenith-sky observations by the SPATRAM spectrometer with highly accurate TOC estimates retrieved from direct-sun recordings by a well-calibrated Brewer instrument. On average, SPATRAM TOC estimates are (1.69 +/- 0.18)% smaller than collocated Brewer TOC estimates. Significant relative differences between SPATRAM and Brewer total ozone data are detected in some periods caused by the influence of high aerosol load episodes over the SPATRAM data. Satellite data recorded by the ozone monitoring instrument (OMI)-DOAS algorithm aboard the NASA EOS-Aura satellite have been used to quantify the natural spatial variability of TOC over the region of interest as a potential source of uncertainty in the SPATRAM-Brewer comparison. Our results show that the differences observed between both systems is not associated with the natural spatial variability of TOC. In addition, the OMI -DOAS total ozone data have been compared to the ground-based spectroradiometer data showing an excellent agreement for the Brewer spectroradiometer (R-2 similar to 0.95) and a fairly good agreement for the SPATRAM instrument (R-2 similar to 0.89).

This article focuses on the comparison of the total ozone column data from the Ozone Monitoring Instrument (OMI) flying aboard the NASA EOS‐Aura satellite platform with ground‐based measurement recorded by Brewer spectroradiometers located at five Spanish remote sensing ground stations between January 2005 and December 2007. The satellite data are derived from two algorithms: OMI Total Ozone Mapping Spectrometer (OMI‐TOMS) and OMI Differential Optical Absorption Spectroscopy (OMI‐DOAS). The largest relative differences between these OMI total ozone column estimates reach 5% with a significant seasonal dependence. The agreement between OMI ozone data and Brewer measurements is excellent. Total ozone columns from OMI‐TOMS are on average a mere 2.0% lower than Brewer data. For OMI‐DOAS data the bias is a mere 1.4%. However, the relative difference between OMI‐TOMS and Brewer measurements shows a notably lower seasonal dependence and variability than the differences between OMI‐DOAS and ground‐based data. For both OMI ozone data products these relative differences show significant dependence on the satellite ground pixel solar zenith angle for cloud‐free cases as well as for cloudy conditions. However, the OMI ozone data products are shown to reveal opposite behavior with respect to the two antagonistic sky conditions. No significant dependency of the ground‐based to satellite‐based differences with respect to the satellite cross‐track position is seen for either OMI retrieval algorithm.

[1] This paper is an overview of the validation of the total column ozone data products from the Ozone Monitoring Instrument (OMI) on board the NASA EOS-Aura satellite. OMI is an imaging UV/visible spectrometer that maps global ozone on a daily basis. There are two ozone products from OMI, one derived using the traditional TOMS retrieval algorithm and another derived using a Differential Optical Absorption Spectroscopy algorithm that is being developed to take advantage of the hyperspectral capabilities of OMI. Validation is primarily performed through comparison with a network of Dobson and Brewer ground stations and secondarily through campaigns conducted specifically to validate Aura. Comparison with an ensemble of 76 Northern Hemisphere ground stations shows that OMI-TOMS total column ozone averages 0.4% higher than the station average, with station-to-station standard deviation of ±0.6%. The comparison shows that the OMI-TOMS ozone was stable over the 2-year period with no evidence of drift relative to the ground network. The OMI-DOAS product is also stable but with a 1.1% offset and a seasonal variation of ±2%. During four aircraft validation campaigns using the NASA DC-8 and WB-57 aircraft, ozone above the aircraft was measured using an actinic flux instrument and compared with OMI ozone. These comparisons showed agreement within 2% over a broad range of latitude and viewing conditions. Only during the high-latitude flights did the OMI-DOAS ozone show the effects of a solar zenith angle dependent error.

The Ozone Monitoring Instrument (OMI) project team uses two total ozone retrieval algorithms in order to maintain the long‐term record established with Total Ozone Mapping Spectrometer (TOMS) data as well as to improve the ozone column estimate using the hyperspectral capability of OMI. The purpose of this study is to assess where the algorithms produce comparable results and where the differences are significant. Starting with the same set of Earth reflectance data, the total ozone data used in this study have been derived using OMI‐TOMS and OMI–Differential Optical Absorption Spectroscopy (DOAS) algorithms. OMI‐TOMS is based on the TOMS version 8 algorithm that has been used to process TOMS data taken since November 1978. The OMI‐DOAS retrieval algorithm was developed specifically for OMI. It takes advantage of the hyperspectral feature of the OMI instrument to reduce errors due to aerosols, clouds, surface, and sulfur dioxide from volcanic eruptions. The OMI‐DOAS algorithm also has improved correction for cloud height. The mean differences in the ozone column derived from the two algorithms vary from 0 to 9 DU (0–3%), and their correlation coefficients vary between 0.89 and 0.99 with latitude and season. The largest differences occur in the polar regions and over clouds. Some of the differences are due to stray light, dark current, and other instrumental errors that have been corrected in the new version of the OMI radiance/irradiance data set (collection 3). Other differences are algorithmic. OMI‐DOAS algorithmic errors identified through this analysis are also being corrected in collection 3 reprocessing. However, for consistency with the long‐term TOMS record, OMI‐TOMS collection 3 data will still be based on the TOMS V8 algorithm. Preliminary analysis shows much better agreement in the two total ozone data sets after reprocessing. Reprocessed collection 3 data from both algorithms will be available before the end of 2007. Continuing the TOMS total ozone column data record that dates back to November 1978 is the primary OMI mission goal that is achievable with either OMI total ozone column data product.

A comprehensive data set of partial ozone columns was derived from the charge‐coupled device (CCD) Actinic Flux Spectroradiometer (CAFS) measurements taken during the Polar 2005, Houston 2005, and Costa Rica 2006 Aura Validation Experiments (AVE). It was used to validate the colocated daytime Aura Microwave Limb Sounder (MLS) partial ozone columns along the aircraft tracks over diverse geophysical conditions. Results show that the MLS v.1.5 and CAFS ozone columns agree to better than 3% at pressure levels of 100 and 146 hPa, and to better than 5% at 215 hPa level. The partial ozone column differences between the two systems were the largest during the Polar AVE (PAVE) 2005 campaign (polar region, ∼250 hPa pressure level), and the smallest during the CRAVE 2006 campaign (tropics, ∼100 hPa pressure level). Overall, the averaged bias between the MLS and CAFS partial ozone column is about 2%, and the standard deviation of the differences is about 2%. The v.2.2 update of the MLS data tends to reduce the bias to less than 1%. In addition, the AVE 2005 campaign uncovered an altitude‐dependent bias, where the MLS partial ozone columns above 100 and 146 hPa pressure levels were about 1% higher than the CAFS derived columns, while the bias increased to about 3 % in partial columns integrated above 215 hPa. However, the MLS and CAFS data track each other closely over a wide range of atmospheric conditions, and the differences lie within the combined uncertainties of the two data sets.

We review the standard nitrogen dioxide (NO2) data product (Version 1.0.), which is based on measurements made in the spectral region 415–465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System‐Aura satellite. A number of ground‐ and aircraft‐based measurements have been used to validate the data product's three principal quantities: stratospheric, tropospheric, and total NO2 column densities under nearly or completely cloud‐free conditions. The validation of OMI NO2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO2 over its field of view (minimum 340 km2), while a ground‐based instrument samples at a single point. The tropospheric NO2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from <1015 cm−2 over remote, rural areas to >1016 cm−2 over urban and industrial areas. Because of OMI's areal averaging, when validation measurements are made near NO2 sources the OMI measurements are expected to underestimate the ground‐based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO2 amounts; the correlations between nearby instruments is 0.8–0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground‐based measurements, with OMI stratospheric NO2 underestimated by about 14% and total and tropospheric columns underestimated by 15–30%. Typical correlations between OMI NO2 and ground‐based measurements are generally >0.6.

We present a sensitivity analysis of the tropospheric NO2 retrieval from the Ozone Monitoring Instrument (OMI) using measurements from the Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIAMACHY (DANDELIONS) and Intercontinental Chemical Transport Experiment-B (INTEX-B) campaigns held in 2006. These unique campaigns covered a wide range of pollution conditions and provided detailed information on the vertical distribution of NO2. During the DANDELIONS campaign, tropospheric NO2 profiles were measured with a lidar in a highly polluted region of the Netherlands. During the INTEX-B campaign, NO2 profiles were measured using laser-induced fluorescence onboard an aircraft in a range of meteorological and polluted conditions over the Gulf of Mexico and the east Pacific. We present a comparison of measured profiles with a priori profiles used in the OMI tropospheric NO2 retrieval algorithm. We examine how improvements in surface albedo estimates improve the OMI NO2 retrieval. From these comparisons we find that the absolute average change in tropospheric columns retrieved with measured profiles and improved surface albedos is 23% with a standard deviation of 27% and no trend in the improved being larger or smaller than the original. We show that these changes occur in case studies related to pollution in the southeastern United States and pollution outflow in the Gulf of Mexico. We also examine the effects of using improved Mexico City terrain heights on the OMI NO2 product.

Ground‐based UV‐visible instruments for NO2 vertical column measurements have been operating at Issyk‐Kul station, in Kyrgyzstan, and Observatoire de Haute‐Provence (OHP), in France, since 1983 and 1992, respectively. These measurements have already been used for validation of ERS‐2 Global Ozone Monitoring Experiment (GOME) and Envisat Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) NO2 column data. Building upon the successful missions of GOME and SCIAMACHY, the Ozone Monitoring Experiment (OMI) was launched by NASA onboard the EOS Aura satellite in July 2004. Here we present the results of recent comparisons between OMI NO2 operational data (standard product) and correlative ground‐based twilight measurements in midlatitudes, at Issyk‐Kul and OHP, in 2004–2006. The stratospheric NO2 columns, observed by OMI and our ground‐based instruments, have been corrected for NO2 diurnal change and normalized to local noon values using a photochemical box model. According to our comparison, OMI stratospheric NO2 columns underestimate ground‐based measurements by (0.3 ± 0.3) × 1015 molecules/cm2 and (0.7 ± 0.6) × 1015 molecules/cm2 at Issyk‐Kul and OHP, respectively. The effect of tropospheric pollution on the NO2 measurements in both regions of observations has been identified and discussed.