Ozone Monitoring Instrument Retrievals Research Articles

WRF-Chem modeling of sulfur dioxide emissions from the 2008 Kasatochi Volcano

We use the Weather Research Forecasting with Chemistry (WRF-Chem) model to simulate the evolution, dispersion and conversion of the sulfur dioxide (SO<sub>2</sub>) plume generated by the 2008 eruption of Kasatochi Volcano in Alaska, USA. About 1.7 Tg of SO<sub>2</sub> were dispersed into the atmosphere during three distinct explosive events. Stratospheric sulfur dioxide conversion chemistry is detailed and model output is compared to remote sensing retrievals from the Ozone Monitoring Instrument (OMI). WRF-Chem generated SO<sub>2</sub> column densities and plume locations similar to those from OMI retrievals as the plume traveled from the North Pacific through the continental United States and Canada. Analysis of SO<sub>2</sub> conversion established an eight day lifetime of SO<sub>2</sub> for the Kastaochi plume, which is a slightly shorter lifetime than derived by other modeling methods.

Evaluation of OMI operational standard NO<sub>2</sub> column retrievals using in situ and surface-based NO<sub>2</sub> observations

Abstract. We assess the standard operational nitrogen dioxide (NO2) data product (OMNO2, version 2.1) retrieved from the Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite using a combination of aircraft and surface in~situ measurements as well as ground-based column measurements at several locations and a bottom-up NOx emission inventory over the continental US. Despite considerable sampling differences, NO2 vertical column densities from OMI are modestly correlated (r = 0.3–0.8) with in situ measurements of tropospheric NO2 from aircraft, ground-based observations of NO2 columns from MAX-DOAS and Pandora instruments, in situ surface NO2 measurements from photolytic converter instruments, and a bottom-up NOx emission inventory. Overall, OMI retrievals tend to be lower in urban regions and higher in remote areas, but generally agree with other measurements to within ± 20%. No consistent seasonal bias is evident. Contrasting results between different data sets reveal complexities behind NO2 validation. Since validation data sets are scarce and are limited in space and time, validation of the global product is still limited in scope by spatial and temporal coverage and retrieval conditions. Monthly mean vertical NO2 profile shapes from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) used in the OMI retrievals are highly consistent with in situ aircraft measurements, but these measured profiles exhibit considerable day-to-day variation, affecting the retrieved daily NO2 columns by up to 40%. This assessment of OMI tropospheric NO2 columns, together with the comparison of OMI-retrieved and model-simulated NO2 columns, could offer diagnostic evaluation of the model.

Space-based retrieval of NO<sub>2</sub> over biomass burning regions: quantifying and reducing uncertainties

Abstract. The accuracy of space-based nitrogen dioxide (NO2) retrievals from solar backscatter radiances critically depends on a priori knowledge of the vertical profiles of NO2 and aerosol optical properties. This information is used to calculate an air mass factor (AMF), which accounts for atmospheric scattering and is used to convert the measured line-of-sight "slant" columns into vertical columns. In this study we investigate the impact of biomass burning emissions on the AMF in order to quantify NO2 retrieval errors in the Ozone Monitoring Instrument (OMI) products over these sources. Sensitivity analyses are conducted using the Linearized Discrete Ordinate Radiative Transfer (LIDORT) model. The NO2 and aerosol profiles are obtained from a 3-D chemistry-transport model (GEOS-Chem), which uses the Fire Locating and Monitoring of Burning Emissions (FLAMBE) daily biomass burning emission inventory. Aircraft in situ data collected during two field campaigns, the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) and the Dust and Biomass-burning Experiment (DABEX), are used to evaluate the modeled aerosol optical properties and NO2 profiles over Canadian boreal fires and West African savanna fires, respectively. Over both domains, the effect of biomass burning emissions on the AMF through the modified NO2 shape factor can be as high as −60%. A sensitivity analysis also revealed that the effect of aerosol and shape factor perturbations on the AMF is very sensitive to surface reflectance and clouds. As an illustration, the aerosol correction can range from −20 to +100% for different surface reflectances, while the shape factor correction varies from −70 to −20%. Although previous studies have shown that in clear-sky conditions the effect of aerosols on the AMF was in part implicitly accounted for by the modified cloud parameters, here it is suggested that when clouds are present above a surface layer of scattering aerosols, an explicit aerosol correction would be beneficial to the NO2 retrieval. Finally, a new method that uses slant column information to correct for shape-factor-related AMF error over NOx emission sources is proposed, with possible application to near-real-time OMI retrievals.

Retrieving tropospheric nitrogen dioxide from the Ozone Monitoring Instrument: effects of aerosols, surface reflectance anisotropy, and vertical profile of nitrogen dioxide

Abstract. Retrievals of tropospheric nitrogen dioxide (NO2) from the Ozone Monitoring Instrument (OMI) are subject to errors in the treatments of aerosols, surface reflectance anisotropy, and vertical profile of NO2. Here we quantify the influences over China via an improved retrieval process. We explicitly account for aerosol optical effects (simulated by nested GEOS-Chem at 0.667° long. × 0.5° lat. and constrained by aerosol measurements), surface reflectance anisotropy, and high-resolution vertical profiles of NO2 (simulated by GEOS-Chem). Prior to the NO2 retrieval, we derive the cloud information using consistent ancillary assumptions. We compare our retrieval to the widely used DOMINO v2 product, using MAX-DOAS measurements at three urban/suburban sites in East China as reference and focusing the analysis on the 127 OMI pixels (in 30 days) closest to the MAX-DOAS sites. We find that our retrieval reduces the interference of aerosols on the retrieved cloud properties, thus enhancing the number of valid OMI pixels by about 25%. Compared to DOMINO v2, our retrieval better captures the day-to-day variability in MAX-DOAS NO2 data (R2 = 0.96 versus 0.72), due to pixel-specific radiative transfer calculations rather than the use of a look-up table, explicit inclusion of aerosols, and consideration of surface reflectance anisotropy. Our retrieved NO2 columns are 54% of the MAX-DOAS data on average, reflecting the inevitable spatial inconsistency between the two types of measurement, errors in MAX-DOAS data, and uncertainties in our OMI retrieval related to aerosols and vertical profile of NO2. Sensitivity tests show that excluding aerosol optical effects can either increase or decrease the retrieved NO2 for individual OMI pixels with an average increase by 14%. Excluding aerosols also complexly affects the retrievals of cloud fraction and particularly cloud pressure. Employing various surface albedo data sets slightly affects the retrieved NO2 on average (within 10%). The retrieved NO2 columns increase when the NO2 profiles are taken from MAX-DOAS retrievals (by 19% on average) or TM4 simulations (by 13%) instead of GEOS-Chem simulations. Our findings are also relevant to retrievals of other pollutants (e.g., sulfur dioxide, ormaldehyde, glyoxal) from UV–visible backscatter satellite instruments.

Volcanic sulfur dioxide plume forecasts based on UV satellite retrievals for the 2011 Grímsvötn and the 2010 Eyjafjallajökull eruption

AbstractThe sulfur dioxide plumes released by the eruptions of the Icelandic volcanoes Eyjafjallajökull in May 2010 and Grímsvötn in May 2011 were studied using satellite observations from the Second Global Ozone Monitoring Experiment (GOME‐2), Ozone Monitoring Instrument (OMI), and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography and modeled with the Integrated Forecasting System of the European Centre for Medium‐Range Weather Forecasts (ECMWF). The retrievals of SO2 total columns (TCSO2) were (i) used to estimate emission rate and injection height of the two eruptions and (ii) assimilated with ECMWF's four‐dimensional variational data assimilation algorithm to obtain initial conditions for subsequent forecasts. The OMI retrievals provided the highest plume observation values, and GOME‐2 had the best coverage. The emission parameters were estimated by comparing TCSO2 observations with an ensemble of test tracers injected at different heights. The applied methodology led to emission estimates of 0.25 Tg over 20 days in May 2010 and 0.32 Tg over 2 days in May 2011. The SO2 analyses produced by assimilating GOME‐2 TCSO2 retrievals captured the plume maxima well but exaggerated the plume area. The injection height estimate was used during the assimilation to determine the height of the assimilated plume. Plume forecasts were evaluated by means of hit‐rate and plume‐size statistics for different TCSO2 thresholds. Plume forecasts using either the emission parameters or the SO2 analyses as initial conditions agreed reasonably with the observations, but using both led to the best forecast performance. The initialization with SO2 analysis fields improved, in particular, the forecast of the Grímsvötn plume after the end of the eruption. The developed forecast and assimilation system can be applied for near–real‐time forecasting of volcanic SO2 plumes.

Improvement of OMI ozone profile retrievals in the upper troposphere and lower stratosphere by the use of a tropopause-based ozone profile climatology

Abstract. Motivated by the need of obtaining a more accurate global ozone distribution in the upper troposphere and lower stratosphere (UTLS), we have investigated the use of a tropopause-based (TB) ozone climatology in ozone profile retrieval from the Ozone Monitoring Instrument (OMI). Due to the limited vertical ozone information in the UTLS region from OMI backscattered ultraviolet radiances, better climatological a priori information is important for improving ozone profile retrievals. We present the new TB climatology and evaluate the result of retrievals against previous work. The TB climatology is created using ozonesonde profiles from 1983 through 2008 extended with climatological ozone data above sonde burst altitude (~35 km) with the corresponding temperature profiles used to identify the thermal tropopause. The TB climatology consists of the mean states and 1σ standard deviations for every month for each 10° latitude band. Compared to the previous TB climatology by Wei et al. (2010), three additional processes are applied in deriving our climatology: (1) using a variable shifting offset to define the TB coordinate, (2) separating ozonesonde profiles into tropical and extratropical regimes based on a threshold of 14 km in the thermal tropopause height, and (3) merging with an existing climatology from 5–10 km above the tropopause. The first process changes the reference of profiles to a variable position between local and mean tropopause heights within ±5 km of the tropopause and to the mean tropopause elsewhere. The second helps to preserve characteristics of either tropical or extratropical ozone structures depending on tropopause height, especially in the subtropical region. The third improves the climatology above ozonesonde burst altitudes and in the stratosphere by using climatology derived from many more satellite observations of ozone profiles. With aid from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) tropopause height, the new climatology and retrieval can better represent the dynamical variability of ozone in the tropopause region. The new retrieval result demonstrates significant improvement of UTLS ozone, especially in the extratropical UTLS, when evaluated using ozonesonde measurements and the meteorological data. The use of TB climatology significantly enhances the spatial consistency and the statistical relationship between ozone and potential vorticity/tropopause height in the extratropical UTLS region. Comparisons with ozonesonde measurements show substantial improvements in both mean biases and their standard deviations over the extratropical lowermost stratosphere and upper troposphere. Overall, OMI retrievals with the TB climatology show improved ability in capturing ozone gradients across the tropopause found in tropical/extratropical ozonesonde measurements.

Evaluation of ozone profile and tropospheric ozone retrievals from GEMS and OMI spectra

Abstract. South Korea is planning to launch the GEMS (Geostationary Environment Monitoring Spectrometer) instrument into the GeoKOMPSAT (Geostationary Korea Multi-Purpose SATellite) platform in 2018 to monitor tropospheric air pollutants on an hourly basis over East Asia. GEMS will measure backscattered UV radiances covering the 300–500 nm wavelength range with a spectral resolution of 0.6 nm. The main objective of this study is to evaluate ozone profiles and stratospheric column ozone amounts retrieved from simulated GEMS measurements. Ozone Monitoring Instrument (OMI) Level 1B radiances, which have the spectral range 270–500 nm at spectral resolution of 0.42–0.63 nm, are used to simulate the GEMS radiances. An optimal estimation-based ozone profile algorithm is used to retrieve ozone profiles from simulated GEMS radiances. Firstly, we compare the retrieval characteristics (including averaging kernels, degrees of freedom for signal, and retrieval error) derived from the 270–330 nm (OMI) and 300–330 nm (GEMS) wavelength ranges. This comparison shows that the effect of not using measurements below 300 nm on retrieval characteristics in the troposphere is insignificant. However, the stratospheric ozone information in terms of DFS decreases greatly from OMI to GEMS, by a factor of ∼2. The number of the independent pieces of information available from GEMS measurements is estimated to 3 on average in the stratosphere, with associated retrieval errors of ~1% in stratospheric column ozone. The difference between OMI and GEMS retrieval characteristics is apparent for retrieving ozone layers above ~20 km, with a reduction in the sensitivity and an increase in the retrieval errors for GEMS. We further investigate whether GEMS can resolve the stratospheric ozone variation observed from high vertical resolution Earth Observing System (EOS) Microwave Limb Sounder (MLS). The differences in stratospheric ozone profiles between GEMS and MLS are comparable to those between OMI and MLS below ~3 hPa (~40 km), except with slightly larger biases and larger standard deviations by up to 5%. At pressure altitudes above ~3 hPa, GEMS retrievals show strong influence of a priori and large differences with MLS, which, however, can be sufficiently improved by using better a priori information. The GEMS-MLS differences show negative biases of less than 4% for stratospheric column ozone, with standard deviations of 1–3%, while OMI retrievals show similar agreements with MLS except for 1% smaller biases at middle and high latitudes. Based on the comparisons, we conclude that GEMS will measure tropospheric ozone and stratospheric ozone columns with accuracy comparable to that of OMI and ozone profiles with slightly worse performance than that of OMI below ~3 hPa.

SO2 over central China: Measurements, numerical simulations and the tropospheric sulfur budget

SO2in central China was measured in situ from an aircraft and remotely using the Ozone Monitoring Instrument (OMI) from the Aura satellite; results were used to develop a numerical tool for evaluating the tropospheric sulfur budget ‐ sources, sinks, transformation and transport. In April 2008, measured ambient SO2 concentrations decreased from ∼7 ppbv near the surface to ∼1 ppbv at 1800 m altitude (an effective scale height of ∼800 m), but distinct SO2 plumes were observed between 1800 and 4500 m, the aircraft's ceiling. These free tropospheric plumes play a major role in the export of SO2 and in the accuracy of OMI retrievals. The mean SO2 column contents from aircraft measurements (0.73 DU, Dobson Units) and operational OMI SO2 products (0.63 ± 0.26 DU) were close. The OMI retrievals were well correlated with in situ measurements (r = 0.84), but showed low bias (slope = 0.54). A new OMI retrieval algorithm was tested and showed improved agreement and bias (r = 0.87, slope = 0.86). The Community Multiscale Air Quality (CMAQ) model was used to simulate sulfur chemistry, exhibiting reasonable agreement (r = 0.62, slope = 1.33) with in situ SO2 columns. The mean CMAQ SO2 loading over central and eastern China was 54 kT, ∼30% more than the estimate from OMI SO2 products, 42 kT. These numerical simulations, constrained by observations, indicate that ∼50% (35 to 61%) of the anthropogenic sulfur emissions were transported downwind, and the overall lifetime of tropospheric SO2 was 38 ± 7 h.

Interpreting elevated space-borne HCHO columns over the Mediterranean Sea using the OMI sensor

Abstract. Formaldehyde (HCHO) is an oxidation product of a wide range of volatile organic compounds (VOCs) and important atmospheric constituent found in both the polluted urban atmosphere and remote background sites. In this study, remotely sensed data of HCHO vertical column densities are analyzed over the Mediterranean Sea using the Ozone Monitoring Instrument (OMI). Data analysis indicates a marked seasonal cycle with a summer maximum and winter minimum confined to the marine environment during a three year period (2005–2007) examined. A possible retrieval artifact associated with Saharan dust transport over the region is explored by changing intensity of Saharan dust sources in GEOS-Chem following the recommendation of Generoso et al. (2008). Recalculated air mass factors (AMF), based on the new values of aerosol loadings, lead to a reduction of the summertime "hot spot" in OMI retrieval of HCHO vertical columns over the Mediterranean Sea; however, even after the correction, enhanced values are still present in this region. To explain these values, marine biogenic sources of VOCs are examined. Calculations indicate that emission of phytoplankton-produced isoprene is not likely to explain the enhanced HCHO vertical columns over the Mediterranean Sea. Model simulations in conjunction with measurements studies may be required to fully explore the complex mechanism of HCHO formation over the Mediterranean and its implications for the air quality in the region.

Evaluation of stratospheric NO2retrieved from the Ozone Monitoring Instrument: Intercomparison, diurnal cycle, and trending

[1] A 5+ year record of satellite measurements of nitrogen dioxide columns from the Ozone Monitoring Instrument (OMI) is evaluated to establish the quality of the OMI retrievals and to test our understanding of stratospheric NO2. The use of assimilation techniques to retrieve stratospheric vertical columns of NO2 from OMI slant column observations is described in detail. Over remote areas the forecast model state is generally within 0.15 × 1015 molecules/cm2 of the analysis. Dutch OMI NO2 (DOMINO) and Standard Product (SP) stratospheric NO2 columns agree within 0.3 × 1015 molecules/cm2 (13%) with independent, ground-based measurements. This is comparable to the level of consistency (15–20%) among ground-based techniques. On average, DOMINO stratospheric NO2 is higher than SP by 0.2 × 1015 molecules/cm2, but larger differences occur on the synoptic scale. Overlapping OMI orbits poleward of 30° enabled us to extract information on the diurnal variation in stratospheric NO2. We find that in the Arctic, the daytime increase of NO2 has a distinct seasonal dependence that peaks in spring and fall. Daytime increase rates inside the denoxified Arctic polar vortex are low, but we find high rates (>0.4 × 1015 molecules/cm2/h) outside the vortex. A multilinear regression to the DOMINO record shows a distinct quasi-biennial oscillation (QBO) signal in stratospheric NO2 columns over the tropics. The QBO's amplitude is comparable to the annual cycle and stronger over the Southern Hemisphere than over the Northern Hemisphere. We infer near-identical trends from DOMINO observations (+0.4%/decade) as from ground-based instrumentation over Lauder (+0.6%/decade) in the 2004–2010 period.

Comparison of UV irradiances from Aura/Ozone Monitoring Instrument (OMI) with Brewer measurements at El Arenosillo (Spain) – Part 2: Analysis of site aerosol influence

Abstract. Several validation studies have shown a notable overestimation of the clear sky ultraviolet (UV) irradiance at the Earth's surface derived from satellite sensors such as the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) with respect to ground-based UV data at many locations. Most of this positive bias is attributed to boundary layer aerosol absorption that is not accounted for in the TOMS/OMI operational UV algorithm. Therefore, the main objective of this study is to analyse the aerosol effect on the bias between OMI erythemal UV irradiance (UVER) and spectral UV (305 nm, 310 nm and 324 nm) surface irradiances and ground-based Brewer spectroradiometer measurements from October 2004 to December 2008 at El Arenosillo station (37.1° N, 6.7° W, 20 m a.s.l.), with meteorological conditions representative of the South-West of Spain. The effects of other factors as clouds, ozone and the solar elevation over this intercomparison were analysed in detail in a companion paper (Antón et al., 2010). In that paper the aerosol effects were studied making only a rough evaluation based on aerosol optical depth (AOD) information at 440 nm wavelength (visible range) without applying any correction. We have used the precise information given by single scattering albedo (SSA) from AERONET for the determination of absorbing aerosols which has allowed the correction of the OMI UV data. An aerosol correction expression was applied to the OMI operational UV data using two approaches to estimate the UV absorption aerosol optical depth, AAOD. The first approach was based on an assumption of constant SSA value of 0.91. This approach reduces the OMI UVER bias against the reference Brewer data from 13.4% to 8.4%. Second approach uses daily AERONET SSA values reducing the bias only to 11.6%. Therefore we have obtained a 37% and 12% of improvement respectively. For the spectral irradiance at 324 nm, the OMI bias is reduced from 10.5% to 6.98% for constant SSA and to 9.03% for variable SSA. Similar results were obtained for spectral irradiances at 305 nm, and 310 nm. Contrary to what was expected, the constant SSA approach has a greater bias reduction than variable SSA, but this is a reasonable result according to the discussion about the reliability of SSA values. Our results reflect the level of accuracy that may be reached at the present time in this type of comparison, which may be considered as satisfactory taking into account the remaining dependence on other factors. Nevertheless, improvements must be accomplished to determine reliable absorbing aerosol properties, which appear as a limiting factor for improving OMI retrievals.

Lightning‐generated NOx seen by the Ozone Monitoring Instrument during NASA's Tropical Composition, Cloud and Climate Coupling Experiment (TC4)

We present case studies identifying lightning‐generated upper tropospheric NOx (LNOx) observed during NASA's Tropical Composition, Cloud and Climate Coupling Experiment (TC4) in July and August 2007. In the campaign, DC‐8 aircraft missions, flown from Costa Rica, recorded in situ NO2 profiles near active storms and in relatively quiet areas. We combine these TC4 DC‐8 data with satellite data from the Ozone Monitoring Instrument (OMI) to estimate the lightning‐generated NO2 (LNO2), above background levels, in the observed OMI NO2 fields. We employ improved off‐line processing techniques to customize the OMI retrieval for LNO2. Information on lightning flashes (primarily cloud‐to‐ground) observed by the Costa Rica Lightning Detection Network operated by the Instituto Costarricense de Electricidad and the World Wide Lightning Location Network were examined over storms upwind of regions where OMI indicates enhanced LNO2. These flash data are compared with Tropical Rainfall Measuring Mission/Lightning Imaging Sensor satellite data to estimate total flashes. Finally, using [NOx]/[NO2] ratios from NASA's Global Modeling Initiative model, we estimate LNOx production per flash for four cases and obtain rates of ∼100‐250 mol/flash. These are consistent with rates derived from previous studies of tropical and subtropical storms and below those from modeling of observed midlatitude storms. In our study, environments with stronger anvil‐level winds were associated with higher production rates. LIS flash footprint data for one of the low‐LNOx production cases with weak upper tropospheric winds suggest below‐average flash lengths for this storm. LNOx enhancements over background determined from the OMI data were in less than, but roughly proportional to, aircraft estimates.

Dispersion and lifetime of the SO2 cloud from the August 2008 Kasatochi eruption

Hemispherical dispersion of the SO2 cloud from the August 2008 Kasatochi eruption is analyzed using satellite data from the Ozone Monitoring Instrument (OMI) and the Goddard Trajectory Model (GTM). The operational OMI retrievals underestimate the total SO2 mass by 20–30% on 8–11 August, as compared with more accurate offline Extended Iterative Spectral Fit (EISF) retrievals, but the error decreases with time due to plume dispersion and a drop in peak SO2 column densities. The GTM runs were initialized with and compared to the operational OMI SO2 data during early plume dispersion to constrain SO2 plume heights and eruption times. The most probable SO2 heights during initial dispersion are estimated to be 10–12 km, in agreement with direct height retrievals using EISF algorithm and IR measurements. Using these height constraints a forward GTM run was initialized on 11 August to compare with the month‐long Kasatochi SO2 cloud dispersion patterns. Predicted volcanic cloud locations generally agree with OMI observations, although some discrepancies were observed. Operational OMI SO2 burdens were refined using GTM‐predicted mass‐weighted probability density height distributions. The total refined SO2 mass was integrated over the Northern Hemisphere to place empirical constraints on the SO2 chemical decay rate. The resulting lower limit of the Kasatochi SO2 e‐folding time is ∼8–9 days. Extrapolation of the exponential decay back in time yields an initial erupted SO2 mass of ∼2.2 Tg on 8 August, twice as much as the measured mass on that day.

Validation of ozone monitoring instrument SO2 measurements in the Okmok volcanic cloud over Pullman, WA, July 2008

The ozone monitoring instrument (OMI), launched on the EOS/Aura satellite in July 2004, makes daily global observations of natural and anthropogenic SO2 emissions with unprecedented spatial resolution. Here we present the first robust comparison of OMI volcanic SO2 retrievals with ground−based instrumentation, using direct Sun observations of the Okmok volcanic cloud from Washington State University (WSU) in Pullman, WA on 18–20 July 2008. These measurements were made by the multifunction differential optical absorption spectroscopy (MFDOAS) instrument developed at WSU, as the Okmok cloud drifted over Pullman in the upper troposphere and lower stratosphere (UTLS). Observation conditions were favorable with cloud‐free skies and a relatively homogeneous volcanic cloud distribution on OMI ground pixel scales (∼20–50 km). Movement of the Okmok cloud north and south of Pullman over a period of several days permitted comparison with three OMI overpasses with SO2 column amounts above the SO2 background level. The total SO2 columns measured by MFDOAS during OMI overpasses were 3.11 ± 0.23 Dobson units (DU), 1.75 ± 0.16 DU and 1.22 ± 0.18 DU (1 DU = 2.69 × 1016 molecules/cm2 = 0.029 g/m2). Comparison of ground‐based direct Sun and operational and off‐line OMI retrievals show an excellent agreement, providing the first validation of OMI measurements of volcanic SO2 in the UTLS.

Evaluation of AIRS, IASI, and OMI ozone profile retrievals in the extratropical tropopause region using in situ aircraft measurements

We evaluate ozone profile retrievals from the Atmospheric Infrared Sounder (AIRS), the Infrared Atmospheric Sounding Interferometer (IASI), and the Ozone Monitoring Instrument (OMI) using in situ measurements collected on board the NSF/NCAR Gulfstream‐V aircraft during the Stratosphere‐Troposphere Analyses of Regional Transport 2008 (START08) experiment. The focus of this study is to examine how well the satellite retrieval products capture the ozone gradients and variability in the extratropical upper troposphere lower stratosphere (UTLS). The AIRS retrieval examined is version 5, while IASI and OMI retrievals are research products. All satellite instruments show excellent ability in capturing synoptic‐scale ozone gradients associated with strong potential vorticity (PV) gradients. The positive ozone‐PV correlation near the tropopause is also well represented in the satellite data in comparison to collocated aircraft measurements. During aircraft cruise legs, more than 90% of collocated satellite retrievals agree with aircraft measurements within ±50% for ozone mixing ratios greater than 200 ppbv. Below 200 ppbv, AIRS and IASI retrievals show significant positive biases, while OMI shows both positive and negative biases. Ozone gradients across the tropopause are well‐captured, with median values within 30% (positive for AIRS and IASI, negative for OMI) and variances within ±50%. Ozone variability in the UTLS is captured by the satellite retrievals at the 80% level. In the presence of high clouds, however, the infrared retrievals show the largest positive biases. Despite the limited vertical information content, the high horizontal coverage and long‐term data availability make these satellite data sets a valuable asset for UTLS research.

Validation of OMI‐TOMS and OMI‐DOAS total ozone column using five Brewer spectroradiometers at the Iberian peninsula

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.

NO2columns in the western United States observed from space and simulated by a regional chemistry model and their implications for NOxemissions

There are many isolated sources of NOxemissions across the western United States, including electrical power generation plants and urban areas. In this manuscript, two satellite instruments measuring NO2vertical columns over these sources and an atmospheric chemical‐transport model are used to evaluate bottom‐up NOxemission inventories, model assumptions, and satellite retrieval algorithms. We carried out simulations with the Weather Research and Forecasting‐Chemistry (WRF‐Chem) model for the western U.S. domain during the summer of 2005 using measured power plant NOxemissions. Model NO2vertical columns are compared with a retrieval of the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite instrument data by the University of Bremen and retrievals of the Ozone Monitoring Instrument (OMI) data by the U.S. National Aeronautics and Space Administration (NASA) and a modified version of the NASA OMI retrieval produced by the University of Bremen. For areas dominated by power plant NOxemissions, the model NO2columns serve as a comparison standard for satellite retrievals because emissions are continuously monitored at all large U.S. power plants. An extensive series of sensitivity tests of the assumptions in both the satellite retrievals and the model are carried out over the Four Corners and San Juan power plants, two adjacent facilities in the northwest corner of New Mexico that together represent the largest NOxpoint source in the United States. Overall, the SCIAMACHY and OMI NO2columns over western U.S. power plants agree well with model NO2columns, with differences between the two being within the variability of the model and satellite. In contrast to regions dominated by power plant emissions, model NO2columns over large urban areas along the U.S. west coast are approximately twice as large as satellite NO2columns from SCIAMACHY and OMI retrievals. The discrepancies in urban areas are beyond the sensitivity ranges in the model simulations and satellite observations, implying overestimates of these cities' bottom‐up NOxemissions, which are dominated by motor vehicles. Taking the uncertainties in the satellite retrievals into account, our study demonstrates that the tropospheric columns of NO2retrieved from space‐based observations of backscattered solar electromagnetic radiation can be used to evaluate and improve bottom‐up emission inventories.

Improved assessment of aerosol absorption using OMI‐MODIS joint retrieval

The Ozone Monitoring Instrument (OMI) aboard EOS‐Aura and the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard EOS‐Aqua fly in formation as part of the A‐train. Though OMI retrieves aerosol optical depth (AOD) and aerosol absorption, it must assume aerosol layer height. The MODIS cannot retrieve aerosol absorption, but MODIS aerosol retrieval is not sensitive to aerosol layer height and with its smaller pixel size is less affected by subpixel clouds. Here we demonstrate an approach that uses MODIS‐retrieved AOD to constrain the OMI retrieval, freeing OMI from making an a priori estimate of aerosol height and allowing a more direct retrieval of aerosol absorption. To predict near‐UV optical depths using MODIS data we rely on the spectral curvature of the MODIS‐retrieved visible and near‐IR spectral AODs. Application of an OMI‐MODIS joint retrieval over the north tropical Atlantic shows good agreement between OMI and MODIS‐predicted AODs in the UV, which implies that the aerosol height assumed in the OMI‐standard algorithm is probably correct. In contrast, over the Arabian Sea, MODIS‐predicted AOD deviated from the OMI‐standard retrieval, but combined OMI‐MODIS retrievals substantially improved information on aerosol layer height (on the basis of validation against airborne lidar measurements). This implies an improvement in the aerosol absorption retrieval, but lack of UV absorption measurements prevents a true validation. Our study demonstrates the potential of multisatellite analysis of A‐train data to improve the accuracy of retrieved aerosol products and suggests that a combined OMI‐MODIS‐CALIPSO retrieval has large potential to further improve assessments of aerosol absorption.

Comparison of Ozone Monitoring Instrument UV Aerosol Products with Aqua/Moderate Resolution Imaging Spectroradiometer and Multiangle Imaging Spectroradiometer observations in 2006

An assessment of the consistency of the Ozone Monitoring Instrument (OMI) aerosol products with the results from other satellite aerosol sensors is performed in this paper. The OMI aerosol products include the UltraViolet Aerosol Index (UVAI), Aerosol Absorption Optical Depth (AAOD), and Aerosol Extinction Optical Depth (AOD). The OMI AOD is compared with that from Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) and Terra/Multiangle Imaging Spectroradiometer (MISR). OMI‐retrieved AOD values are generally biased high with respect to MODIS measurements, likely as a result of a calibration offset. Subpixel cloud contamination is the second most important source of error. Other sources of error may contribute to the noise in the retrieval but not necessarily to the systematic bias. In spite of the bias, OMI retrievals show a high degree of correlation with MODIS observations. The analyses of daily cloud‐free collocated AOD data between OMI and MODIS show about 0.15 root mean square error on average relative to the linear fit. OMI also shows a reasonable agreement with MODIS and MISR observations in seasonal annual cycles of aerosols over most of the major emission sources of carbonaceous aerosols from biomass burning and mineral dust from deserts. The AAOD is less sensitive to cloud contamination than the AOD. In addition, the information content in the UVAI is well expressed as AAOD with higher correlation than AOD. However, subpixel cloud contamination of OMI footprint (13 × 24 km2 at nadir) is unavoidable, and retrieved AOD values tend to be overestimated with varying degrees of sensitivity as a function of viewing geometry and aerosol types selected.