Radiative Transfer Model SCIATRAN Research Articles

An improved retrieval of tropospheric NO<sub>2</sub> from space over polluted regions using an Earth radiance reference

Abstract. A novel tropospheric NO2 DOAS retrieval algorithm optimised for a nadir-viewing satellite instrument imaging polluted areas is proposed in this work. Current satellite DOAS retrievals have relied on using a solar reference spectrum to derive a total slant column, then using either model assimilation or spatial filtering to derive the tropospheric component. In the ERrs-DOAS (Earth radiance reference sector DOAS) algorithm, tropospheric NO2 slant columns are derived using spectra averaged from measurements over unpolluted regions, thus removing the need for post-hoc separation techniques, though some residual stratospheric biases may still remain. To validate the ERrs-DOAS algorithm, DOAS retrievals were performed on modelled spectra created by the radiative transfer model SCIATRAN, as well as L1B Earth radiance data measured by the NASA/KNMI Ozone Monitoring Instrument (OMI). It was found that retrievals using an Earth radiance reference produce spatial distributions of tropospheric NO2 over eastern China during June 2005 that highly correlate with those derived using existing retrieval algorithms. Comparisons with slant columns retrieved by the operational NO2 retrieval algorithm for OMI (OMNO2A) show that the ERrs-DOAS algorithm greatly reduces the presence of artificial across-track biases (stripes) caused by calibration errors, as well as the removal of path length enhancement in off-nadir pixels. Analysis of Pacific SCDs suggests that the algorithm also produces a 27% reduction in retrieval uncertainty, though this may be partially due to biases introduced by differences in the retrieval algorithm settings. The ERrs-DOAS technique also reveals absorption features over the Sahara and similar regions characteristic of sand and liquid H2O absorption, as first discovered in the analysis of GOME-2 NO2 retrievals.

Effect of surface BRDF of various land cover types on geostationary observations of tropospheric NO<sub>2</sub>

Abstract. We investigated the effect of surface reflectance anisotropy, bidirectional reflectance distribution function (BRDF), on satellite retrievals of tropospheric NO2. We assume the geometry of geostationary measurements over Tokyo, which is one of the worst air-polluted regions in East Asia. We calculated air mass factors (AMF) and box AMFs (BAMF) for tropospheric NO2 to evaluate the effect of BRDF by using the radiative transfer model SCIATRAN. To model the BRDF effect, we utilized the Moderate Resolution Imaging Spectroradiometer (MODIS) products (MOD43B1 and MOD43B2), which provide three coefficients to express the RossThick–LiSparse reciprocal model, a semi-empirical and kernel-based model of BRDF. Because BRDF depends on the land cover type, we also utilized the High Resolution Land-Use and Land-Cover Map of the Advanced Land Observing Satellite (ALOS)/Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2), which classifies the ground pixels over Tokyo into six main types: water, urban, paddy, crop, deciduous forest, and evergreen forest. We first develop an empirical model of the three BRDF coefficients for each land cover type over Tokyo and then apply the model to the calculation of land-cover-type-dependent AMFs and BAMFs. Results show that the variability of AMF among the land types is up to several tens of percent, and if we neglect the reflectance anisotropy, the difference with AMFs based on BRDF reaches 10% or more. The evaluation of the BAMFs calculated shows that not considering BRDF will cause large errors if the concentration of NO2 is high close to the surface, although the importance of BRDF for AMFs decreases for large aerosol optical depth (AOD).

Determination of Cloud Optical Thickness Over Snow Using Satellite Measurements in the Oxygen A-Band

We present our newly developed Cloud optical thickness Retrieval Over Snow (CROS) algorithm, which makes use of the sensitivity of top-of-atmosphere (TOA) reflectance in the oxygen A-band to the cloud optical thickness. The CROS algorithm applies forward simulations for clouds over snow using the radiative transfer model SCIATRAN in order to find the cloud optical thickness (and cloud bottom height and effective underlying surface albedo) representing the measured TOA radiance for a given cloud top height and solar zenith angle. We present results for synthetic retrievals as well as an error analysis with respect to errors in cloud top height. It was demonstrated that the retrievals are most robust for thin high clouds. The retrievals for low-level clouds require an accurate estimation of the cloud altitude ahead of the cloud optical thickness retrieval. Also, it is demonstrated that the retrievals become less accurate for the cases of low Sun.

Measurement of atmospheric CO2 vertical column density using weighting function modified differential optical absorption spectroscopy

In this paper, we demonstrate a method of atmospheric CO2 vertical column density (VCD) retrieval by using weighting function modified differential optical absorption spectroscopy (WFM-DOAS). Direct sun measurement serves as an example for WFM-DOAS fitting. The atmosphere is divided into 50 layers in the process of WFM-DOAS retrieval, while the simulated sun-normalized radiance and the total weighting functions (CO2, H2O and CH4) for each measurement are computed in terms of the radiative transfer model (RTM) SCIATRAN by using the HITRAN data base and the solar spectrum from Livingston and Wallace (1991). The CO2 vertical column density time series were obtained by utilizing WFM-DOAS fitting method to all direct sun measurement spectra. Fitting errors are all less than 3%. Fitting results for two different DOAS algorithms are compared with each other. It is indicated that the WFM-DOAS technique has the potential of being applied in passive gas remote sensing in the infrared region.

Determination of tropospheric NO2 by airborne multi axis differential optical absorption spectroscopy

The airborne multi axis differential optical absorption spectroscopy (AMAXDOAS) is employed to retrieve tropospheric vertical columns of NO2. The differential slant columns of NO2 are retrieved combined with the spectra in different viewing directions by the DOAS method and the vertical columns are calculated with the radiative transfer model SCIATRAN. The dependences of AMF on altitude, surface albedo, and solar zenith angle are studied. The measurement campaign in pearl region delta region on December 10 in 2008 is reported. The NO2 vertical column distribution is obtained in zenith and nadir directions. The results from airborne multi axis DOAS are compared with those from a ground-based multi axis DOAS in Zhuhai, with a deviation of 0.08. The results show that AMAXDOAS can be used to measure the tropospheric NO2 in a large area within a short time.

Fast retrieval of aerosol optical depth and its sensitivity to surface albedo using remote sensing data

Aerosol remote sensing over land is still a great challenge. The retrieval of optical and micro-physical aerosol properties usually requires comprehensive and computationally extensive retrieval algorithms. An efficient approach for the retrieval of the aerosol optical depth (AOD) is proposed in this paper. It is based on a fast and simple atmospheric radiative transfer model using approximations and analytical equations. The proposed algorithm is validated on both synthetic data from the benchmark radiative transfer model SCIATRAN and on real data from the airborne imaging spectrometer APEX. The promising results confirm the feasibility of retrievals using single wavelength nadir observations under constrained conditions. The AOD retrieval is found to be very sensitive to uncertainties in the surface albedo. In the current experimental setup, 0.01 surface albedo uncertainty leads to approximately 0.2 AOD retrieval uncertainty. Generally, larger AOD leads to the increase of reflectance R at sensor level for underlying “dark” surfaces and to a decrease of R for “bright” surfaces. R does not depend strongly on AOD for a surface albedo in the range of approximately 0.2–0.4. Third and higher orders of aerosol scattering, polarization and three-dimensional effects at the surface–atmosphere interface are not taken into account by this study. Nevertheless, a brief validation shows the feasibility with promising results of the proposed simple and fast AOD retrieval algorithm from remote sensing data.

Ozone profiles retrieval from SCIAMACHY Chappuis-Wulf limb scattered spectra using MART

The Scanning Imaging Absorption spectroMeter for Atmospheric ChartographY (SCIAMACHY) instrument, launched on the Envisat satellite in March 2002, measures the earthshine radiance, simultaneously from the ultraviolet (UV) to the near infrared (NIR), in the three viewing geometries: nadir, limb, and occultation. These measurements are used to retrieve both the total amount and vertical profiles of a large number of atmospheric constituents. In this paper, stratospheric ozone profiles between 15 and 40 km altitude are retrieved on 3 km grids from SCIAMACHY limb scattered radiance in the Chappuis-Wulf band. The study employs a new multiplicative algebraic reconstruction technique (MART) coupled with the radiative transfer model SCIATRAN. This technique is outstanding in that more than one measurement vector element can be used to retrieve the ozone density at any altitude. Furthermore, it is straightforward to understand, easy to implement and likely to produce stable results. Radiance normalization and wavelength pairing is applied to radiance as an intermediate step, using the wavelengths 525 nm, 600 nm and 675 nm. The sensitivity of ozone retrieval by this method to tangent altitude pointing, surface albedo, aerosol and cloud parameters is studied, and the results show that the retrieval impact due to tangent altitude pointing bias is the biggest up to 75% with 1 km shift, and the impact of albedo is limited within 5%. The effect of boundary visibility and cloud parameters can be ignored since their impact is too small. The effectiveness of the retrieval is demonstrated using a set of coincident SCIAMACHY products at Hefei that shows a mean bias of less than 12% between 15 and 40 km, and with a better accuracy of 5% from 16 to 36 km.

On the improvement of NO<sub>2</sub> satellite retrievals – aerosol impact on the airmass factors

Abstract. The accurate determination of nitrogen dioxide (NO2) tropospheric vertical columns from satellite measurements depends strongly on the airmass factor (AMF) used. A sensitivity study was performed with the radiative transfer model SCIATRAN to better understand the impact of aerosols on the calculation of NO2 AMFs. This influence was studied by varying the NO2 and aerosol vertical distributions, as well as physical and optical properties of the particles. In terms of aerosol definitions, the key factors for these calculations were identified as the relation between trace gas and aerosol vertical profiles, the optical depth of the aerosol layer, and single scattering albedo. In addition, surface albedo also has a large impact on the calculations. Overall it was found that particles mixed with the trace gas increases the measurements' sensitivity, but only when the aerosol is not very absorbing. The largest change, a factor of ~2 relative to the situation without aerosols, was found when a low layer of aerosol (600 m) was combined with a homogenous NO2 layer of 1.0 km. A layer of aerosol above the NO2 usually reduces the sensitivity of the satellite measurement. This situation is found mostly for runs with discrete elevated aerosol layers (representative for long-range transport) that can generate a decrease of the AMF values of up to 70%. The use of measured aerosol profiles and modelled NO2 resulted, generally, in much smaller changes of AMF relative to the pure Rayleigh case. Exceptions are some events of elevated layers with high aerosol optical depth that lead to a strong decrease of the AMF values. These results highlight the importance of aerosols in the retrieval of tropospheric NO2 columns from space and indicate the need for detailed information on aerosol properties and vertical distribution.

MAX-DOAS measurements of atmospheric trace gases in Ny-Ålesund - Radiative transfer studies and their application

Abstract. A new approach to derive tropospheric concentrations of some atmospheric trace gases from ground-based UV/vis measurements is described. The instrument, referred to as the MAX-DOAS, is based on the well-known UV/vis instruments, which use the sunlight scattered in the zenith sky as the light source and the method of Differential Optical Absorption Spectroscopy (DOAS) to derive column amounts of absorbers like ozone and nitrogen dioxide. Substantial enhancements have been applied to this standard setup to use different lines of sight near to the horizon as additional light sources (MAX - multi axis). Results from measurements at Ny-Ålesund (79° N, 12° E) are presented and interpreted with the full-spherical radiative transfer model SCIATRAN. In particular, measurements of the oxygen dimer O4 which has a known column and vertical distribution in the atmosphere are used to evaluate the sensitivity of the retrieval to parameters such as multiple scattering, solar azimuth, surface albedo and refraction in the atmosphere and also to validate the radiative transfer model. As a first application, measurements of NO2 emissions from a ship lying in Ny-Ålesund harbour are presented. The results of this study demonstrate the feasibility of long term UV/vis multi axis measurement that can be used to derive not only column amounts of different trace gases but also some information on the vertical location of these absorbers.

Trace gas column retrieval from IR nadir spectra – a model study for SCIAMACHY

The SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) is part of the European satellite ENVISAT launched at 1st March 2002. It is a follow-on instrument of the Global Ozone Monitoring Experiment (GOME) flying on ERS-2 and, compared to GOME, has extended capabilities. Using measurements of the direct extra-terrestrial solar spectrum and sun-light reflected and scattered by the earth atmosphere or surface SCIAMACHY detects atmospheric absorption of several trace species absorbing in spectral regions from the ultraviolet to the near infrared (240–2380 nm). Vertical columns of H 2O, N 2O, CO, and CH 4 can be retrieved using SCIAMACHY Channel 8 near-infrared nadir measurements. In this study, selected atmospheric and instrument specific errors relevant for the retrieval are analysed. Spectral windows of Channel 8 are considered, which are currently used for the operational near-real-time processing. For this purpose, spectral error patterns have been simulated as well as sun-normalised model radiances for nadir scanning mode, the latter using the radiative transfer model SCIATRAN. Focus are the polarisation sensitivity and dark signals of the instrument. Further on, accuracy estimates for a number of different atmospheric scenarios are presented.

Air mass factor calculations for GOME measurements of lightning-produced NO 2

The quantification of lightning-produced nitrogen oxide is difficult as the NO x-measurement must be concurrent with the thunderstorm. Also the interpretation of the measurements is complicated by the inhomogeneity of the clouds. Currently, the global NO x-production by lightning is estimated to be 2–20 TgN/a. This study analyses the possibility of the detection of NO 2-production by thunderstorms via satellite. Lightning can be detected by NASA/NASDA-satellite project Lightning Imaging Sensor (LIS). The European satellite spectrometer Global Ozone Monitoring Experiment (GOME) can be used for the coincident NO 2-measurements. For a quantitative analysis of NO 2 measurements, the air mass factor (AMF) is needed. AMF are calculated using the radiative transfer model SCIATRAN and indicate the sensitivity of GOME for NO 2 produced by flashes. The results of the AMF calculations show, that the sensitivity of GOME is largest at the top of a thundercloud, where most of the flash produced NO x is released. The measurement is insensitive to NO 2 under the thundercloud. For a quantitative analysis, the height of the cloud is needed, but the dependencies of the sensitivity to the height of the NO 2 outside the cloud and the cloud particles density are negligible.