. We present an improved tropospheric nitrogen dioxide column retrieval algorithm (DOMINO v2.0) for OMI based on better air mass factors (AMFs) and a correction for across-track stripes resulting from calibration errors in the OMI backscattered reflectances. Since October 2004, NO2 retrievals from the Ozone Monitoring Instrument (OMI), a UV/Vis nadir spectrometer onboard NASA's EOS-Aura satellite, have been used with success in several scientific studies focusing on air quality monitoring, detection of trends, and NOx emission estimates. Dedicated evaluations of previous DOMINO tropospheric NO2 retrievals indicated their good quality, but also suggested that the tropospheric columns were susceptible to high biases (by 0–40%), probably because of errors in the air mass factor calculations. Here we update the DOMINO air mass factor approach. We calculate a new look-up table (LUT) for altitude-dependent AMFs based on more realistic atmospheric profile parameters, and include more surface albedo and surface pressure reference points than before. We improve the sampling of the TM4 model, resulting in a priori NO2 profiles that are better mixed throughout the boundary layer. We evaluate the NO2 profiles simulated with the improved TM4 sampling as used in the AMF calculations and show that they are highly consistent with in situ NO2 measurements from aircraft during the INTEX-A and INTEX-B campaigns in 2004 and 2006. Our air mass factor calculations are further updated by the implementation of a high-resolution terrain height and a high-resolution surface albedo climatology based on OMI measurements. Together with a correction for across-track stripes, the overall impact of the improved terrain height and albedo descriptions is modest (

/pdf/an-improved-tropospheric-no-2-column-retrieval-algorithm-for-h9jq96uyix.pdf

An improved tropospheric NO 2 column retrieval algorithm for the Ozone Monitoring Instrument

The collisions between two oxygen molecules give rise to O4 absorption in the Earth atmosphere. O4 absorption is relevant to atmospheric transmission and Earth's radiation budget. O4 is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O4 absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203–295 K), and at 820 mbar pressure. We characterize the temperature dependent O4 line shape and provide high precision O4 absorption cross section reference spectra that are suitable for atmospheric O4 measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within <3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH250 = −0.12 ± 0.12 kJ mol−1, which is essentially zero, and supports previous assignments of O4 as collision induced absorption (CIA). At 203 K, van der Waals complexes (O2-dimer) contribute less than 0.14% to the O4 absorption in air. We conclude that O2-dimer is not observable in the Earth atmosphere, and as a consequence the atmospheric O4 distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10−3 from knowledge of the oxygen concentration profile.

Temperature dependent absorption cross-sections of O2–O2 collision pairs between 340 and 630 nm and at atmospherically relevant pressure

SCIATRAN is a comprehensive software package for the modeling of radiative transfer processes in the terrestrial atmosphere and ocean in the spectral range from the ultraviolet to the thermal infrared ( 0.18 – 40 μ m ) including multiple scattering processes, polarization, thermal emission and ocean–atmosphere coupling. The software is capable of modeling spectral and angular distributions of the intensity or the Stokes vector of the transmitted, scattered, reflected, and emitted radiation assuming either a plane-parallel or a spherical atmosphere. Simulations are done either in the scalar or in the vector mode (i.e. accounting for the polarization) for observations by space-, air-, ship- and balloon-borne, ground-based, and underwater instruments in various viewing geometries (nadir, off-nadir, limb, occultation, zenith-sky, off-axis). All significant radiative transfer processes are accounted for. These are, e.g. the Rayleigh scattering, scattering by aerosol and cloud particles, absorption by gaseous components, and bidirectional reflection by an underlying surface including Fresnel reflection from a flat or roughened ocean surface. The software package contains several radiative transfer solvers including finite difference and discrete-ordinate techniques, an extensive database, and a specific module for solving inverse problems. In contrast to many other radiative transfer codes, SCIATRAN incorporates an efficient approach to calculate the so-called Jacobians, i.e. derivatives of the intensity with respect to various atmospheric and surface parameters. In this paper we discuss numerical methods used in SCIATRAN to solve the scalar and vector radiative transfer equation, describe databases of atmospheric, oceanic, and surface parameters incorporated in SCIATRAN, and demonstrate how to solve some selected radiative transfer problems using the SCIATRAN package. During the last decades, a lot of studies have been published demonstrating that SCIATRAN is a valuable tool for a wide range of remote sensing applications. Here, we present some selected comparisons of SCIATRAN simulations to published benchmark results, independent radiative transfer models, and various measurements from satellite, ground-based, and ship instruments. Methods for solving inverse problems related to remote sensing of the Earth's atmosphere using the SCIATRAN software are outside the scope of this study and will be discussed in a follow-up paper. The SCIATRAN software package along with a detailed User's Guide is freely available for non-commercial use via the webpage of the Institute of Environmental Physics (IUP), University of Bremen: http://www.iup.physik.uni-bremen.de/sciatran .

Radiative transfer through terrestrial atmosphere and ocean: Software package SCIATRAN

. Tropospheric NO2, a key pollutant in particular in cities, has been measured from space since the mid-1990s by the GOME, SCIAMACHY, OMI, and GOME-2 instruments. These data provide a unique global long-term dataset of tropospheric pollution. However, the observations differ in spatial resolution, local time of measurement, viewing geometry, and other details. All these factors can severely impact the retrieved NO2 columns. In this study, we present three ways to account for instrumental differences in trend analyses of the NO2 columns derived from satellite measurements, while preserving the individual instruments' spatial resolutions. For combining measurements from GOME and SCIAMACHY into one consistent time series, we develop a method to explicitly account for the instruments' difference in ground pixel size (40 × 320 km2 vs. 30 × 60 km2). This is especially important when analysing NO2 changes over small, localised sources like, e.g. megacities. The method is based on spatial averaging of the measured earthshine spectra and extraction of a spatial pattern of the resolution effect. Furthermore, two empirical corrections, which summarise all instrumental differences by including instrument-dependent offsets in a fitted trend function, are developed. These methods are applied to data from GOME and SCIAMACHY separately, to the combined time series, and to an extended dataset comprising also GOME-2 and OMI measurements. All approaches show consistent trends of tropospheric NO2 for a selection of areas on both regional and city scales, for the first time allowing consistent trend analysis of the full time series at high spatial resolution. Compared to previous studies, the longer study period leads to significantly reduced uncertainties. We show that measured tropospheric NO2 columns have been strongly increasing over China, the Middle East, and India, with values over east-central China tripling from 1996 to 2011. All parts of the developed world, including Western Europe, the United States, and Japan, show significantly decreasing NO2 amounts in the same time period. On a megacity level, individual trends can be as large as +27.2 ± 3.9% yr−1 and +20.7 ± 1.9% yr−1 in Dhaka and Baghdad, respectively, while Los Angeles shows a very strong decrease of −6.00 ± 0.72% yr−1. Most megacities in China, India, and the Middle East show increasing NO2 columns of +5 to 10% yr−1, leading to a doubling to tripling within the study period.

/pdf/long-term-changes-of-tropospheric-no-2-over-megacities-2r988rdtla.pdf

Long-term changes of tropospheric NO 2 over megacities derived from multiple satellite instruments

The Bremen Optimal Estimation differential optical absorption spectroscopy (DOAS) (BESD) algorithm for satellite based retrievals of
 XCO_2 (the column-average dry-air mole fraction of atmospheric CO_2) has
 been applied to Scanning Imaging Absorption Spectrometer for
 Atmospheric Cartography (SCIAMACHY) data. It uses measurements in the
 O_2-A absorption band to correct for scattering of undetected clouds
 and aerosols. Comparisons with precise and accurate ground-based
 Fourier transform spectrometer (FTS) measurements at four Total Carbon
 Column Observing Network (TCCON) sites have been used to quantify the
 quality of the new SCIAMACHY XCO_2 data set. Additionally, the results
 have been compared to NOAA's assimilation system CarbonTracker. The
 comparisons show that the new retrieval meets the expectations from
 earlier theoretical studies. We find no statistically significant
 regional XCO_2 biases between SCIAMACHY and the FTS instruments.
 However, the standard error of the systematic differences is in the
 range of 0.2 ppm and 0.8 ppm. The XCO_2 single-measurement precision of
 2.5 ppm is similar to theoretical estimates driven by instrumental
 noise. There are no significant differences found for the year-to-year
 increase as well as for the average seasonal amplitude between
 SCIAMACHY XCO_2 and the collocated FTS measurements. Comparison of the
 year-to-year increase and also of the seasonal amplitude of
 CarbonTracker exhibit significant differences with the corresponding
 FTS values at Darwin. Here the differences between SCIAMACHY and
 CarbonTracker are larger than the standard error of the SCIAMACHY
 values. The difference of the seasonal amplitude exceeds the
 significance level of 2 standard errors. Therefore, our results suggest
 that SCIAMACHY may provide valuable additional information about XCO_2,
 at least in regions with a low density of in situ measurements.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=6802&context=scipapers

Retrieval of atmospheric CO2 with enhanced accuracy and precision from SCIAMACHY: validation with FTS measurements and comparison with model results

A successor version of the SCIATRAN radiative transfer model (RTM) has been developed to perform radiative transfer modeling in any observation geometry appropriate to measurements of the scattered solar radiation in the Earth’s atmosphere. The model is designed to be used as a forward model in the retrieval of atmospheric constituents from measurements of scattered solar light by satellite, ground-based, or airborne instruments in UV–Vis–NIR spectral region. Furthermore, it can be used to calculate air mass factors or fluxes. The new generation of the SCIATRAN model comprises all features of the latest SCIATRAN 1.2 RTM supporting additionally radiative transfer calculations in a spherical atmosphere. The program is written in FORTRAN 95 and suitable for parallel execution using the OpenMP standard. The wavelength range covered by the radiative transfer model is extended to 175–2380 nm including Schuman-Runge and Herzberg absorption bands of oxygen. The SCIATRAN 2.0 model exhibits the following new capabilities: (i) modeling of the scattered solar radiation in limb viewing geometry as well as any kind of measurements of the scattered radiation within the atmosphere, (ii) corresponding quasi-analytical calculation of weighting functions of atmospheric parameters, (iii) airmass factor calculations for ground-based, space and airborne measurements including off-axis geometry, (v) accounting for photochemically active species, i.e., radiative transfer calculations can be performed using solar zenith angle dependent vertical distributions of atmospheric species, (iv) height resolved radiation fluxes, including actinic fluxes for photolysis rate calculations, (vi) inelastic rotational Raman scattering in any supported viewing geometry, (vii) new effective approximations for radiative transfer modeling in presence of clouds. The SCIATRAN model is freely available via the world wide web for non-commercial scientific applications.

SCIATRAN 2.0 - A new radiative transfer model for geophysical applications in the 175 - 2400 nm spectral region.

SCIATRAN 2.0 – A new radiative transfer model for geophysical applications in the 175–2400 nm spectral region

A recent development in ground-based remote sensing of atmospheric constituents by UV-visible absorption measurements of scattered light is the simultaneous use of several horizon viewing directions in addition to the traditional zenith-sky pointing. The different light paths through the atmosphere enable the vertical distribution of some atmospheric absorbers, such as NO2, BrO, or O3, to be retrieved. This approach has recently been implemented on an airborne platform. This novel instrument, the airborne multiaxis differential optical absorption spectrometer (AMAXDOAS), has been flown for the first time. In this study, the amount of profile information that can be retrieved from such measurements is investigated for the trace gas NO2. Sensitivity studies on synthetic data are performed for a variety of representative measurement conditions including two wavelengths, one in the UV and one in the visible, two different surface spectral reflectances, various lines of sight (LOSs), and for two different flight altitudes. The results demonstrate that the AMAXDOAS measurements contain useful profile information, mainly at flight altitude and below the aircraft. Depending on wavelength and LOS used, the vertical resolution of the retrieved profiles is as good as 2 km near flight altitude. Above 14 km the profile information content of AMAXDOAS measurements is sparse. Airborne multiaxis measurements are thus a promising tool for atmospheric studies in the troposphere and the upper troposphere and lower stratosphere region.

/pdf/retrieval-of-profile-information-from-airborne-multiaxis-uv-2v57ikjoah.pdf

Retrieval of profile information from airborne multiaxis UV-visible skylight absorption measurements

Beyond SCIAMACHY’s capability to perform global nadir and limb measurements of Earth’s atmosphere in the ultraviolett, visible and near-infrared spectral range, solar occultation measurement are performed during sunrise at higher northern latitudes. Using an optimal estimation approach with the radiative transfer code SCIATRAN as forward model, these measurements are used to derive ozone and NO2 profiles. The sun as well known target in space is used to derive the exact tangent heights for the solar occultation measurements. Here we present a dataset for the time period 2002 to 2004, showing validation results with independent measurements from the HALOE instruments.

Alexej Rozanov

Papers

SCIATRAN 2.0 - A new radiative transfer model for geophysical applications in the 175 - 2400 nm spectral region.

SCIATRAN 2.0 – A new radiative transfer model for geophysical applications in the 175–2400 nm spectral region

Retrieval of profile information from airborne multiaxis UV-visible skylight absorption measurements

Sciamachy solar occultation: ozone and no2 profiles

SCIAMACHY limb ozone and NO2 current validation results