AMTD - Latest Articles

Remote sensing of water vapour profiles in the framework of the Total Carbon Column Observing Network (TCCON)

2010-08-31T00:00:00+02:00

Remote sensing of water vapour profiles in the framework of the Total Carbon Column Observing Network (TCCON)

Atmospheric Measurement Techniques Discussions, 3, 3987-4007, 2010

Author(s): M. Schneider, E. Sepúlveda, O. García, F. Hase, and T. Blumenstock

We show that the near infrared solar absorption spectra recorded in the framework of the Total Carbon Column Observing Network (TCCON) can be used to derive the vertical distribution of tropospheric water vapour. Using spectral H₂O signatures in the 4500–4700 cm⁻¹ region one can well distinguish lower from middle/upper tropospheric water vapour concentrations. The vertical resolution is about 3 and 6 km, for the lower and middle/upper troposphere, respectively. We document the quality of the remotely-sensed profiles by comparisons with coincident in-situ Vaisala RS92 radiosonde measurements. The agreement of both techniques is very satisfactory. Due to the long-term strategy of the network and the high measurement frequency, the TCCON water vapour profile data offer novel opportunities for estimating the water vapour variability at different time scales and altitudes.

Observation of the exhaust plume from the space shuttle main engine using the Microwave Limb Sounder

2010-08-30T00:00:00+02:00

Observation of the exhaust plume from the space shuttle main engine using the Microwave Limb Sounder

Atmospheric Measurement Techniques Discussions, 3, 3971-3986, 2010

Author(s): H. C. Pumphrey, A. Lambert, and N. J. Livesey

A space shuttle launch deposits 700 t of water in the atmosphere. Some of this water is released into the upper mesosphere and lower thermosphere where it may be directly detected by a limb sounding satellite instrument. We report measurements of water vapour plumes from shuttle launches made by the Microwave Limb Sounder (MLS) on the Aura satellite. Approximately 50% of shuttle launches are detected by MLS. The signal appears at a similar level across the upper 10 km of the MLS limb scan, suggesting that the bulk of the observed water is above the top of the scan. Only a small fraction at best of smaller launches (Ariane, Proton) are detected. We conclude that the sensitivity of MLS is only just great enough to detect a shuttle sized launch, but that a suitably designed instrument of the same general type could detect the exhausts from a large proportion of heavy-lift launches.

Airborne DOAS limb measurements of tropospheric trace gas profiles: case study on the profile retrieval of O₄ and BrO

2010-08-30T00:00:00+02:00

Airborne DOAS limb measurements of tropospheric trace gas profiles: case study on the profile retrieval of O₄ and BrO

Atmospheric Measurement Techniques Discussions, 3, 3925-3969, 2010

Author(s): C. Prados-Roman, A. Butz, T. Deutschmann, M. Dorf, L. Kritten, A. Minikin, U. Platt, H. Schlager, H. Sihler, N. Theys, M. Van Roozendael, T. Wagner, and K. Pfeilsticker

A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals (e.g., O₃, BrO, IO, HONO) was deployed on the DLR-Falcon (Deutsches Zentrum für Luft- und Raumfahrt) aircraft and tested during the ASTAR 2007 campaign (Arctic Study of Tropospheric Aerosol, Clouds and Radiation) that took place at Svalbard (78° N) in spring 2007. Our main objectives during this campaign were to test the instrument, and to perform spectral and profile retrievals of tropospheric trace gases, with particular interest on investigating the distribution of halogen compounds (e.g., BrO) during the so-called ozone depletion events (ODEs). In the present work, a new method for the retrieval of vertical profiles of tropospheric trace gases from tropospheric DOAS limb observations is presented. Major challenges arise from modeling the radiative transfer in an aerosol and cloud particle loaded atmosphere, and from overcoming the lack of a priori knowledge of the targeted trace gas vertical distribution (e.g., unknown tropospheric BrO vertical distribution). Here, those challenges are tackled by a mathematical inversion of tropospheric trace gas profiles using a regularization approach constrained by a retrieved vertical profile of the aerosols extinction coefficient ε_M. The validity and limitations of the algorithm are tested with in situ measured ε_M, and with an absorber of known vertical profile (O₄). The method is then used for retrieving vertical profiles of tropospheric BrO. Results indicate that, for aircraft ascent/descent observations, the limit for the BrO detection is roughly 1.5 pptv (pmol/mol), and the BrO profiles inferred from the boundary layer up to the upper troposphere and lower stratosphere have around 10 degrees of freedom.

For the ASTAR 2007 deployments during ODEs, the retrieved BrO vertical profiles consistently indicate high BrO mixing ratios (~15 pptv) within the boundary layer, low BrO mixing ratios (≤1.5 pptv) in the free troposphere, occasionally enhanced BrO mixing ratios (~1.5 pptv) in the upper troposphere, and increasing BrO mixing ratios with altitude in the lowermost stratosphere. These findings are well in agreement with satellite and balloon-borne soundings of total and partial BrO atmospheric column densities.

Ceilometer-lidar inter-comparison: backscatter coefficient retrieval and signal-to-noise ratio determination

2010-08-30T00:00:00+02:00

Ceilometer-lidar inter-comparison: backscatter coefficient retrieval and signal-to-noise ratio determination

Atmospheric Measurement Techniques Discussions, 3, 3907-3924, 2010

Author(s): B. Heese, H. Flentje, D. Althausen, A. Ansmann, and S. Frey

The potential of a new generation of ceilometer instruments for aerosol monitoring has been studied in the Ceilometer-Lidar Inter- Comparison (CLIC) study. The ceilometer is of type CHM15k from Jenoptik, Germany, which uses a solid state laser at the wavelength of 1064 nm and an avalanche photodiode for photon counting detection. The German Meteorological Service is in progress of setting up a ceilometer network for aerosol monitoring in Germany. The intercomparison study was performed to determine whether the ceilometers are capable to deliver quality assured particle backscatter coefficient profiles. For this, the derived ceilometer profiles were compared to simultaneously measured lidar profiles at the same wavelength. The lidar used for this intercomparison was IfTs multi-wavelengths Raman lidar Polly^XT. During the EARLINET lidar intercomparison campaign EARLI 09 in Leipzig, Germany, a new type of the Jenoptik ceilometer, the CHM15k-X, took part. This new ceilometer has a new optical setup resulting in a complete overlap at 150 m. The derived particle backscatter profiles were compared to profiles derived from Polly^XTs measurements, too. The elastic daytime particle backscatter profiles as well as the less noisy night-time Raman particle backscatter profiles compare well with the ceilometers profiles in atmospheric structures like aerosol layers or the boundary layer top height. The calibration of the ceilometer profiles by an independent measurement of the aerosol optical depth (AOD) by a sun photometer is necessary to determine the correct magnitude of the particle backscatter coefficient profiles. A comprehensive signal-to-noise ratio study was carried out to characterize the ceilometers signal performance with increasing altitude.

Retrieval of macrophysical cloud parameters from MIPAS: algorithm description and preliminary validation

2010-08-26T00:00:00+02:00

Retrieval of macrophysical cloud parameters from MIPAS: algorithm description and preliminary validation

Atmospheric Measurement Techniques Discussions, 3, 3877-3906, 2010

Author(s): J. Hurley, A. Dudhia, and R. G. Grainger

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard ENVISAT has the potential to be particularly useful for studying high, thin clouds, which have been difficult to observe in the past. This paper details the development, implementation and testing of an optimal-estimation-type retrieval for three macrophysical cloud parameters (cloud top height, cloud top temperature and cloud extinction coefficient) from infrared spectra measured by MIPAS, employing additional information derived to improve the choice of a priori. The retrieval is applied and initially validated on MIPAS data. From application to MIPAS data, the retrieved cloud top heights are assessed to be accurate to within 50 m, the cloud top temperatures to within 0.5 K and extinction coefficients to within a factor of 15%. This algorithm has been adopted by the European Space Agency's ''MIPclouds'' project, which itself recognises the potential of MIPAS beyond monitoring atmospheric chemistry and seeks to study clouds themselves rigorously using MIPAS.

Dry deposition of NaCl aerosols: theory and method for a modified leaf-washing technique

2010-08-25T00:00:00+02:00

Dry deposition of NaCl aerosols: theory and method for a modified leaf-washing technique

Atmospheric Measurement Techniques Discussions, 3, 3851-3876, 2010

Author(s): A. Reinap, B. L. B. Wiman, S. Gunnarsson, and B. Svenningsson

Within the framework of aerosol deposition to vegetation we present a specially designed leaf wash-off method used in a wind-tunnel based study, where leaves of Quercus robur L. were exposed to NaCl aerosols. We summarise the principles and illustrate the method for two types of substances, the chloride ion and the sodium ion, and for two levels of aerosol exposure prior to leaf washing. On the average, in the low-exposure experiments (S1), the 1st (2nd) wash-off step provided 90% (96%) of the amount of Cl⁻ on the leaves. In the high-exposure experiments (S2) the corresponding values were 96% and 99%. For sodium, the general dynamics resembles that of chloride, but the amounts washed off were, in both series, on the average below what would be expected if the equivalent ratio in the tunnel aerosol were to be preserved. Na⁺ showed adsorption and/or absorption at the leaf surfaces. The difference between the mean values of the amounts of chloride and of sodium washed off in S1 was not statistically significant, the mean Na⁺ to Cl⁻ difference as a fraction of Cl⁻ being minus 18%±27%; corresponding values for S2 were minus 16%±9%, however (p<0.05). In the latter case, 101±57 μequiv Na⁺ per m² of leaf area were missing for the equivalent relationship 1:1 with Cl⁻ to be met. Although uncertainties are thus large, this indicates the magnitude of the Na⁺-retention. The method is suitable not only for chloride, an inexpensive and easy-to-handle tracer, but also for sodium under exposure at high aerosol concentrations. Our findings will help design further studies of aerosol/forest interactions.

In situ detection of atomic and molecular iodine using resonance and off-resonance fluorescence by lamp excitation: ROFLEX

2010-08-25T00:00:00+02:00

In situ detection of atomic and molecular iodine using resonance and off-resonance fluorescence by lamp excitation: ROFLEX

Atmospheric Measurement Techniques Discussions, 3, 3803-3849, 2010

Author(s): J. C. Gómez Martín, J. Blahins, U. Gross, T. Ingham, A. Goddard, A. S. Mahajan, A. Ubelis, and A. Saiz-Lopez

We demonstrate a new instrument for in situ detection of atmospheric iodine atoms and molecules based on atomic and molecular resonance and off-resonance ultraviolet fluorescence excited by lamp emission. The instrument combines the robustness, light weight, low power consumption and efficient excitation of radio-frequency discharge light sources with the high sensitivity of the photon counting technique. Calibration of I₂ fluorescence is achieved via quantitative detection of the molecule by incoherent broad band cavity-enhanced absorption spectroscopy. Atomic iodine fluorescence signal is calibrated by controlled broad band photolysis of known I₂ concentrations in the visible spectral range at atmospheric pressure. The instrument has been optimised in laboratory experiments to reach detection limits of 1.2 pptv for I atoms and 20 pptv for I₂, for S/N=1 and 10 min of integration time. The ROFLEX system has been deployed in a field campaign in Northern Spain, representing the first concurrent observation of ambient mixing ratios of iodine atoms and molecules in the 1–350 pptv range.

A new optimal estimation retrieval scheme for carbon monoxide using IASI spectral radiances – Part 1: Sensitivity analysis, error budget and simulations

2010-08-24T00:00:00+02:00

A new optimal estimation retrieval scheme for carbon monoxide using IASI spectral radiances – Part 1: Sensitivity analysis, error budget and simulations

Atmospheric Measurement Techniques Discussions, 3, 3747-3802, 2010

Author(s): S. M. Illingworth, J. J. Remedios, H. Boesch, D. P. Moore, H. Sembhi, A. Dudhia, and J. C. Walker

This paper presents a new retrieval scheme for tropospheric carbon monoxide (CO), using measured radiances from the Infrared Atmospheric Sounding Interferometer (IASI) onboard the MetOp-A satellite. The University of Leicester IASI Retrieval Scheme (ULIRS) is an optimal estimation retrieval scheme, which utilises equidistant pressure levels and a floating pressure grid based on topography. It makes use of explicit digital elevation and emissivity information, and incorporates a correction for solar surface reflection in the daytime with a high resolution solar spectrum. The retrieval scheme has been assessed through a formal error analysis, via the simulation of surface effects and by an application to real IASI data over a region in Southern Africa. The ULIRS enables the retrieval of between 1 and 2 pieces of information about the tropospheric CO vertical profiles, with peaks in the sensitivity at approximately 5 and 12 km. Typical errors for the African region relating to the profiles are found to be ~20% at 5 and 12 km, and on the total columns to range from 18 to 34%. Finally the performance of the ULIRS is shown for a range of simulated geophysical conditions.

Laboratory evaluation of the effect of nitric acid uptake on frost point hygrometer performance

2010-08-24T00:00:00+02:00

Laboratory evaluation of the effect of nitric acid uptake on frost point hygrometer performance

Atmospheric Measurement Techniques Discussions, 3, 3725-3745, 2010

Author(s): T. Thornberry, T. Gierczak, R. S. Gao, H. Vömel, L. A. Watts, J. B. Burkholder, and D. W. Fahey

Chilled mirror hygrometers (CMH) are widely used to measure water vapour in the troposphere and lower stratosphere from balloon-borne sondes. Systematic discrepancies among in situ water vapour instruments have been observed at low water vapour mixing ratios (<5 ppm) in the upper troposphere and lower stratosphere (UT/LS). Understanding the source of the measurement discrepancies is important for a more accurate and reliable determination of water vapour abundance in this region. We have conducted a laboratory study to investigate the potential interference of gas-phase nitric acid (HNO₃) with the measurement of frost point temperature, and consequently the water vapour mixing ratio, determined by CMH under conditions representative of operation in the UT/LS. No detectable interference in the measured frost point temperature was found for HNO₃ mixing ratios of up to 4 ppb for exposure times up to 150 min. HNO₃ was observed to co-condense on the mirror frost, with the adsorbed mass increasing linearly with time at constant exposure levels. Over the duration of a typical balloon sonde ascent (90–120 min), the maximum accumulated HNO₃ amounts were comparable to monolayer coverage of the geometric mirror surface area, which corresponds to only a small fraction of the actual frost layer surface area. This small amount of co-condensed HNO₃ is consistent with the observed lack of HNO₃ interference in the frost point measurement because the CMH utilizes significant reductions (>10%) in surface reflectivity by the condensate to determine H₂O.

Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO₂, CH₄ and CO over concentrations representative of ''clean air'' and ''polluted plumes''

2010-08-23T00:00:00+02:00

Absolute accuracy and sensitivity analysis of OP-FTIR retrievals of CO₂, CH₄ and CO over concentrations representative of ''clean air'' and ''polluted plumes''

Atmospheric Measurement Techniques Discussions, 3, 3675-3723, 2010

Author(s): T. E. L. Smith, M. J. Wooster, M. Tattaris, and D. W. T. Griffith

When compared to established point-sampling methods, Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy can provide path-integrated concentrations of multiple gases simultaneously, in situ and near-continuously. Concentrations can be retrieved from the measured IR spectra using a forward model coupled to a non-linear least squares fitting procedure, without requiring ''background'' spectral measurements unaffected by the gases of interest. However, few studies have investigated the accuracy of such retrievals for CO₂, CH₄ and CO, particularly across a broad concentration range covering ambient to highly polluted air (e.g. from biomass burning or industrial plumes). Here we perform such an assessment using data collected by a field-portable FTIR spectrometer. The FTIR was positioned to view a fixed IR source placed at the other end of an IR-transparent cell filled with the gases of interest, whose target concentrations were varied by up to two orders of magnitude. Retrievals made using the forward model are complicated by absorption line pressure broadening, the effects of temperature on absorption band shape and by convolution of the gas absorption lines and the instrument line shape (ILS). Despite this, with optimal forward model parameterisation (i.e. the wavenumber range used in the retrieval, gas temperature, pressure and ILS), concentration retrievals for all gases were able to be made to within 5% of the true value. Sensitivity to the aforementioned model inputs was also investigated. CO retrievals were shown to be most sensitive to the ILS (a function of the assumed instrument FOV), which is due to the narrow nature of CO absorption lines and their consequent sensitivity to convolution with the ILS. Conversely, CO₂ retrievals were most sensitive to assumed atmospheric parameters, particularly temperature. The analysis suggests that trace gas concentration retrieval errors can remain well below 10%, even with the uncertainties in atmospheric pressure and temperature that might arise when studying plumes in field situations (e.g. at uncertain altitudes or temperatures). Our findings provide confidence that FTIR-derived trace gas retrievals of CO₂, CH₄ and CO based on forward modeling can yield accurate results, even over very large concentration ranges that can prove difficult to retrieve via standard classical least squares (CLS) techniques.

Aerosol profiling using the ceilometer network of the German Meteorological Service

2010-08-23T00:00:00+02:00

Aerosol profiling using the ceilometer network of the German Meteorological Service

Atmospheric Measurement Techniques Discussions, 3, 3643-3673, 2010

Author(s): H. Flentje, B. Heese, J. Reichardt, and W. Thomas

The German Meteorological Service (DWD) operates about 52 lidar ceilometers within its synoptic observations network, covering Germany. These affordable low-power lidar systems provide spatially and temporally high resolved aerosol backscatter profiles which can operationally provide quasi 3-D distributions of particle backscatter intensity. Intentionally designed for cloud height detection, recent significant improvements allow following the development of the boundary layer and to detect denser particle plumes in the free tropospere like volcanic ash, Saharan dust or fire smoke. Thus the network builds a powerful aerosol plume alerting and tracking system. If auxiliary aerosol information is available, the particle backscatter coefficient, the extinction coefficient and even particle mass concentrations may be estimated, with however large uncertainties. Therefore, large synergistic benefit is achieved if the ceilometers are linked to existing lidar networks like EARLINET or integrated into WMO's envisioined Global Aerosol Lidar Observation Network GALION. To this end, we demonstrate the potential and limitations of ceilometer networks by means of three representative aerosol episodes over Europe, namely Sahara dust, Mediterranean fire smoke and, more detailed, the Icelandic Eyjafjoll volcano eruption from mid April 2010 onwards. The DWD (Jenoptik CHM15k) lidar ceilometer network tracked the Eyjafjoll ash layers over Germany and roughly estimated peak extinction coefficients and mass concentrations on 17 April of 4–6(± 2) 10^-4 m⁻¹ and 500–750(± 300) μg/m⁻³, respectively, based on co-located aerosol optical depth, nephelometer (scattering coefficient) and particle mass concentration measurements. Though large, the uncertainties are small enough to let the network suit for example as aviation advisory tool, indicating whether the legal flight ban threshold of presently 2 mg/m³ is imminent to be exceeded.

Synergetic cloud fraction determination for SCIAMACHY using MERIS

2010-08-19T00:00:00+02:00

Synergetic cloud fraction determination for SCIAMACHY using MERIS

Atmospheric Measurement Techniques Discussions, 3, 3601-3642, 2010

Author(s): C. Schlundt, A. A. Kokhanovsky, W. von Hoyningen-Huene, T. Dinter, L. Istomina, and J. P. Burrows

Since clouds play an essential role in the Earth's climate system, it is important to understand the cloud characteristics as well as their distribution on a global scale using satellite observations. The main scientific objective of SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) onboard the ENVISAT satellite is the retrieval of vertical columns of trace gases.

On the one hand, SCIAMACHY has to be sensitive to low variations in trace gas concentrations which means the ground pixel size has to be large enough. On the other hand, such a large pixel size leads to the problem that SCIAMACHY spectra are often contaminated by clouds. SCIAMACHY spectral measurements are not well suitable to derive a reliable sub-pixel cloud fraction that can be used as input parameter for subsequent retrievals of cloud properties or vertical trace gas columns. Therefore, we use MERIS/ENVISAT spectral measurements with its high spatial resolution as sub-pixel information for the determination of MerIs Cloud fRation fOr Sciamachy (MICROS). Since MERIS covers an even broader swath width than SCIAMACHY, no problems in spatial and temporal collocation of measurements occur. This enables the derivation of a SCIAMACHY cloud fraction with an accuracy much higher as compared with other current cloud fractions that are based on SCIAMACHY's PMD (Polarization Measurement Device) data.

We present our new developed MICROS algorithm, based on the threshold approach, as well as a qualitative validation of our results with MERIS satellite images for different locations, especially with respect to bright surfaces such as snow/ice and sands. In addition, the SCIAMACHY cloud fractions derived from MICROS are intercompared with other current SCIAMACHY cloud fractions based on different approaches demonstrating a considerable improvement regarding geometric cloud fraction determination using the MICROS algorithm.

Determination of aerosol properties from satellite observations of the Ring effect

2010-08-19T00:00:00+02:00

Determination of aerosol properties from satellite observations of the Ring effect

Atmospheric Measurement Techniques Discussions, 3, 3535-3599, 2010

Author(s): T. Wagner, S. Beirle, T. Deutschmann, and M. P. de Vries

In this study we explore the potential of satellite observations of the Ring effect (at various wavelengths) for the retrieval of atmospheric aerosol properties. Compared to clouds, aerosols have a rather weak influence on the Ring effect, thus the requirements on the accuracy of the measurements and the radiative transfer simulations are high. In this study, we show that for moderate and high aerosol optical depth (AOD), Ring effect observations are sensitive enough to yield information not only on the AOD, but also on the absorbing properties of aerosols and the aerosol layer height. The latter two quantities are especially important for the determination of the radiative effects of aerosols.

Our investigations are based on observations by the satellite instrument SCIAMACHY on ENVISAT (2004–2008) and on model simulations using the Monte-Carlo radiative transfer model McArtim. In addition to the Ring effect we investigate the impact of aerosols on the absorptions of the oxygen molecule (O₂) and dimer (O₄) as well as the radiance. In general good consistency between measured and simulated quantities is found. In some cases also systematic differences occurred, which are probably mainly related to the strong polarisation sensitivity of the SCIAMACHY instrument.

Our study indicates that Ring effect observations have important advantages for aerosol retrievals: in contrast to O₂ and O₄ absorptions they are only weakly affected by the surface albedo; they can be analysed with high accuracy in various wavelength ranges; and depending on the wavelength range, they show different sensitivities on aerosol properties like single scattering albedo, optical depth or layer height. The results of this study are of particular interest for future satellite instruments with reduced polarisation sensitivity and smaller ground pixels, capable of measuring the Ring effect with higher accuracy.

A geostationary thermal infrared sensor to monitor the lowermost troposphere: O₃ and CO retrieval studies

2010-08-18T00:00:00+02:00

A geostationary thermal infrared sensor to monitor the lowermost troposphere: O₃ and CO retrieval studies

Atmospheric Measurement Techniques Discussions, 3, 3489-3534, 2010

Author(s): M. Claeyman, J.-L. Attié, V.-H. Peuch, L. El Amraoui, W. A. Lahoz, B. Josse, P. Ricaud, T. von Clarmann, M. Höpfner, J. Orphal, J.-M. Flaud, D. P. Edwards, K. Chance, X. Liu, F. Pasternak, and R. Cantié

This paper describes the capabilities of a nadir thermal infrared (TIR) sensor proposed for embarkation onboard a geostationary platform to monitor ozone (O₃) and carbon monoxide (CO) for air quality (AQ) purposes. To assess the capabilities of this sensor we perform idealized retrieval studies considering typical atmospheric profiles of O₃ and CO over Europe with different instrument configurations (signal to noise ratio and spectral sampling interval) using the KOPRA forward model and the KOPRA-fit retrieval scheme based on the Tikhonov-Phillips regularization. We then select a configuration, referred to as GEO-TIR, optimized for providing information in the lowermost troposphere (LmT; 0–3 km in height). For the GEO-TIR configuration we obtain around 1.5 degrees of freedom for O₃ and 2 for CO at altitudes between 0 and 15 km. The error budget of GEO-TIR, calculated taking account of the principal contributions to the error (namely, temperature, measurement error, smoothing error) shows that information in the LmT can be achieved by GEO-TIR. We also retrieve analogous profiles from another geostationary infrared instrument with characteristics similar to the Meteosat Third Generation Infrared Sounder (MTG-IRS) which is dedicated to numerical weather prediction, referred to as GEO-TIR2. Comparison between GEO-TIR and GEO-TIR2 allows us to quantify the added value of GEO-TIR, a mission complementing the AQ observing system. To better characterize the information provided by GEO-TIR and GEO-TIR2 in the LmT, we retrieve two typical profiles of O₃ and CO for different thermal contrast ranging from –10 K to 10 K. The shape of the first averaging kernel (corresponding to the surface level) confirms that GEO-TIR has good sensitivity to CO in the LmT and also to O₃ for high positive thermal contrast. GEO-TIR2 has very low sensitivity in the LmT to O₃ but can have sensitivity to CO with high positive thermal contrast. To quantify these results for a realistic atmosphere, we simulate it using the chemical transport model MOCAGE (MOdèle de Chimie Atmospherique à Grande Echelle) – this is the nature run. We simulate the O₃ and CO spatial and temporal distributions from GEO-TIR observations in the LmT in July 2009 over Europe by sampling the nature run. Results show that GEO-TIR is able to capture well the spatial and temporal variability in the LmT for both O₃ and CO, particularly during periods with high positive thermal contrast near the ground and high surface temperature, which results in active photochemistry and a raised planetary boundary layer. These results also provide evidence of the significant added value in the LmT of GEO-TIR compared to GEO-TIR2 by showing GEO-TIR is closer to the nature run than GEO-TIR2 for various statistical parameters (correlation, bias, standard deviation).

Fine-scale turbulence soundings in the stratosphere with the new balloon-borne instrument LITOS

2010-08-18T00:00:00+02:00

Fine-scale turbulence soundings in the stratosphere with the new balloon-borne instrument LITOS

Atmospheric Measurement Techniques Discussions, 3, 3455-3487, 2010

Author(s): A. Theuerkauf, M. Gerding, and F.-J. Lübken

We have developed a new compact balloon payload called LITOS (Leibniz-Institute Turbulence Observations in the Stratosphere) for high resolution wind turbulence soundings up to 35 km altitude. The wind measurements are performed applying a constant temperature anemometer (CTA) with a vertical resolution of ~2.5 mm, i.e. 2 kHz sampling rate at 5 m/s ascent speed. Thereby, for the first time, it is possible to study the entire turbulence spectrum down to the viscous subrange in the stratosphere. Including telemetry, housekeeping, batteries and recovery unit the payload weighs less than 5 kg and can be launched at any radiosonde station. Since autumn 2007 LITOS has been successfully launched several times from the Leibniz-Institute of Atmospheric Physics (IAP) in Kühlungsborn, Germany (54° N, 12° E). Two additional soundings were carried out in 2008 and 2009 at Kiruna, Sweden (67° N, 21° E) as part of the BEXUS program (Balloon-borne EXperiments for University Students). We describe here the basic principle of CTA measurements and prove the validity of this method in the stratosphere. First case studies allow a clear distinction between non-turbulent layers and turbulent layers with a thickness of some tens of meters. Since our measurements cover the transition between the inertial and viscous subrange, energy dissipation rates can be calculated with high reliability.

Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: the problem of dust type selection

2010-08-17T00:00:00+02:00

Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: the problem of dust type selection

Atmospheric Measurement Techniques Discussions, 3, 3425-3453, 2010

Author(s): T. Mielonen, R. C. Levy, V. Aaltonen, M. Komppula, G. de Leeuw, J. Huttunen, H. Lihavainen, P. Kolmonen, K. E. J. Lehtinen, and A. Arola

Aerosol Optical Depth (AOD) and Ångström exponent (AE) values derived with the MODIS retrieval algorithm over land (Collection 5) were compared with ground based sun photometer measurements in Europe, Asia, Africa, North America and South America. In Finland (Jokioinen and Sodankylä) measurements were done with Precision Filter Radiometer (PFR), while in Estonia (Toravere), Italy (Ispra, Rome Tor Vergata), India (Kanpur), China (Xianghe), GSFC (USA), Mexico (Mexico City), Zambia (Mongu) and Brazil (Alta Floresta) Cimel (AERONET, level 2) measurements were used. Comparison results for AOD were generally good, although there seems to be room for improvement in the MODIS aerosol model selection, particularly how dust is taken into account. At all studied sites, the MODIS algorithm often selects the dust aerosol model even when dust does not seem to be present and the air masses are not coming from arid regions. This happens especially when AOD values are relatively small (<0.3). The selection of the dust model reduces the correlation between ground based and MODIS AOD measurements in dust-free situations. Moreover, the current aerosol model selection scheme produces unphysical AE values. Our study suggests that the aerosol model combining is sensitive to the ratio of 660 nm and 2130 nm surface reflectances (slope(660/2130)). Furthermore, the value of the slope in the algorithm is mainly dependent on the Normalized Difference Vegetation Index (NDVI). The current relationship of these two parameters in the algorithm is not supported by the surface albedo climatology derived from MODIS measurements. The use of a more physical relationship improves the AE retrieval at the studied sites. However, at some sites the AOD correspondence deteriorates when the new relationship is used.

Intercomparison of slant column measurements of NO₂ and O₄ by MAX-DOAS and zenith-sky UV and visible spectrometers

2010-08-13T00:00:00+02:00

Intercomparison of slant column measurements of NO₂ and O₄ by MAX-DOAS and zenith-sky UV and visible spectrometers

Atmospheric Measurement Techniques Discussions, 3, 3383-3423, 2010

Author(s): H. K. Roscoe, M. Van Roozendael, C. Fayt, A. du Piesanie, N. Abuhassan, C. Adams, M. Akrami, A. Cede, J. Chong, K. Clémer, U. Friess, M. Gil Ojeda, F. Goutail, R. Graves, A. Griesfeller, K. Grossmann, G. Hemerijckx, F. Hendrick, J. Herman, C. Hermans, H. Irie, P. V. Johnston, Y. Kanaya, K. Kreher, R. Leigh, A. Merlaud, G. H. Mount, M. Navarro, H. Oetjen, A. Pazmino, M. Perez-Camacho, E. Peters, G. Pinardi, O. Puentedura, A. Richter, A. Schönhardt, R. Shaiganfar, E. Spinei, K. Strong, H. Takashima, T. Vlemmix, M. Vrekoussis, T. Wagner, F. Wittrock, M. Yela, S. Yilmaz, F. Boersma, J. Hains, M. Kroon, and A. Piters

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.

Quantitative and enantioselective analysis of monoterpenes from plant chambers and in ambient air using SPME

2010-08-09T00:00:00+02:00

Quantitative and enantioselective analysis of monoterpenes from plant chambers and in ambient air using SPME

Atmospheric Measurement Techniques Discussions, 3, 3345-3381, 2010

Author(s): N. Yassaa, T. Custer, W. Song, F. Pech, J. Kesselmeier, and J. Williams

A solid-phase microextraction (HS-SPME) and gas chromatography/mass spectrometry (GC/MS) system has been developed for quantifying enantiomeric and nonenantiomeric monoterpenes in plant chamber studies and ambient air. Performance of this system was checked using a capillary diffusion system to produce monoterpene standards. The adsorption efficiency, competitive adsorption and chromatographic peak resolution of monoterpene enantiomer pairs were compared for three SPME fibre coatings: 75 μm Carboxen-PDMS (CAR-PDMS), 50/30 μm, divinylbenzene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS) and 65 μm divinylbenzene-polydimethyl-siloxane (DVB-PDMS). Key parameters such as the linearity and reproducibility of the SPME system have been investigated in this work. The best compromise between the enantiomeric separation of monoterpenes and competitive adsorption of the isoprenoids on the solid SPME fibre coating was found for DVB-PDMS fibres. The optimum conditions using DVB-PDMS fibres were applied to measure the exchange rates of monoterpenes in the emission of Quercus ilex using a laboratory whole plant enclosure under light and dark conditions, as well as in ambient air. With 592 and 223 ng m⁻² s⁻¹, respectively, β-myrcene and limonene were the predominant monoterpenes in the emission of Q. ilex. These values were closely comparable to those obtained using a zNose and cartridge GC-FID systems.

Intercomparison of CALIOP and MODIS aerosol optical depth retrievals

2010-08-09T00:00:00+02:00

Intercomparison of CALIOP and MODIS aerosol optical depth retrievals

Atmospheric Measurement Techniques Discussions, 3, 3319-3344, 2010

Author(s): C. Kittaka, D. M. Winker, M. A. Vaughan, A. Omar, and L. A. Remer

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) is carried on the CALIPSO satellite and has acquired global aerosol profiles since June 2006. CALIPSO is flown in formation with the Aqua satellite as part of the A-train satellite constellation, so that a large number of coincident aerosol observations are available from CALIOP and the MODIS-Aqua instrument. This study compares column aerosol optical depth at 0.532 μm derived from CALIOP aerosol profiles with MODIS-Aqua 0.55 μm aerosol optical depth over the period June 2006 through August 2008. The study is based on the CALIOP Version 2 Aerosol Layer Product and MODIS Collection 5. While CALIOP is first and foremost a profiling instrument, this comparison of column aerosol optical depth provides insight into quality of CALIOP aerosol data. It is found that daytime aerosol optical depth from the CALIOP Version 2 product has a small global mean bias relative to MODIS Collection 5. Regional biases, of both signs, are larger and biases are seen to vary somewhat with season. In northern mid-latitudes, aerosol optical depth from CALIOP is lower, on average, than from MODIS. This may be partly due to a latitude-dependent calibration error in Version 2 CALIOP Level 1 daytime 0.532 μm profiles. This comparison of CALIOP and MODIS also provides insight into possible biases in the MODIS aerosol optical depth product due to cloud masking and errors in modeling land surface reflectance.

Fast-response high-resolution temperature sonde aimed at contamination-free profile observations

2010-08-06T00:00:00+02:00

Fast-response high-resolution temperature sonde aimed at contamination-free profile observations

Atmospheric Measurement Techniques Discussions, 3, 3293-3317, 2010

Author(s): K. Shimizu and F. Hasebe

An innovative temperature sonde, equipped with an ultra thin tungsten wire, has been developed to meet the scientific requirements suitable for climate change research. The response time, shorter than 40 ms achieved at the altitude of 30 km, enables the temperature observations with the radiation correction of less than 0.4 K in the whole observation range. Test flights during the development stage reveal significant artificial perturbations in the observed temperature profiles. They are identified as the thermal contamination arising primarily from radiosonde package box with some additional effect from the launching balloon. The modification of the sensor mount successfully removed the contribution from the former effect. On the other hand, some filtering procedure need to be applied to remove the latter, although the use of a long suspension line will be effective to reduce the noise. There remain unavoidable small fluctuations (less than 0.4 K) that are brought about by the solid angle modulation of the illumination against the sensor body in the daytime. While conventional radiation correction may unintentionally have taken a part of such contaminations into account, they may not be properly corrected in existing radiosonde data, as the origin of errors has not been identified. Our tungsten sonde that scarcely relies on the ambiguous correction procedures is ideal for serving as an international reference.

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