[show abstract][hide abstract] ABSTRACT: The particle linear depolarization ratio δp of Saharan dust, marine aerosols and mixtures of biomass-burning aerosols from southern West Africa and Saharan dust was determined at three wavelengths with three lidar systems during the SAharan Mineral dUst experiMent 2 at the airport of Praia, Cape Verde, between 22 January and 9 February 2008. The lidar ratio Sp of these major types of tropospheric aerosols was analysed at two wavelengths. For Saharan dust, we find wavelength dependent mean particle linear depolarization ratios δp of 0.24–0.27 at 355 nm, 0.29–0.31 at 532 nm and 0.36–0.40 at 710 nm, and wavelength independent mean lidar ratios Sp of 48–70 sr. Mixtures of biomass-burning aerosols and dust show wavelength independent values of δp and Sp between 0.12–0.23 and 57–98 sr, respectively. The mean values of marine aerosols range independent of wavelength for δp from 0.01 to 0.03 and for Sp from 14 to 24 sr.
Tellus B 08/2011; 63(4):706 - 724. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: The spatio-temporal evolution of the Saharan dust and biomass-burning plume during the SAMUM-2 field campaign in January and February 2008 is simulated at 28 km horizontal resolution with the regional model-system COSMO-MUSCAT. The model performance is thoroughly tested using routine ground-based and space-borne remote sensing and local field measurements. Good agreement with the observations is found in many cases regarding transport patterns, aerosol optical thicknesses and the ratio of dust to smoke aerosol. The model also captures major features of the complex aerosol layering. Nevertheless, discrepancies in the modelled aerosol distribution occur, which are analysed in detail. The dry synoptic dynamics controlling dust uplift and transport during the dry season are well described by the model, but surface wind peaks associated with the breakdown of nocturnal low-level jets are not always reproduced. Thus, a strong dust outbreak is underestimated. While dust emission modelling is a priori more challenging, since strength and placement of dust sources depend on on-line computed winds, considerable inaccuracies also arise in observation-based estimates of biomass-burning emissions. They are caused by cloud and spatial errors of satellite fire products and uncertainties in fire emission parameters, and can lead to unrealistic model results of smoke transport.
Tellus B 07/2011; 63(4):781 - 799. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: Aerosol Robotic Network (AERONET) Sun photometer observations were carried out at Ouarzazate, Morocco, during the Saharan Mineral Dust Experiment (SAMUM) 2006. Data from one measurement day, 19 May 2006, are used to derive particle optical and microphysical parameters with AERONET's latest version of light‐scattering model for non‐spherical particle geometry. In our analysis we also make use of a novel measurement channel at 1638 nm wavelength. We compare the results to data products obtained by airborne high‐spectral‐resolution lidar, several ground‐based Raman lidar, and airborne and ground‐based in situ measurement platforms. We chose that specific measurement day because the dust plume was vertically well mixed. Extinction coefficients from AERONET Sun photometer and lidar observations and in situ measurements agree well. Ångström exponents from Sun photometer and lidar are in close agreement, too. Airborne in situ measurements of dust particle size distributions show larger effective radii than inferred from the AERONET data. Complex refractive indices that are derived with the AERONET algorithm differ from the values obtained with different independent techniques employed in our study. The single‐scattering albedo was derived from the airborne observations of particle size distributions and complex refractive indices. Single‐scattering albedo differs to the value inferred from the AERONET data. The differences may be attributed to the different effective radii that we obtained from the various techniques. The differences between the data products from the various measurement platforms, however, cannot be generalized, as we could only test data for one measurement day. An analysis of additional measurements is under way.
Journal of Geophysical Research 04/2010; 115(2010):1-18. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nearly pure Saharan dust was observed with an Aerosol Robotic Network (AERONET) Sun photometer, several Raman and high spectral resolution lidars, and airborne in situ instruments during the Saharan Mineral Dust Experiment (SAMUM) 2006
in Morocco. In the framework of a case study we present particle shape‐dependent dust properties, i.e., backscatter coefficients, extinction‐to‐backscatter (lidar) ratios, and linear particle depolarization ratios. These parameters can be inferred from AERONET’s latest version of the mineral dust retrieval algorithm. The parameters can be measured with multiwavelength Raman/depolarization lidar without critical assumptions on particle shape. Lidar ratios inferred from the AERONET Sun photometer data tend to be larger than lidar ratios measured directly with lidar. Linear dust depolarization ratios were
derived for several measurement wavelengths from the data products of the AERONET Sun photometer. The depolarization ratios tend to be smaller than the depolarization ratios measured with lidar. The largest differences exist in the near‐ultraviolet wavelength range.
Particle axis ratios were determined with scanning electron microscopy. The axis ratio distribution differs significantly from the axis ratio distribution that is assumed in the AERONET retrievals. If the axis ratio distributions measured during SAMUM are used, the reproducibility of the lidar data products improves. The differences may in part be caused by an insufficient understanding of the light‐scattering model that is used in the AERONET algorithm. The results of the present study will be used to develop a dust light‐scattering model that will serve as the theoretical basis for the inversion of optical data into dust microphysical properties.
Journal of Geophysical Research. 01/2010; 115(2010-D11207):1-11.
[show abstract][hide abstract] ABSTRACT: Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground–based sites and by airborne High Spectral Resolution Lidar. One of the sites, Tinfou, is located close to the border of Sahara in southern Morocco and was the main in–situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site during SAMUM. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for one day. The retrieved profiles of backscatter and extinction coefficients and particle depolarization
ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions.
Tellus B 01/2009; 61B(2009):195-205. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: The SAMUM field campaign in southern Morocco in May/June 2006 provides valuable data to study the emission, and the horizontal and vertical transports of mineral dust in the Northern Sahara. Radiosonde and lidar observations show differential advection of air masses with different characteristics during stable nighttime conditions and up to 5-km deep vertical mixing in the strongly convective boundary layer during the day. Lagrangian and synoptic analyses of selected
dust periods point to a topographic channel from western Tunisia to central Algeria as a dust source region. Significant emission events are related to cold surges from the Mediterranean in association with eastward passing upper-level waves and lee cyclogeneses south of the Atlas Mountains. Other relevant events are local emissions under a distinct
cut-off low over northwestern Africa and gust fronts associated with dry thunderstorms over the Malian and Algerian Sahara. The latter are badly represented in analyses from the European Centre for Medium–Range Weather Forecasts and in a regional dust model, most likely due to problems with moist convective dynamics and a lack of observations in this region. This aspect needs further study. The meteorological source identification is consistent with estimates of optical and mineralogical properties of dust samples.
Tellus B 01/2009; 61B(2009):12-31. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: The regional dust model system LM-MUSCAT-DES was developed in the framework of the SAMUM project. Using the unique comprehensive data set of near-source dust properties during the 2006 SAMUM field campaign, the performance of the model system is evaluated for two time periods in May and June 2006. Dust optical thicknesses, number size distributions and the position of the maximum dust extinction in the vertical profiles agree well with
the observations. However, the spatio-temporal evolution of the dust plumes
is not always reproduced due to inaccuracies in the dust source placement by
the model. While simulated winds and dust distributions are well matched for
dust events caused by dry synoptic-scale dynamics, they are often misrepresented when dust emissions are caused by moist convection or influenced by small-scale topography that is not resolved by the model. In contrast to long range dust transport, in the vicinity of source regions the model performance strongly depends on the correct prediction of the exact location of sources. Insufficiently resolved vertical grid spacing causes the absence of inversions in the model vertical profiles and likely explains the absence of the observed sharply defined dust layers.
Tellus B 01/2009; 61B(2009):307-324. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m−2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m−2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m−2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.
Tellus B 12/2008; 61(1):252 - 269. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: Lifting of dust particles by dust devils and convective plumes may significantly contribute to the global mineral dust budget. During the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006 vertical profiling of dusty plumes was performed for the first time. Polarization lidar observations taken at Ouarzazate (30.9°N, 6.9°W, 1133 m height above sea level) are analyzed. Two cases with typical and vigorous formation of convective plumes and statistical results of 5 d are discussed. The majority of observed convective plumes have diameters on order of 100–400 m. Most of the plumes (typically 50–95%) show top heights <1 km or 0.3DLH with the Saharan dust layer height DLH of typically 3–4 km. Height-to-diameter ratio is mostly 2–10. Maximum plume top height ranges from 1.1 to 2.9 km on the 5 d. 5–26 isolated plumes and clusters of plumes per hour were detected. A low dust optical depth (<0.3) favours plume evolution. Observed surface, 1 and 2–m air temperatures indicate that a difference of 17–20 K between surface and 2-m air temperature and of 0.9–1 K between the 1 and 2-m temperatures are required before convective plumes develop. Favourable horizontal wind speeds are 2–7 m s−1.
Tellus B 11/2008; 61(1):340 - 353. · 3.20 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] For the first time, a Raman lidar operating simultaneously in the ultraviolet (UV) and in the visible wavelength range was employed to measure vertical profiles of volume extinction coefficients of particles at 355 and 532 nm, the respective Å ngström exponent, and the 355–nm and 532–nm extinction–to–backscatter ratio (lidar ratio) on a routine basis for several years. The long–term observations were performed at Leipzig (52°N, 12°E), Germany, in the framework of the European Aerosol Research Lidar Network (EARLINET) project from May 2000 to March 2003. The lidar data were acquired under nighttime conditions. The main findings describe the mean optical properties of central European haze (anthropogenic particles) and can be summarized as follows. The 3-year mean, planetary boundary layer (PBL) extinction coefficients were 191 Mm À1 at 355 nm and 94 Mm À1 at 532 nm. The respective mean Å ngström exponent (for the 355–532–nm wavelength range) was 1.4 in the upper PBL (above 1000 m). The PBL stretched, on average, to heights of 1300 m (winter) and 2350 m (summer). PBL mean particle optical depths were 0.38 (355 nm, ±0.23 standard deviation) and 0.18 (532 nm, ±0.11 standard deviation). Free tropospheric particles contributed 2% (clean free troposphere) to 88% (major Saharan dust events) to the tropospheric optical depth. The average was 17% (355 nm) and 22% (532 nm) in 2000–2003. Å ngström exponents in the free troposphere of about one reflect the dominant influence of Saharan dust and aged smoke on the optical properties in this height range. Three-year mean lidar ratios of 58 sr (355 nm) and 53 sr (532 nm) were found in the upper part of the PBL. Free tropospheric lidar ratios were on average 52 sr (355 nm) and 53 sr (532 nm). From combined observations with the Aerosol Robotic Network (AERONET) Sun photometer and the EARLINET lidar, extinction–to–backscatter ratios at 1064 nm were estimated to be, on average, 45 sr. A comparison with optical measurements made at the Leipzig University from 1990 to 1994 indicates that the PBL extinction coefficient at 532 nm decreased by a factor of about 2 (summer half year) to almost 5 (winter season) since the unification of Germany (1990), mainly caused by the industrial breakdown in eastern Germany in the early 1990s. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0933 Exploration Geophysics: Remote sensing Citation: Mattis, I., A. Ansmann, D. Müller, U. Wandinger, and D. Althausen (2004), Multiyear aerosol observations with dual-wavelength Raman lidar in the framework of EARLINET, J. Geophys. Res., 109, D13203, doi:10.1029/2004JD004600.
Journal of Geophysical Research 07/2004; 109(D13203). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] We present for the first time a comprehensive data set of vertically resolved physical particle properties, and the single-scattering albedo at 532 nm, derived from multiwavelength lidar observations of pollution plumes advected from India and Southeast Asia out over the tropical Indian Ocean during the northeast monsoon. The parameters follow from the inversion of optical data that were obtained from six-wavelength aerosol lidar observations at Hulhule Island (4.1°N, 73.3°E), Maldives, in the framework of the Indian Ocean Experiment (INDOEX) (February/March 1999) and three follow-up campaigns in July and October 1999 and in March 2000. Effective particle radii were 0.20 ± 0.08 mm for pollution plumes above 1-km height, advected during the northeast monsoon. Volume concentrations ranged from 6 to 44 mm 3 cm À3 , and surface-area concentrations were 95–774 mm 2 cm À3 . The particles showed substantial absorption when advected from northern India. The imaginary part of the wavelength-independent complex refractive index was as large as 0.045i. The respective mean value was 0.022i ± 0.014i for observations during the northeast monsoon. The mean real part was 1.54 ± 0.11. The mean single-scattering albedo at 532 nm was 0.90 ± 0.06. Values were as low as 0.8 during advection of air from northern India. On average, less absorbing particles were advected from southern India and Southeast Asia. The numbers indicate that the major contributor to the observed pollution from northern India is absorbing soot-like material from, e.g., fossil fuel and firewood burning. The effective radius was well correlated with the Å ngström exponents of the underlying extinction spectra. Strong correlation of the single-scattering albedo was found not only with respect to the imaginary part of the complex refractive index but also with respect to the particle extinction-to-backscatter ratio. A comparison to Å ngström exponents and single-scattering albedo obtained from Sun photometer and satellite observations showed good representativity of the lidar-derived quantities. Comparison with results from airborne in situ observations showed substantial deviations of Å ngström exponents and single-scattering albedo.-Asian pollution during INDOEX: Microphysical particle properties and single-scattering albedo inferred from multiwavelength lidar observations, J. Geophys. Res., 108(D19), 4600, doi:10.1029/2003JD003538, 2003.
Journal of Geophysical Research 10/2003; 108(D19-4600). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] Aircraft in situ and Raman lidar profiles of aerosol light extinction (ep) and 180 backscattering (p) are compared for 6 days during the Indian Ocean Experiment (INDOEX). The measurements of ep and p were made from the National Center for Atmospheric Research C-130 aircraft using two integrating nephelometers to measure light scattering and one Radiance Research Particle Soot Absorption Photometer to measure light absorption. Particulate 180 backscattering was measured in situ using a new instrument, the 180 backscatter nephelometer. The Institute for Tropospheric Research Raman lidar was located on the island of Hulule (4.18N, 73.53E), and all of the in situ profiles presented are from descents into the Hulule airport. Aerosol optical depth was also measured from Hulule using a Sun photometer, and these data are included in the intercomparison. On average, the lidar-derived values of ep and p are 30% larger than the in situ-derived values to a 95% confidence interval. Possible reasons for the overall discrepancy are (1) a low bias in the in situ measurements because of losses in the C-130 Community Aerosol Inlet; (2) underestimation of the humidification effect on light extinction in the in situ measurements; (3) overestimation of ep and p in the lidar because of subvisible cloud contamination; (4) errors in data processing that could be biasing either measurement, though the lidar retrievals are especially sensitive to this type of error. Temporal and spatial variability also appear to be the source of at least some of the discrepancy in two of the six cases, none of which are well collocated. intercomparison of aerosol light extinction and 180 backscatter as derived using in situ instruments and Raman lidar during the INDOEX field campaign, J. Geophs. Res., 107(D19), 8014, doi:10.1029/2000JD000035, 2002.
Journal of Geophysical Research 09/2002; 107(D19-8014). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] On the basis of multiwavelength backscatter and 532-nm extinction profiling with lidar at Sagres (37°N, 9°W), southern Portugal, and optical depth observations with a star photometer at the lidar site and a Sun photometer atop a nearby mountain, several European pollution outbreaks were characterized during the Second Aerosol Characterization Experiment (ACE 2) in the summer of 1997. A sophisticated analysis scheme applied to the lidar-photometer data set is described. The observations are mainly presented in terms of profiles of the 532-nm extinction-to-backscatter ratio (lidar ratio) and of Å ngström exponents calculated for the wavelength ranges 400–532 nm and 532–800 nm. The lidar ratio indicates the aerosol type (marine, soil, pollution) whereas the Å ngström exponents are sensitive to changes in the particle size distribution (accumulation mode, coarse mode). Results of an extensive correlation analysis considering all determined optical parameters, relative humidity, and measurement height are discussed. Finally, the spectrally resolved optical depth and the column Å ngström exponents for the lofted outbreak plumes determined from the lidar profiles are compared with respective values derived from the star and Sun photometer measurements.
Journal of Geophysical Research 08/2002; 107(D15-4259). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] We present vertically resolved physical particle properties and the single-scattering albedo at 532 nm of pollution plumes advected from the European continent out over the Atlantic Ocean. The parameters follow from the inversion of optical data, which were obtained from six-wavelength aerosol lidar observations near Sagres (37°N, 9°W), Portugal, in the framework of Aerosol Characterization Experiment 2 (ACE 2) (June–July 1997). Particle effective radii were 0.15 ± 0.06 mm, volume concentrations ranged from 6 to 27 mm 3 cm À3 , and surface area concentrations were 80 to 1200 mm 2 cm À3 . The particles in general showed negligible absorption, with the mean imaginary part of the wavelength-independent complex refractive index being at 0.009i ± 0.010i. The mean real part was 1.56 ± 0.07. A mean value of 0.95 ± 0.06 was obtained for the single-scattering albedo at 532 nm. The numbers indicate that the major contributor to the observed pollution was nonabsorbing ammonium-sulfate-like material. A very small fraction was contributed by absorbing sootlike material. Correlation analysis showed that effective radius was well correlated with the Å ngström exponents of the underlying extinction spectra. Furthermore, correlations of the single-scattering albedo with the particle extinction-to-backscatter ratio and with the imaginary part of the complex refractive index were found. This result shows that the first two parameters contain information about the chemical state of the observed particles.
Journal of Geophysical Research 08/2002; 107(D15-4248). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: 1] For the first time, height profiles of the extinction-to-backscatter ratio (lidar ratio) of desert dust particles were simultaneously measured at 355 and 532 nm. The observations were performed with an advanced Raman lidar during two long-range Saharan dust outbreaks at Leipzig, Germany (51.3°N, 12.4°E), in August and October 2001. Measured desert-dust lidar ratios are needed for a proper profiling of the climate-relevant volume extinction coefficient of the dust particles with widely used standard backscatter lidars. Unexpectedly large lidar ratios, mainly between 50 and 80 sr, were found in the Saharan dust plumes. The lidar ratios at 355 nm were, on average, higher by 10% – 30% than the ones at 532 nm, probably due to enhanced light absorption in the UV. The large lidar ratios can be explained by model calculations, available in the literature for 532 nm, which focus on the deviations between the scattering characteristics of spheres and spheroids. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 1640 Global Change: Remote sensing
Geophysical Research Letters 05/2002; 29(9-1306). · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: Observations of the extinction-to-backscatter ratio (lidar ratio) of South and Southeast Asian aerosol par-ticles are presented for the wavelength of 532 nm. Raman lidar measurements were performed in the Maldives (4.1 • N, 73.3 • E) in the framework of the Indian Ocean Experiment (INDOEX) in 1999/2000. These observations in the tropics are an important contribution to a growing global lidar– ratio climatology which is needed for an improved determi-nation of the particle optical depth with ground–based and spaceborne lidars. The lidar ratio was found to be a use-ful quantity to trace back different pollution sources and to identify less and considerably light–absorbing particles. During the winter/spring seasons heavily polluted air from India and Southeast Asia was advected to the lidar site. Un-der these conditions lidar ratios up to 110 sr were observed in the lofted pollution plumes above 1000 m height. According to backward trajectories the highest lidar ratios were found for airmasses which crossed the eastern and northeastern parts of India. Large lidar ratios >70 sr indicate small, con-siderably absorbing aerosol particles. Below 1000 m height, the lidar ratio typically ranged from 30–60 sr. The marine boundary layer contained a mixture of marine and anthro-pogenic particles. Under clean, marine conditions in Octo-ber 1999, lidar ratios <30 sr were found.
Geophysical Research Letters 12/2001; 28(24):4559-4562. · 3.98 Impact Factor
[show abstract][hide abstract] ABSTRACT: We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral
Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun
photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and
with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the
far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ.
Ångstr¨om exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical
properties of the dust plume with a dust transport model. The model results and the experimental data show similar
times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees
in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust
could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases
we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one
reason for these deviations.
Tellus B - Chemical and Physical Meteorology. 61B(2009):325-339.