[Show abstract][Hide abstract] ABSTRACT: A global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63° N to 52° S and 72° W to 124° E has been achieved within the Raman and polarization lidar network PollyNET. This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. PollyNET is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design and apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at http://polly.tropos.de. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Ångström exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the PollyNET locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of PollyNET to support the establishment of a global aerosol climatology that covers the entire troposphere.
[Show abstract][Hide abstract] ABSTRACT: The atmospheric science community demands for autonomous and quality-assured vertically resolved measurements of aerosol and cloud properties. For this purpose, a portable lidar called Polly was developed at TROPOS in 2003. The lidar system was continuously improved with gained experience from EARLINET, worldwide field campaigns and institute collaborations within the last 10 years. Here we present recent changes of the setup of our portable multiwavelength Raman and polarization lidar PollyXT and the improved capabilities of the system by means of a case study. Our latest developed system includes an additional near-range receiver unit for Raman measurements of the backscatter and extinction coefficient down to 120 m above ground, a water-vapor channel, and channels for simultaneous measurements of the particle linear depolarization at 355 and 532 nm. Quality improvements were achieved by following consequently the EARLINET guidelines and own developments. A modified ship radar ensures measurements in agreement with air-traffic safety regulations and allows 24/7 monitoring of the atmospheric state with PollyXT.
[Show abstract][Hide abstract] ABSTRACT: MODIS retrievals of the aerosol optical depth (AOD) are biased over urban areas, where surface reflectance is not well characterized. Since the operational MODIS aerosol retrieval for dark targets assumes fixed spectral slopes to calculate the surface reflectance at 0.47 μm, the algorithm may fail in urban areas with different spectral characteristics of the surface reflectance. To investigate this bias we have implemented variable spectral slopes into the operational MODIS aerosol algorithms of Collection 5 (C5) and C6. The variation of slopes is based on airborne measurements of surface reflectances over the city of Zhongshan, China. AOD retrieval results of the operational and the modified algorithms were compared for a MODIS measurement over Zhongshan. For this case slightly lower AOD values were derived using the modified algorithm. The retrieval methods were additionally applied to MODIS data of the Beijing area for a period between 2010–2014 when also AERONET data were available. A reduction of the differences between the AOD retrieved using the modified C5 algorithm and AERONET was found, whereby the mean difference from 0.31 ± 0.11 for the operational C5 and 0.18 ± 0.12 for the operational C6 where reduced to a mean difference of 0.09 ± 0.18 by using the modified C5 retrieval. Furthermore, the sensitivity of the MODIS AOD retrieval for several surface types was investigated. Radiative transfer simulations were performed to model reflectances at top of atmosphere for predefined aerosol properties. The reflectances were used as input for the retrieval methods. It is shown that the operational MODIS AOD retrieval over land reproduces the AOD reference input of 0.85 for dark surface types [retrieved AOD = 0.87 (C5)]. An overestimation of AOD = 0.99 is found for urban surfaces, whereby the modified C5 algorithm shows a good performance with a retrieved value of AOD = 0.86.
[Show abstract][Hide abstract] ABSTRACT: Lidar standards are needed to ensure quality and lidar product control at the interface between lidar manufacturers and lidar users. Meanwhile three lidar standards have been published by German and international standardization organizations. This paper describes the cooperation between the lidar technique inventors, lidar instrument constructors, and lidar product users to establish useful standards. Presently a backscatter lidar standard is elaborated in Germany. Key points of this standard are presented here. Two German standards were already accepted as international standards by the International Organization for Standardization (ISO). Hence, German and international organizations for the establishment of lidar standards are introduced to encourage a cooperative work on lidar standards by lidar scientists.
International Laser Radar Conference, New York City; 07/2015
[Show abstract][Hide abstract] ABSTRACT: Lidar techniques are measuring methods for range resolved determination of atmospheric constituents such as clouds, aerosol particles, trace gases and atmospheric state parameters such as temperature and wind. Lidar techniques are based on the interaction of (previously emitted) pulsed electromagnetic radiation with the atmosphere and the subsequent time resolved detection of the backscattered radiation. In this paper, three existing lidar standards and one standard on backscatter lidar that is currently elaborated are briefly introduced. As an example of use of lidar systems, the determination of the mass concentrations in ash clouds, which were advected over Europe by the Icelandic volcano Eyjafjallajökull in 2010 are mentioned. Also associated with this, lidar techniques are more often used by „non lidar experts“ and in routine operation, making the development of further lidar standards mandatory.
Gefahrstoffe Reinhaltung der Luft 06/2015; 2015(6):235-240. · 0.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Observations of the vertical aerosol structure over the Amazon rain forest were performed with the automated multiwavelength-Raman-lidar Polly XT . The almost con-tinuous measurements were taken in Brazil at 2 • S, 60 • W from January to November 2008. With Polly XT vertical profiles of the backscatter coefficient at 355 nm, 532 nm, and 1064 nm, of the extinction coefficient at 355 nm and 532 nm, and of the particle depolarization ratio at 355 nm can be determined. Out of 2500 hours of data, a typical observation case for the wet (November-May) and dry season (June-October) is presented and intensively discussed in terms of optical aerosol prop-erties and geometrical layer structure. Microphysical aerosol properties like effective radius and volume con-centration are also calculated with an inversion algo-rithm. The analysis reveals, that for the wet season case aged biomass-burning aerosol from Africa domi-nated optical aerosol properties above 800 m. For the dry season case, biomass burning smoke from local fires was observed.
[Show abstract][Hide abstract] ABSTRACT: Based on 11 months of polarization lidar observations in the Amazon Basin near Manaus, Brazil (2.3° S, 60° N), the relationship between temperature and heterogeneous ice formation efficiency in stratiform clouds was evaluated in the cloud-top temperature range between −40 and 0 °C. Between −30 and 0 °C ice-containing clouds are a factor of 1.5 to 2 more frequent during the dry season. Free-tropospheric aerosol backscatter profiles revealed a two- to tenfold increase in aerosol load during the dry season and a MACC reanalysis dataset implies that the aerosol composition during the dry season is strongly influenced by biomass burning aerosol whereas other components such as mineral dust do not vary strongly between the seasons. The injection of smoke accompanied by the likely dispersion of biological material, soil dust, or ash particles was identified as possible source for the increased ice formation efficiency during the dry season.
Geophysical Research Letters 05/2015; 42(13). DOI:10.1002/2015GL064068 · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  We present combined Raman and elastic backscatter lidar observations that were carried out in Zhongshan, PRD (Pearl River Delta), China, during two periods in 2009: one haze pollution period and one moderate pollution period. During the haze period , high AOD (0.86 and 1.20 at 355 nm) and medium Ångström exponents (1.23 and 1.35 at 355 nm/532 nm) were observed. In the moderate pollution period, the corresponding parameters were comparatively lower with values of 0.83 and 0.74 at 355 nm for AOD and 1.108 and 0.98 at 355 nm/532 nm for Ångström exponent.The mean lidar ratios in the two periods were 64 ± 10sr and 56 ±9 sr, respectively, at 355 nm. The lidar ratio during haze phase was a bit higher compared to that in previous observations in this area. The calculated values of the lidar ratio and Ångström exponent were also used to help identify aerosol types. The Ångström exponent was calculated for the extinction from the wavelength pair 355 nm/532 nm, with high values of around 1.35 for the haze event. The particle size distribution (PSD) and single scattering albedo (SSA) derived from sunphotometer measurements indicate the presence of rather small particles. The three-day back trajectories from a HYSPLIT model in the haze period indicate that the air masses in the lower layer were advected from the southeast coast of China, where incomplete combustion of carbonaceous fuels and straw burning are frequently found in Shanghai during the heating period in winter. In the moderate pollution period, the air mass passed through western China, indicating a combination of some pollution from South Asia in case of strong convection, local aerosol aging, and smoke from adjacent fire burning spots in the PRD region.
[Show abstract][Hide abstract] ABSTRACT: In the CALIPSO data analysis the surface type (land/ocean) is used to augment the aerosol characterization. However, this surface-dependent aerosol typing prohibits a correct classification of sea-breeze-related marine aerosol over land. This might result in a systematic overestimation of the particle extinction coefficient and of the aerosol optical thickness (AOT) of up to a factor of 3.5 over land in coastal areas. We present a long-term comparison of CALIPSO and ground-based lidar observations of the aerosol conditions in the coastal environment of southern Latin America (Punta Arenas, Chile, 53° S), performed in December 2009-April 2010. Punta Arenas is almost entirely influenced by marine particles throughout the year, indicated by a rather low AOT of 0.02-0.04. However, we found an unexpectedly high fraction of continental aerosol in the aerosol types inferred by means of CALIOP observations and, correspondingly, too high particle extinction values. Similar features of the CALIOP data analysis are presented for four other coastal areas around the world. Since CALIOP data serve as important input for global climate models, the influence of this systematic error was estimated by means of simplified radiative-transfer calculations.
[Show abstract][Hide abstract] ABSTRACT: At present one of the largest uncertainties in our understanding of global climate concerns the interaction of
aerosols with clouds and atmospheric dynamics. In the climate system, mineral dust aerosol is of key importance,
because mineral dust contributes to about half of the global annual particle emissions by mass. Although our
understanding of the effects of mineral dust on the atmosphere and the climate improved during the past decade, many questions such as the change of the dust size distribution during transport across the Atlantic Ocean and the associated impact on the radiation budget, the role of wet and dry dust removal mechanisms during transport, and the complex interaction between mineral dust and clouds remain open.
The Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE:
http://www.pa.op.dlr.de/saltrace) was conducted in June/July 2013 to investigate the transport and transformation of Saharan mineral dust during long-range transport from the Sahara across the Atlantic Ocean into
the Caribbean. SALTRACE is a German initiative combining ground-based and airborne in-situ and lidar measurements with meteorological data, long-term measurements, satellite remote sensing and modeling which involved many national and international partners.
During SALTRACE, the DLR Falcon research aircraft was based at Sal, Cape Verde, between 11 and 17 June 2013, and at Barbados between 18 June and 11 July 2013. The Falcon was equipped with a suite of in-situ instruments for the measurement of microphysical and optical aerosol properties, with sampling devices for offline particle analysis, with a nadir-looking 2-�m wind lidar, with dropsondes and instruments for standard meteorological parameters. Ground-based lidar and in-situ instruments were deployed in Cape Verde, Barbados and Puerto Rico.
During SALTRACE, mineral dust from five dust outbreaks was studied by the Falcon research aircraft between
Senegal, the Caribbean and Florida under different atmospheric conditions. On the eastern side of the
Atlantic, dust plumes were quite homogenous and extended up to 6-7 km altitude. In contrast, the dust layers in the Caribbean showed three layers with different dust characteristics and were mainly below 4.5 km altitude. In the upper part of the dust layers in the Caribbean, the aerosol properties were similar to the observations near
Africa. In contrast, much more variability in the dust microphysical and optical properties was observed between
0.7 and 2.5 km altitude. The aerosol optical thickness of the dust outbreaks studied in the Barabados area ranged
from 0.2 to 0.6 at 500 nm. Highlights during SALTRACE included the Lagrangian sampling of a dust plume in
the Cape Verde area on 17 June which was again measured with the same instrumentation on 21 and 22 June 2013 near Barbados. The event was also captured by the ground-based lidar and in-situ instrumentation. Another
highlight was the formation of tropical storm Chantal in the dusty environment.
In our presentation, we give an overview of the SALTRACE study and investigate the impact of dust aging
processes between the Cape Verde region and the Caribbean on dust microphysical and optical properties. We
show vertical profiles of dust size distributions, CCN and dust optical properties and compare our results with
the ground-based in-situ, sun photometer and lidar measurements. In particular, we show the results from the
trans-Atlantic Lagrangian dust study and discuss similarities and differences of the dust plumes observed over Cape Verde and in the Caribbean.
[Show abstract][Hide abstract] ABSTRACT:  The direct solar radiative effect of aerosols over the Atlantic Ocean was investigated on the basis of aerosol Raman/polarization lidar observations aboard the research vessel Polarsternbetween Germany (50°N) and either South America (50°S) or South Africa (40°S) in 2009 and 2010. First, a case study of complex aerosol conditions with marine aerosol, dust, and smoke particles in the boundary layer and free troposphere is presented to demonstrate that detailed knowledge of aerosol layering (boundary layer, free troposphere) and aerosol mixing state is required for an accurate determination of the resulting radiative effects. A statistical analysis based on all lidar observations revealed the highest daily mean radiative effect (−43±59 W m−2at the surface, −14±18 W m−2at top of atmosphere) in the latitudinal belt from 0°N–15°N in the Saharan dust outflow region. Mean aerosol radiative effects of the polluted northern and clean southern midlatitudes were contrasted. In the northern midlatitudes, the averaged aerosol radiative effect of all simulations was −24±33 W m−2at the surface which is a factor of 1.6 higher than at similar southern hemispheric latitudes. The simulations based on the lidar observations are in good agreement with colocated pyranometer measurements.
[Show abstract][Hide abstract] ABSTRACT: The study of interactions between aerosol particles, atmospheric
dynamics and clouds and their resulting corresponding indirect effects
on precipitation and radiative transfer demand new measurement
strategies combining the strength of lidar, radar, and in-situ
instrumentation. To match this challenge the Leipzig Aerosol and Cloud
Remote Observations System (LACROS) has been set up at TROPOS, combining
the strengths of a unique set of active and passive remote sensing and
in-situ measurement systems.
Proceedings of SPIE - The International Society for Optical Engineering 10/2013; 8890:02-. DOI:10.1117/12.2030911 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: PollyNET is a growing global network of automatized
multiwavelength polarization Raman lidars of type Polly (Althausen et
al., 2009). The goal of this network is to conduct advanced remote
measurements of aerosol profiles and clouds by the same type of
instrument. Since 2006 this network assists the controlling and
adjustment activities of Polly systems. A central facility receives the
data from the Polly measurements. The observational data are displayed
in terms of quicklooks at http://polly:tropos.de in near real time. In
this way, the network serves as a central information platform for
inquisitive scientists. PollyNET comprises permanent stations
at Leipzig (Germany), Kuopio (Finland), Evora (Portugal), Baengnyeong
Island (South Korea), Stockholm (Sweden), and Warsaw (Poland).
Non-permanent stations have been used during several field experiments
under both urban and very remote conditions - like the Amazon
rainforest. These non-permanent stations were lasting from several weeks
up to one year and have been located in Brazil, India, China, South
Africa, Chile, and also aboard the German research vessels Polarstern
and Meteor across the Atlantic. Within PollyNET the
interaction and knowledge exchange is encouraged between the Polly
operators. This includes maintenance support in system calibration
procedures and distribution of latest hardware and software
improvements. This presentation introduces the PollyNET. Main
features of the Polly systems will be presented as well as recent
instrumental developments. Some measurement highlights achieved within
PollyNET are depicted.
Proceedings of SPIE - The International Society for Optical Engineering 10/2013; 8894. DOI:10.1117/12.2028921 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Airborne measurements of spectral upward radiance and irradiance over the city of Zhongshan, China, were used to examine the heterogeneity of reflection properties of urban surfaces. After the surface reflectivity was derived from the airborne measurements, ratios of surface reflectivity at 466 and 644 nm wavelengths were calculated for the present case study. Implementing the derived ratios into the operational aerosol algorithm over land for the moderate resolution imaging spectroradiometer, enabled us to quantify the influence of the assumptions made for the estimation of the surface reflectivity at 466 nm on the retrieved aerosol
International Radiation Symposium on Radiation Processes in the; 05/2013
[Show abstract][Hide abstract] ABSTRACT: Ground-based Raman lidar measurements during the second Saharan Mineral Dust Experiment (SAMUM-2) in 2008 were used for validation of measurements of the lidar aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite within the dusty environment of the Cape Verde region. SAMUM-2 featured two one-month campaigns in January/February and May/June 2008 to cover different modes of aerosol transport to the tropical Atlantic: dust from northern Africa and biomass-burning smoke from western Africa during winter, and pure Saharan dust during summer. During the investigated time period, 33 CALIPSO overflights occurred at a distance of less than 500 km from the location of the ground-based lidar. Fifteen out of these 33 cases were found suitable for comparing the findings of the two instruments. The parameters for this comparison are the particle backscatter coefficient at 532 and 1064 nm, the extinction coefficient, the lidar ratio (aerosol type), and the particle depolarization ratio at 532 nm, as well as the backscatter-related Ångström exponent for the wavelength pair 532/1064 nm. Best agreement was found for the 532 nm backscatter coefficient, while the 532 nm extinction coefficient is underestimated by up to 30%. The latter is due to the use of an effective dust lidar ratio that gives reliable backscatter coefficients but is not suitable to transform these to extinction coefficients. CALIPSO particle depolarization ratios provided in the current (version 3.01) aerosol profile product were found to be affected by a computing error and should be calculated from the perpendicular and total particle backscatter coefficients provided in the same data file. CALIPSO aerosol classification was found to be mostly correct but a demand for homogeneous aerosol layers could improve the retrieval. Suggestions for the improvement of the CALIPSO retrieval by introducing iterative procedures are provided.
Journal of Geophysical Research Atmospheres 04/2013; 118(7):2889-2902. DOI:10.1002/jgrd.50248 · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT:  Shipborne aerosol lidar observations were performed aboard the research vessel Polarstern in 2009 and 2010 during three north-south cruises from about 50°N to 50°S. The aerosol data set provides an excellent opportunity to characterize and contrast the vertical aerosol distribution over the Atlantic Ocean in the polluted northern and relatively clean southern hemisphere. Three case studies, an observed pure Saharan dust plume, a Patagonian dust plume east of South America, and a case of a mixed dust/smoke plume west of Central Africa are exemplarily shown and discussed by means of their optical properties. The meridional transatlantic cruises were used to determine the latitudinal cross section of the aerosol optical thickness (AOT). Profiles of particle backscatter and extinction coefficients are presented as mean profiles for latitudinal belts to contrast northern- and southern-hemispheric aerosol loads and optical effects. Results of lidar observations at Punta Arenas (53°S), Chile, and Stellenbosch (34°S), South Africa, are shown and confirm the lower frequency of occurrence of free-tropospheric aerosol in the southern hemisphere than in the northern hemisphere. The maximum latitudinal mean AOT of 0.27 was found in the northern tropics (0– 15°N) in the Saharan outflow region. Marine AOT is typically 0.05 ± 0.03. Particle optical properties are presented separately for the marine boundary layer and the free troposphere. Concerning the contrast between the anthropogenically influenced midlatitudinal aerosol conditions in the 30– 60°N belt and the respective belt in the southern hemisphere over the remote Atlantic, it is found that the AOT and extinction coefficients for the vertical column from 0–5km (total aerosol column) and 1–5km height (lofted aerosol above the marine boundary layer) are a factor of 1.6 and 2 higher at northern midlatitudes than at respective southern midlatitudes, and a factor of 2.5 higher than at the clean marine southern-hemispheric site of Punta Arenas. The strong contrast is confined to the lowermost 3km of the atmosphere.