European Aerosol Research Lidar Network Research Articles

EARLINET observations of the 14–22-May long-range dust transport event during SAMUM 2006: validation of results from dust transport modelling

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. Ångström 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.

Ten years of multiwavelength Raman lidar observations of free‐tropospheric aerosol layers over central Europe: Geometrical properties and annual cycle

We present geometrical properties and seasonal variations of appearance of aerosol particle pollution in the free troposphere over the central European lidar site at Leipzig, Germany. The data set has been acquired with Raman lidar in the past 10 years in the framework of the German Lidar Network (1997–2000) and since 2000 in the framework of the European Aerosol Research Lidar Network (EARLINET). In summary we analyzed 1028 measurements. Geometrical depth of the pollution layers was ≤1 km in 33% of all cases. Geometrical depths >5 km were found in 10% of all cases. Traces of particle pollution were detected up to the height of the tropopause. Forest‐fire burning in North America causes intrusion of particles into the stratosphere. Seven hundred seventeen of all observations were carried out on the basis of a regular measurement schedule which allows us to establish a statistic on the frequency of particle transport in the free troposphere. In 43% of the regular measurements we observed pollution above the continental boundary layer. The lofted particle layers largely result from intercontinental long‐range transport. We use backward trajectory analysis to identify the main source regions of the lofted pollution layers. In 19% of all regular measurements, free‐tropospheric pollution was advected from North America. Forest‐fire smoke from Canada and anthropogenic pollution from urban areas of the United States of America and Canada were the sources of the particle layers. We find a strong seasonal dependence of occurrence of these layers with a peak in June–August of each year. In a few cases we observed forest‐fire smoke advected from Siberia and east Asia with winds from westerly directions. Pollution advected from areas north of 70°N presents another transport channel. That pollution consists of Arctic haze or mixtures of haze with anthropogenic pollution. The main occurrence of such particle layers is around springtime of each year. Import of mineral dust from the Sahara represents another transport path. Most of such cases are observed during late springtime and summertime. Free‐tropospheric pollution advected from east and southeast Europe and Russia presents one transport channel from within the Euro‐Asian continent.

In Situ Samplings and Remote Sensing Measurements to Characterize Aerosol Properties over Southeast Italy

Abstract Ground-based particulate matter (PM) samplers, an XeF Raman lidar operating in the framework of the European Aerosol Research Lidar Network (EARLINET), and a sun/sky radiometer operating in the framework of the Aerosol Robotic Network (AERONET) have been used to characterize vertical profiles, optical and microphysical properties, and chemical composition of aerosols during the 29 June–1 July 2005 dust outbreak that occurred over the central-eastern Mediterranean. Aerosol backscatter coefficient, total depolarization, and lidar ratio vertical profiles revealed that a well-mixed dust layer extending from ∼0.5 to 6 km was present over southeastern Italy on 30 June. Sun/sky radiometer measurements revealed a bimodal lognormal size distribution during all measurement days. The particle volume distribution was found to be well correlated either to the PM mass distribution measured at ground by a seven-stage cascade impactor and to the fine to total suspended PM mass ratio measured by ground-based PM samplers. Scanning electron microscopy and ion chromatography analyses on PM samples revealed that coarse-mode aerosols were mainly made of carbonate, aluminum-silicate, and sea salt particles. Carbon, sulfate, and nitrate particles were the main components of fine-mode aerosols representing more than 50% of the total aerosol load; the significant role of fine-mode anthropogenic particles during a dust event is highlighted. Finally, the potential capabilities of complementary measurements by passive and active remote sensing techniques and in situ observations to retrieve the vertical distribution of the particle number and mass concentration are analyzed and discussed.

Systematic lidar observations of Saharan dust over Europe in the frame of EARLINET (2000–2002)

More than 130 observation days of the horizontal and vertical extent of Saharan dust intrusions over Europe during the period May 2000 to December 2002 were studied by means of a coordinated lidar network in the frame of the European Aerosol Research Lidar Network (EARLINET). The number of dust events was greatest in late spring, summer, and early autumn periods, mainly in southern (S) and southeastern (SE) Europe. Multiple aerosol dust layers of variable thickness (300–7500 m) were observed. The center of mass of these layers was located in altitudes between 850 and 8000 m. However, the mean thickness of the dust layer typically stayed around 1500–3400 m and the corresponding mean center of mass ranged from 2500 to 6000 m. In exceptional cases, dust aerosols reached northwestern (NW), northern (N), or northeastern (NE) Europe, penetrating the geographical area located between 4°W–28°E (longitude) and 38°N–58°N (latitude). Mean aerosol optical depths (AOD), extinction‐to‐backscatter ratios (lidar ratios, LR), and linear depolarization ratios of desert aerosols ranged from 0.1 to 0.25 at the wavelength of 355 or 351 nm, 30 to 80 sr at 355 or 351 nm, and 10 to 25% at 532 nm, respectively, within the lofted dust plumes. In these plumes typical Saharan dust backscatter coefficients ranged from 0.5 to 2 Mm−1sr−1. Southern European stations presented higher variability of the LR values and the backscatter‐related Ångström exponent values (BRAE) (LR: 20–100 sr; BRAE: −0.5 to 3) than northern ones (LR: 30–80 sr; BRAE: −0.5 to 1).

Ill-posed retrieval of aerosol extinction coefficient profiles from Raman lidar data by regularization

In the analysis of Raman lidar measurements of aerosol extinction, it is necessary to calculate the derivative of the logarithm of the ratio between the atmospheric number density and the range-corrected lidar-received power. The statistical fluctuations of the Raman signal can produce large fluctuations in the derivative and thus in the aerosol extinction profile. To overcome this difficult situation we discuss three methods: Tikhonov regularization, variational, and the sliding best-fit (SBF). Three methods are performed on the profiles taken from the European Aerosol Research Lidar Network lidar database simulated at the Raman shifted wavelengths of 387 and 607 nm associated with the emitted signals at 355 and 532 nm. Our results show that the SBF method does not deliver good results for low fluctuation in the profile. However, Tikhonov regularization and the variational method yield very good aerosol extinction coefficient profiles for our examples. With regard to, e.g., the 532 nm wavelength, the L2 errors of the aerosol extinction coefficient profile by using the SBF, Tikhonov, and variational methods with respect to synthetic noisy data are 0.0015(0.0024), 0.00049(0.00086), and 0.00048(0.00082), respectively. Moreover, the L2 errors by using the Tikhonov and variational methods with respect to a more realistic noisy profile are 0.0014(0.0016) and 0.0012(0.0016), respectively. In both cases the L2 error given in parentheses concerns the second example.

EARLINET: the European aerosol research lidar network

Present knowledge concerning aerosols and their distribution patterns in the earth’s atmosphere is far from adequate to properly assess their role in changing climate and environmental conditions, both regionally and globally. Information about vertical concentrations is particularly lacking. Lidar (light detection and ranging) remote sensing is the most appropriate tool to close this observational gap, and networks that deploy this technology are fundamental to the scaled-up study of aerosol concentrations, transport, and modification.1 Established in 2000, the EuropeanAerosol Research LidarNetwork (EARLINET) currently comprises 25 stations and has as its main objective the development of a qualitatively and quantitatively significant database that details the horizontal, vertical, and temporal distribution of atmospheric aerosol over the entire continent (see Figure 1). Station observation capabilities vary. EARLINET currently includes 10 single backscatter stations and 8 stations with a UV Raman channel for independent measurements of aerosol extinction and backscatter. In addition, for retrieval of aerosol microphysical properties, 7 multiwavelength Raman stations include elastic channels at 1064, 532, and 355nm, with Raman channels at 532 and 355nm, and a depolarization channel at 532nm. Network-wide observations take place systematically on a fixed schedule. For collection of unbiased data, all stations perform measurements three fixed days per week. Lidar surveillance adheres to a regular timetable: once each week around noon when the boundary layer is usually well developed, and twice weekly at night under low background light conditions in order to perform Raman extinction measurements. Other network activities monitor special events such as Saharan dust outbreaks (see Figure 2), forest fires, photochemical smog, and volcanic eruptions. A rigorous quality assurance program applies to both instruments and evaluation algorithms,2–4 and a standardFigure 1. Stations in the European Aerosol Research Lidar Network (EARLINET).

Observed and simulated vertically resolved optical properties of continental aerosols over southeastern Italy: A closure study

A 2.5‐year data set of Raman lidar measurements performed in Lecce within the European Aerosol Research Lidar Network and a numerical model are used to characterize continental aerosols over southeast Italy. Continental conditions are selected by means of back trajectories, limiting the data set to cases of advection from the European continent. The model is used to constrain the relevant aerosol microphysical parameters needed to reproduce the altitude‐resolved optical properties measured by lidar, namely, aerosol extinction (α), backscatter (β), and lidar ratio (LR = α/β) at 351 nm. The observed variability of α, β, and LR is simulated by randomly varying, within appropriate ranges, all the microphysical parameters describing the aerosol composition and size distribution (assumed as bimodal). An optimal model configuration capable of reproducing within ±10% the mean optical properties retrieved by lidar, including their vertical pattern, is presented and discussed with sensitivity studies. Results indicate rather clean continental aerosols to characterize the site, with water‐soluble fine‐mode aerosols (dry modal radius of 0.05–0.10 μm) accounting for 98–99% of the total number of particles. The (dry) imaginary refractive index of fine‐mode particles is extended up to 0.02 to better fit the lidar data. This suggests some mixing of water‐soluble substances with more absorbing material. Coarse‐mode particles (modal radius 0.4–0.5 μm) are shown to play a crucial role in determining the observed vertical profile of β and LR. Finally, an altitude‐resolved relationship linking α to β is derived. This could be usefully employed in continental conditions to invert the lidar measurements when only the elastic signal is available (for example, in daylight).

Saharan dust intrusions in the Mediterranean area: Three years of Raman lidar measurements

A multiyear climatological study of Saharan dust intrusions in the central Mediterranean in terms of aerosol optical parameters vertical profiles is carried out for the first time. Observations are performed at Istituto di Metodologie per l'Analisi Ambientale (IMAA) Raman/elastic lidar station located in Tito Scalo, Potenza (40°36′N, 15°44′E), from May 2000 to April 2003, in the framework of European Aerosol Research Lidar Network (EARLINET). Desert dust aerosols are observed between 1.8 and 9 km in 112 days. Mean values within the desert dust layer of 76 Mm−1, 1.0 Mm−1 sr−1 and 0.54 Mm−1 sr−1 are observed for aerosol extinction at 355 nm and aerosol backscatter at 355 and 532 nm. Desert dust layer optical depth at 355 nm ranges between 0.001 and 0.68, with a mean of 0.13. The source origin is the central Sahara in about 65% of the cases, the western Sahara in about 31%, and only in four cases the eastern Sahara. The most extended database of Saharan dust lidar ratio data was collected: Values range between 6 and 126 sr following a 3‐modal Gaussian distribution centered at 22, 37 and 57 sr. A mean value of 37 sr is found around the center of the Saharan dust layer. At its extremes, where dust particles are mixed to PBL and free troposphere background aerosols, a mean value of 57 sr is found. Finally, low lidar ratio values of about 22 sr are observed when large amount of dust is transported at low altitudes over the Mediterranean Sea.

Height and seasonal dependence of aerosol optical properties over southeast Italy

Systematic Raman lidar measurements performed from May 2000 to August 2002 in the framework of the European Aerosol Research Lidar Network are analyzed and discussed. Main results on height dependence and seasonal cycle of extinction α(z) and backscatter β(z) coefficients, aerosol optical depths AOD(z), and lidar ratios S(z) are presented. Yearly‐averaged vertical profiles of extinction and backscatter coefficients reveal that both α(z) and β(z) decrease more than 75% within the lowermost 2 km. Yearly‐averaged lidar ratios increase from about 42–48 sr in the lowermost 2 km to between 67–72 sr in the air masses above 2 km. The changes in lidar ratios are indicative of changes in the physical characteristics of the monitored aerosols and suggest that over the monitoring site, the lowermost air masses (z < 2 km) are on average characterized by a larger concentration of particles leading to S values typical of marine aerosols, while the elevated air masses (z > 2 km) are characterized by lidar ratios typical of continental aerosol type. The seasonal cycle analysis reveals that the average maximum height zM at which aerosols are detected by the lidar that is about 5 km in spring‐summer, and decreases up to about 3 km in autumn‐winter. Monthly AODs span the 0.16–0.26 range in autumn‐winter and the 0.26–0.4 range in spring‐summer. The scarce renovation of air masses, occurring in spring‐summer as a consequence of the weather stability, favours the accumulation of the atmospheric aerosol particles and may account for the larger AODs and zM values.

Potential of lidar backscatter data to estimate solar aerosol radiative forcing

The potential to estimate solar aerosol radiative forcing (SARF) in cloudless conditions from backscatter data measured by widespread standard lidar has been investigated. For this purpose 132 days of sophisticated ground-based Raman lidar observations (profiles of particle extinction and backscatter coefficients at 532 nm wavelength) collected during two campaigns [the European Aerosol Research Lidar Network (EARLINET) and the Indian Ocean Experiment (INDOEX)] were analyzed. Particle extinction profiles were used as input for radiative transfer simulations with which to calculate the SARF, which then was plotted as a function of the column (i.e., height-integrated) particle backscatter coefficient (beta(c)). A close correlation between the SARF and beta(c) was found. SARF-beta(c) parameterizations in the form of polynomial fits were derived that exhibit an estimated uncertainty of +/-(10-30)%. These parameterizations can be utilized to analyze data of upcoming lidar satellite missions and for other purposes. The EARLINET-based parameterizations can be applied to lidar measurements at mostly continental, highly industrialized sites with limited maritime influence (Europe, North America), whereas the INDOEX parameterizations rather can be employed in polluted maritime locations, e.g., coastal regions of south and east Asia.

Mixed-Layer Depth Determination in the Barcelona Coastal Area From Regular Lidar Measurements: Methods, Results and Limitations

Regular aerosol backscatter measurements using an elastic-backscatter lidar were performed between May 2000 and December 2002 in Barcelona (Spain), within the framework of the European project EARLINET (European Aerosol Research Lidar Network). The mixed-layer depth was one of the major parameters to be retrieved. Three derivative methods have been tested in this complex coastal area using the range-squared-corrected lidar signal: (1) the minimum of its first derivative, (2) the minimum of its second derivative, and (3) the minimum of the first derivative of its logarithm. The second method was found to give statistically the best results when compared to radiosoundings, and was used to process the whole dataset. A number of 162 days and 660 profiles averaged over 30 min have been examined. Between 1000 and 1500 UTC, the mixed-layer depth oscillates between 300 and 1450 m in summer and between 390 and 1420 m in winter. The standard deviation for this portion of the day is 180 and 256 m, respectively, in summer and winter. In summer, low heights (mainly limited to 400–800 m) are associated with large mesoscale compensatory subsidence over the sea and to the thermal internal boundary-layer formation. The strong coastal and orographic influences and the climatological settling of Barcelona determine the complexity of the boundary-layer dynamics and the high heterogeneity of the lidar signals. In many cases, single lidar analyses do not allow an unambiguous determination of the mixed-layer depth. Two diurnal cycle measurements are discussed together with synoptic maps, backtrajectories and radiosoundings in order to outline the complexity of the area and the limitations of the methods.

Four‐year aerosol observations with a Raman lidar at Thessaloniki, Greece, in the framework of European Aerosol Research Lidar Network (EARLINET)

Regular aerosol extinction and backscatter measurements using a UV Raman lidar have been performed from January 2001 to December 2004 at Thessaloniki, Greece (40.5°N, 22.9°E), in the framework of the European Aerosol Research Lidar Network (EARLINET). Profiles of the aerosol extinction coefficient, backscatter coefficient, and extinction‐to‐backscatter ratio (so‐called “lidar ratio”) were acquired under nighttime conditions and have been used for statistical investigations. The statistical analysis was made both for the planetary boundary layer and for the free troposphere. It was found that the 4‐year mean boundary layer particle optical depth at 355 nm was 0.44 ± 0.18, and the total aerosol optical depth was 0.63 ± 0.27. Free tropospheric particles account on the average for 30% of the total aerosol optical depth, ranging from 5% (clean free troposphere conditions) to 55% (mainly Saharan dust events). For the cases examined, the integral of the lidar‐derived extinction coefficient was in good agreement with colocated aerosol optical depth measurements at 355 nm obtained with a Brewer spectroradiometer. The mean value of the lidar ratio at 355 nm derived, for the period of measurements at Thessaloniki, was 40 sr with a standard deviation of 21 sr. Mean height profiles of the particle lidar ratio, extinction, and backscatter coefficients are shown along with their seasonal dependence, showing a significant seasonal variability in the free troposphere. An analysis of the data using back trajectories showed also a dependence of the aerosol optical depth and the lidar ratio on the origin of the air mass, with higher values mostly corresponding to air masses originating from the northeast Balkans and eastern Europe.

Air mass modification over Europe: EARLINET aerosol observations from Wales to Belarus

For the first time, the vertically resolved aerosol optical properties of western and central/eastern European haze are investigated as a function of air mass transport. Special emphasis is put on clean maritime air masses that cross the European continent from the west and become increasingly polluted on their way into the continent. The study is based on observations at seven lidar stations (Aberystwyth, Paris, Hamburg, Munich, Leipzig, Belsk, and Minsk) of the European Aerosol Research Lidar Network (EARLINET) and on backward trajectory analysis. For the first time, a lidar network monitored continent‐scale haze air masses for several years (since 2000). Height profiles of the particle backscatter coefficient and the particle optical depth of the planetary boundary layer (PBL) at 355‐nm wavelength are analyzed for the period from May 2000 to November 2002. From the observations at Aberystwyth, Wales, the aerosol reference profile for air entering Europe from pristine environments was determined. A mean 355‐nm optical depth of 0.05 and a mean PBL height of 1.5 km was found for clean maritime summer conditions. The particle optical depth and PBL height increased with increasing distance from the North Atlantic. Mean summer PBL heights were 1.9–2.8 km at the continental sites of Leipzig, Belsk, and Minsk. Winter mean PBL heights were mostly between 0.7 and 1.3 km over the seven EARLINET sites. Summer mean 355‐nm optical depths increased from 0.17 (Hamburg, northwesterly airflow from the North Sea) and 0.21 (Paris, westerly flow from the Atlantic) over 0.33 (Hamburg, westerly flow) and 0.35 (Leipzig, westerly flow) to 0.59 (Belsk, westerly flow), and decreased again to 0.37 (westerly flow) at Minsk. Winter mean optical depths were, on average, 10–30% lower than the respective summer values. PBL‐mean extinction coefficients were of the order of 200 Mm−1 at 355 nm at Hamburg and Leipzig, Germany, and close to 600 Mm−1 at Belsk, Poland, in winter for westerly flows. Whereas the optical depth for westerly flows was typically <0.35 during the summer halfyear, it increased to values of 0.5–0.7 over most of the central European sites during easterly flows. Compared to aerosol sources in Poland and southeastern Europe, the highly industrialized and populated western European region was found to contribute only moderately to the European aerosol burden. Comparably clean conditions (low particle optical depth) prevailed at the Munich site, indicating a sensitive influence of the orography on haze conditions. Estimates of the mean effective (upward minus downward) aerosol mass flux into the atmosphere along the way from the Atlantic Ocean to central Europe (Leipzig), and from Leipzig to Belsk, that are consistent with the optical depth increase, yield values of 0.11–0.17 μg/(m2s) and 0.25 μg/(m2s), respectively. These values correspond to mean effective aerosol mass fluxes for 50 km × 50 km grid cells of the order of 10000 Mg/year and 20000 Mg/year, respectively. The estimates are in reasonable agreement with EMEP emission data.

Aerosol lidar intercomparison in the framework of the EARLINET project 3 Raman lidar algorithm for aerosol extinction, backscatter, and lidar ratio

An intercomparison of the algorithms used to retrieve aerosol extinction and backscatter starting from Raman lidar signals has been performed by 11 groups of lidar scientists involved in the European Aerosol Research Lidar Network (EARLINET). This intercomparison is part of an extended quality assurance program performed on aerosol lidars in the EARLINET. Lidar instruments and aerosol backscatter algorithms were tested separately. The Raman lidar algorithms were tested by use of synthetic lidar data, simulated at 355, 532, 386, and 607 nm, with realistic experimental and atmospheric conditions taken into account. The intercomparison demonstrates that the data-handling procedures used by all the lidar groups provide satisfactory results. Extinction profiles show mean deviations from the correct solution within 10% in the planetary boundary layer (PBL), and backscatter profiles, retrieved by use of algorithms based on the combined Raman elastic-backscatter lidar technique, show mean deviations from solutions within 20% up to 2 km. The intercomparison was also carried out for the lidar ratio and produced profiles that show a mean deviation from the solution within 20% in the PBL. The mean value of this parameter was also calculated within a lofted aerosol layer at higher altitudes that is representative of typical layers related to special events such as Saharan dust outbreaks, forest fires, and volcanic eruptions. Here deviations were within 15%.

Vertical aerosol distribution over Europe: Statistical analysis of Raman lidar data from 10 European Aerosol Research Lidar Network (EARLINET) stations

Since 2000, regular lidar observations of the vertical aerosol distribution over Europe have been performed within the framework of EARLINET, the European Aerosol Research Lidar Network. A statistical analysis concerning the vertical distribution of the volume light extinction coefficients of particles derived from Raman lidar measurements at 10 EARLINET stations is presented here. The profiles were measured on a fixed schedule with up to two measurements per week; they typically covered the height range from 500 m to 6000 m above ground level (agl). The analysis is made for the planetary boundary layer (PBL) as well as for several fixed layers above ground. The results show typical values of the aerosol extinction coefficient and the aerosol optical depth (AOD) in different parts of Europe, with highest values in southeastern Europe and lowest values in the northwestern part. Annual cycles and cumulative frequency distributions are also presented. We found that higher aerosol optical depths in southern Europe compared to the northern part are mainly attributed to larger amounts of aerosol in higher altitudes. At 9 of the 10 sites the frequency distribution of the aerosol optical depth in the planetary boundary layer follows a lognormal distribution at the 95% significance level.

Multiyear aerosol observations with dual‐wavelength Raman lidar in the framework of EARLINET

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 Ångström 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−1 at 355 nm and 94 Mm−1 at 532 nm. The respective mean Ångström 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. Ångström 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.

Optical properties of Saharan dust layers as detected by a Raman lidar at Thessaloniki, Greece

Measurements during Saharan dust transport events were performed at several stations of the European Aerosol Research Lidar Network. During the period 2001–2002, 12 cases were captured at Thessaloniki, Greece (40.5°N, 22.9°E) with a 355 nm Raman lidar. For these cases the vertical profiles of the extinction and backscatter coefficients were determined and examined, as well as profiles of extinction‐to‐backscatter ratio (lidar ratio) and the backscatter‐related Ångström exponent between 355 and 532 nm (color index). Model calculations from the DREAM model confirmed the existence of Saharan dust in all cases examined. Within the dust plumes the lidar ratios ranged from 20 to 100 sr and the color index ranged from −1 to 3. An anticorrelation was found between the lidar ratio and the color index during the Saharan dust events. In certain cases the results suggest that the mixing of dust particles with boundary layer aerosols reduces the dust ‘signature’ and thus, makes difficult the separation of the dust particles from those of mostly urban origin.

Summertime re-circulations of air pollutants over the north-eastern Iberian coast observed from systematic EARLINET lidar measurements in Barcelona

Abstract Regular aerosol backscatter measurements using an elastic-backscatter lidar were performed from May 2000 to December 2002 in Barcelona (Spain) in the frame of EU EARLINET Project (European Aerosol Research Lidar Network). Vertical profiles retrieved in a regular schedule confirmed the presence of multiple aerosol layers of regional origin above the mixing layer during numerous measurement days. Analysis of the meteorological situation of the corresponding days showed common synoptic regimes, which are typical in summertime around the region. As documented by earlier studies, under strong insolation and weak synoptic forcing, sea breezes and mountain-induced winds develop to create re-circulations of pollutants along the eastern Iberian coast. Layers are formed when aerosols are injected from the mountains into the return flow at various heights and distances from the coast. Aerosol layers can be found above the mixing layer up to 4000 m with variable thickness typically ranged between 100 and 1000 m. The mixing height mainly oscillates between 400 and 800 m asl in periods of maximum insolation. Four selected episodes are analysed combining lidar profiles, radiosoundings and synoptic meteorology information. One of them includes regional re-circulation aerosols at low levels and upper Saharan dust layers. Maximum backscatter coefficients ranged from 1×10−6 to 2×10−6 (m sr)−1 at the wavelength of 1064 nm. Assuming a lidar ratio of 30 sr (60 sr for Saharan dust), aerosol optical depths above the mixing layer ranged from 0.016 to 0.073. Detailed mesoscale analysis of one episode is performed by means of high resolution modelling with the PSU/NCAR Mesoscale Model 5 (MM5).

Aerosol lidar intercomparison in the framework of the EARLINET project. 1. Instruments.

In the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET), 19 aerosol lidar systems from 11 European countries were compared. Aerosol extinction or backscatter coefficient profiles were measured by at least two systems for each comparison. Aerosol extinction coefficients were derived from Raman lidar measurements in the UV (351 or 355 nm), and aerosol backscatter profiles were calculated from pure elastic backscatter measurements at 351 or 355, 532, or 1064 nm. The results were compared for height ranges with high and low aerosol content. Some systems were additionally compared with sunphotometers and starphotometers. Predefined maximum deviations were used for quality control of the results. Lidar systems with results outside those limits could not meet the quality assurance criterion. The algorithms for deriving aerosol backscatter profiles from elastic lidar measurements were tested separately, and the results are described in Part 2 of this series of papers [Appl. Opt. 43, 977-989 (2004)]. In the end, all systems were quality assured, although some had to be modified to improve their performance. Typical deviations between aerosol backscatter profiles were 10% in the planetary boundary layer and 0.1 x 10(-6) m(-1) sr(-1) in the free troposphere.

Aerosol lidar intercomparison in the framework of the EARLINET project. 2. Aerosol backscatter algorithms.

An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.