[show abstract][hide abstract] ABSTRACT: The planetary boundary-layer (PBL) is defined as the height of the inversion level separating the free troposphere from the boundary-layer [Stull, 1988]. PBL height is an important parameter in applications ranging from climate studies to air quality modeling. Lidars with high spatial (< 30 m) and temporal (< 5 min) resolutions can be employed to monitor the PBL height using aerosols as tracers.
This study evaluates an approach using an Extended Kalman Filter (EKF) to estimate the PBL height in different atmospheric situations over Barcelona, Spain. PBL heights estimated by the EKF method are compared with classic methods (gradient, Haar wavelet, radiosondes, etc.) under 7 atmospheric conditions.
Atmospheric situations are selected using a cluster analysis of 2-day backtrajectories from the NOAA HYSPLIT model.
[show abstract][hide abstract] ABSTRACT: Pesticide spray drift entails a series of risks and costs in terms of human, animal and environmental well-being. A proper understanding of this phenomenon is essential to minimise these risks. However, most conventional methods used in drift measurement are based on point collectors which are unable to obtain information concerning the temporal or spatial evolution of the pesticide cloud. Such methods are also costly, labour-intensive, and require a considerable amount of time. The aim of this paper is to propose a method to measure the spray drift based on lidar (LIght Detection And Ranging) and to prove that it can be an alternative to passive collectors. An analytical model is proposed to relate the measurements obtained through passive collectors and those obtained with lidar systems considering several spray application and meteorological parameters. The model was tested through an experimental campaign involving multiple ground spray tests. A lidar system and two types of passive collectors (nylon strings and water-sensitive paper) were used simultaneously to measure the drift. The results showed for each test a high coefficient of determination (R2 ≈ 0.90) between the lidar signal and the tracer mass captured by the nylon strings. This coefficient decreased (R2 = 0.77) when all tests were considered together. Lidar measurements were also used to study the evolution of the pesticide cloud with high range (1.5 m) and temporal resolution (1 s) and to estimate its velocity. Furthermore, a very satisfactory adjustment (R2 = 0.89) was observed between the tracer mass collected by the nylon lines and the coverage on water-sensitive paper sheets. These results are in accordance with the proposed analytical model and allow the conclusion that the application and meteorological parameters can be considered spatially invariant for a given test but are not invariant for different tests.
[show abstract][hide abstract] ABSTRACT: The depth of the planetary boundary-layer (PBL) is defined as the height of the inversion level separating the free troposphere (FT) from the boundary-layer (Stull, 1988). Reliable representation of PBL height is important in applications ranging from climate studies to air quality modeling. Convective turbulent mixing processes are dominant in the mixing layer of the PBL and have a major influence on the growth and transport of atmospheric pollutants. In recent years, lidar (laser radar) has proven to be a useful operational tool for nearly continuous monitoring of the lowest levels of the atmosphere with high spatial (~ 3.75 m) and temporal (< 5 min) resolutions. Four Raman-elastic multi-wavelength lidar stations from EARLINET (European Aerosol Research Lidar Network) conducted a 72-hr campaign of continuous observations over Spain (Barcelona, Granada, Madrid) and Portugal (Evora) in early July 2012. This study systematically exploits 1-min averaged, range-squared-corrected lidar signals (RSCS) from the 532 nm analog reception channel of the instruments. Several methods that have been applied in previous literature to derive PBL height from vertical aerosol backscatter profiles are compared. Most widely used are derivative techniques such as the gradient method (GM), inflection point method (IPM), and logarithm gradient method (LGM) and covariance techniques such as the wavelet covariance transform (WCT) method using a Haar wavelet. The methods function by detecting steep gradients in the aerosol backscatter profile, a proxy for the transition zone between the PBL and FT. It is found that all the methods provide comparable results. However, it is determined that WCT is an optimal method as it is more computationally efficient than the derivative techniques. In summer, PBL heights over the Iberian Peninsula are typically between 1-3 km. In addition, spatial patterns and diurnal variation of the PBL height and an analysis of the meteorological situation over the study area are also conducted. Backward trajectories from the NOAA HYSPLIT model indicate aerosols arrived from tropical maritime origins over the eastern Atlantic Ocean in the previous 24-48 hours of the campaign. Overall, it is shown that lidar can be an effective means of obtaining accurate PBL heights on a nearly continuous basis.
2013 Fall Meeting of the American Geophysical Union, San Francisco, CA, USA; 12/2013
[show abstract][hide abstract] ABSTRACT: A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by a ground-based elastic-backscatter tropospheric lidar is presented. An erf-like profile is used to model the mixing-layer top and the entrainment-zone thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to analyze atmospheric scenes with low signal-to-noise ratios (SNRs) without the need to resort to long-time averages or range-smoothing techniques, as well as to pave the way for future automated detection solutions. First, EKF results based on oversimplified synthetic and experimental lidar profiles are presented and compared with classic ABL estimation quantifiers for a case study with different SNR scenarios.
IEEE Transactions on Geoscience and Remote Sensing 09/2013; PP(99):1-12. · 3.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: The problem of overlap factor (OVF) computation and its near-range
sensitivity for medium-size aperture (f/10, f/11) bi-axial tropospheric
lidar systems using ray-tracing simulation software is presented. The
method revisits both detector and fiber optics coupling alternatives at
the telescope focal-plane along with the insertion of a field lens. A
sensitivity analysis is carried out as a function of laser divergence,
field lens, and detector/fiber positions, detector size, and the fiber's
core diameter and numerical aperture. The ray-tracing approach presented
here is straightforward and a comparatively much simpler solution than
analytical-based methods. Parametric simulations are carried out to show
that both approaches are coincident. Insertion of a field lens proves to
be an elegant and low sensitivity solution for OVF enhancement,
particularly, in the near-range of the lidar.
[show abstract][hide abstract] ABSTRACT: Total backscatter-coefficient inversion error bounds for the two-component lidar inversion algorithm (so-called Fernald's or Klett-Fernald-Sasano's method) are derived in analytical form in response to the following three error sources: 1) the measurement noise; 2) the user uncertainty in the backscatter-coefficient calibration; and 3) the aerosol extinction-to-backscatter ratio. The following two different types of error bounds are presented: 1) approximate error bounds using first-order error propagation and 2) exact error bounds using a total-increment method. Both error bounds are formulated in explicit analytical form, which is of advantage for practical physical sensitivity analysis and computational implementation. A Monte Carlo approach is used to validate the error bounds at 355-, 532-, and 1064-nm wavelengths.
IEEE Transactions on Geoscience and Remote Sensing 11/2012; 50(11):4791-4803. · 3.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: Although routinely monitored by ground based air quality networks, the particulate
matter distribution could be eventually better described with remote sensing techniques.
However, valid relationships between ground level and columnar ground based quantities
should be known beforehand. In this study we have performed a comparison between
particulate matter measurements at ground level at different cut sizes (10, 2.5 and 1.0 mm),
and the aerosol optical depth obtained by means of a ground based sunphotometer during
a multiinstrumental field campaign held in El Arenosillo (Huelva, Spain) from 28 June to
4 July 2006. All the PM fractions were very well correlated with AOD with correlation
coefficients that ranged from 0.71 to 0.81 for PM10, PM2.5 and PM1. Furthermore, the
influence of the mixing layer height in the correlations was explored. The improvement in
the correlation when the vertical distribution is taken into account was significant for days
with a homogeneous mixing layer. Moreover, the chemical analysis of the individual size
fractions allowed us to study the origin of the particulate matter. Secondary components
were the most abundant and also well correlated in the three size fractions; but for PM10
fraction, chemical species related to marine origin were best correlated. Finally, we obtained
a relationship between MODIS L3 AOD from collection 5.1 and the three PM cut sizes.
In spite of being a relatively clean environment, all the techniques were able to capture
similar day to day variations during this field campaign.
Journal of Geophysical Research 02/2012; 117(D04201). · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: A method to estimate wind velocity using the backscatter lidar signals of transported aerosol particles is presented. The lidar signal is correlated along range and time dimensions for each line of sight in which the lidar is pointing. The method is based on the analysis of contours of these range-time auto-correlation functions. This analysis obtains ellipse curve parameters which are related to the radial velocity and to the square wind speed, locally retrieved for a given line of sight. The combination of data from several angular positions enables the wind direction retrieval. Two possible implementations of the method are considered and tested with an actual measurement with the lidar of the Universitat PolitÃ¨cnica de Catalunya (UPC).
[show abstract][hide abstract] ABSTRACT: Minimization of the risk associated with spray applications requires a proper understanding of the spray drift phenomenon. This fact has led to the development of several techniques to measure the deposition on horizontal surfaces as well as the airborne spray profiles. Assessment of airborne spray drift is particularly difficult because this phenomenon is subject to variable micrometeorological conditions. However the monitoring of airborne drift has a great importance since it can be carried over long distances. This paper reviews main sampling techniques currently used to asses the airborne spray drift, based on passive collectors and tracers. Theoretical principles that determine the efficiency of passive samplers are studied as well as the performance of different types of tracers. On the other hand, this paper shows new airborne spray drift assessment techniques based on lidar technology, reviewing its principle of operation as well as its practical application in several spray drift trials. It is concluded that the lidar technique has significant advantages over conventional methods, especially in terms of time consumption and monitoring capabilities. However, the future adoption of lidar technology for airborne spray drift studies will be subjected to the development of lidar instruments really adapted to this application.
[show abstract][hide abstract] ABSTRACT: The bidimensional auto-correlation for the lidar signal measured along a single line of sight (LOS) is spectrally analyzed to estimate the wind turbulence variance, while a correlation contour analysis retrieves the mean wind speed. A combined model of a turbulent wind field plus an isotropic gaussian aerosol concentration field is used to 1) estimate the wind field variance causing diffusion on the aerosol structures and 2) to extend the contour autocorrelation analysis to include the turbulence effect and correct the mean wind estimation. The present analysis is intended to be used on a single LOS sensing, that can provide an estimation of the relative turbulence.
[show abstract][hide abstract] ABSTRACT: This paper presents a method to compute the overlap function of a lidar system in which a step-index optical fiber (or a bundle of such fibers) is used to carry the light collected by the telescope to the photoreceiver and a field lens is placed between the telescope and the optical fiber to increase the receiver field of view (FOV). The use of field lenses is a classical way to increase the FOV of radiometric systems (such as the receiving part of a lidar) when there is no numerical aperture (NA) limitation after the lens. However, when such a limitation exists, as in the case studied here, it will place a limit on the maximum attainable FOV. In the case of lidars, which have range-resolution capabilities, the limited FOV has an effect on the fraction of power coming from scattering volumes at different ranges that actually reaches the photodetector. This fraction is a function (the so-called overlap function) of the range of the scattering volume and its behavior has an impact on the accuracy of the retrievals. The application of the method developed in this paper shows that, in spite of the fiber NA limit, in practical situations the goal is attained of making the overlap function steeper and reaching higher values by using a field lens.
[show abstract][hide abstract] ABSTRACT: A 6-channel dichroic-based polychromator is presented as the spectrally selective unit for the U.P.C. elastic/Raman lidar. Light emission is made at 355-nm (ultraviolet, UV), 532-nm (visible, VIS) and 1064-nm (near infrared, NIR) wavelengths. In reception, the polychromator is the spectral separation unit that separates the laser backscattered composite return into 3 elastic (355, 532, 1064-nm wavelengths) and 3 Raman channels (386.7, 607.4 and 407.5-nm (water-vapor) wavelengths). The polychromator houses photo-multiplier tubes (PMT) for all the channels except for the NIR one, which is avalanche photodiode (APD) based. The optomechanical design uses 1-inch optics and Eurorack standards. The APD-based receiver uses a XY-axis translation/elevation micro-positioning stage due to its comparatively small active area and motorised neutral density filters are used in all PMT-based channels to avoid detector saturation. The design has been specially optimized to provide homogeneous spatial light distribution onto the photodetectors and good mechanical repeatability. All channels are acquired in mixed analog and photon-counting mode using LicelÂ® transient recorders, which are controlled by means of a user friendly LabVIEWTM interface. The paper focuses on the main polychromator optical design parameters, that is, light collimation trade-offs, end-to-end transmissivity, net channel responsivity, light distribution and spot size onto the photodetectors. The polychromator along with the rest of the U.P.C. lidar system has successfully been tested during a recent lidar system intercomparison campaign carried out in Madrid (Spain) during Oct. 2010.
[show abstract][hide abstract] ABSTRACT: The annual and seasonal variability of aerosol optical properties observed by means of a Raman lidar over Northeastern Spain has been assessed. The lidar representativeness has first been checked against sun-photometer measurements in terms of aerosol optical thickness. Then the annual cycle and the seasonal variability of the planetary boundary layer aerosol optical thickness and its fraction compared to the columnar optical thickness, the lidar ratio, the backscatter-related Ångström exponent and the planetary boundary layer height are analyzed and discussed. Winter and summer mean profiles of extinction, backscatter and lidar ratio retrieved with the Raman algorithm are presented. The analysis shows the impact of most of the natural events (Saharan dust intrusions, wildfires, etc.) and meteorological situations (summer anticyclonic situation, the formation of the Iberian thermal low, winter long-range transport from North Europe and/or North America, re-circulation flows, etc.) occurring in the Barcelona area. A detailed study of a special event including a combined intrusion of Saharan dust and biomass-burning particles proves the suitability of combining the retrieval of aerosol optical properties from Raman and pure elastic lidar measurements to discriminate spatially different types of aerosols and to follow their spatial and temporal evolution.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present a comparison of aerosol properties derived from in situ and remote sensing instruments during DAMOCLES campaign, aimed at investigating the equivalence between the instrumentation and methodologies employed by several Spanish groups to study atmospheric aerosols at a regional background site. The complete set of instruments available during this closure experiment allowed collecting a valuable high-resolution aerosol measurement data set. The data set was augmented with airborne in situ measurements carried out in order to characterize aerosol particles during the midday of 29 June 2006. This work is focused on aerosol measurements using different techniques of high-quality instruments (ground-based remote sensing and aircraft in situ) and their comparisons to characterize the aerosol vertical profiles. Our results indicate that the variability between the detected aerosol layers was negligible in terms of aerosol optical properties and size distributions. Relative differences in aerosol extinction coefficient profiles were less than 20% at 355 and 532 nm and less than 30% at 1064 nm, in the region with high aerosol concentration. Absolute differences in aerosol optical depth (AOD) were below 0.01 at 532 and 1064 nm and less than 0.02 at 355 nm, less than the uncertainties assumed in the AOD obtained from elastic lidar. Columnar values of the lidar ratio revealed some discrepancies with respect to the in situ aircraft measurements, caused fundamentally by the lack of information in the lowest part of the boundary layer.
Journal of Geophysical Research 01/2011; 116(D2):D02209. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Lidar and sun-photometer measurements were performed intensively over the Iberian Peninsula (IP) during
the eruption of the Eyjafjallaj¨okull volcano (Iceland) in April–May 2010. The volcanic plume reached all the IP stations for the first time on 5 May 2010. A thorough study of the event was conducted for the period 5–8 May. Firstly, the spatial and temporal evolution of the plume was described by means of lidar and sun-photometer measurements supported with backtrajectories. The volcanic aerosol layers observed over the IP were rather thin (<1000 m) with a top height up to 11–12 km. However, in some cases at the beginning of the period the thickness of those layers reached several kilometers in Evora and Madrid. The optical thicknesses associated to those layers were rather low (between 0.013 and 0.020 in average over the whole period), with peak values near 0.10 detected on 7 May. Secondly, the volcanic aerosols were characterized in terms of extinction and backscatter coefficients, lidar ratios, Angstrom exponents and linear particle depolarization ratio. Lidar ratios at different sites varied between 30 and 50 sr without a marked spectral dependency. Similar extinction-related Angstrom exponents varying between 0.6 and 0.8 were observed at different sites. The temporal evolution of the backscatter-related Angstrom exponents points out a possible decrease of the volcanic particle size as the plume moved from west to east. Particle depolarization ratios on the order of 0.06–0.08 confirmed the coexistence of both ash and non-ash particles. Additionally, profiles of mass concentration were obtained with a method using the opposite depolarizing effects of ash particles (strongly depolarizing), non-ash particles (very weakly depolarizing), and sun-photometer observations. In Granada the ash mass concentration was found to be approximately 1.5 times higher than that of non-ash particles, and probably did not exceed the value of 200 μgm−3 during the whole event.
ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2011; 11:29681–29721. · 5.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: A solution based on a Kalman filter to trace the evolution of the atmospheric boundary layer (ABL) sensed by an elastic backscatter lidar is presented. An erf-like profile is used to model the mixing layer top and the entrainment zone thickness. The extended Kalman filter (EKF) enables to retrieve and track the ABL parameters based on simplified statistics of the ABL dynamics and of the observation noise present in the lidar signal. This adaptive feature permits to analyze atmospheric scenes with low signal-to-noise ratios without need to resort to long time averages or rangesmoothing techniques, as well as to pave the way for an automated detection method. First EKF results based on synthetic lidar profiles are presented and compared with a typical least-squares inversion for different SNR scenarios.