Study of aerosols and molecular extinction effects in ultraviolet DIAL remote sensing in the lower atmosphere

Light passing through the atmosphere is scattered and absorbed by the molecules and particles in the atmosphere. This can adversely restrict and limit not only the signal-to-noise ratio (SNR) but also the accuracy and sensitivity of measurements, especially in long-path remote sensing. Usually, in differential absorption lidar (DIAL) techniques, errors are increased mainly because of the different extinction and backscattering properties of the atmosphere at the DIAL probe wavelengths. In this work we have investigated the effects of background aerosol and molecular extinction in DIAL remote sensing in the lower atmosphere for several visibilities at ultraviolet (UV) wavelengths by taking into account the dependence of refraction on the air temperature and pressure. For simplicity, we neglected the spatial inhomogeneity of aerosols in the lower atmosphere. Because of the weak attenuation produced by oxygen and other gaseous atmospheric constituents in this region compared with that from ozone, only ozone is considered as significant among the expected errors. Values for the total attenuation (km^?1) at wavelengths 200–400 nm are tabulated for several values of visibility. The acquired results show that the absorption and scattering by the molecules and aerosols vary with wavelength and visibility. The aerosol attenuation in the UV region varies smoothly and thereby errors caused by aerosol scattering can be neglected in remote sensing by UV-DIAL. In addition, aerosols play a very important role in lidar remote sensing in the lower atmosphere by scattering and absorption of radiation, which is considered as a significant factor. At high altitudes, the aerosol concentration is lower than at the ground; the molecular scattering is important, especially for wavelengths greater than 310 nm, where ozone attenuation is not important. The obtained results are important for accurate UV-DIAL measurements of concentration as well as when real-world signals are not available, for example when designing lidar and simulating or when access to real-world signals is not possible.     

Aerosol optical and microphysical properties observed by the lidar technique from a forest-fire smoke event over Madrid

On 27 August 2012, a wildfire occurred in the western zone of the Madrid region. Consequently, a significant release of smoke and aerosols was injected into the free troposphere and advected by the synoptic circulation in trajectories that passed over the capital. This event was detected by the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT; Research Centre for Energy, Environment and Technology), Madrid lidar station at about 5 km height. While the main aerosol plume did not pass over the lidar station, the eastern portion of the dispersed emissions did. No aerosol layers were detected by lidar in the free-troposphere for the next few days. Since few observations of fresh biomass burning aerosol have been carried out by lidar so far, this study contributes to previous studies on biomass burning aerosols, in particular regarding the first stages of the smoke transport.

This article studies the optical and microphysical properties of the smoke aerosols retrieved by the lidar technique along with the recently developed Lidar/Radiometer Inversion Code (LiRIC), which combines both lidar and sun–sky photometer data. The sun–sky photometer data are provided by the Agencia Estatal de Meteorología (AEMET; Spanish Meteorological Agency) station close to the CIEMAT site. We suggest that the aerosol properties retrieved might be linked to the water uptake phenomenon in view of the fact that simultaneous water vapour lidar measurements exhibited a close relationship with backscatter coefficient profiles. This article is concerned not only with showing the capability of combining different remote-sensing techniques and the LiRIC code to provide aerosol vertical distribution for fine and coarse modes, but also with providing signs about the plausible interaction between aerosols and humidity that leads fresh biomass burning aerosols to act as cloud condensation nuclei. This key role, known as the indirect effect, remains the major source of uncertainty when the global radiation budget is assessed.     

The EOLE Project: A multiwavelength laser remote sensing (lidar) system for ozone and aerosol measurements in the troposphere and the lower stratosphere. Part II: Aerosol measurements over Athens,Greece

Ground-based measurements of suspended aerosols in the lower troposphere (0.6-3.5 km height) in Athens, Greece were performed for a period of 10 days in November 1994. The measurements were performed in a suburban area in the northern part of the Athenian basin, using a two-wavelength backscattering lidar system operating at 355 nm and 532 nm, simultaneously. The lidar system was pointing vertically and was operated under different meteorological conditions. A lidar inversion algorithm was applied to determine the vertical profile of the aerosol backscattering coefficient in the 0.6-3.5 km altitude range. The analysis of the bi-dimensional cross-section of the aerosol backscattering coefficient and slope rate, for two selected cases, in conjunction with radiosonde, solar UV-B irradiance and in situ air pollution data, gave the possibility to study air pollution characteristics inside the Athens basin.     

Retrieval and analysis of aerosol optical characteristics in Hubei Province,China, based on CALIPSO

The aerosol optical characteristics in Hubei Province, China, derived from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), are investigated. Through this spaceborne lidar, some limitations of airborne and ground-based systems can be overcome and a global survey of clouds and aerosol can be provided. First, a lidar ratio selection algorithm is introduced. Since the lidar ratio (the aerosol extinction-to-backscatter ratio, S _a) is an important parameter used in aerosol retrieval, it is required for a reliable aerosol model. After retrieving the original signal, aerosol optical parameters are obtained. Based on these steps, the impacts of atmospheric motion on aerosol diffusion in different layers, the distribution of these aerosols, as well as their time-changing characteristic in Hubei Province are discussed. The studies show that in areas of plains aerosols are easily influenced by wind; furthermore, aerosol distribution in the western Hubei Province is usually influenced by biomass burning and in eastern Hubei by industrial emission. Although errors still exist, the aerosol optical characteristics in different regions are strongly related to the ground surface, wind direction and visible distinctions exist in urban and country areas.     

Remote sensing of aerosols over the oceans using AVHRR data Theory,practice and applications

Abstract

The basic features of single- and dual-channel aerosol retrieval algorithms based on matching radiances measured in AVHRR channels -1 (～0.58-0.68 μm) and -2( ～0.73-1.10μm) with model computations will be described. The use of the NOAA/NESDIS single-channel algorithm will be illustrated with examples of detection and mapping of enhanced atmospheric turbidity over the oceans. The effects of variations in the physical and radiative properties of atmospheric aerosols, and in atmospheric ozone and water vapour, will be briefly discussed in the light of model sensitivity analyses.     

The effect of aerosol vertical profiles on satellite-estimated surface particle sulfate concentrations

The aerosol vertical distribution is an important factor in determining the relationship between satellite retrieved aerosol optical depth (AOD) and ground-level fine particle pollution concentrations. We evaluate how aerosol profiles measured by ground-based lidar and simulated by models can help improve the association between AOD retrieved by the Multi-angle Imaging Spectroradiometer (MISR) and fine particle sulfate (SO4) concentrations using matched data at two lidar sites. At the Goddard Space Flight Center (GSFC) site, both lidar and model aerosol profiles marginally improve the association between SO4 concentrations and MISR fractional AODs, as the correlation coefficient between cross-validation (CV) and observed SO4 concentrations changes from 0.87 for the no-scaling model to 0.88 for models scaled with aerosol vertical profiles. At the GSFC site, a large amount of urban aerosols resides in the well-mixed boundary layer so the column fractional AODs are already excellent indicators of ground-level particle pollution. In contrast, at the Atmospheric Radiation Measurement Program (ARM) site with relatively low aerosol loadings, scaling substantially improves model performance. The correlation coefficient between CV and observed SO4 concentrations is increased from 0.58 for the no-scaling model to 0.76 in the GEOS-Chem scaling model, and the model bias is reduced from 17% to 9%. In summary, despite the inaccuracy due to the coarse horizontal resolution and the challenges of simulating turbulent mixing in the boundary layer, GEOS-Chem simulated aerosol profiles can still improve methods for estimating surface aerosol (SO4) mass from satellite-based AODs, particularly in rural areas where aerosols in the free troposphere and any long-range transport of aerosols can significantly contribute to the column AOD.     

Comparative study of aircraft- and satellite-derived aerosol and cloud microphysical parameters during CAIPEEX-2009 over the Indian region

This article presents the spatial and vertical distribution of aerosols and cloud microphysical parameters from the combined data sets of aircraft and satellites. The aircraft-based Cloud Aerosol Interactions and Precipitation Enhancement Experiment (CAIPEEX) was conducted in India during May to September 2009. During the experimental period, 3 days were identified on which space-borne lidar (CALIPSO) and radar (CloudSat) were nearby/over passed the observational regions, which covered north, south central, and southern parts of the Indian subcontinent. The results obtained from these three cases are explored. Similar features of aerosol layering and water/ice cloud signatures are observed by both aircraft and CALIPSO. In addition, events where dust aerosols acting as ice nuclei and polluted aerosols increase the depth of warm rain initiation are observed. The CloudSat profiles of liquid water content, droplet number concentration, and effective radii are underestimated when compared with the corresponding aircraft profiles. The aircraft measurements are able to bring out fine variability in vertical distribution, which would be more useful for regional parameterization schemes and model evaluation.     

Tropospheric aerosol observations in São Paulo,Brazil using a compact lidar system

A backscattering light detection and ranging (lidar) system, the first of this kind in the country, has been set up in a suburban area in the city of São Paulo, Brazil (23°33′?S, 46°44′?W) to provide the vertical profile of the aerosol backscatter and extinction coefficients at 532?nm and up to 4–5?km height above sea level (asl). The measurements have been carried out during the second half of the so‐called Brazilian dry season, September and October in the year of 2001. When possible, the lidar measurements were complemented with aerosol optical thickness measurements obtained by a CIMEL Sun‐tracking photometer in the visible spectral region, not only to validate the lidar data, but also to provide an input value of the so‐called extinction‐to‐backscatter ratio (lidar ratio). The lidar data were also used to retrieve the Planetary Boundary Layer (PBL) height and low troposphere structural features over the city of São Paulo. Three‐dimensional air mass back trajectory analysis was also conducted to determine the source regions of aerosols observed during this study. These first lidar measurements over the city of São Paulo during the second half of the dry season showed a significant variability of the aerosol optical thickness (AOT) in the lower troposphere (0.5–5?km) at 532?nm. It was also found that the aerosol load is maximized in the 1–3?km height region and this load represents about 20–25% of the lower tropospheric aerosol.     

Ground-, satellite- and simulation-based analysis of a strong dust event over Abastumani,Georgia, during May 2009

A strong dust event over Abastumani, Georgia, during May 2009 was studied using light detection and ranging (lidar), satellite and sun photometric measurements. High aerosol optical depth (AOD) values (0.45–0.57) at 500 nm were measured over the closest Aerosol Robotic Network (AERONET) site (Erdemli, Turkey), whereas over Georgia, the AOD measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) was about 0.9 at 550 nm. The AERONET data analysis showed a mean aerosol effective radius of about 2.5 μm, whereas the mean value of the Ångström exponent (α) (wavelength pair 440/870 nm) was smaller than 1, indicating the dominance of large aerosols. The aerosol lidar over Abastumani showed the existence of a strong particle load from the near ground up to a height of 3.5 km. The BSC-DREAM8b forecast model showed that the dust aerosols travelled from the Saharan and the Arabic deserts to the studied area, even reaching southern Russia, covering a total distance of about 5500 km, in the height region from about 2 to 11.5 km.     

A Dempster Shafer-based fuzzy multisensor fusion system using airborne lidar and hyperspectral imagery

ABSTRACT

Nowadays, accurate spectral reflectance information is provided by hyperspectral (HS) data while light detection and ranging (lidar) data provides precise information about the height and geometrical properties of the surfaces. In the most research papers, data fusion of disparate sensors significantly improves object classification performance compared to that of just an individual sensor. Previous researches on fusion of these two sensors had problems such as crisp classifiers or simple fuzzy decision-making systems. This article tries to overcome these weaknesses by accurate support vector machine (SVM) and Fuzzy SVM as classifiers in crisp and fuzzy decision fusion system and fusion of two sensors by two different methods based on precise theories of Bayesian and Shafer. Also, the proposed method tries to compare the results of fusion of both data using decision fusion system with stacked features strategy. This study focuses on HS and lidar fusion through three main phases. The first phase is based on the using of Noise Weighted Harsanyi-Farrand-Chang method and principal component analysis to overcome the high dimensionality problem of HS data. The second phase is based on the feature extraction and selection strategy on lidar data. Finally, fuzzy SVM and Dempster Shafer methods are applied as fuzzy classification and fuzzy decision fusion strategies on the feature spaces. A co-registered HS and lidar data set from Houston of U.S.A. by 15 classes was available to examine the effectiveness of the proposed method. The results of this study highlight that the combination of HS and lidar data enable reliable mapping of land cover.     

The relationship between aerosol backscatter coefficient and atmospheric relative humidity in an urban area over Athens,Greece, using Raman lidar and radiosonde data

In this article a statistical assessment concerning the relationship between the aerosol backscatter coefficient (β_aer) and the relative humidity (RH) in the lower and middle troposphere, over Athens (Greece), is presented. For the purpose of this study, correlative radiosonde and aerosol backscatter lidar data were analysed for a period of 4 years (January 2003–December 2006), as obtained in the framework of the European Aerosol Lidar Network (EARLINET) project. The vertical profiles of the aerosol backscatter coefficients were measured by a combined Raman/elastic lidar system at ultraviolet (355 nm) and visible (532 nm) wavelengths. The correlation coefficient (R) of the vertical profiles of the RH against the backscatter coefficient of aerosols was investigated in altitudes within the free troposphere (0–6000 m). The altitude range was divided into three areas: 0 m up to the top of the planetary boundary layer (PBL); PBL up to PBL?+?2000 m; and PBL?+?2000 m up to 6000 m. The properties and seasonal variations of the height of the PBL were also studied. The annual mean PBL height over Athens was found to be (1320 ± 480) m, while during the warm period of the year (spring–summer) the PBL was higher than during the cold period (autumn–winter). Regarding the correlation coefficient (R), low (0–0.5) and medium (0.5–0.8) R values were mostly observed during the warm months of the year. For the aerosols originating from the Balkan area the highest correlation was observed at both wavelengths (R?=?0.71 at 355 nm and R?=?0.41 at 532 nm), especially during the years 2003 and 2005 (R?=?0.61 at 355 nm and R?=?0.93 at 532 nm). The almost linear correlation of this type of aerosols can be attributed to the fact that these remained for a longer time in a coherently alternating atmosphere, therefore having the tendency to become homogenized.     

Automated extraction of street-scene objects from mobile lidar point clouds

Mobile laser scanning or lidar is a new and rapid system to capture high-density three-dimensional (3-D) point clouds. Automatic data segmentation and feature extraction are the key steps for accurate identification and 3-D reconstruction of street-scene objects (e.g. buildings and trees). This article presents a novel method for automated extraction of street-scene objects from mobile lidar point clouds. The proposed method first uses planar division to sort points into different grids, then calculates the weights of points in each grid according to the spatial distribution of mobile lidar points and generates the geo-referenced feature image of the point clouds using the inverse-distance-weighted interpolation method. Finally, the proposed method transforms the extraction of street-scene objects from 3-D mobile lidar point clouds into the extraction of geometric features from two-dimensional (2-D) imagery space, thus simplifying the automated object extraction process. Experimental results show that the proposed method provides a promising solution for automatically extracting street-scene objects from mobile lidar point clouds.     

Application of ADIFOR for air pollution model sensitivity studies

Typical computational methods of sensitivity analysis are discussed. Automatic differentiation addresses the need for computing derivatives of large codes accurately, regardless of the complexity of the model. Automatic differentiation in FORTRAN (ADIFOR) is a source transformation technique that accepts FORTRAN coded program for the computation of a function and generates portable FORTRAN code for the computation of the derivatives of that function. ADIFOR is introduced and applied to a comprehensive atmospheric chemistry/transport/radiative-transfer model to study the sensitivity of photochemical ozone production with respect to aerosol. The modeling results indicate that aerosol interaction with ozone may be as important as NO_x and non-methane hydrocarbon (NMHC) emissions in determining ozone production. The presence of scattering and/or absorbing aerosols in the atmosphere can cause significant differences in calculated ozone levels. Normalized sensitivity coefficients show that ozone and other photochemical oxidants are most sensitive to the aerosol single scattering albedo, which determines the scattering efficiency of the aerosol. ADIFOR is demonstrated to be an effective tool for sensitivity analysis in air pollution modeling.     

Remote sounding of minor constituents in the stratosphere and heterogeneous reactions of gases at solid interfaces

Various remote sounding techniques of minor constituents of the stratosphere may be carried out, especially during the Antarctic and Arctic winters, where heterogeneous reactions of gases at solid interfaces play an important role in ozone destruction. After the El Chichon volcanic eruption in 1982 the significant decreases in NO₃, NO₂, and increases in HCI were not consistent with known chemistry unlessCIN0₃ could be converted to HCl on the solid NaCI (observed to be present in the El Chichon aerosols). Recently the eruption of Mount Pinatubo in 1991 has led to a large (factor of ten) increase in stratospheric aerosol. The biggest chemical impact evident in the measurements made during the European Arctic Stratospheric Ozone Experiment (EASOE) was the reduction of NO and NO₂,. Given that the chemical behaviour of the solid NaCI is most likely associated with its mechanical properties, a recently proposed thermodynamical model is employed to show the important role of the heterogeneous reactions with NaCI in the volcanic aerosols. These reactions might also prove to be critical in the inversion algorithms usually applied to satellite measurements.     

Atmospheric aerosol measurements and the reliability problem: new results

Atmospheric aerosol measurements and the reliability problem have been discussed previously. New results of the inverse problem solutions are discussed here. The relationship between the backscattering coefficient and the extinction coefficient was found for different approximations of the integral method using finite duration probe pulses. The optical thickness of the atmosphere was determined by methods of multiposition probing without numerical integration. The integral algorithms for the determination of the extinction coefficients can be more accurate than the algorithms for the determination of the optical thicknesses. New algorithms were developed for the determination of the relationship between the optical parameters of aerosols. New lidar equation solutions developed for weak signal processing were investigated. The errors of the extinction coefficient for different minimizing procedures were considered based on the weighting coefficients. A method was developed to determine the weighting coefficients. The preliminary calculated parameters were used for more accurate linearization of the inverse problem. The model of spherical particles with the radially variable refractive index was considered. The effectiveness of the proposed model is based on the possibility of choosing the thickness of inhomogeneous coating depending on the particle size. The proposed model can explain the significant discrepancy found between the photoelectric and filter aspiration results and can be used to minimize the error of particle size optical determination.     

Contribution of EARLINET/ACTRIS to the summer 2013 Special Observing Period of the ChArMEx project: monitoring of a Saharan dust event over the western and central Mediterranean

In the framework of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx; http://charmex.lsce.ipsl.fr/) initiative, a field campaign took place in the western Mediterranean Basin between 10 June and 5 July 2013 within the ADRIMED (Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) project. The scientific objectives of ADRIMED are the characterization of the most common ‘Mediterranean aerosols’ and their direct radiative forcing (column closure and regional scale). During 15–24 June a multi-intrusion dust event took place over the western and central Mediterranean Basin. Extra measurements were carried out by some EARLINET/ACTRIS (European Aerosol Research Lidar Network /Aerosols, Clouds, and Trace gases Research InfraStructure Network, http://www.actris.net/) lidar stations in Spain and Italy, in particular on 22 June in support to the flight over southern Italy of the Falcon 20 aircraft involved in the campaign. This article describes the physical and optical properties of dust observed at the different lidar stations in terms of dust plume centre of mass, optical depth, lidar ratio, and particle depolarization ratio. To link the differences found in the origin of dust plumes, the results are discussed on the basis of back-trajectories and air- and space-borne lidars. This work puts forward the collaboration between a European research infrastructure (ACTRIS) and an international project (ChArMEx) on topics of interest for both parties, and more generally for the atmospheric community.     

Lidar remote sensing of savanna biophysical attributes: opportunities,progress, and challenges

Despite occupying almost a fifth of the global terrestrial vegetation system, savanna ecosystems are relatively understudied in the Earth observation field. As a result, their contribution to global socioecological functions, such as carbon sequestration, habitat provision, watershed protection, biodiversity, and communal supply of timber and non-timber products, is inadequately accounted for. Since lidar remote sensing has been proved to estimate accurately the three-dimensional structural attributes of vegetation, the author found it insightful to synthesize the application of this technique in the savannas as one of the steps towards addressing this knowledge gap. The synthesis evaluated the progress of current studies that primarily use lidar data in the savannas and identified the associated opportunities and challenges. For each selected application, three main questions are asked: (1) what is typically needed from lidar remote sensing? (2) what have we achieved already? And (3) what is the current status? The last question was further split into two: (a) what is lacking, if any? (b) what challenges need to be addressed? This article concludes by looking into the potential future of lidar remote sensing in the savannas and some recommendations are put forward accordingly.     

The influence of a south Asian dust storm on aerosol radiative forcing at a high-altitude station in central Himalayas

The impact of long-range transported dust aerosols, originating from the Thar Desert region, to a high-altitude station in the central Himalayas was studied with the help of micro-pulse lidar (MPL) observations. A drastic change in lidar back-scatter profile was observed on a dust day as compared with that on a pre-dust day. The back-scatter coefficient on a dust day revealed that the dust layer peaked at an altitude ～1300 m above ground level (AGL) and extended up to ～3000 m AGL, with maximum value ～3?×?10^–5 m^–1 sr^–1. Aerosol Index (AI) and air mass back-trajectory analysis substantiate the transport of dust aerosols from the far-off Thar Desert region to the experimental site. A significant effect of dust aerosols was also observed over the station on the spectral aerosol optical depths (AODs), measured using a Microtops-II Sunphotometer. It showed significantly different spectral behaviour of AOD on a dust day as compared with that on a pre-dust day. The Ångström exponent (α) showed a marked decrease from 0.42 to 0.04 from the pre-dust day to the dust day. The aerosol radiative forcing estimated using the Santa Barbara DISORT (discrete ordinate radiative transfer) atmospheric radiative transfer (SBDART) model, in conjunction with the optical properties of aerosol and cloud (OPAC) model, showed values of about –30, –45 and?+15 W m^–2, respectively, at top-of-atmosphere (TOA), surface and in the atmosphere on the dust day. The positive atmosphere forcing caused an estimated heating of the lower atmosphere by ～0.4 K day^–1.     

Benchmarking stereo-derived DSMs to lidar by land cover and slope for resource development in northern Ontario,Canada

Topographic and elevation data are essential in the development of supporting infrastructure around mining sites. The de facto standard for acquiring elevation data is through light detection and ranging (lidar). The high labour and monetary cost of acquiring lidar has fostered more cost-effective approaches for creating elevation models that use stereo photogrammetry. To assess the accuracy of stereo-photogrammetry-derived elevation models and their potential application, we benchmark satellite (Worldview-2) and aircraft (South Central Ontario Orthoimagery Project; SCOOP) stereo-derived digital surface models (DSMs) against a lidar-derived DSM. Our results show that both stereo-derived DSMs have strong monotonic correlations with lidar across a range of land-cover types and slopes. The overall vertical accuracy of Worldview-2 and SCOOP DSMs are similar and do not meet the United States National Digital Elevation Program (NDEP) standards. However, accuracy assessment across land-cover types and slope categories show that specific land cover types (i.e. grass, row crops/pasture, sparse vegetation and marsh) on gently sloping terrain compare well to lidar data and meet NDEP accuracy standards. We situate the presented research in the context of northern resource development and discuss opportunities to improve the vertical accuracy of stereo-derived DSMs, for example, through unmanned aerial systems.