Aerosol Backscatter Coefficient Research Articles

Improved method to retrieve aerosol optical properties from combined elastic backscatter and Raman lidar data

An improved method that has the potential to improve the retrieval of aerosol optics properties (back- scatter/extinction coefficients) from elastic-Raman lidar data is presented. Aerosol backscatter coefficients can be retrieved by choosing the reference height at near-range rather than conventional far-range when the signal-to-noise ratios are low at the far-range or aloft aerosol layers and clouds appear there. Significant retrieval errors in aerosol backscatter coefficients caused by large uncertainties of the aerosol reference value at far-range can be reduced. To avoid the ill-posed retrievals of aerosol extinction from the conventional Raman method, the new method derives the aerosol extinction and lidar ratio with the constrained Fernald inversions by independent aerosol backscatter coefficients from above proposed method. The numerical simulations demonstrated that the proposed method pro- vides good accuracy and resolution of aerosol profile ret- rievals. And the method is also applied to elastic-Raman lidar measurements at the Hampton University, Hampton, Virginia.

A study on the use of radar and lidar for characterizing ultragiant aerosol

AbstractFrom 19 April to 19 May 2010, volcanic aerosol layers originating from the Eyjafjallajökull volcano were observed at the Institute of Methodologies for Environmental Analysis of the National Research Council of Italy Atmospheric Observatory, named CIAO (40.60°N, 15.72°E, 760 m above sea level), in Southern Italy with a multiwavelength Raman lidar. During this period, ultragiant aerosols were also observed at CIAO using a colocated 8.45 mm wavelength Doppler radar. The Ka‐band radar signatures observed in four separate days (19 April and 7, 10, and 13 May) are consistent with the observation of nonspherical ultragiant aerosols characterized by values of linear depolarization ratio (LDR) higher than –4 dB. Air mass back trajectory analysis suggests a volcanic origin of the ultragiant aerosols observed by the radar. The observed values of the radar reflectivity (Ze) are consistent with a particle effective radius (r) larger than 50–75 µm. Scattering simulations based on the T‐matrix approach show that the high LDR values can be explained if the observed particles have an absolute aspect ratio larger than 3.0 and consist of an internal aerosol core and external ice shell, with a variable radius ratio ranging between 0.2 and 0.7 depending on the shape and aspect ratio. Comparisons between daytime vertical profiles of aerosol backscatter coefficient (β) as measured by lidar and radar LDR reveal a decrease of β where ultragiant particles are observed. Scattering simulations based on Mie theory show how the lidar capability in typing ultragiant aerosols could be limited by low number concentrations or by the presence of an external ice shell covering the aerosol particles. Preferential vertical alignment of the particles is discussed as another possible reason for the decrease of β.

Annual variations of the altitude distribution of aerosols and effect of long-range transport over the southwest Indian Peninsula

Annual variations of the altitude distribution of aerosols and the effect of long-range transport in modulating the aerosol loading over Thiruvananthapuram (8.5°N, 77°E), a relatively clean tropical station located in the southwest coast of Peninsular India, are investigated using dual polarization Micro Pulse Lidar observations carried out during March 2008–May 2011. Combined analysis of these lidar observations with the spatial distribution of aerosols derived from satellite data shows the occurrence of elevated layers of highly non-spherical aerosols in the 1.5–4 km altitude region, which are associated with the wide-spread aerosol plumes over the Arabian Sea during the pre-monsoon and summer-monsoon seasons. In contrast, ∼90% of the column integrated aerosol backscatter coefficient (βa) (below 5 km altitude) occurs below ∼1.5 km during winter. Seasonal variation of mean βa below ∼1 km altitude is <20%. Altitude profiles of βa above ∼1 km during January – characterised by the smallest values of βa, absence of elevated aerosol layers, and weak atmospheric winds – may be considered as the upper limit of the contribution by locally produced aerosols for quantifying the effect of long-range transport during the other months. Compared to January, a 3–10 fold increase in βa occurs in the 2–4 km altitude region during April–May and July–August. The elevated layers contribute ∼20–30% of the total aerosol loading during the above months.

Aerosol lidar for continuous atmospheric monitoring

A design is proposed of an eye-safe high spectral resolution lidar operating at a wavelength of 532 nm. Absolute calibration is ensured by a molecular channel where aerosol signals are filtered in an iodine-filled cell. Laser beam expansion in a transmitter via a receiving telescope ensures high thermo-mechanical stability of the design, which allows a small field-of-view and substantial reduction of the background noise level. A detailed optical circuit of the transceiver is shown, where the transmitter and the receiver are located on different sides of the optical bench for better stability. Specifications of the laser and the system are given. Lidar returns are calculated, and measurement errors are estimated. It is shown that the time of averaging should be no longer than 1 min to attain 10% accuracy when calculating the aerosol backscattering coefficient and optical depth in the troposphere. The system proposed is to operate continuously and unattended.

Continuous atmospheric boundary layer observations in the coastal urban area of Barcelona during SAPUSS

Abstract. Continuous measurements of surface mixed layer (SML), decoupled residual/convective layer (DRCL) and aerosol backscatter coefficient were performed within the Barcelona (Spain) boundary layer from September to October 2010 (30 days) in the framework of the SAPUSS (Solving Aerosol Problems by Using Synergistic Strategies) field campaign. Two near-infrared ceilometers (Jenoptik CHM15K), vertically and horizontally probing (only vertical profiles are herein discussed), were deployed. Ceilometer-based DRCLs (1761 ± 363 m a.g.l.) averaged over the campaign duration were twice as high as the mean SML (904 ± 273 m a.g.l.). Both DRCL and SML showed a marked SML diurnal cycle. Ceilometer data were compared with potential temperature profiles measured by daily radiosounding (twice a day, midnight and midday) to interpret the boundary layer structure in the coastal urban area of Barcelona. The overall agreement (R2 = 0.80) between the ceilometer-retrieved and radiosounding-based SML heights (h) revealed overestimation of the SML by the ceilometer (Δh=145 ± 145 m). After separating the data in accordance with different atmospheric scenarios, the lowest SML (736 ± 183 m) and DRCL (1573 ± 428 m) were recorded during warm North African (NAF) advected air mass. By contrast, higher SML and DRCL were observed during stagnant Regional (REG) (911 ± 234 m and 1769 ± 314 m, respectively) and cold Atlantic (ATL) (965 ± 222 m and 1878 ± 290 m, respectively) air masses. In addition to being the lowest, the SML during the NAF scenario frequently showed a flat upper boundary throughout the day possibly because of the strong winds from the Mediterranean Sea limiting the midday SML convective growth. The mean backscatter coefficients were calculated at two selected heights representative of middle and top SML portions, i.e. β500 = 0.59 ± 0.45 Mm−1 sr−1 and β800 = 0.87 ± 0.68 Mm−1 sr−1 at 500 m and 800 m a.g.l., respectively. The highest backscatter coefficients were observed during NAF (β500 = 0.77 ± 0.57 Mm−1 sr−1) when compared with ATL (β500 = 0.51 ± 0.44 Mm−1 sr−1) and REG (β500 = 0.64 ± 0.39 Mm−1 sr−1). The relationship between the vertical change in backscatter coefficient and atmospheric stability (∂θ/∂z) was investigated in the first 3000 m a.g.l., aiming to study how the unstable, stable or neutral atmospheric conditions of the atmosphere alter the distribution of aerosol backscatter with height over Barcelona. A positive correlation between unstable conditions and enhanced backscatter and vice versa was found.

Pure rotational-Raman channels of the Esrange lidar for temperature and particle extinction measurements in the troposphere and lower stratosphere

Abstract. The Department of Meteorology at Stockholm University operates the Esrange Rayleigh/Raman lidar at Esrange (68° N, 21° E) near the Swedish city of Kiruna. This paper describes the design and first measurements of the new pure rotational-Raman channel of the Esrange lidar. The Esrange lidar uses a pulsed Nd:YAG solid-state laser operating at 532 nm as light source with a repetition rate of 20 Hz and a pulse energy of 350 mJ. The minimum vertical resolution is 150 m and the integration time for one profile is 5000 shots. The newly implemented channel allows for measurements of atmospheric temperature at altitudes below 35 km and is currently optimized for temperature measurements between 180 and 200 K. This corresponds to conditions in the lower Arctic stratosphere during winter. In addition to the temperature measurements, the aerosol extinction coefficient and the aerosol backscatter coefficient at 532 nm can be measured independently. Our filter-based design minimizes the systematic error in the obtained temperature profile to less than 0.51 K. By combining rotational-Raman measurements (5–35 km height) and the integration technique (30–80 km height), the Esrange lidar is now capable of measuring atmospheric temperature profiles from the upper troposphere up to the mesosphere. With the improved setup, the system can be used to validate current lidar-based polar stratospheric cloud classification schemes. The new capability of the instrument measuring temperature and aerosol extinction furthermore enables studies of the thermal structure and variability of the upper troposphere/lower stratosphere. Although several lidars are operated at polar latitudes, there are few instruments that are capable of measuring temperature profiles in the troposphere, stratosphere, and mesosphere, as well as aerosols extinction in the troposphere and lower stratosphere with daylight capability.

Statistical analysis of aerosol optical properties retrieved by Raman lidar over Southeastern Spain

In this work, a statistical study of aerosol optical properties retrieved from Raman lidar profiles has been addressed at the EARLINET station of Granada, Spain, during the period 2008–2010. Lidar measurements were performed during day- and night-time. Mean values and variances of the aerosol extinction and backscatter coefficient profiles in the troposphere have been computed. These profiles evidenced that during autumn–winter, most of the particles are confined to the first kilometres above the surface (below 3500 m above sea level), while a major presence of aerosol at higher altitudes is observed during spring–summer. Moreover, a study of the planetary boundary layer (PBL) height and aerosol stratification has been performed for the whole studied period. Monthly mean β-related Angström exponent values have been obtained for aerosols in the PBL and in the free troposphere. Furthermore, monthly mean lidar ratio values at 532 nm have been retrieved from Raman profiles during night-time. A detailed study of these intensive properties has allowed characterizing the aerosol present over our station. The results evidenced a predominance of large and scattering particles during spring and summer and an increase of small and absorbing particles during autumn and winter.

Method for determining microphysical parameters of atmospheric aerosol from the results of satellite and ground-based multifrequency sounding

A method for reconstructing the parameters of postvolcanic stratospheric aerosol from the results of joint measurements of the aerosol backscattering coefficient with lidar systems based on the Nd:YAG laser and the aerosol extinction coefficient with the SAGE III satellite instrumentation is developed. The most informative set of optical characteristics is determined for each of the microphysical parameters under consideration (concentration, surface area, volume, and effective size of particles of the fine and coarse aerosol fractions). Multiple polynomial regressions between optical and microphysical characteristics of aerosol are obtained. These regressions make it possible to determine the microphysical characteristics of aerosol within a wide range without solving incorrect inverse problems. The results are compared with independent experimental data. The errors of reconstructing microphysical parameters of aerosol are estimated for different situations in the stratosphere. The influence of the shape of particles of the dust fraction on the results of a reconstruction of the microphysical parameters of aerosol is considered.

A New Method for Measuring Atmospheric Temperature and Aerosol Backscattering Coefficient Using a Pure Rotational Raman Lidar

AbstractIt has been proved to be a high precision method to derive atmospheric temperature with a pure rotational Raman signal of atmospheric molecules and many pure rotational Raman lidars have been established around the world. The sum of all pure rotation Raman spectral lines is independent of temperature. By using this feature, we can determine the aerosol extinction without any assumption. This paper presents a new method for measuring atmospheric temperature and aerosol backscattering coefficient based on the two single lines of pure rotational Raman spectrum (J = 4 and 14) detected by a pure rotation Raman lidar with an additional Mie channel. The concept and mathematical derivation are given in the paper.

Two-wavelength lidar characterization of atmospheric aerosol fields at low altitudes over heterogeneous terrain

The possibilities for applying multiwavelength elastic lidar probing of the atmosphere to help monitor air-quality over large industrial and densely populated areas, based predominantly on the use and analysis of commonly obtainable backscatter-related lidar quantities, are examined. Presented are two-wavelength (1064/532 nm ) lidar observations on the spatial distribution, structure, composition, and temporal evolution of close-to-surface atmospheric aerosol fields over heterogeneous orographic areas (adjacent city, plain, and mountain) near Sofia, Bulgaria. Selected winter-time evening lidar measurements are described. Range profiles, histograms, and evolutional range-time diagrams of the aerosol backscatter coefficients, range-corrected lidar signals, normalized standard deviations, and backscatter-related Ångström exponents (BAE) are analyzed. Near-perfect correlation between the aerosol density distribution and orographic differentiation of the underlying terrain is established, finding expression in a sustained horizontal stratification of the probed atmospheric domains. Distinctive features in the spatial distribution and temporal evolution of both the fine- and coarse aerosol fractions are revealed in correlation with terrain's orography. Zonal aerosol particle size distributions are qualitatively characterized by using an approach based on BAE occurrence frequency distribution analysis. Assumptions are made about the aerosol particle type, origin, and dominating size as connected (by transport-modeling data) to local pollution sources.

Evolution of planetary boundary layer under different weather conditions, and its impact on aerosol concentrations

A field experiment was conducted in Tianjin, China from September 9–30, 2010, focused on the evolution of Planetary Boundary Layer (PBL) and its impact on surface air pollutants. The experiment used three remote sensing instruments, wind profile radar (WPR), microwave radiometer (MWR) and micro-pulse lidar (MPL), to detect the vertical profiles of winds, temperature, and aerosol backscattering coefficient and to measure the vertical profiles of surface pollutants (aerosol, CO, SO2, NOx), and also collected sonic anemometers data from a 255-m meteorological tower. Based on these measurements, the evolution of the PBL was estimated. The averaged PBL height was about 1000–1300m during noon/afternoon-time, and 200–300m during night-time. The PBL height and the aerosol concentrations were anti-correlated during clear and haze conditions. The averaged maximum PBL heights were 1.08 and 1.70km while the averaged aerosol concentrations were 52 and 17μg/m3 under haze and clear sky conditions, respectively. The influence of aerosols and clouds on solar radiation was observed based on sonic anemometers data collected from the 255-m meteorological tower. The heat flux was found significantly decreased by haze (heavy pollution) or cloud, which tended to depress the development of PBL, while the repressed structure of PBL further weakened the diffusion of pollutants, leading to heavy pollution. This possible positive feedback cycle (more aerosols→lower PBL height→more aerosols) would induce an acceleration process for heavy ground pollution in megacities.

Vertical versus scanning lidar measurements in a horizontally homogeneous atmosphere

This study compares the aerosol backscatter and extinction coefficients retrieved from vertical elastic and Raman channels with those derived from measurements with multiangle elastic channels. Retrievals from simulated vertical signals at 355 nm, 387 nm, 532 nm, and 607 nm are compared with those from multiangle measurements (at 15 elevation angles) at 355 nm and 532 nm. The atmosphere is considered horizontally homogeneously stratified. For the backscatter coefficient, the Raman backscatter solution and the multiangle solution are considered. For the extinction coefficient, retrievals from the Raman channel and multiangle measurements are compared. The comparison shows that in the presence of horizontal homogeneity, multiangle measurements provide more reliable results, especially for the aerosol extinction coefficient. The uncertainty in the measured signals is considered in an alternative approach to quantify the relative error of the retrieved profiles with respect to the models (linear regression between retrieval and model).

Notes on Rayleigh scattering in lidar signals

Classical and quantum formulations are used to estimate Rayleigh scattering within lidar signals. Within the classical approach, three scenarios are used to characterize atmospheric molecular composition: 2-component atmosphere (N2 and O2), 4-component atmosphere (N2, O2, Ar, and CO2), and 5-component atmosphere (N2, O2, Ar, CO2, and water vapor). First, analysis focuses on Rayleigh scattering, showing the relative difference between the three scenarios within classical approach. The relative difference in molecular scattering between 2(4)-component atmosphere and 5-component atmosphere is below ~1%. The second analysis focuses on the lidar retrieval of aerosol backscatter and extinction coefficients showing the effect of different molecular formulations. A relative difference of ±3% was found between the molecular formulation of 2-component atmosphere and the molecular formulation of 5-component atmosphere. Consideration of the Raman rotational lines blocked by the interference filter is important for the elastic channels, but of little significance in the N2 Raman channel. For lidar retrieval of aerosol profiles, the 5-component approximation is the best when the water vapor profile is known, but 2-component is still adequate and quite accurate when water vapor is only poorly known.

라이다를 이용한 지역 대기중 꽃가루의 광학적 두께 산출

Air-borne pollen, biogenically created aerosol particle, influences Earth's radiative balance, visibility impairment, and human health. The importance of pollens has resulted in numerous experimental studies aimed at characterizing their dispersion and transport, as well as health effects. There is, however, limited scientific information concerning the optical properties of airborne pollen particles contributing to total ambient aerosols. In this study, for the first time, optical characteristics of pollen such as aerosol backscattering coefficient, aerosol extinction coefficient, and depolarization ratio at 532 nm and their effect to the atmospheric aerosol were studied by lidar remotes sensing technique. Dual-Lidar observations were carried out at the Gwangju Institute of Science & Technology (GIST) located in Gwagnju, Korea (35.15Š E, 126.53Š N) for a spring pollen event from 5 to 7 May 2009. The pollen concentration was measured at the rooftop of Gwangju Bohoon hospital where the building is located 1.0 km apart from lidar site by using Burkard trap sampler. During intensive observation period, high pollen concentration was detected as 1360, 2696, and 1952 m _ 3 in 5, 6, and 7 May, and increased lidar return signal below 1.5km altitude. Pollen optical depth retrieved from depolarization ratio was 0.036, 0.021, and 0.019 in 5, 6, and 7 May, respectively. Pollen particles mainly detected in daytime resulting increased aerosol optical depth and decrease of Angstrom exponent.

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 (βaer) 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.

Retrieval of optical and microphysical characteristics of postvolcanic stratospheric aerosol from the results of three-frequency lidar sensing

A method is developed for retrieving the altitude profiles of optical and microphysical parameters (MPPs) of postvolcanic stratospheric aerosol (SA) from atmospheric remote sensing data at wavelengths of 355, 532 and 1064 nm. The method uses robust multiple regressions between optical SA characteristics derived from the statistical optical-microphysical SA model to retrieve the profiles of the aerosol backscattering coefficient (ABSC) at the mentioned wavelengths. The inverse problem is solved using polynomial multiple regressions between aerosol integral MPPs and spectral ABSC values. Agreement with independent experimental data confirms the reliability of the derived regressions. We present the results of numerical experiments on retrieving ABSC and aerosol MPP profiles corresponding to different states of the postvolcanic stratosphere.

Annual cycle of aerosol backscatter coefficient and aerosol mixed layer height above Neuchâtel (Switzerland, 47.00°N, 6.95°E)

AbstractDue to the complexity of the direct and indirect effects of the atmospheric aerosols on the air quality and atmospheric radiation balance, complementary methods are necessary to assess the aerosol optical properties. In this sense the backscatter lidar methods are critical in aerosol vertical distribution study. This work presents the annual cycle of aerosol backscatter coefficient (ABC) and aerosol mixed layer (AML) height over Neuchâtel, Switzerland (47.00 N; 6.950 E, 485 m asl). The study is performed by a ground based backscatter lidar operating at 532 nm wavelength as part of EARLINET program of EU. Atmospheric layering for this study is based on a classification of lower atmosphere from 1 km to 2 km above sea level (asl) and from 2 km to 5 km asl, respectively.

Annual cycle of aerosol backscatter coefficient and aerosol mixed layer height above Neuchâtel (Switzerland, 47.00°N, 6.95°E)

Due to the complexity of the direct and indirect effects of the atmospheric aerosols on the air quality and atmospheric radiation balance, complementary methods are necessary to assess the aerosol optical properties. In this sense the backscatter lidar methods are critical in aerosol vertical distribution study. This work presents the annual cycle of aerosol backscatter coefficient (ABC) and aerosol mixed layer (AML) height over Neuchâtel, Switzerland (47.00 N; 6.950 E, 485 m asl). The study is performed by a ground based backscatter lidar operating at 532 nm wavelength as part of EARLINET program of EU. Atmospheric layering for this study is based on a classification of lower atmosphere from 1 km to 2 km above sea level (asl) and from 2 km to 5 km asl, respectively.

Calibration of the 1064 nm lidar channel using water phase and cirrus clouds

Calibration is essential to derive aerosol backscatter coefficients from elastic scattering lidar. Unlike the visible UV wavelengths where calibration is based on a molecular reference, calibration of the 1064 nm lidar channel requires other approaches, which depend on various assumptions. In this paper, we analyze two independent calibration methods which use (i) low-altitude water phase clouds and (ii) high cirrus clouds. In particular, we show that to achieve optimal performance, aerosol attenuation below the cloud base and cloud multiple scattering must be accounted for. When all important processes are considered, we find that these two independent methods can provide a consistent calibration constant with relative differences less than 15%. We apply these calibration techniques to demonstrate the stability of our lidar on a monthly scale, along with a natural reduction of the lidar efficiency on an annual scale. Furthermore, our calibration procedure allows us to derive consistent aerosol backscatter coefficients and angstrom coefficient profiles (532-1064 nm) along with column extinction-to-backscatter ratios which are in good agreement with sky radiometer inversions.

Consistency between backscatter lidar products and visibility range

We present a consistency of the following values: the aerosol backscatter coefficient (ABC) and top of Atmospheric Boundary Layer (ABL), derived from backscatter lidar measurements from one side, and the visually determined Visibility Range (VR) from the other. The VR is determined towards long-range reference topographic targets in horizontal or slant path, while the lidar measurement is performed in vertical. The mean extinction coefficient along line-of-sights to reference topographic objects is calculated from the lidar derived back-scatter coefficient with model aerosol extinction-to-backscatter ratio (EBR), when necessary, taking into account the ABL top. The mean extinction coefficient along the line-of-sight to the reference target is also determined from the VR via Koschmieder equation. The correlation coefficient between the two data sets is R2 = 0.86 for all data points and R2 = 0.91 when selecting out the points with possible VR systematic error at the farthest reference target.