Spectral Aerosol Optical Thickness Research Articles

Regional validation of the solar irradiance tool SolaRes in clear-sky conditions, with a focus on the aerosol module

Abstract. The Solar Resource estimate (SolaRes) tool based on the Speed-up Monte Carlo Advanced Radiative Transfer code using GPU (SMART-G) has the ambition to fulfil both research and industrial applications by providing accurate, precise, and high-time-resolution simulations of the solar resource. We investigate the capacity of SolaRes to reproduce the radiation field, relying on 2 years of ground-based measurements by pyrheliometers and pyranometers acquired in northern France (Lille and Palaiseau). Our main objective is to provide, as a first step in clear-sky conditions, a thorough regional validation of SolaRes, allowing us to investigate aerosol impacts on solar resource. We perform comparisons between SolaRes-simulated and clear-sky-measured global horizontal irradiance (GHI), direct normal irradiance (DNI), diffuse horizontal irradiance (DifHI), and global and diffuse irradiance on a tilted plane (GTI, DifTI), and we even consider the circumsolar contributions. Using spectral aerosol optical thickness (AOT) data sets as input, which are delivered by the AErosol RObotic NETwork (AERONET) and the Copernicus Atmosphere Monitoring Service (CAMS), we examine the influence of aerosol input data sets in SolaRes on the comparison scores. Two aerosol models are mixed to compute aerosol optical properties. We also perform a sensitivity study on the aerosol parametrisation and investigate the influence of applying more or less strict cloud-screening methods to derive ground-based proof data sets of clear-sky moments. SolaRes is validated with the (relative) root mean square difference (RMSD) in GHI as low as 1 % and a negligible mean bias difference (MBD). The impact of the cloud-screening method in GHI is 0.5 % of RMSD and 0.3 % of MBD. SolaRes also estimates the circumsolar contribution, which improves MBD in DNI and DifHI by 1 % and 4 %, respectively, and RMSD by ∼ 0.5 %. MBD in DNI is around −1 % and RMSD around 2 %, and MBD in DifHI is 2 % and RMSD around 9 %. RMSD and MBD in both DNI and DifHI are larger than in GHI because they are more sensitive to the aerosol and surface properties. DifTI measured on a vertical plane facing south is simulated by SolaRes with an RMSD of 8 %, comparable to that obtained for DifHI. Our results suggest a strong influence of reflection by not only ground surface but also surrounding buildings. The sensitivity studies on the aerosol parameterisation show that the spectral AOT contains enough information for high performance in DNI simulations, with low influence of the choice of the aerosol models on the RMSD. However, choosing a model with smaller aerosol single scattering albedo significantly decreases SolaRes DifHI and GHI. The best combination in Lille and Palaiseau consists of continental clean mixed with desert dust. Also, complementary information on angular scattering and aerosol absorption provided by the AERONET-inverted model further improves simulated clear-sky GHI by reducing RMSD by ∼ 0.5 % and MBD by ∼ 0.8 %. Eventually, the choice of the data source has a significant influence. Indeed, using CAMS AOT instead of AERONET AOT increases the RMSD in GHI by ∼ 1 % and MBD by ∼ 0.4 % and RMSD in DNI by 5 %. The RMSD in GHI remains slightly smaller than state-of-the-art methods.

Cloud top heights and aerosol columnar properties from combined EarthCARE lidar and imager observations: the AM-CTH and AM-ACD products

Abstract. The Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) is a combination of multiple active and passive instruments on a single platform. The Atmospheric Lidar (ATLID) provides vertical information of clouds and aerosol particles along the satellite track. In addition, the Multi-Spectral Imager (MSI) collects multi-spectral information from the visible to the infrared wavelengths over a swath width of 150 km across the track. The ATLID–MSI Column Products processor (AM-COL) described in this paper combines the high vertical resolution of the lidar along track and the horizontal resolution of the imager across track to better characterize a three-dimensional scene. ATLID Level 2a (L2a) data from the ATLID Layer Products processor (A-LAY), MSI L2a data from the MSI Cloud Products processor (M-CLD) and the MSI Aerosol Optical Thickness processor (M-AOT), and MSI Level 1c (L1c) data are used as input to produce the synergistic columnar products: the ATLID–MSI Cloud Top Height (AM-CTH) and the ATLID–MSI Aerosol Column Descriptor (AM-ACD). The coupling of ATLID (measuring at 355 nm) and MSI (at ≥670 nm) provides multi-spectral observations of the aerosol properties. In particular, the Ångström exponent from the spectral aerosol optical thickness (AOT 355/670 nm) adds valuable information for aerosol typing. The AOT across track, the Ångström exponent and the dominant aerosol type are stored in the AM-ACD product. The accurate detection of the cloud top height (CTH) with lidar is limited to the ATLID track. The difference in the CTH detected by ATLID and retrieved by MSI is calculated along track. The similarity of MSI pixels across track with those along track is used to transfer the calculated CTH difference to the entire MSI swath. In this way, the accuracy of the CTH is increased to achieve the EarthCARE mission's goal of deriving the radiative flux at the top of the atmosphere with an accuracy of 10 W m−2 for a 100 km2 snapshot view of the atmosphere. The synergistic CTH difference is stored in the AM-CTH product. The quality status is provided with the products. It depends, e.g., on day/night conditions and the presence of multiple cloud layers. The algorithm was successfully tested using the common EarthCARE test scenes. Two definitions of the CTH from the model truth cloud extinction fields are compared: an extinction-based threshold of 20 Mm−1 provides the geometric CTH, and a cloud optical thickness threshold of 0.25 describes the radiative CTH. The first CTH definition was detected with ATLID and the second one with MSI. The geometric CTH is always higher than or equal to the radiative CTH.

Aerosol optical properties and precipitable water vapor column in the atmosphere of Norway

Between February 2012 and April 2014, we measured and analyzed direct solar radiances at a ground-based station in Bergen, Norway. We discovered that the spectral aerosol optical thickness (AOT) and precipitable water vapor column (PWVC) retrieved from these measurements have a seasonal variation with highest values in summer and lowest values in winter. The highest value of the monthly median AOT at 440 nm of about 0.16 was measured in July and the lowest of about 0.04 was measured in December. The highest value of the monthly median PWVC of about 2.0 cm was measured in July and the lowest of about 0.4 cm was measured in December. We derived Ångström exponents that were used to deduce aerosol particle size distributions. We found that coarse-mode aerosol particles dominated most of the time during the measurement period, but fine-mode aerosol particles dominated during the winter seasons. The derived Ångström exponent values suggested that aerosols containing sea salt could have been dominating at this station during the measurement period.

Retrieval of Aerosol Size Distribution from Microtops II Sunphotometer in Hong Kong

A new method has been developed for retrieving Aerosol Size Distribution (ASD) from spectral aerosol optical thickness (AOT) measured by Microtops II sunphotometer. The rationale of this method is based on a Spectral Extinction Fitting Technique (SEFT) between measured and model-simulated aerosol extinctions. Numerical simulation of aerosol extinctions includes seven aerosol microphysical properties and the resulting total of 64,324,260 datasets were used to compare with Microtops II sunphotometer AOT measurements at five wavelengths (340, 500, 675, 870, and 1020 nm). This SEFT method works directly on AOT measurements and does not require any specific inputs of aerosol properties. The accuracy was examined using AERONET inversion products, and the results show a difference between SEFT and AERONET inversion products of bimodal lognormal volume size concentrations, of ± 10%. The SEFT algorithm was applied for two specific conditions (hazy and clear atmospheres) in Hong Kong, and the corresponding ASD retrievals show reasonable agreements with AERONET inversion products.

A Case Study of Aerosol Characteristics During a Haze Episode Over Beijing

This work presents the variation of aerosol characteristics obtained before, during and after the haze episode in October, 2010 in Beijing, a highly industrialized city which located in the northwest of North China Plain with more than 21 million inhabitants. Surface solar irradiance was measured by pyranometer which showed great differences between hazy and non-hazy days. Aerosol scattering coefficients were obtained by integrated nephelometer and the influence of relative humidity on aerosol scattering coefficient was investigated by means of the Weather Research and Forecasting Model Wave Watch III (WRF-WW3) imagery. Spectral aerosol optical thickness was measured by Sun photometer and Angstrom exponent were calculated to investigate the variation of aerosol loading and modes under different weather conditions. Haze weather process favoured the formation of the secondary aerosol, which contributed to the increase of fine mode particles.

Night-Time Ground Hyperspectral Imaging for Urban-Scale Remote Sensing of Ambient PM. I. Aerosol Optical Thickness Acquisition

Aerosol loadings in vertical atmospheric columns are commonly measured by satellite-borne or ground instruments that remotely sense the spectral extinction through the optical path. However, correlations of these integrated measurements with ground-level particulate matter concentrations are highly influenced by local meteorology, seasonality, and the surface albedo. Moreover, as most measurements are based on solar radiation, they are limited to daytime. To account for these limitations, we study the feasibility of using a ground hyperspectral camera for acquiring images of artificial light sources through horizontal urban-scale open paths during the night, in order to retrieve the apparent spectral aerosol optical thickness. Laboratory-scale measurements demonstrated a linear response of the camera and set the spectral operational range. A procedure for night-time imaging of illuminating targets through ambient open paths has been developed to enable a consistent selection of pixels for analysis, provid...

Variation of aerosol optical thickness with atmospheric water vapour : A case study over a continental station Mysore, India

Atmospheric measurements in a continental, low latitude station Mysore (12.3° N) has been carried out, for the period December 2003 to June 2006. Measurements were made using a sunphotometer with five bands in the visible and near-infrared range of the solar spectrum. To bring out the wavelength dependence of Aerosol Optical Thickness (AOT) on atmospheric water vapour, typically two wavelength channels are being used, one at 500 nm and the other at 1020 nm. A linear dependence between AOT and water vapour on meteorologically calm days is the important observation made. Growth rate of AOT is found to be larger at shorter wavelength (500 nm) than that of the longer wavelength (1020 nm). A mass-plot representation is followed on monthly basis, which is nothing but the graphical plot of spectral AOT versus water vapour of the scans for all the clear sky days of a particular month. Further investigations reveal that some months exhibit a single trend of growth of AOT with water vapour whereas double trend is the scenario for other months. These results provide insight into the changes in the atmospheric aerosol characteristics with precipitable water vapour, which is the subject matter of this paper.

Seasonal variability in the optical properties of Baltic aerosols

A five-year dataset of spectral aerosol optical thickness was used to analyse the seasonal variability of aerosol optical properties (the aerosol optical thickness (AOT) at wavelength λ = 500nm, AOT(500) and the Ångström exponent for the 440–870nm spectral range, α(440, 870)) over the Baltic Sea and dependence of these optical properties on meteorological factors (wind direction, wind speed and relative humidity). The data from the Gotland station of the global radiometric network AERONET (Aerosol Robotic Network, http://aeronet.gsfc.nasa.gov) were taken to be representative of the Baltic Sea conditions. Meteorological observations from Fårosund were also analysed.Analysis of the data from 1999 to 2003 revealed a strong seasonal cycle in AOT(500) and α(440, 870). Two maxima of monthly mean values of AOT(500) over the Baltic were observed. In April, an increase in the monthly mean aerosol optical thickness over Gotland most probably resulted from agricultural waste straw burning, mainly in northern Europe and Russia as well as in the Baltic states, Ukraine and Belarus. During July and August, the aerosol optical thickness was affected by uncontrolled fires (biomass burning). There was a local minimum of AOT(500) in June.Wind direction, a local meteorological parameter strongly related to air mass advection, is the main meteorological factor influencing the variability of aerosol optical properties in each season. The highest mean values of AOT(500) and α(440, 870) occurred with easterly winds in both spring and summer, but with southerly winds in autumn.

Aerosol layer properties over Kyiv from AERONET/PHOTONS sunphotometer measurements during 2008–2009

The AErosol RObotic NETwork (AERONET) is one of the most developed ground-based networks for aerosol monitoring. Radiative measurements (solar irradiance extinction, aureole brightness and sky light polarization) are used by the AERONET inversion retrieval technique to derive a wide variety of aerosol particle properties and parameters that are important for estimations of aerosol influences on climate change. Within the framework of scientific cooperation between Université de Lille 1 and Taras Shevchenko National University (TSNU) of Kyiv (Kiev), a CIMEL CE 318-2 sunphotometer has been operating at Kyiv since March 2008. This article presents a preliminary analysis of the columnar aerosol properties retrieved from Kyiv sunphotometer measurements and one case study with unusual behaviour of the aerosol parameters. Version 2 of the AERONET inversion products (spectral aerosol optical thickness (AOT), the Ångström parameter (α), single-scattering albedo (SSA), size distribution and refractive index, and aerosol radiative forcing estimations) is considered in the analysis, demonstrating variability of aerosol properties and radiative forcing at the Kyiv observational site during a 1-year period.

Retrieval of spectral aerosol optical thickness over land using ocean color sensors MERIS and SeaWiFS

Abstract. For the determination of aerosol optical thickness (AOT) Bremen AErosol Retrieval (BAER) has been developed. Method and main features on the aerosol retrieval are described together with validation and results. The retrieval separates the spectral aerosol reflectance from surface and Rayleigh path reflectance for the shortwave range of the measured spectrum of top-of-atmosphere reflectance for wavelength less than 0.670 μm. The advantage of MERIS (Medium Resolution Imaging Spectrometer on the Environmental Satellite – ENVISAT – of the European Space Agency – ESA) and SeaWiFS (Sea viewing Wide Field Sensor on OrbView-2 spacecraft) observations is the availability of several spectral channels in the blue and visible range enabling the spectral determination of AOT in 7 (or 6) channels (0.412–0.670 μm) and additionally channels in the NIR, which can be used to characterize the surface properties. A dynamical spectral surface reflectance model for different surface types is used to obtain the spectral surface reflectance for this separation. The normalized differential vegetation index (NDVI), taken from the satellite observations, is the model input. Further surface bi-directional reflectance distribution function (BRDF) is considered by the Raman-Pinty-Verstraete (RPV) model. Spectral AOT is obtained from aerosol reflectance using look-up-tables, obtained from radiative transfer calculations with given aerosol phase functions and single scattering albedos either from aerosol models, given by model package "optical properties of aerosol components" (OPAC) or from experimental campaigns. Validations of the obtained AOT retrieval results with data of Aerosol Robotic Network (AERONET) over Europe gave a preference for experimental phase functions derived from almucantar measurements. Finally long-term observations of SeaWiFS have been investigated for 11 year trends in AOT. Western European regions have negative trends with decreasing AOT with time. For the investigated Asian region increasing AOT have been found.

A new method for deriving aerosol solar radiative forcing and its first application within MILAGRO/INTEX-B

Abstract. We introduce a method for deriving aerosol spectral radiative forcing along with single scattering albedo, asymmetry parameter, and surface albedo from airborne vertical profile measurements of shortwave spectral irradiance and spectral aerosol optical thickness. The new method complements the traditional, direct measurement of aerosol radiative forcing efficiency from horizontal flight legs below gradients of aerosol optical thickness, and is particularly useful over heterogeneous land surfaces and for homogeneous aerosol layers where the horizontal gradient method is impractical. Using data collected by the Solar Spectral Flux Radiometer (SSFR) and the Ames Airborne Tracking Sunphotometer (AATS-14) during the MILAGRO (Megacity Initiative: Local and Global Research Observations) experiment, we validate an over-ocean spectral aerosol forcing efficiency from the new method by comparing with the traditional method. Retrieved over-land aerosol optical properties are compared with in-situ measurements and AERONET retrievals. The spectral forcing efficiencies over ocean and land are remarkably similar and agree with results from other field experiments.

The inter-comparison of major satellite aerosol retrieval algorithms using simulated intensity and polarization characteristics of reflected light

Abstract. Remote sensing of aerosol from space is a challenging and typically underdetermined retrieval task, requiring many assumptions to be made with respect to the aerosol and surface models. Therefore, the quality of a priori information plays a central role in any retrieval process (apart from the cloud screening procedure and the forward radiative transfer model, which to be most accurate should include the treatment of light polarization and molecular-aerosol coupling). In this paper the performance of various algorithms with respect to the of spectral aerosol optical thickness determination from optical spaceborne measurements is studied. The algorithms are based on various types of measurements (spectral, angular, polarization, or some combination of these). It is confirmed that multiangular spectropolarimetric measurements provide more powerful constraints compared to spectral intensity measurements alone, particularly those acquired at a single view angle and which rely on a priori assumptions regarding the particle phase function in the retrieval process.

Determination of particulate matter vertical columns using satellite observations

Abstract. A new technique to retrieve the particulate matter vertical columns from spaceborne observations is described. The method is based on the measurements of the spectral aerosol optical thickness (AOT). The spectral slope of the derived aerosol optical thickness is used to infer the size of particles, which is needed (along with the absolute value of AOT) to determine corresponding vertical columns. The technique is applied to the case of a cloudless atmosphere over Germany and results are compared with ground-based observations. Several assumptions are made in the retrieval process such as the prescribed phase function, single scattering albedo, the refractive index of aerosol, and the half-width of the size distribution.

Aerosol Optical Thickness Measurements on Tsunami Day at a Continental Station, Mysore

Daily atmospheric measurements of spectral Aerosol Optical Thickness (AOT), precipitable water vapor (W), and irradiance were being carried out (ISRO-RESPOND Project) at Mysore, India (12.3oN), a continental station, on the day a massive tsunami struck India’s coast, December 26, 2004. A comparison was made of that day’s data with two other days: December 25, 2004, the day before tsunami and December 27, 2004, the day after. Compared to the values of AOT on December 25, 2004 and December 27, 2004, AOTs on December 26, 2004 were very low throughout the day. Values were almost one-third the magnitude of the other two days. The values measured on December 26, 2004 are unique because no other day in this month showed such low values. Comparison of the results with some other location is not possible at this point, because there appears to be no other study of this kind to support the observed results. Possible conclusions are drawn based on the available information.

Measurements of desert dust optical characteristics at Porte au Sahara during SAMUM

Main optical characteristics of desert dust, such as phase function and single scattering albedo, have been derived from combinations of sun-/sky-radiometer and satellite measurements during the SAMUM experiment (10 May–10 June 2006) at the site Porte au Sahara (30.237◦N, 5.607◦W) in South Morocco. Scattering phase functions have been retrieved using combined data of spectral aerosol optical thickness (AOT) and spectral sky brightness in the almucantar, considering non-spherical light scattering. Intercomparisons of modelled top-of-atmosphere (TOA) reflectance with satellite observations of the Medium Resolution Imaging Spectrometer (MERIS) and Scanning Imaging Absorption Spectrometer for Atmospheric Chartography () instrument have been used for the estimation of spectral single scattering albedo. For the radiative transfer calculations scattering phase functions and AOT from ground-based observations have been used. The spectral single scattering albedo ranges from 0.93 in the blue to 0.98 at 753 nm.

Validation of UV-visible aerosol optical thickness retrieved from spectroradiometer measurements

Abstract. Global and diffuse UV-visible solar irradiances are routinely measured since 2003 with a spectroradiometer operated by the Laboratoire d'Optique Atmosphérique (LOA) located in Villeneuve d'Ascq, France. The analysis of the direct irradiance derived by cloudless conditions enables retrieving the aerosol optical thickness (AOT) spectrum in the 330–450 nm range. The site hosts also sunphotometers from the AERONET/PHOTONS network performing routinely measurements of the AOT at several wavelengths. On one hand, comparisons between the spectroradiometer and the sunphotometer AOT at 440 nm as well as, when available, at 340 and 380 nm, show good agreement: in 2003–2005 at 440 nm the correlation coefficient, the slope and the intercept of the regression line are [0.97, 0.95, 0.025], and in 2006 at 440, 380 and 340 nm they are [0.97, 1.00, −0.013], [0.97, 0.98, −0.007], and [0.98, 0.98, −0.002] respectively. On the other hand, the AOT's spectral variations have been compared using the Angström exponents derived from AOT data at 340 and 440 nm for both instruments. The comparisons show that this parameter is difficult to retrieve accurately due to the small wavelength range and due to the weak AOT values. Thus, AOT derived at wavelengths outside the spectroradiometer range by means of an extrapolation using the Angström parameter would have large uncertainties, whereas spectroradiometer's spectral AOT could be used for direct validation of other AOT, such as those provided by satellite instruments.

Variations of aerosol optical thickness of the atmosphere in Russia in 1976–2003

Space-time variations of aerosol optical thickness of the atmospheric column are analyzed on the basis of observational data of the Russian actinometric network in 1976–2003. From the measured direct solar radiation flux and air humidity for 44 stations in European and Asian Russia, spectral aerosol optical thickness (AOT) of the atmosphere is estimated for a wavelength λ −0.55 μm. Interannual, seasonal, and space variations of AOT are estimated. Effects of eruptions of the volcanoes El Chichon (April 1982) and Pinatubo (June 1991) are considered. For most stations, a negative tendency is found in the AOT interannual changes, which is more pronounced in spring and summer. It is shown that the period after the Pinatubo eruption is characterized by “purification” of the atmosphere from aerosol.

Changes in the spectral aerosol optical thickness in Estonia (1951–2004)

Changes in the spectral aerosol optical thickness in Estonia (1951–2004)

Ability of multiangle remote sensing observations to identify and distinguish mineral dust types: 2. Sensitivity over dark water

We explore the ability of multiangle, multispectral measurements, such as those from the Multiangle Imaging SpectroRadiometer (MISR), to retrieve mineral dust properties. We first investigate MISR sensitivity theoretically, to a selection of medium‐mode particle shapes, to single‐scattering albedo (SSA), and to the ratio of medium‐ to large‐mode aerosol optical thickness (AOT). Favorable, but not ideal, viewing conditions over dark water are assumed, and new dust optical models are adopted. This study shows sensitivity to differences in the angular spectral signals of medium‐mode shapes when column midvisible AOT is greater than ∼0.15 for components contributing 15–20% or more to the AOT. MISR‐like retrievals can also distinguish strongly absorbing (red channel SSA ∼ 0.94) from less absorbing (SSA ∼ 0.98–0.99) dust particles. Bimodal size sensitivity results suggest that MISR can distinguish the ratio of medium to larger components in roughly 20% midvisible AOT increments. For five Saharan dust field events over homogeneous, cloud‐free dark water near the Cape Verde AERONET station, MISR research retrieval results were dominated by weakly absorbing medium grains. MISR‐retrieved spectral AOT was within 0.02 of the Aerosol Robotic Network (AERONET), except for one spatially heterogeneous AOT > 1 case, and one where AERONET retrieved a significant component <0.1 μm in radius. In two cases where AERONET ran retrievals assuming nonspherical (spheroidal) particles, SSA fell within 0.02 of MISR. When included in the MISR Standard Aerosol Retrieval Algorithm, the new dust optical models improve both the coverage of successful retrievals and the spectral AOT agreement with near‐coincident AERONET measurements. Large‐mode dust models and available field constraint limitations are being addressed in continuing work.

Atmospheric aerosol load as derived from space

The paper is devoted to the introduction of a new technique to derive the aerosol mass load and particle number concentration from spaceborne observations. It is based on the simultaneous determination of the spectral aerosol optical thickness (AOT) τ and the spectral effective Lambertian ground reflectance A for a given scene using the aerosol retrieval algorithm developed by us. The aerosol mass is proportional to the measured AOT with the coefficient of proportionality dependent on the effective radius a ef of aerosol particles. The value of a ef can be estimated from the slope of the function τ( λ), where λ is the wavelength. The developed technique is applied to SeaWiFs imagery data over Moscow.