•More accurate night sky brightness predictions requires information on aerosols.•A portable turbidity analyzer is developed for in-situ aerosol characterization.•Multi-wavelength radiometry provides ...information on AOD and size distribution.•An experiment was performed to test the system and the method under limit conditions.
Scattering by aerosols and gases cause a certain fraction of artificial light emitted upwards is redirected to the ground. Of all atmospheric constituents just the aerosols are most important modulators of night-sky brightness under cloudless conditions. Unlike most of the previous we highlight a crucial role of solar radiometry for determining the atmospheric optical depth before night-time observation is to be made. Aerosol optical depth at visible wavelengths extracted from the data measured provides then the information on size distribution or mean refractive index of aerosol particles that in turn are both necessary to make night sky brightness prediction more accurate. Therefore, combining daytime and night-time radiometry we can achieve accuracy much higher than ever before. This is due to significantly reduced uncertainty in aerosol properties.
The aerosol data are retrieved from a new portable multi-wavelength optical analyzer that operates Ocean Optics spectrometer. The equipment provides the radiance data from 350 nm to 1000 nm with spectral resolution of 1 nm. Due to high sun radiance levels we use a system of mirrors each reducing the signal to about 4%, while keeping the integration time short. The minimum integration time of 3 ms allows for detection of direct sunlight. The system developed is sensitive to small changes in the aerosol system, while showing a good detection limit even under low turbidity conditions. The system performance is demonstrated in field experiment conducted shortly after front passage when most of aerosol particles is effectively removed by rain.
ABSTRACT
The emission spectrum of a light-pollution source is a determining factor for modelling artificial light at night. The spectral composition of skyglow is normally derived from the initial ...spectra of all artificial light sources contributing to the diffuse illumination of an observation point. However, light scattering in the ambient atmosphere imposes a wavelength-specific distortion on the optical signals captured by the measuring device. The nature of the emission, the spectra and the light-scattering phenomena not only control the spectral properties of the ground-reaching radiation, but also provide a unique tool for remote diagnosis and even identification of the emission spectra of the light-polluting sources. This is because the information contained in the night-sky brightness is preferably measured in directions towards a glowing dome of light over the artificial source of light. We have developed a new method for obtaining the emission spectra using remote terrestrial sensing of the bright patches of sky associated with a source. Field experiments conducted in Vienna and Bratislava have been used to validate the theoretical model and the retrieval method. These experiments demonstrate that the numerical inversion is successful even if the signal-to-noise ratio is small. The method for decoding the emission spectra by the light-scattering spectrometry of a night sky is a unique approach that enables for (i) a systematic characterization of the light-pollution sources over a specific territory, and (ii) a significant improvement in the numerical prediction of skyglow changes that we can expect at observatories.
Determining the nighttime aerosol extinction coefficient profile in the lower atmosphere through optical measurements is a necessary but challenging task in environmental science due, among other ...factors, to the lack of a suitable extra-atmospheric night light source. In this work we present a method for retrieving the vertical distribution of the aerosol extinction coefficient via light scattered from a low-divergence light beam. We demonstrate its practical implementation using a low-power, portable green laser light source to sample the air column and an off-the-shelf, wide-angle digital camera located a few hundred meters away as detector. The light scattered off the laser beam in the direction toward the camera contains enough information to retrieve the aerosol extinction profile by inverting the integral equation relating the radiance of the recorded beam image to the aerosol distribution. An efficient and robust numerical inversion procedure is developed and demonstrated here. This approach can be used both in permanent measurement facilities or in mobile stations, and it facilitates the task of characterizing nighttime aerosols both on-demand or in the framework of routine environmental monitoring protocols.
•Measuring the vertical aerosol concentration profile at nighttime is challenging.•A simple system (laser plus digicam) enables sampling the nighttime atmosphere.•The scattered laser beam radiance contains information on the aerosol profile.•The aerosol concentration profile may be determined by inverting the radiance data.
•Excess charge (EC) initiates unexpectedly amplified optical resonances in nanoparticles.•EC can have significant consequences for the optical characterization of particle systems.•Charge-controlled ...resonances can be used in novel optical devices.•The conductivity models for non-spherical particles remain unexplored until now.
In spite of the enormous progress in electromagnetic (EM) scattering since Mie, subsequent theories assume that particle optical properties are not perturbed by surface currents. Given that the premise remained largely unexamined, major concerns were raised after preliminary results obtained very recently for a charged spherical particle showed that it resonates with the EM radiation in a wide range of modes. The mechanism of this phenomenon is still poorly described for idealized particle models and completely unknown for arbitrarily shaped particle aggregates.
The purpose of this mini-review is to briefly highlight the recent advances, open problems, their possible treatment and the potential directions of future development in the field. We need to improve upon a fundamental understanding and satisfactory treatment of the effect of charge on EM scattering from arbitrary particles in order to proceed with novel formulations and solutions to the EM scattering by arbitrarily sized/shaped/composed electrically charged particles, to carry out computer simulations on distinct types of materials and particles that have the potential to be used as optical devices, and to identify a fashion in which inter-particle separation controls collective resonances and amplitude/phase changes in transmitted or scattered radiation. There is no doubt that such research can have profound consequences for new important applications in EM scattering, remote sensing, engineering, but also optical diagnostics, and it also can lead to the acquisition of knowledge necessary to explain anomalous features that appear in the EM responses of charged particles, e.g. extinction peaks that have no relation to material properties or widening of resonance bands.
•Electromagnetic waves resonate with surface excitations of a charged dielectric particle.•Physics interpretation of resonance mode is an excitation of the odd surface plasmon.•Net electric charge ...enhances scattering efficiency and absorption of electromagnetic field.•Electric charge reduces scattering and absorption in very small particles.•The effect can have important consequences for remote sensing of particulate systems.
When a particle is charged, electrons can move freely along its surface and influence its optical properties in the same way as a thin, nonuniform metallic layer. These electrons contribute to scattering phenomena, including resonances. We model the light scattering from charged particles and demonstrate that these resonances result from excitation of an anti-symmetric surface plasmon at the layer interfaces. The modeling explains suppression of absorption when the size of the charged particle decreases, as well as differences in the light-scattering efficiencies of as much as a factor of 2 that occur before and after the resonance. These light-scattering properties must be taken into consideration when performing remote-sensing studies of charged particles, like those in the interstellar medium and dust storms.
•A sky scanner for measuring extremely low night-sky brightness has been developed.•The device is well suited for a narrow-band dark sky spectroradiometry.•The scanner benefits from fast readout, ...high dynamic range, and tunable sensitivity.•The spectral radiance characterizes spectral power distribution with high accuracy.•The minimum detectable signal threshold is a few µcd⋅m−2 (airglow level).
A new portable sky scanner designed for low-light-level detection at night is developed and employed in night sky brightness measurements in a rural region. The fast readout, adjustable sensitivity and linear response guaranteed in 5–6 orders of magnitude makes the device well suited for narrow-band photometry in both dark areas and bright urban and suburban environments. Quasi-monochromatic night-sky brightness data are advantageous in the accurate characterization of spectral power distribution of scattered and emitted light and, also allows for the possibility to retrieve light output patterns from whole-city light sources. The sky scanner can operate in both night and day regimes, taking advantage of the complementarity of both radiance data types. Due to its inherent very high sensitivity the photomultiplier tube could be used in night sky radiometry, while the spectrometer-equipped system component capable of detecting elevated intensities is used in daylight monitoring. Daylight is a source of information on atmospheric optical properties that in turn are necessary in processing night sky radiances. We believe that the sky scanner has the potential to revolutionize night-sky monitoring systems.
The sun and sky photometry made concurrently or, alternatively, simultaneous measurements of extinction and scattering data both represent a valuable tool for gathering the information on aerosol ...particles. Most typically the size distribution and/or refractive index of aerosol particles can be inferred from multispectral and/or multiangle optical data. Extraction of size-dependent aspect ratio of aerosol particles from optical data is a highly non-trivial task since the kernel of the particular integral equation is a non-linear function of the sought solution. The iterative solution to this problem is introduced and demonstrated on synthetically generated data. It is shown that retrieval of size-dependent aspect ratio is possible even for complex morphologies, most typically for irregularly shaped dust particles.
•A novel method for inversion of scattering and extinction data is developed.•Size distribution and aspect ratio of nonspherical particles can be retrieved.•Aerosol aspect ratio is determined iteratively if refractive index is a-priori known.•The method is effective for large scattering angles including side and back scatter.
•Optical properties of nonspherical charged particles are computed using DDA.•The modeling is at wavelengths where charge-induced resonances typically occur.•Surface charge distribution is simulated ...by a multi-level material distribution.•The method is validated for a pseudosphere which allows for an analytical solution.•The method takes advantage of the interfaces to DDSCAT.
The optical properties of non-spherical particles have been studied for decades and there are a number of solution techniques to model distinct geometries. Of these methods, the Discrete Dipole Approximation (DDA) is known to compute electromagnetic scattering from irregularly shaped, heterogeneous particles. DDSCAT, which is the numerical implementation of DDA, is identified as a potential solver for electrically charged particles; however, there is a set of limitations and shortcomings to be addressed. The main concern is the conductivity, which introduces an infinitesimally thin charged monolayer on the particle surface. The side effect of this concept is a steep increase of refractive index and impedance. The DDSCAT can have trouble converging to a solution when the thin shell at the particle surface produces large losses due to having electromagnetic properties significantly different from the media on either side of the particle interface.
In this paper we introduce a method to calculate the optical response of charged particles at wavelengths where charge-induced resonances typically occur. The method takes advantage of the interfaces to DDSCAT, while the conductive shell is simulated by a multi-level material distribution. The discretization involves both the 3D model of the particle and the charge distribution. The latter is determined numerically by solving the Laplace equation. The surface- and volume-averaging parameter is used to avoid large refractive indices, but the results are still accurate within a few percent. The method is validated for sensitivity and specificity in modeling optical resonances that are analytically retrievable for ideal spheres. The applicability to an irregularly shaped particle is demonstrated consequently. Implementation algorithms and numerical solvers are made publicly available, which allows light-scattering modelers to study particles in different media under different conditions.
•Radiative transfer modeling was used to predict skyglow changes resulting from an LED conversion.•Measurements of skyglow were made in epochs approximately bracketing the conversion effort.•Results ...of modeling and observations were compared against satellite “night lights”.•Some evidence supports the hypothesis that dimming LED lights reduced skyglow.•A clear need exists for further investigation of this phenomenon including constant monitoring.
The transition from earlier lighting technologies to white light-emitting diodes (LEDs) is a significant change in the use of artificial light at night. LEDs emit considerably more short-wavelength light into the environment than earlier technologies on a per-lumen basis. Radiative transfer models predict increased skyglow over cities transitioning to LED unless the total lumen output of new lighting systems is reduced. The City of Tucson, Arizona (U.S.), recently converted its municipal street lighting system from a mixture of fully shielded high- and low-pressure sodium (HPS/LPS) luminaires to fully shielded 3000 K white LED luminaires. The lighting design intended to minimize increases to skyglow in order to protect the sites of nearby astronomical observatories without compromising public safety. This involved the migration of over 445 million fully shielded HPS/LPS lumens to roughly 142 million fully shielded 3000 K white LED lumens and an expected concomitant reduction in the amount of visual skyglow over Tucson. SkyGlow Simulator models predict skyglow decreases on the order of 10–20% depending on whether fully shielded or partly shielded lights are in use. We tested this prediction using visual night sky brightness estimates and luminance-calibrated, panchromatic all-sky imagery at 15 locations in and near the city. Data were obtained in 2014, before the LED conversion began, and in mid-2017 after approximately 95% of ∼ 18,000 luminaires was converted. Skyglow differed marginally, and in all cases with valid data changed by < ± 20%. Over the same period, the city’s upward-directed optical radiance detected from Earth orbit decreased by approximately 7%. While these results are not conclusive, they suggest that LED conversions paired with dimming can reduce skyglow over cities.
•An experiment was conducted involving the dimming of nearly 20,000 municipally owned street lights in a U.S. city of half a million inhabitants over ten nights in the spring of 2019.•The signal of ...the dimming tests was successfully observed in measurements of zenith night sky radiance obtained at various radii from the city center.•Observedzenith radiance changes of ≤−5% during the tests imply that known street lights account for (26 ± 4)%•The results can be used to inform future optimization of the city’s street lighting system to further reductions in skyglow while preserving public safety.
Anthropogenic skyglow dominates views of the natural night sky in most urban settings, and the associated emission of artificial light at night (ALAN) into the environment of cities involves a number of known and suspected negative externalities. One approach to lowering consumption of ALAN in cities is dimming or extinguishing publicly owned outdoor lighting during overnight hours; however, there are few reports in the literature about the efficacy of these programs. Here we report the results of one of the largest municipal lighting dimming experiments to date, involving ~ 20,000 roadway luminaires owned and operated by the City of Tucson, Arizona, U.S. We analyzed both single-channel and spatially resolved ground-based measurements of broadband night sky radiance obtained during the tests, determining that the zenith sky brightness during the tests decreased by (−5.4±0.9)% near the city center and (−3.6±0.9)% at an adjacent suburban location on nights when the output of the street lighting system was dimmed from 90% of its full power draw to 30% after local midnight. Modeling these changes with a radiative transfer code yields results suggesting that street lights account for about (14 ± 1)% of light emissions resulting in skyglow seen over the city. A separate derivation from first principles implies that street lighting contributes only 2−3% of light seen at the zenith over Tucson. We discuss this inconsistency and suggest routes for future work.