Vertical Aerosol Optical Depth Research Articles

Aerosol measurements at sub-percent precision using wide-field stellar photometry

Wide-field stellar photometry – the process of simultaneous comparison of apparent and catalog brightness of a large number of stars – has long been a promising method of atmospheric monitoring, as it can provide atmospheric transparency measurements with high temporal and spatial resolution without any artificial illumination of the sky which would disturb nearby optical experiments. However, only recently have we developed methods of data analysis that allow an overall uncertainty in the Vertical Aerosol Optical Depth (VAOD) better than 0.01 optical depths. Such a feat requires precise laboratory measurements of spectral and signal responses of the instruments as well as careful analysis of all systematic effects that can appear during the extraction of stellar signals from the images. We will present a summary of the method with emphasis on the reliability of the estimates of uncertainties. Furthermore, we will discuss various possible applications of the method in different modes of operation, a comparison with other methods of aerosol measurement, and a selection of open questions and areas for further improvement.

Improved Point Spread Function Correction for Aerosol Measurements Using Wide-field Stellar Photometry

When using wide-field stellar photometry to measure Vertical Aerosol Optical Depth (VAOD), we model the dependence of measured stellar fluxes on the star color and position within the field of view of the imaging system in order to control systematic uncertainties introduced through those dependencies. In wide-field imagers, the Point Spread Function (PSF) varies significantly across the instrument’s field of view (FOV) as the deformation of star images increases with the distance from the center of the FOV. While such dependence may be compensated using a synthetic flat-field correction created through the simultaneous analysis of many images, such an approach fails to account for the image-to-image changes in this correction due to minute changes in focus over time. This effect is believed to be the main reason for fluctuations in the conversion factor between measured photometric flux and actual star brightness (also known as zero-point) as determined by self-calibration scans, which is the dominant source of uncertainty for single-image VAOD measurements. We study the possibilities and limitations of using the PSFEx (PSF Extractor) code to extract the precise model of the PSF from wide-field images and compare the modeled PSF with the actual star shapes and search for optimal choices of global PSF extraction methods, including dividing the frame into smaller segments, across which the PSF is more stable.

A New Method for Aerosol Measurement Using Wide-field Photometry

Abstract We present a new method to measure the vertical aerosol optical depth (VAOD) during clear nights using a wide-field imager—a CCD camera with a photographic lens on an equatorial mount. A series of 30 s exposures taken at different altitudes above the horizon can be used to measure the VAOD with a precision better than 0.008 optical depths within a few minutes. Such a measurement does not produce any light and is thus suitable for use at sites where other astronomical instruments are located. The precision of the VAOD measurement depends on the laboratory calibration of spectral properties of the system and of the response of the camera electronics to varying illumination levels, as well as careful considerations of the details of stellar photometry and modeling of the dependence of the measured stellar flux, the star color, and the position within the field of view. The results obtained with robotic setups at the future sites of the Cerenkov Telescope Array show good internal consistency and agreement with simultaneous measurements from a Sun/Moon photometer located at the same sites.

Atmospheric aerosol attenuation effect on FD data analysis at the Pierre Auger Observatory

The atmospheric aerosol monitoring system of the Pierre Auger Observatory has been operating smoothly since 2004. Two laser facilities (Central Laser Facility, CLF and eXtreme Laser Facility, XLF) fire sets of 50 shots four times per hour during FD shifts to measure the highly variable hourly aerosol attenuation to correct the longitudinal UV light profiles of the Extensive Air Showers detected by the Fluorescence Detector. Hourly aerosol attenuation loads (Vertical Aerosol Optical Depth) are used to correct the measured profiles. Two techniques are used to determine the aerosol profiles, which have been proven to be fully compatible. The uncertainty in the VAOD profiles measured consequently leads to an uncertainty on the energy and on the estimation of the depth at the maximum development of a shower (Xmax) of the event in analysis. To prove the validity of the aerosol attenuation measurements used in FD event analysis, the flatness of the ratio of reconstructed SD to FD energy as a function of the aerosol transmission to the depth of shower maximum has been verified.

New developments in aerosol measurements using stellar photometry

The idea of using stellar photometry for atmospheric monitoring for optical experiments in highenergy astrophysics is seemingly straightforward, but reaching high precision of the order of 0.01 in the determination of the vertical aerosol optical depth (VAOD) has proven difficult. Wide-field photometry over a large span of altitudes allows a fast determination of VAOD independently of the absolute calibration of the system, while providing this calibration as a useful by-product. Using several years of data taken by the FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre Auger Observatory in Argentina and about a year of data taken by a similar instrument deployed at the planned future Southern site of the Cherenkov Telescope Array in Chile, we have developed methods to improve the precision of this measurement technique towards and possibly beyond the 0.01 mark. Detailed laboratory measurements of the response of the whole system to both the spectrum and intensity of incoming light have proven indispensable in this analysis as the usual assumption of linearity of the CCD detectors is not valid anymore for the conditions of the observations.

FRAM for the Cherenkov Telescope Array: an update

The Cherenkov Telescope Array (CTA) is a project to build a new generation ground-based gammaray observatory. The FRAM – F(/Ph)otometric Robotic Atmospheric Monitor – is a device, which can provide it with the Vertical Aerosol Optical Depth (VAOD) maps in the field of view of the CTA in almost real time (delay in minutes). The FRAM is a small fast robotic astronomical telescope with a large field of view and a sensitive CCD camera. It uses stellar photometry to measure atmospheric extinction across the field of view and uses it to calculate the VAOD. In this contribution, results of first few months of operations of the prototype and outlook for deployment at the CTA site will be presented.

Aerosol Measurements with the FRAM Telescope

Precision stellar photometry using a telescope equipped with a CCD camera is an obvious way to measure the total aerosol content of the atmosphere as the apparent brightness of every star is affected by scattering. Achieving high precision in the vertical aerosol optical depth (at the level of 0.01) presents a series of interesting challenges. Using 3.5 years of data taken by the FRAM instrument at the Pierre Auger Observatory, we have developed a set of methods and tools to overcome most of these challenges. We use a wide-field camera and measure stars over a large span in airmass to eliminate the need for absolute calibration of the instrument. The main issues for data processing include camera calibration, source identification in curved field, catalog deficiencies, automated aperture photometry in rich fields with lens distortion and corrections for star color. In the next step, we model the airmass-dependence of the extinction and subtract the Rayleigh component of scattering, using laboratory measurements of spectral sensitivity of the device. In this contribution, we focus on the caveats and solutions found during the development of the methods, as well as several issues yet to be solved. Finally, future outlooks, such as the possibility for precision measurements of wavelength dependence of the extinction are discussed.

The atmospheric transparency measured with a LIDAR system at the Telescope Array experiment

An atmospheric transparency was measured using a LIDAR with a pulsed UV laser (355nm) at the observation site of Telescope Array in Utah, USA. The measurement at night for two years in $2007\sim 2009$ revealed that the extinction coefficient by aerosol at the ground level is $0.033^{+0.016}_{-0.012} \rm km^{-1}$ and the vertical aerosol optical depth at 5km above the ground is $0.035^{+0.019}_{-0.013}$. A model of the altitudinal aerosol distribution was built based on these measurements for the analysis of atmospheric attenuation of the fluorescence light generated by ultra high energy cosmic rays.

Aerosol Optical Depth over Oceans: High Space- and Time-Resolution Retrieval and Error Budget from Satellite Radiometry

A method to retrieve aerosol vertical optical depth at 0.64 mm from satellite observations of cloud-free scenes over oceans with high spatial resolution (;18) and instantaneous temporal resolution is described and evaluated. The observed radiance is treated as the linear sum of contributions to path radiance by different scattering processes in the atmosphere‐ocean system. This treatment allows examination of errors in the retrieved vertical aerosol optical depth contributed by each process and approximation. Random error in retrieved aerosol optical depth is typically 0.03. The systematic error due to absolute calibration uncertainty in the measured radiance is 0.01. The largest errors and biases are due to radiative transfer approximations (122%) and assumptions regarding aerosol microphysical and optical properties (220%). The latter errors, which are due to the optical properties (e.g., phase function), vary systematically with latitude and season because of the variation of the mean observing geometry. This method is applied to Advanced Very High Resolution Radiometer global area coverage data, and example maps of aerosol optical depth are presented for specific dates in July and October 1986. The aerosol optical depth derived from the satellite data is suitable for examining large aerosol signatures by instantaneous comparison of the amplitude and location of aerosol plumes with model predictions based on meteorological conditions at and preceding the time of observation.

Features of aerosol optical depth observed at Barrow, March 10‐20, 1983

Total vertical aerosol optical depth over Barrow, Alaska, during March 1983 was up to four times greater than the average for recent years, with part of the excess being due to stratospheric debris from El Chichon. The variability in optical depth during a 10‐day period spanning the aircraft flights of the Arctic Gas and Aerosol Sampling Program (AGASP) suggests a major tropospheric aerosol event on March 12 and 13, which accounts for the maximum observed optical depths. Occurrence of the tropospheric event is substantiated with independent aerosol data from aircraft, surface sampling, and synoptic scale meteorological data. Analysis of the Barrow optical depth data yields information on the climatic effects of both the stratospheric aerosol from El Chichon and the tropospheric aerosol commonly called Arctic haze.

Galilean satellite eclipse studies II. Jovian stratospheric and tropospheric aerosol content

The Galilean satellite eclipse technique for measuring the aerosol distribution in the Jovian lower stratosphere and upper troposphere is described and applied using 30 color observations of 12 natural satellite eclipses obtained with the 200-in Hale telescope. These events probe the North and South Polar Regions, the North Temperate Belt, the South Equatorial Belt, the South Tropical Zone, the South Temperate Zone, and the Great Red Spot. Aerosol is found above the visible cloud tops in all locations. It is very tenuous and varies with altitude, increasing rapidly with downward passage through the tropopause. The aerosol extinction coefficient at 1.05 μm is 1.0 ± 0.05 × 10 −8 cm −1 at the tropopause and the mass density is a few times 10 −13 g cm −3. The observations require some aerosol above the tropopause but do not clearly determine its structure. The present analysis emphasizes an extended haze distribution, but the alternate possibility that the stratospheric aerosol resides in a thin layer is not excluded. The vertical aerosol optical depth above the tropopause at 1.05 μm exceeds 0.04 in the NPR, SPR, NTB, SEB, and StrZ, is ∼0.006 ± 0.003 in the STZ, and is ∼ 0.003 ± 0.001 above the GRS. The aerosol extinction increases with decreasing wavelength in the STZ and NTB and indicates a particle radius of 0.2–0.5 μm; a radius of ∼0.9 μm is indicated in the STrZ.