Long-term records of aerosol optical depth (AOD) with high quality, suitable temporal continuity and spatial coverage are of immense interest to climate-related research activities. Both satellite- and ground-based measurements of AOD are typically provided by instruments with different designs, and distinct data acquisition and processing schemes. Thus, the corresponding AOD records likely have different accuracy, spatial coverage, and temporal resolution. Several studies have been focused on the synergy of multi-sensor satellite AOD products. Here we combine multi-year (1997-2018) AOD records available from four collocated ground-based instruments deployed at the mid-continental Southern Great Plains (SGP) Central Facility supported by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program. We demonstrate how to minimize drawbacks (patchy spots) and to maintain benefits (high quality) of these records. Our demonstration finds a combined AOD obtained at two wavelengths (500 and 870 nm), with high temporal resolution (1-min), and provides the user with an estimate of the AOD uncertainty. Finally, we highlight expected applications of the merged dataset and its future extensions.
We introduce and evaluate a simple retrieval of areal-averaged surface albedo using ground-based measurements of atmospheric transmission alone at five wavelengths (415, 500, 615, 673 and 870nm), under fully overcast conditions. Our retrieval is based on a one-line semi-analytical equation and widely accepted assumptions regarding the weak spectral dependence of cloud optical properties, such as cloud optical depth and asymmetry parameter, in the visible and near-infrared spectral range. To illustrate the performance of our retrieval, we use as input measurements of spectral atmospheric transmission from the Multi-Filter Rotating Shadowband Radiometer (MFRSR). These MFRSR data are collected at two well-established continental sites in the United States supported by the U.S. Department of Energy’s (DOE’s) Atmospheric Radiation Measurement (ARM) Program and National Oceanic and Atmospheric Administration (NOAA). The areal-averaged albedos obtained from the MFRSR are compared with collocated and coincident Moderate Resolution Imaging Spectroradiometer (MODIS) white-sky albedo. In particular, these comparisons are made at four MFRSR wavelengths (500, 615, 673 and 870nm) and for four seasons (winter, spring, summer and fall) at the ARM site using multi-year (2008-2013) MFRSR and MODIS data. Good agreement, on average, for these wavelengths results in small values (≤0.015) of the corresponding root mean square errors (RMSEs) for these two sites. The obtained RMSEs are comparable with those obtained previously for the shortwave albedos (MODIS-derived versus tower-measured) for these sites during growing seasons. We also demonstrate good agreement between tower-based daily-averaged surface albedos measured for “nearby” overcast and non-overcast days. Thus, our retrieval originally developed for overcast conditions likely can be extended for non-overcast days by interpolating between overcast retrievals.
Ground-based remote sensing and in situ observations of aerosol microphysical and optical properties have been
collected during summertime (June-August, 2012) as part of the Two-Column Aerosol Project (TCAP;
http://campaign.arm.gov/tcap/), which was supported by the U.S. Department of Energy’s (DOE’s) Atmospheric
Radiation Measurement (ARM) Program (http://www.arm.gov/). The overall goal of the TCAP field campaign is to
study the evolution of optical and microphysical properties of atmospheric aerosol transported from North America to
the Atlantic and their impact on the radiation energy budget. During TCAP, the ground-based ARM Mobile Facility
(AMF) was deployed on Cape Cod, an arm-shaped peninsula situated on the easternmost portion of Massachusetts
(along the east coast of the United States) and that is generally downwind of large metropolitan areas. The AMF site was
equipped with numerous instruments for sampling aerosol, cloud and radiative properties, including a Multi-Filter
Rotating Shadowband Radiometer (MFRSR), a Scanning Mobility Particle Sizer (SMPS), an Aerodynamic Particle Sizer
(APS), and a three-wavelength nephelometer. In this study we present an analysis of diurnal and day-to-day variability
of the column and near-surface aerosol properties obtained from remote sensing (MFRSR data) and ground-based in situ
measurements (SMPS, APS, and nephelometer data). In particular, we show that the observed diurnal variability of the
MFRSR aerosol optical depth is strong and comparable with that obtained previously from the AERONET climatology
in Mexico City, which has a larger aerosol loading. Moreover, we illustrate how the variability of aerosol properties
impacts the direct aerosol radiative forcing at different time scales.
The fifth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held June 13 to 21, 2003 at Table Mountain outside of Boulder, Colorado, USA. The main purpose of the Intercomparison was to assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks. This Intercomparison was coordinated by NOAA and included participants from six national and international agencies. The UV measuring instruments included scanning spectroradiometers, spectrographs, and multi-filter radiometers. Synchronized spectral scans of the solar irradiance were performed between June 16 and 20, 2003. The spectral responsivities were determined for each instrument using the participants' lamps and calibration procedures and with NOAA/CUCF standard lamps. This paper covers the scanning spectroradiometers and the one spectrograph. The solar irradiance measurements from the different instruments were deconvolved using a high resolution extraterrestrial solar irradiance and reconvolved with a 1-nm triangular band-pass to account for differences in the bandwidths of the instruments. The measured solar irradiance from the spectroradiometers using the rivmSHIC algorithm on a clear-sky day on DOY 172 at 17.0 UTC (SZA = 30o) had a relative 1- standard deviation of +/-2.6 to 3.4% for 300- to 360-nm using the participants' calibration.
The USDA ultraviolet radiation network currently includes four high-resolution spectroradiometers, located at Table Mountain, Colorado (deployed November 1998); the Atmospheric Radiation Measurement Climate Research Facility in Oklahoma (October 1999); Beltsville, Maryland (November 1999); and Fort Collins, Colorado (October 2002). These spectroradiometers contain Jobin Yvon's 1-m Czerny-Turner double additive spectrometers. The instruments measure total horizontal radiation in the 290- to 371-nm range, once every 30 min, with a nominal FWHM of 0.1 nm. We describe data quality control techniques as well as the data processing required to convert the raw data into calibrated irradiances. The radiometric calibration strategies using Central UV Calibration Facility FEL lamps that are directly NIST-traceable, portable field calibrators, and vicarious calibrations using data from UV multifilter rotating shadowband radiometers (MFRSRs) are discussed. Using direct-to-diffuse ratios from UV MFRSRs, we derive direct and diffuse high-resolution horizontal spectra from the collocated UV spectroradiometers of the USDA network. The direct-beam spectra can be used in a Langley regression that leads to spectroradiometric in situ calibration and to ozone column and aerosol optical depth retrievals. The high-resolution direct spectra are used to obtain the ozone column and aerosol optical depth in the 290- to 360-nm range at 0.1-nm resolution. A statistical summary of network performance is presented.
The Central UV Calibration Facility (CUCF) annually calibrates and characterizes 47 Ultraviolet Multi-Filter Rotating Shadow-band Radiometers (UV-MFRSR) for the USDA UV Monitoring and Research Program (UVMRP). The UV-MFRSR instrument has seven 2-nm wide channels with nominal centroids at 300, 305, 311, 317, 325, 332, and 368 nm. The first two channels 300 and 305 nm use silicon-carbide (SiC) photodiodes, and in the original design the remaining five channels used gallium-phosphide (GaP) photodiodes. Because of the high rate of failure in the channels with GaP photodiodes, channels 3 through 7 were replaced with silicon (Si) photodiodes starting in June 2000 by the manufacturer Yankee Environmental Systems, Inc. The newer design radiometers were tested for out-of-band rejection with two sources, in the laboratory using a 1000W FEL quartz tungsten halogen lamp and in the field using the sun. Out-of-band light measurements were completed in the field on all 47 radiometers and show there is no appreciable signal from out-of-band light contributing to the total solar horizontal irradiance in each of the seven wavelength bands. However, in the calibration procedure, using a 1000W FEL quartz- tungsten-halogen lamp there is significant out-of-band signal contributing to the measured signal. The out-of-band signal is measured at the time of the calibration and corrections are applied to the calibration factors of the radiometer in each channel. At the Table Mountain Test Facility, solar irradiance from a calibrated filter radiometer with and without the out-of-band correction factors are compared to filter weighted solar irradiance from the U111 reference spectroradiometer.
KEYWORDS: Atmospheric modeling, Ultraviolet radiation, Data modeling, Shortwaves, Visible radiation, Received signal strength, Aerosols, Radiometry, Absorption, Data acquisition
Broadband shortwave diffuse horizontal irradiance models overestimate measurements by between 7 and 14% using the most reliable input data for the models and the best available broadband measurements of diffuse irradiance. This paper uses spectral irradiance measurements and models as opposed to broadband measurements and models to investigate the contributions to this difference from various regions of the spectrum. The data are from the first Atmospheric Radiation Measurement (ARM) diffuse irradiance intensive observation period (IOP) held in September and October of 2001 at the Oklahoma ARM site near Ponca City. Visible and ultraviolet (UV) rotating shadowband spectroradiometers (RSS) acquired data during the IOP. Diffuse measurements with conventional broadband diffuse pyranometers and direct irradiance measurements using an absolute cavity radiometer are also available for analysis. Integrated spectral measurements are consistent with broadband measurements and, therefore, confirm the earlier results that models over predict diffuse. The wavelength dependent differences in models and measurements are illustrated and discussed.
The Rotating Shadowband Spectroradiometer (RSS) is a tandem-prisms spectrograph that uses a CCD array to measure solar direct and diffuse irradiances. Two versions of the RSS were designed at the Atmospheric Sciences Research Center at the State University of New York at Albany to measure UV from 295-370 nm and VIS-NIR from 360-1050 nm. A number of prototypes have been deployed at two sites of DOE's Atmospheric Radiation Measurement program since 1996. The first commercial UV RSS built by Yankee Environmental Systems, Inc. was deployed in 2001 and the VIS-NIR RSS is slated for permanent installation at the ARM SGP site in 2002. The paper describes instrument characterization procedures, spectral and radiometric calibrations. Mathematical algorithms applied to the spectra to correct wavelength shifts, to reduce stray light effects, and to correct drifts in radiometric calibration are described.
The UV rotating shadowband spectroradiometer (UV-RSS) is capable of measuring direct, diffuse and total horizontal irradiances simultaneously with spectral resolution of 0.25- 0.45 nm in the 290-380 nm range. It is based on a two-prism spectrograph that has very high out-of-band rejection of 2*10-6) as defined by the 325 nm HeCd laser line. Without moving parts, the radiometric stability is limited by the stability of the diffuser throughput and the stability of the cooled CCD. The wavelength stability is maintained by temperature control of the fused silica prisms and air pressure in the spectrograph. The current signal-to- noise ratio allows optical depth retrievals in the 305-360 nm range at mid latitudes in summer for typical ozone loading of 300 DU. This signal-to-noise can be increased by a factor of 5 within a one-minute shadowbanding cycle by means of multiple exposures. The UV-RSS permits ozone retrieval from diffuse irradiance using the DOAS method or from direct irradiance via Langley regression. Either method is robust as the UV-RSS provides 205 pixels of data within 310-330 nm range.
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