George Janson

Aerosol ultraviolet absorption experiment (2002 to 2004), part 1: ultraviolet multifilter rotating shadowband radiometer calibration and intercomparison with CIMEL sunphotometers

Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance.

The 1997 North American Interagency Intercomparison of Ultraviolet Spectroradiometers Including Narrowband Filter Radiometers

The fourth North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 15 to 25, 1997 at Table Mountain outside of Boulder, Colorado, USA. Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. 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 NIST and NOAA, and included participants from the ASRC, EPA, NIST, NSF, SERC, USDA, and YES. The UV measuring instruments included scanning spectroradiometers, spectrographs, narrow band multi-filter radiometers, and broadband radiometers. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity. The spectral irradiance responsivity was determined two to three times outdoors to assess temporal stability. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST traceable standard lamp, and a simple convolution technique with a Gaussian slit-scattering function to account for the different bandwidths of the instruments, the measured solar irradiance from the spectroradiometers excluding the filter radiometers at 16.5 h UTC had a relative standard deviation of ±4 % for wavelengths greater than 305 nm. The relative standard deviation for the solar irradiance at 16.5 h UTC including the filter radiometer was ±4 % for filter functions above 300 nm.

Aerosol column absorption measurements using co-located UV-MFRSR and AERONET CIMEL instruments

Column aerosol absorption properties in the visible wavelengths are measured routinely in worldwide locations by NASA AERONET network (http://aeronet.gsfc.nasa.gov), while similar optical properties in UV can be derived from diffuse and global irradiance measurements measured with Multifilter Rotating Shadowband Radiometer (MFRSR) instruments of the USDA UV-MFRSR network (http://uvb.nrel.colostate.edu). To enable direct comparisons between the two techniques, we have modified our UV-MFRSR by replacing standard 300nm filter with 440nm filter used in AERONET network. The modified UV/VIS-MFRSR has been mostly deployed at AERONET calibration site at NASA GSFC in Greenbelt, MD, but also at number of field campaigns. While the UV-MSFRSR instrument is highly susceptible to calibration drifts, these drifts can be accurately assessed using co-located AERONET direct-sun AOT data. In 2006 quartz dome has been installed atop the MFRSR diffuser, which stabilized calibration drifts in 2007-2009. After correcting for remaining calibration changes, the AOT and single scattering albedo (SSA) at the UV wavelengths can be accurately inferred by fitting the measurements of global and diffuse atmospheric transmittances with the forward RT model at each UV-MFRSR spectral channel. Derived AOT and SSA at common wavelength 440nm by two different techniques are generally in good agreement. We also found that SSA becomes smaller in the UV wavelengths and has strong wavelength dependence across blue and near-UV spectral range. The measured enhanced UV absorption might suggest the presence of selectively UV absorbing aerosols. High spectral resolution SSA measurements in UV-VIS wavelengths are called for.

Dependence of erythemally weighted UV radiation on geographical parameters in the United States

The relationship between solar ultraviolet (UV) radiation reaching the Earths surface and geographical parameters is helpful in estimating the spatial distribution of UV radiation, which provides useful information to evaluate the potential impacts of enhanced UV levels on human health, agriculture, environment, and ecosystems for sustainable development. Measurements of erythemally weighted UV radiation at the sites of the United States Department of Agriculture UV-B Monitoring and Research Program (UVBMRP) monitoring network were analyzed to investigate the geographical distribution and seasonal variations. Twenty nine observation sites, which had continuous measurements during the recent six years, are selected for this study; twenty seven of them are distributed in the United States, including one in Hawaii and one in Alaska, and two of them are located in Canada along the United States border. The measurements were taken using the Yankee Environmental Systems Inc. (YES) UVB-1 ultraviolet pyranometer. This work focuses the data from the recent six years of 2001-2006 and the measurements during summer months (June-August) are emphasized. For each day, the measurements are integrated from sunrise to sunset to produce the daily UV dosage, which is then averaged for different seasons or for the whole year over the six years to generate the average daily UV dosage. A multivariable regression technique is exploited to characterize the dependence of UV dosages on geographical parameters, including latitude and altitude. The results show that, although there are many factors, such as clouds, ozone, aerosols, air pollutants, and haze, that affect the UV radiation intensity at a location, the latitude and altitude of the site are the primary factors that regulate the average daily UV dosage. On average over the last six years in the United States, more than 95% of the variability in averaged daily UV dosages can be explained by the latitude and altitude. Longitude is not statistically significant in predicting UV irradiance. Nonlinear relationships can be statistically established between averaged daily UV dosage and latitude and altitude. The effects of latitude on UV radiation are much more significant than the altitude. The average daily UV dosages decrease exponentially with the latitude. While an increase of one degree in latitude may lead to a decrease of more than 350 Jm-2day-1 in the averaged daily dosage in the low latitudes, the decrease is around 100 Jm-2day-1 in the mid latitudes and less than 50 Jm-2day-1 in the high latitudes. The averaged daily UV dosage increases with altitude almost linearly until up to 1500 meters. Then it increases gradually and no significant increases can be detected above 2600 meters. Although the regression against latitude and altitude is statistically highly significant, notable deviations from the regression predictions are observed in the lower and mid latitudes and lower altitudes. These discrepancies are most likely due to the intense anthropogenic activities and natural events occurring in this area, including natural fire, industrial production, driving, and farming. These locally dependent activities will generate more UV absorbers into the air.

Long-term stability of UV multifilter rotating shadowband radiometers: part 2. Lamp calibrations versus the Langley method

This is the second continuation of work begun by Dave Bigelow and James Slusser in their study of the same name published in 2000 in J. Geophys. Res., 105, 4833-4840, which studied only a few instruments over a limited in-service time span. Part 1 expanded the Langley stability analysis by using 42 instruments over 5 years of field service. This part 2 expands stability as expressed with repeated laboratory lamp calibrations of the instruments, and compares these to the prior Langley analysis. 115 cases representing 44 instruments covering seven years of deployment are studied. Complicating this analysis are the four versions of the UV-MFRSR instrument that span the analysis time frame, and the results are presented as such. These results show the mean annual drift in sensitivity for the seven nominal wavelengths of the UV-MFRSR instrument are: pre-Rev.M: 300nm -8.8%, 305nm -8.1%, 311nm -7.4%, 317nm -8.3%, 325nm -7.3%, 332nm -7.6%, 368nm -7.2%; Rev.M: 300nm -7.5%, 305nm -7.1%, 311nm -6.5%, 317nm -5.6%, 325nm -5.8%, 332nm -5.3%, 368nm -5.1%; Rev.N and P; 300nm -10.1%, 305nm -7.2%, 311nm -8.3%, 317nm -4.3%, 325nm -3.6%, 332nm -3.7%, 368nm -3.5%; and Rev.Q: 300nm -5.6%, 305nm -5.8%, 311nm -3.8%, 317nm -4.4%, 325nm -4.8%, 332nm -4.6%, 368nm -3.5%.

Quality assurance of the UV irradiances of the UV-B Monitoring and Research Program: the Mauna Loa test case

The USDA UV-B Monitoring and Research Program (UVMRP) is an ongoing effort aiming to establish a valuable, longstanding database of ground-based ultraviolet (UV) solar radiation measurements over the US. Furthermore, the program aims to achieve a better understanding of UV variations through time, and develop a UV climatology for the Northern American section. By providing high quality radiometric measurements of UV solar radiation, UVMRP is also focusing on advancing science for agricultural, forest, and range systems in order to mitigate climate impacts. Within these foci, the goal of the present study is to investigate, analyze, and validate the accuracy of the measurements of the UV multi-filter rotating shadowband radiometer (UV-MFRSR) and Yankee (YES) UVB-1 sensor at the high altitude, pristine site at Mauna Loa, Hawaii. The response-weighted irradiances at 7 UV channels of the UV-MFRSR along with the erythemal dose rates from the UVB-1 radiometer are discussed, and evaluated for the period 2006-2015. Uncertainties during the calibration procedures are also analyzed, while collocated groundbased measurements from a Brewer spectrophotometer along with model simulations are used as a baseline for the validation of the data. Besides this quantitative research, the limitations and merits of the existing UVMRP methods are considered and further improvements are introduced.

Ground-based monitoring of UV radiation

High levels of UV radiation (UV-B) from the Sun have harmful effects on agricultural crops, forest ecosystems, humans, and livestock.1 In recent years the level of UV-B reaching Earth’s surface has increased because the protective stratospheric ozone layer has been eroded by man-made chemicals, such as chlorofluorocarbons and methyl bromide.2 Although the rapid decline in ozone concentration is now showing improvement, the US Department of Agriculture (USDA) remains concerned about possible harmful effects of increased UV-B on plants and ecosystems. UV-B levels, however, are not typically monitored at weather stations in the United States. For this reason, the USDA initiated the UV-B Monitoring and Research Program (UVMRP) in 1992.3 This program represents the only active source of complete ground-based UV measurements (since 1996) in the United States.4 The UVMRP network collects ambient ground-level solar radiation data, including UV-B and photosynthetically active radiation, from 37 climatological and three long-term research sites (see Figure 1). Data is downloaded from the network sites every night, and then subjected to automated quality control and calibration processes, so that it is available to researchers in several disciplines on our website the following morning.5 The primary UV instruments at our network’s stations use independent interference filter-photodiode detectors and automated rotating shadow bands to measure the direct-normal, total-horizontal, and diffuse-horizontal UV solar irradiance at seven wavelengths. The diffuser is an integrating cavity, with thin Teflon walls, that protrudes above the top of the detector head and is surrounded by an artificial horizon that improves the angular response of the instrument. Two diaphragms made of frosted WG-280 glass in the integrating cavity act as transmission diffusers. Light that exits the bottom of the diffuser is incident on seven photodiodes with interference filters, which Figure 1. The location of UV-B Monitoring and Research Program (UVMRP) monitoring sites in 2013. The 37 climatological sites are intended for long-term monitoring of different ecoregions across the United States. The three research sites are used for shorter-term and specific studies.

Long-term evaluation of the calibration of YES UVB-1 broadband radiometers of the Central UV Calibration Facility (1994-2005) and the suite of UV radiometers in the USDA UV Monitoring Network

The U.S. Central UV Calibration Facility (CUCF) at the National Oceanic and Atmospheric Administration (NOAA) of the Earth Systems Laboratory calibrates Yankee Environmental System (YES) UVB-1 broadband radiometers for the USDA UV Monitoring Program. The CUCF has three reference YES UVB-1 broadband radiometers that operate in the field at the CUCFs Table Mountain Test Facility (TMTF). These three reference broadband radiometers are run simultaneously against a reference U111 Spectroradiometer developed by Atmospheric Science Research Center (ASRC) at SUNY. The temporal stability will be shown of the erythema calibration factors of the CUCF reference YES UVB-1 radiometers under clear skies from 1994 until 2005. The USDA UV Monitoring Program has 51 UV broadband radiometers that are characterized and calibrated approximately once every 1-2 years by the CUCF starting in 1997. The average annual changes in the calibration are given for the 51 USDA YES UVB broadband radiometers.

Out-of-band rejection studies of the UV multi-filter rotating shadow-band radiometers

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.