The USDA Ultraviolet Radiation Monitoring Program

Transcription

1 The USDA Ultraviolet Radiation Monitoring Program D. S. Bigelow,* J. R. Slusser,* A. F. Beaubien, + and J. H. Gibson* ABSTRACT The U.S. Department of Agriculture s Ultraviolet (UV) Radiation Monitoring Program has been measuring UV radiation since The initial network of 12 stations employed broadband meters to measure UVB irradiance and included ancillary measurements of temperature, humidity, and irradiance at seven wavelengths in the visible produced by a Multi-Filter Rotating Shadowband Radiometer (MFRSR). Since that beginning the network has expanded to more than 20 stations and the broadband meters have been supplemented with a seven-wavelength Ultraviolet Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR). The network has been designed to include 30 stations, each with a full complement of instrumentation. Annual characterizations of the network s filter radiometers indicate that gradual shifts in instrument response are manageable but must be accounted for to achieve accurate and precise measurements of UV irradiance. The characterization and calibration of the filter instruments is discussed along with filter stability and instrument precision. Broadband instruments are shown to be quite stable and collocated instruments are shown to agree to within 2.3% for zenith angles less than 80 under all sky conditions. Preliminary investigations into the accuracy of the UV-MFRSR calibrated with the Langley method are presented and successful column ozone retrievals are demonstrated with the UV-MFRSR under clear skies. 1. Introduction *Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado. + Yankee Environmental Systems, Inc., Turners Falls, Massachusetts. Corresponding author address: David S. Bigelow, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO daveb@nrel.colostate.edu In final form 29 December American Meteorological Society Since the discovery of the Antarctic ozone hole (Farman et al. 1985), there has been a growing awareness and concern within the scientific community of the potential for ecological changes resulting from the destruction of stratospheric ozone (Worrest et al. 1989). There is also growing evidence of increasing levels of ultraviolet radiation as a result of ozone losses over the Arctic (Santee et al. 1995) as well as over midlatitudes (Bojkov and Fioletov 1995). The concern that increasing ultraviolet radiation may alter agricultural production (Caldwell et al. 1986) prompted the U.S. Department of Agriculture (USDA) to cosponsor a series of workshops to determine what type of response might be necessary to address this potential threat to U.S. agriculture (Gibson 1991, 1992; Science and Policy Associates 1992; O Hara and O Hara 1993). A recommendation of these workshops was that the USDA establish a network to determine the geographical and temporal climatology of UVB radiation to support plant and animal effects research. The USDA Ultraviolet Radiation Monitoring Program was established in 1992 to provide the USDA with the information necessary to determine if changing levels of UV irradiance will impact food and fiber production in the United States. Prior to the establishment of the program only limited long-term information was available to make such an assessment (Scotto et al. 1988; Correl et al. 1992; Frederick and Erlick 1995) and the geographic distribution and spectral quality of this information was deemed insufficient to address the requirements of the USDA (Science and Policy Associates 1992). The primary objective of the monitoring network is to provide information to the agricultural research community about the geographic and temporal climatology of UVB irradiance and to assist this commu- 601

2 nity in relating changes in stratospheric ozone abundance to changes in UV irradiance. It is recognized, however, that the network s data will also improve the understanding of other factors that control UV irradiance (clouds, aerosols, etc.) and be of primary use to model developers, human health effects researchers, ecosystem scientists, and those seeking ground truth measurements for satellite measurement systems. To meet the USDA objectives, UV instrumentation was chosen that could be operated essentially unattended under a variety of field conditions. Additional instrumentation was selected to provide ancillary data for interpretative purposes. Because there has been little guidance to date from the agriculture effects research community as to their requirements for accuracy of UVB irradiance measurements, the approach of the program has been to establish the uncertainties in the measurements, to determine their source, and to minimize these uncertainties through the establishment of a quality assurance program. By considering the current limitations of instrument stability, cosine and spectral characterizations, and the availability of absolute standard references, the network is documenting the data precision, accuracy, and completeness that might be achieved and sustained with current unattended measurement technologies. This is an important and necessary first step in identifying more specific measurement quality objectives for agriculture effects researchers. A discussion of the network and the spectral and operational characteristics of this instrumentation, with emphasis on calibration and data quality, is the subject of this paper. States. Sites are chosen to occupy each grid rectangle based upon their long-term viability; proximity to agricultural, biological, and ongoing UV research; field of view; and availability of phone and power. Twenty sites had been established by the end of Their locations are listed in Table 1 and displayed in Fig. 1. b. Instrumentation A fully instrumented USDA network site, shown in Fig. 2, contains a Yankee Environmental Systems (YES) UV Multi-Filter Rotating Shadowband Radi- TABLE 1. Locations of USDA ultraviolet radiation monitoring sites. Station name Lat N Long W Elev (m) Baton Rouge, LA Big Bend, TX Bondville, IL CPER, CO Davis, CA Douglas Lake, MI Geneva, NY Grand Canyon, AZ Grand Rapids, MN Griffin, GA Jornada, NM Lake Dubay, WI Logan, UT Mead, NE The climate network a. Location of sites The climate network is designed to establish an approximate equal-area representation of the conterminous United States with additional coverage of Alaska, Hawaii, and Puerto Rico. The design is underpinned by a 5 lat 10 long grid covering the continental United Oxford, OH Presque Isle, ME Pullman, WA Table Mountain, CO Underhill, VT Wye, MD Vol. 79, No. 4, April 1998

3 FIG. 1. Locations of sites in the USDA Ultraviolet Radiation Monitoring Program. FIG. 2. Typical USDA Ultraviolet Radiation Monitoring Network instrument array. ometer (UV-MFRSR); a YES UVB-1 broadband meter; a Pacific Northwest National Laboratory (PNNL), or YES visible Multi-Filter Rotating Shadowband Radiometer (VIS-MFRSR); a Vaisala temperature humidity probe; and a downward-looking LI-COR photometer. 1 Data is captured by an onboard data-logging system integral to the VIS- MFRSR and UV-MFRSR instruments, and returned daily to the network s administrative office and data management facility at Colorado State University via dedicated telephone lines. All measurements, except the UV-MFRSR, are made as 15-s snapshots, aggregated to 3-min averages in near-real time and stored in the onboard data logger. The UV-MFRSR measurements are also stored as 3-min averages but are derived from measurements made at 20-s intervals. The 3-min averages are the fundamental measurements stored by the network. The UV-MFRSR is a seven-channel ultraviolet version of the Multi-Filter Rotating Shadowband Radiometer described by Harrison et al. (1994). This new shadowbanded instrument contains separate solid-state detectors that utilize Barr Associates, Inc., ion-assisteddeposition filters, each with a nominal 2-nm full-width at half-maximum (FWHM) bandwidth. The seven filters have nominal center wavelengths at each of 300, 305, 311, 317, 325, 332, and 368 nm. Each detector shares a common diffuser, thereby allowing total horizontal (no blocking) and diffuse horizontal (direct 1 Products listed in this document are identified for the convenience of the reader and are not specifically endorsed or recommended by the USDA. beam blocked) irradiance to be measured simultaneously at each passband. Direct normal irradiance is derived in near-real time by firmware included within the data logging component of the instrument. All three measurements are returned for each 3-min interval. The UVB-1 meter measures broadband UV from approximately 280 to 360 nm. The instrument employs a black glass UV-transmitting optical filter to block the sun s visible radiation and a UV sensitive phosphor to convert UV light to green light. This light is then passed through a green filter where it is measured with a photodiode. The VIS-MFRSR is a 7-channel, 10-nm (FWHM) filter shadowbanded radiometer that measures visible irradiance (total horizontal, diffuse, and direct normal) at 415-, 500-, 610-, 665-, 860-, and 940-nm nominal center wavelengths and with an additional unfiltered silicon diode (Harrison et al. 1994). Initial models deployed by the network were manufactured by PNNL. Newer models are now manufactured by YES. Both instruments are similar to the UV-MFRSR. The Vaisala model HMP 35E temperature humidity probe and LI-COR model 210SZ photometric sensor are maintained as auxiliary instrumentation by the network. The LI-COR, however, is mounted to look at the ground and is used solely for the purpose of indicating the presence or absence of snow cover. 3. Quality assurance The viability of any long-term program is dependent upon data quality assurance and the availability of this quality assurance information to the scientific 603

4 community. The failure to provide quality assurance results concurrent with measurement results often leads to surprises during a posteriori reviews of longterm datasets (Myers 1989; Weatherhead et al. 1997). The USDA UVB monitoring network addresses five essential elements of quality assurance. Completeness of its datasets at each site is tracked as the percent of possible 3-min averages captured on an annual basis (> 95% at all sites). Representativeness of each site is documented through annual photographs and through the network s adherence to a common set of siting goals. Annual visits to sites confirm that siting goals continue to be met and document any changes to the local area surrounding the site. Comparability, bias, and precision of network data are established through periodic and permanent collocated instrumentation studies and through the annual remeasurement of cosine and spectral responses of each of the network s spectral instruments. Absolute calibration (accuracy) of the USDA instruments is being established through the network s participation in the National Oceanic Atmospheric Administration National Institute Standards and Technology (NOAA NIST) jointly sponsored annual North American spectroradiometer intercomparisons held in Boulder, Colorado. Further details of the network s quality assurance practices are given below. routine reevaluation of the cosine responses of each detector. In general, cosine responses of VIS-MFRSR instruments are more uniform than those of UV- MFRSRs. This is largely due to the differences in diffuser material VIS-MFRSRs use Spectralon while UV-MFRSRs are constructed with a Teflon diffuser. The use of Teflon in the UV-MFRSR is necessary to optimize light throughput in the UV, especially at the shorter wavelengths. 2) SPECTRAL CHARACTERIZATIONS OF VIS- AND UV- MFRSRS The USDA procedures include an annual evaluation of each instrument s spectral response, FWHM passbands and out-of-band leakage. To determine the relative spectral response function (SRF) of the detector, a 150-W Xenon arc lamp is focused onto the entrance slit of an Acton Spectra-Pro 275 monochromator equipped with a 1200 grooves mm 1 grating. The monochromatic beam passes through the exit slits and strikes a beam-splitter that directs half of the light to a reference NIST-traceable photodiode and the a. Multifilter VIS- and UV-MFRSR measurements 1) COSINE RESPONSE OF VIS- AND UV-MFRSRS The Lambertian response of MFRSR instruments has been described by Harrison et al. (1994) and Michalsky et al. (1995). Precise angular corrections are applied to the direct-beam measurement (but not the diffuse) based upon an independent radiometric characterization of individual detectors made through the diffuser along two orthogonal planes (Michalsky et al. 1995). This adjustment must be made to correct for the instrument s cosine response. Figure 3 displays an initial and remeasured cosine response of a VIS-MFRSR. The first response was measured in June 1994 and the second in March Each line on the graph represents a single detector s response to a collimated beam passed in two orthogonal directions as the detector is rotated from 90 to +90. The flattening of the responses between 20 and +20 SZAs (solar zenith angles) and the sagging of the responses at the higher zeniths (50 80 ) in the remeasured response is believed to be indicative of diffuser soiling and underscores the need for FIG. 3. Typical cosine response of a VIS-MFRSR instrument (a) 30 June 1994, and (b) 29 March Vol. 79, No. 4, April 1998

5 other half to the detector (the diffuser filter photodiode combination). The irradiance determined by the reference photodiode is used to adjust the signal for the spectral characteristics of the lamp in order to yield the detector s relative spectral response at that wavelength. It is assumed that the differences between the spectral response of the two photodiodes is negligible over the narrow passband being measured. The measurement is repeated over all wavelengths within the passband by scanning the wavelength of the monochromator in 0.1-nm steps in the UV and 1.0-nm steps in the visible. Center wavelengths and passbands are determined by integrating all responses greater than 3% of the normalized peak response and then identifying the first and second moments of the integral. The wavelength that corresponds to one-half the integral value is reported as the effective center wavelength (ECW). The ECW is typically near but not always at the wavelength where the peak response of the filter s normalized response function occurs. The FWHM passband corresponds to the difference between the points where the instrument s response function is at 50% of the maximum normalized response. All filter functions are maintained by the network and are available in a standardized format to data users. For the UV-MFRSR an evaluation of out-of-band rejection was made as follows. The total irradiance from a UV-MFRSR was measured on a sunny day (SZA ~45 ) first with and then without Schott glass cut-on filters. With a 2.5-cm square Schott WG 320 glass filter, 0.3 cm thick, covering the diffuser, the magnitude of the signal was less than 0.5% of the unfiltered value for channels 300, 305, and 311 nm. This indicates that red light leakage for wavelengths greater than 320 nm was less than 0.5% of the total signal in these channels. With a 0.3-cm-thick WG 385 filter covering the diffuser, all channels returned values of less than 0.5% of their unfiltered values, indicating that red light leakage from light with wavelengths greater than 385 nm was less than 0.5% of the total signal at each of the 7 channels. 3) CALIBRATION OF VIS AND UV MFRSRS To determine the absolute response of a detector, the SRF of each filter channel is measured as described previously. Next the voltage of the detector (diffuser filter photodiode combination) is recorded with the detector exactly 50 cm from a 1000-W NIST-traceable FEL lamp spectral irradiance standard. The SRF is convolved with the known absolute lamp spectral irradiance to give the effective power incident on the detector. Dividing the measured voltage by the effective power produces the spectral responsivity for each individual detector in volts/watts per meter squared per nanometer (Yankee Environmental Systems, Inc., 1994). Presently this is the method being used to supply USDA data clients with calibrated VIS and UV- MFRSR data. The limitations of calibrations based upon standard lamps having different specrtral distributions than those of the solar spectra being measured have been discussed by Hickey (1970) and Booth et al. (1994). An alternate method suggested by Booth et al. (1994) based upon comparisons of the filter instrument to a spectroradiometer in the field, however, becomes somewhat ambitious for routine network calibrations. The circulation of the network s more than 40 instruments from the field to the spectroradiometer installation, on a schedule that minimizes gaps in the monitoring record at each site and provides for an adequate collocation period, is difficult to sustain. This is especially true in light of the uncertainties associated with establishing and maintaining an absolute calibration of a spectroradiometer. Another technique that has potential for routine network calibrations is the use of the Langley method to extrapolate voltages of each channel to their value outside the earth s atmosphere and relate this voltage to the extraterrestrial irradiance (Shaw 1982; Harrison and Michalsky 1994; Schmid and Wehrli 1995; Wilson and Forgan 1995). The attenuation of the direct radiation as it passes through the earth s atmosphere is described by the Beer Lambert Law (Craig 1965): ( ) I = I exp τ m, (1) λ o, λ λ, i where I λ is the direct irradiance at the ground, I o,λ is the extraterrestrial irradiance, τ is the optical depth for the ith absorber, and m is the slant path (or air mass) through the atmosphere. Taking the natural log of both sides ln Iλ = ln Io, λ mτ (2) and, if instead of irradiance we measure uncalibrated voltages, the equation is lnvλ = ln Vo, λ mτ, (3) 605

6 where V λ is the measured voltage of a particular channel and V o,λ is the extrapolated voltage intercept at zero air mass. Plotting the natural log of the direct beam voltage at one filter wavelength versus the optical pathlength or air mass (the secant of the solar zenith angle for angles up to 75 ) results in a straight line whose slope is the optical depth of the atmosphere at that wavelength and whose intercept at zero air mass is the voltage the detector would register if it were pointed toward the sun at the top of the atmosphere. The irradiance at the ground (I λ ) is retrieved by multiplying the measured voltage at the ground (V λ ) by the calibration factor k. This factor k is the integral multiplication of the extraterrestrial solar irradiance I o,λ (Vanhoosier et al. 1988) and the filter function or SRF of the filter photodetector F λ, divided by the product of the extrapolated Langley voltage intercept V o,λ and the integral of the filter function (Tüg and Baumann 1994). The relationship of the transformation is given in Eq. (4). This assumes that the direct component has been cosine corrected: I λ Vλ Io, λfλdλ = Vλk =. (4) V F dλ o, λ λ The precision of the calibration is measured by the variance of V o,λ, determined from a number of Langley events. The Langley technique has typically been used at clean, high-elevation sites such as Mauna Loa where assumptions of unchanging atmospheric optical conditions throughout the day and uniform horizontal mixing of the absorbers can be met. Under these ideal conditions Shaw (1982) reported extrapolations of V o,λ to within a standard deviation of 1.2% at 380 nm and 0.4% or smaller at eight of nine 7-nm passbands between and nm. Comparable results have also been reported by Schmid and Wehrli (1995) at six 5-nm passbands between 500 and 1024 nm. More encouraging, however, are the results of Harrison and Michalsky (1994) who reported that standard deviations of extrapolated V o,λ measurements could be reduced to less than 1% at Boulder, Colorado, over a 3-month period using a robust objective Langley algorithm of their own design. Their work demonstrates that Langley analysis can be utilized at more difficult sites under less than ideal conditions by aggregating and analyzing discontinuous Langley events. Successful application of the Langley technique to a less than ideal site has also been reported by Wilson and Forgan (1995), who extended the technique into the ultraviolet down to 304 nm using spectroradiometric data. Extrapolations to zero air mass in the ultraviolet are expected to be more variable than those in the visible region as a consequence of the variations in the ozone and aerosol optical depths during the Langley event. Rapidly changing ozone optical depths with wavelength over the 2.0-nm passband of the UV-MFRSR results in the failure of the Beer Lambert law adding to the uncertainty of the Langley-derived calibrations (Wilson and Forgan 1995). Since the shorter wavelengths are attenuated more than the longer as the SZA increases, the effective center wavelength will shift increasingly to the red at larger SZAs. Further complications result from the increased scattered light around the sun s disk at these shorter wavelengths (McKenzie and Johnston 1995). The USDA network is actively working toward implementing a Langley technique as an alternate method of calibrating both the VIS- and UV-MFRSR. Preliminary results from applying the technique to the more difficult UV-MFRSR wavelengths are presented below. Langley plots for a single UV-MFRSR first located at the Central Plains Experimental Range (CPER) during July and August 1996 and then at Table Mountain (see Table 1) during September and October 1996 were made using the Harrison and Michalsky (1994) algorithm. The voltage intercepts were used along with the SUSIM extraterrestrial solar flux (Vanhoosier et al. 1988) and the measured filter functions in Eq. (4) to develop wavelength-specific calibration factors for global irradiances measured by a UV-MFRSR. Table 2 gives the average Langley voltage intercept for the six channels studied along with the standard deviation and percent standard deviation of the 16 Langley events that were recorded during this time period. No Langley plots were available for the 300-nm channels because of insufficient signal to noise. An example Langley plot of log voltage versus air mass for a 2-nm FWHM passband at nm channel for 10 October 1996 at the Table Mountain site near Boulder is displayed in Fig. 4. Langley-calibrated UV-MFRSR irradiances, measured on a clear day at Table Mountain, Colorado, at local noon 10 October 1996, were checked by comparing them to both the Stamnes et al. (1988) radiative transfer model using the extraterrestrial spectrum of Vanhoosier (1988), with appropriate input conditions (total column ozone measured by a Dobson 606 Vol. 79, No. 4, April 1998

7 TABLE 2. Average of 16 Langley voltage intercepts from a single instrument located at two Colorado sites, July October Effective center wavelengths (nm) V o Std dev % Std dev 7% 7% 8% 8% 10% 8% within 15 km, SZA, estimated surface albedo, aerosol optical depth of measured at 368 nm and elevation), and to irradiances measured by a collocated Brewer spectrophotometer operated by the University of Georgia (Bais et al. 1996) and calibrated using a 1000-W FEL lamp. The response function of each UV- MFRSR channel was then passed over the model and Brewer irradiances to give irradiance at each of the UV-MFRSR passbands. The comparison is summarized in Table 3. Because the Brewer only measures out to 365 nm no comparison is made at the nm channel. The ratio of irradiances of the radiative transfer model to the UV-MFRSR using the Langley calibration factors ranged from 1.00 at nm to 1.11 at nm with an average of 1.06 for all six channels. The ratio of the Brewer to the UV-MFRSR ranged from 0.92 at nm to 0.82 at nm with an average of 0.88 for the five channels. To further compare the UV-MFRSR Langley-derived irradiances with those of the Brewer spectrophotometer, data from an entire clear day (11 October 1996) were analyzed. Because Brewer scans took 5.4 min and were made only approximately every 20 min, they were linearly interpolated to conform to 30-min intervals that could be directly compared to the UV-MFRSR measurements. The ratio of irradiances for the two instruments is shown in Fig. 5. As noted previously, there were no data for the 300- and 368-nm channels. Between 1600 UTC (60.8 SZA) and 2200 UTC (65.7 ), the ratios of Brewer to UV- MFRSR irradiances were between 0.8 and 0.99 for all five channels. This indicates a systematic bias of 1% 20% between the instruments. Without further comparisons with other instruments it is not possible to determine which instrument is nearer the truth. Calibrations using the same source could improve the agreement but have not been done. At a large SZA during the morning (80.9 ) the ratios of the Brewer to the UV-MFRSR were greater than 1 for the 305, 311, and 317 channels. This indicates the two instruments had different cosine or angular responses. The ratios late in the day as the sun is setting are not symmetric for ratios at the same SZAs in the morning indicating that the cosine responses of the two instruments have azimuthal differences. Since the UV-MFRSRs direct beam is corrected for its measured angular response, it is either its response to the diffuse or the unmeasured overall an- FIG. 4. Langley plot for the channel 10 October 1996 at Table Mountain. The range of the air masses is 2 6. The slope is the atmospheric optical depth τ at this wavelegth. FIG. 5. Comparison of UV-MFRSR to Brewer spectroradiometer data. 607

8 TABLE 3. Comparison of UV-MFRSR using Langley calibration with model and Brewer spectrophotometer 10 October 1996 at Table Mountain, SZA = Effective center wavelengths UV-MFRSR Model Brewer (nm) (W m 2 nm 1 ) (W m 2 nm 1 ) (W m 2 nm 1 ) gular response (diffuse plus direct) of the Brewer that account for the diverging irradiance ratios for SZAs larger than 60. Though the results were somewhat less precise than those reported in the visible by Shaw (1982) and others (Harrison and Michalsky 1994; Schmid and Wehrli 1995), they are nonetheless promising. Further work with the Langley algorithms will be necessary to determine how much improvement can be made in the UV calibrations. 4) FILTER PASSBAND STABILITY OF VIS- AND UV-MFRSRS The gradual degradation of filters in narrowband instruments has been reported by a number of investigators (Bruegge et al. 1992; Russell et al. 1993; Schmid and Wehrli 1995; Blumthaler et al. 1997) and is considered to be a general problem as a consequence of exposing the filters to the sun and other factors such as temperature and humidity. Using Langley analysis software supplied by Harrison and Michalsky (1994), the network examined all of its data for suitable Langley events and then, as described by Schmid and Wehrli (1995), arranged the returned intercepts (V o ) by head and time for further analysis. Figure 6 illustrates the results of this stability check for VIS-MFRSR head This head was first located at the network s Colorado site and then at its Utah site hence the break in the data record. The downward drift of V o at each wavelength suggests that the filters are exhibiting some instability, especially early in their life. Interestingly, the 610- and 665-nm wavelengths typically exhibit the strongest downward trends, a pattern also noted by Schmid and Wehrli (1995). Reasons for the disproportionate decline of these passbands are unknown but it is speculated that the wavelengthspecific bonding agents used in these particular filter stacks are deteriorating more rapidly. Though most of the USDA instruments do not as yet have as extensive an operating history as the 8725 head, most appear to exhibit similar behavior. It appears therefore that the network will need to apply a time series correction, similar to that described by Schmid and Wehrli (1995), to its temporal record to compensate for these drifts. Changes in returned intercepts can be the result of both changes in filter transmission and changes in the spectral shape of the passband. Changes in the instrument s diffuser may contribute as well. The network s annual recharacterizations (cosine, calibration, and relative spectral response) of its instruments will allow investigation of these effects in the future. FIG. 6. Time series of Langley regression intercepts for USDA VIS-MFRSR head Vol. 79, No. 4, April 1998

9 5) PRECISION OF VIS- AND UV-MFRSRS A preliminary precision study was conducted by the network collocating two early prototype UV- MFRSRs during the fall of One of the instruments was then maintained in regular service for almost two years while the other was used in laboratory tests. The units were again collocated briefly in early summer of Results from the initial 51-day collocation are displayed as boxplots in Fig. 7. The figure presents the distribution of differences in the daily integral of each instrument s total horizontal irradiances expressed as a percentage of one of the instruments. The plot includes all sky conditions and SZAs during the collocation. For the initial 51 days that the instruments were collocated there is a clear bias between the two instruments. Median percent differences range from approximately 10% on the 311-nm channel to +3% on the 317-nm channel. Precision is much better within an individual channel however. For instance, 80% of the differences in the 311-nm passband are within 3% of one another. Uncertainties in the lamp calibrations of each instrument along with differences in the filter s center wavelengths (Table 4) contribute to the observed differences. Spatial and temporal comparisons between instruments will need to account for these differences if more precise comparisons are to be made. In the future, the network will be pursuing more automated methods for adjusting instrument responses to a single common wavelength for each channel. These might include both ratio and modeling techniques. b. UVB-1 broadband meters FIG. 7. Precision of the daily integrals from two UV-MFRSRs. Shaded areas represent the interquartile range of differences. Hats designate the 10th and 90th percentile values. 1) COSINE RESPONSE OF THE UVB-1 BROADBAND PYRANOMETERS Figure 8 displays the variance and departure from an ideal cosine response of 10 network broadband meters that were characterized by the NIST. The characterizations were done at the same time prior to the instrument s deployment to the field. While the variability between the different meters is quite small (< 0.1%, Fig. 8c) the departure from the ideal cosine response (Figs. 8a and 8b) exceeds 10% beyond 60 SZAs. These results are typical of those reported by others (Grainger et al. 1993; Mayer and Seckmeyer 1996). The repeatability among the cosine responses of the USDA broadband meters, however, suggests that a single generalized cosine correction may be applied to all broadband meters in the network. Because the cosine response is largely a function of the design geometry and manufacturing of the meter, it is anticipated that the cosine response characteristic of an individual instrument will not change unless the instrument becomes damaged. The stability of cosine response should only be dependent upon maintaining it in a TABLE 4. Effective center wavelengths by channel for two UV-MFRSRs. Effective center wavelengths by channel (nm) Unit Difference

10 FIG. 9. Spectral response of 13 broadband UVB-1 meters characterized by NIST. FIG. 8. (a) Actual versus ideal (dotted) cosine response of 10 UVB-1, (b) ratio of each meters response to the ideal cosine response, and (c) variance of cosine responses. level plane and the instrument should require only periodic characterization. 2) SPECTRAL RESPONSE OF THE UVB-1 BROADBAND PYRANOMETERS Spectral response functions (SRF) of 13 UVB-1 pyranometers characterized by NIST are presented in Fig. 9. Measurements were made at 2-nm intervals and responses were normalized to 1.0 at the wavelength of each instrument s response. The shapes of the SRFs are similar to those reported by DeLuisi et al. (1992) but with peaks between 296 and 298 nm. Differences in the SRF of the various meters are minimized during calibration by adjusting each instrument s signal based upon the ratio of its daily integral to that of a reference instrument maintained at the manufacturer s calibration facility (YES). The utility of this technique is explained by DeLuisi et al. (1992). Drift in the long-term response of the meters can also be handled by this adjustment technique. An examination of the calibration history of two UVB-1 s that have cycled four times each through the YES calibration facility for routine annual recalibration suggests that except for the initial adjustment, additional adjustments are small and for the most part unnecessary. Figure 10 displays the calibration history of two UVB-1 meters, serial numbers and Each dashed line of the dot plot represents a single day that the instrument was exposed to sunlight during the evaluation phase of the calibration step. Circles represent the difference between the daily integral of the voltages returned by the instrument and that of a reference instrument in this case unit expressed as a percentage of the reference instrument. The solid vertical line represents the response of the reference instrument. In the case of unit it can be seen that it was returned for calibration in 1993, 1995, and once in An initial evaluation was done in 1992 at the time the instrument was manufactured. When the instrument was evaluated over an initial 6-day period it returned values approximately 0% 9% high. The instrument was adjusted and then sent to NIST for evaluation. It was returned to YES in 1993 and reevaluated prior to its deployment to the field. The integrals returned by the unit are about 1.5% higher than the reference. It is unknown if it was adjusted before it returned to the field, but, since the manufacturer only guarantees an accuracy of 5%, it is unlikely that it was adjusted. In 1995 the unit was returned to YES for an 11-day evaluation. This time the unit returned values on average that were 1.8% lower than the reference. Again it is unclear whether the instrument was adjusted. As in 1993, however, it was well within the specifications stated by the manufacturer. In the spring of 1996 the unit was again returned for recalibration and exhibited readings that were 5.4% higher than the reference. For unknown 610 Vol. 79, No. 4, April 1998

11 reasons the unit was returned to the field without adjustment. A similar pattern of stability is shown for the unit, indicating that instrument drift does not appear to be a significant problem. This is reaffirmed in the precision study given below. 3) PRECISION OF THE UVB-1 BROADBAND METER The network maintains an array of three broadband meters at its Central Plains Experimental Range site in Colorado. An examination of the variance from each 3-min time period in 1995 ( triads) demonstrates that the precision of the broadband meters is good (< 2.5%) but, as reported by others (DeLuisi et al. 1992; Mayer and Seckmeyer 1996), is subject to angular dependencies. Because the dependencies result from the cosine response of the meter (Fig. 8), they correlate well with solar declination and zenith angles and may be a factor in seasonal or diurnal stratifications of the data. Figure 11 displays the standard error (square root of the variance divided by each triads average) of collocated broadband meters as a function of zenith angle. Precision decreases as the sun moves away from the spring and fall equinox and as the sun deviates from solar noon. These solarrelated perturbations, however, are small and as shown in Table 5, 95% of the triad s 3-min time periods had standard errors of less than 2.3% at SZAs less than 80. For individual meters, precision might even be better. During 1995 and 1996 the triad s meters were replaced as a part of the network s practice of returning meters annually to the manufacturer for recalibration. To minimize the amount and seasonality of incompleteness in the network s temporal record, the network has chosen to simply replace the returned meter with a newly calibrated one rather than wait for the meter to be returned. Unfortunately this introduces an increased amount of random error into the network s measurement process. Figure 12 displays the standard error in the daily integral for the 2-yr time period. The distinct step changes in the plots correspond to the annual meter s replacements. While these FIG. 10. Recalibration history of two UVB-1 broadband pyranometers. Zero represents the unit value. changes at first appear alarming, it should be noted that the manufacturer only claims an accuracy of 5%. Our data suggest that as a group the meters are performing well within that specification. The range in the displayed error of individual meter groups further suggests that with a meter permanently assigned to a site, precision might be improved by 50%. Further investigation of the network s frequent calibration results will permit a more complete understanding of the differences between individual broadband meters. c. Accuracy of network instrumentation The accuracy of both filter and broadband detectors has been the subject of a number of recent investigations employing either a comparison to a more trusted spectroradiometer (Grainger et al. 1993; Booth et al. 1994; Mayer and Seckmeyer 1996; Leszczynski et al. 1998) or a comparison to the sun (Schmid and 611

12 TABLE 5. Standard error of broadband measurements as a function of solar zenith angle. Percentile of standard errors SZA 25% 50% 75% 90% 95% FIG. 11. Contributions of angular dependencies to standard errors in broadband pyranometers Wehrli 1995; Wilson and Forgan 1995). Neither method provides the network with a universally accepted absolute reference, and therefore neither method is preferentially endorsed by the network. The network is committed to establishing the accuracy of its detectors and so does and will continue to support instrument collocation studies and participate in toplevel intercomparisons. It is believed however, that the accuracy of the network s instrumentation will only be resolved by independent efforts outside of the scope of routine network operations. Because of this, only a preliminary accuracy investigation has been conducted by the network to date. 4. Applications of network data FIG. 12. Standard error in the daily integrals of three broadband meters at the Central Plains Experimental Range: a. Aerosol and ozone optical depths via the Langley method of calibration The relative contribution of clouds, ozone, and aerosols to the attenuation of UVB is an active area of radiation research that can be explored with network data. A preliminary investigation into the estimation of column ozone was conducted for clear skies using the new UV-MFRSR. The UV-MFRSR returns the direct normal irradiance I at seven ultraviolet wavelengths. 612 Vol. 79, No. 4, April 1998

13 A plot of the natural log of V versus air mass yields a straight line (Langley plot) with a slope of the optical depth τ, which is composed of τ = τ ray + τ ozo + τ aer, (5) where τ ray is the Rayleigh or scattering optical depth, τ ozo is the ozone optical depth, and τ aer is the aerosol optical depth. Two channels with center wavelengths close together (311 and 317 nm) were chosen, and total optical depths, τ 1 and τ 2, at these wavelengths were computed using the Langley method outlined previously. The aerosol optical depth for both channels were assumed to be equal and the Rayleigh optical depth was computed (Stephens 1994) allowing a solution for (τ ozo1 τ ozo2 ): τ 1 = τ ray1 + τ ozo1 + τ aer1 (6) (τ 2 = τ ray2 + τ ozo2 + τ aer2 ) (7) τ 1 τ 2 = (τ ray1 τ ray2 ) (τ ozo1 τ ozo2 ). (8) Next, the effective ozone cross section α was computed, weighted by the extraterrestrial solar flux S(λ), and the filter function of the photometer F(λ): α = ( ) ( ) ( ) ( ) ( ) αλs λf λdλ S λ F λ dλ. (9) The ozone optical depth τ ozo is the product of the ozone column χ and the effective ozone cross section α: τ ozo = χα. (10) Combining Eqs. (9), (10), and (11) the ozone column χ can be determined: χ = ozo [ 1 2 ( 1 ray2) ] ( τ τ ) τ ray τ α α ozo1 ozo2. (11) Ozone column abundances determined from direct normal solar irradiances measured by the UV-MFRSR located at the Central Plains Experimental Range, Colorado, on the mornings of 11, 22, and 23 July 1996 were compared with those measured at 1000 LT on the same dates by the NOAA Dobson spectrophotometer located at Boulder about 100 km southwest of the site (Table 6). The agreement is within an average of 7%. The network is expanding its work with ozone retrievals to include improved algorithms under cloudy skies. Preliminary retrievals of column ozone using a discrete ordinate radiative transfer code (Stamnes et al. 1991) under all sky conditions and over a 3-month period, suggests agreements to within 4% of the Dobson can be achieved. b. Cloud properties With the ability to measure the global transmission, cloud optical depth for totally overcast conditions can be determined using the VIS-MFRSR (Min and Harrison 1996; Leontyeva and Stamnes 1996). This method has the advantage of requiring only a previously determined Langley intercept and the raw voltage for a particular passband to determine transmittance, thereby avoiding the uncertainties of absolute calibrations. With the rapid development of all-sky imaging cameras (C. Long 1997, personal communication), UV- and VIS-MFRSR data will allow the study of the spectral dependence of diffuse/ direct radiation in broken cloud fields. A simple model such as that of Lantz et al. (1996) might then be used to estimate the decrease (sun occluded) and increase (sun not occluded) in actinic UV fluxes based on cloud fraction. c. Model validation: A TOMS example NASA has expressed a desire to utilize the USDA UV-MFRSR measurements as a set of ground-based measurements with which they can evaluate their model-based calculations of UVB in their TOMS satellite program. As an early indication as to how the comparisons might be made, a number of tests were made using the Stamnes et al. (1988) model in parallel with the NASA model. The models were run with TABLE 6. Comparison of column ozone derived from a UV- MFRSR and a nearby Dobson spectrophotometer. Date UV-MFRSR (DU) Dobson (DU) 11 July July July

14 identical input parameters (column ozone, scattering optical depth, albedo, and SZA) and agreement of total irradiances to within 1.5% was achieved for SZAs up to 54 at 300 nm. These preliminary results that used data from the Table Mountain site further suggest that TOMS overflights may be used for quality assurance for both the TOMS and USDA instruments. 5. Summary and conclusions The USDA Ultraviolet Radiation Monitoring Program furnishes both the scientific agricultural and radiation community with a standard suite of radiation measurements at more than 20 locations throughout the United States. The availability of regular characterization and calibration data for all of the network s radiation instrumentation additionally provides a wealth of information for studying long-term instrument stability and quality. In addition to UV and visible irradiance measurements, column ozone, total, and aerosol optical depths will be made available. The need for detailed instrument characterization information has been demonstrated for both broadband and filter instruments, and it is apparent that a posteriori corrections must be applied to the data if spatial and temporal interpretations are to be accurate. Much more work needs to be completed before the network can make these corrections on a routine basis. Nonetheless, the information to make these adjustments are made available to the network s data users data that apparently has not been available to most researchers in the past. Acknowledgments. The USDA Ultraviolet Radiation Monitoring Program is supported by a grant to Colorado State University (USDA Agreement ). The authors additionally wish to acknowledge the dedication of Bill Durham and George Janson in installing and maintaining all of the monitoring equipment in the USDA UVB Monitoring Network and Bill Davis and Becky Olson for maintaining a steady stream of daily measurement data from the network. 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