Data Collection and Processing

Two data loggers are installed at each site to collect and transfer the direct measurements from the instruments referred to in Table 8.1. One data logger is used to collect the data every 15 seconds from the VIS-MFRSR, UVB-1 pyranometer, temperature/humidity sensors, and the downward looking LI-COR 210SA photometer. The other is for the UV-MFRSR, PAR Quantum sensor, and barometer (and UV-A radiometer where applicable) that are sampled every 20 seconds. The difference between the sampling frequencies results from a longer required dwell time of the UV-MFRSR instrument. The data are aggregated into 3-min averages and stored in each data logger. Figure 8.2 shows an example of a UVMRP station.

Figure 8.2 The layout of instruments at a typical climatological site in the UVMRP monitoring network. (1) UV-B Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR); (2) Visible Multi-Filter Rotating Shadowband Radiometer (vis-MFRSR); (3) UVB-1 Pyranometer (erythemal); (4) Photosynthetically Active Radiation (PAR or Quantum Sensor); (5) UV-A biometer (only at 7 sites); (6) Downward-looking photometer; (7) Air temperature and relative humidity sensor; (8) Barometric pressure (14 sites, inside datalogger enclosure); (9) UV-MFRSR Datalogger; (10) Vis-MFRSR Datalogger

Figure 8.2 The layout of instruments at a typical climatological site in the UVMRP monitoring network. (1) UV-B Multi-Filter Rotating Shadowband Radiometer (UV-MFRSR); (2) Visible Multi-Filter Rotating Shadowband Radiometer (vis-MFRSR); (3) UVB-1 Pyranometer (erythemal); (4) Photosynthetically Active Radiation (PAR or Quantum Sensor); (5) UV-A biometer (only at 7 sites); (6) Downward-looking photometer; (7) Air temperature and relative humidity sensor; (8) Barometric pressure (14 sites, inside datalogger enclosure); (9) UV-MFRSR Datalogger; (10) Vis-MFRSR Datalogger

The instruments are located at sites with unobstructed horizons and are positioned at a level of 1.5 m above the ground. The temperature and relative humidity measurements are intended to be used in applications that require these data to be collocated with the radiation measurements and are not intended as substitutes for the National Weather Service (NWS) data, since these sensors are not installed in conformance to NWS standards. The data loggers are driven by AC power with rechargeable batteries in line to act as a backup in case the AC power fluctuates or outages occur for a short period of time. The "raw voltage" measurements from the station data loggers are transferred to the UVMRP data server every day through dedicated phone lines or the Internet. The raw voltages are processed into their corresponding physical quantities at the UVMRP headquarters located at Colorado State University, Fort Collins, CO, USA. Data quality control measures are applied daily. Discussion in this section will focus on the UV-B data, including UV-MFRSR and UVB-1 measurements.

Before 1997, characterization and calibration of the UV-MFRSR, VIS-MFRSR, and UVB-1 instruments were done annually at YES or at the Atmospheric Science Research Center (ASRC) at the State University of New York (SUNY)-Albany, NY, USA. Since 1997, annual characterization and calibration of the UV-MFRSR and UVB-1 instruments have been performed at the Central UV Calibration Facility

(CUCF), which is housed within NOAA's ESRL. The CUCF was initiated and developed in collaboration with the Optical Technology Division of the National Institute of Standards and Technology (NIST) and works to transfer the NIST standards of spectral irradiance to the U.S. solar UV radiation monitoring community in an accurate, long-term, repeatable, and cost-efficient manner (Disterhoft, 2005). The characterization and calibration of VIS-MFRSR instruments are based on the original YES values. Because many of the VIS-MFRSR instruments have been deployed for many years, the users are warned of this on the UVMRP website and are encouraged to use Langley calibrations, which are updated monthly. Until recently, the UV-MFRSRs were cycled through the calibration process at CUCF on an annual basis. Beginning in 2006, program priorities and fiscal constraints significantly lengthened the period between laboratory calibrations. Until this situation is resolved, the users of these data may wish to consult the most recent deployment date listed for the instrument at the site of interest via the UVMRP web page under "Site Location Deployment History", to check for last lamp calibration date. Then the choice of using lamp or Langley calibrations for spectral UV data may be made. The PAR sensors are sent to LI-COR for recalibration at their recommended two-year interval. The rest of the instruments, LI-COR 210SA photometers, and temperature-humidity probes, are calibrated by the manufacturers before they are deployed.

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