UV Radiation and Climate Monitoring
Ultraviolet (UV) radiation that reaches the lower atmosphere can have severe effects on exposed skin, eyes, plants, etc. Cancers can be caused, some fatal. Recent attention to the changes in stratospheric ozone has generated considerable interest in UV radiation, because as ozone levels decrease the amount of damaging UV-B reaching the surface increases. Ozone is a highly efficient absorber of UV-B radiation. In fact, detailed measurements of UV-B are used by scientists to keep track of the total amount of ozone contained in the atmosphere.
Interest in both atmospheric composition and UV effects motivate the monitoring of UV at the surface. The various US agencies, with their different but interlocking missions, have initiated a number of UV monitoring programs using broad-band, narrow-band and high resolution spectral instruments. Different instruments suit different applications. Questions about variations in human exposure are conveniently answered using broad-band instruments. Questions about specific airborne chemicals (e.g. ozone) require spectral information. Narrow band instruments sometimes constitute an optimal practical solution to the desire to satisfy both communities, and are being increasingly incorporated into monitoring networks.
Links to other agency UV monitoring programs
The SERC UV monitoring program goals are to define short- and long-term variation in spectral UV-B and supports SERC's program of UV-B effects research, which is conducted by the SERC Photobiology/Solar Research Laboratory.
The SERC program uses a narrow-band, multifilter instrument, the SR-18 (link) which continuously monitors incident solar UV-B and short-wavelength UV-A (290-324 nm). We have also developed a extended range instrument, the SR-19, which monitors out to 330 nm. The instruments are designed, constructed and calibrated at SERC to maintain quality control as needed to ensure consistent long-term data.
The SR-18 and SR-19 are the most recent products of long-running program of solar instrument development at the Smithsonian. A previous UV-B instrument was the SR-8. Instruments of this design were in operation in Maryland (first at Rockville then Edgewater) 1975- 1984 (Correll et al. 1992), and at Mauna Loa, Hawaii since 1984-1995 (Neale et al., 1994a; Hofmann et al., 1995). SERC maintains a data base of 12-minute averages through the day for Maryland for the period since 1975, which is available upon request. A electronic distribution system is currently under development in collaboration with the National Institutes of Standards and Technology (NIST).
A priority in the SERC Solar Radiation research program has been the development of an instrument that provides sufficient spectral resolution for evaluation of wavelength-dependent UV-B responses, yet is less complicated and easier to operate than the available high-resolution scanning instruments. A basic objective is to design simple and rugged instruments that are serviceable for continuous monitoring under a wide range of temperature and weather conditions.
Instrument development and monitoring efforts date from the 1970s. At that time the SRL (then part of the SI Radiation Biology Lab) developed an 8-channel filter wheel instrument (SR-8). These instruments have eight interference filters with nominal 5-nm bandwidths and nominal center wavelengths of 290, 295, 300, 305, 310, 315, 320 and 325 nm. The interference filters are mounted in a rotating filter wheel which positions the filters above a solar-blind photomultiplier (PMT) detector. The wheel rotates at 72 RPM so that spectral data can be obtained at high frequency.
The 5-nm bandwidth of the filters in the SR-8, combined with the steep spectral gradient in the short wavelength (<312 nm) UV-B results in the effective center wavelengths of the SR-8 channels varying as a function of solar secant and ozone concentration, especially for the 290 - 310 nm channels (Correll et al. 1992). This disadvantage motivated the design of an instrument with narrower (2-nm) spectral resolution which would also take advantage of 1990's state of the art in optical and electronic components. The result is the SR-18. The optical design consists of 18, 2-nm bandwidth interference filters covering the spectral range from 290 to 324 nm (for detailed technical information see SR-18 Instrument Description - tab below).
The radiometer sensor head unit is a weatherproof housing with cosine-corrected diffuser for light sampling, a filter wheel with 18 2nm-nominal band pass interference filters mounted in the periphery, a collimator, and a solar blind R-1657 photomultiplier tube (PMT).
All of the components within the sensor head unit are environmentally housed for extended year-round field operation. All components of the sensor head, except the PMT, are designed to operate without significant temperature effects over a -25° to +55° C ambient temperature range. The PMT housing is temperature regulated with a thermostated thermoelectric system. A dessicant canister allows filled with indicating silica gel maintains low humidity inside the sensor head. A viewing port in the canister allows the operator to determine when to replace the silica gel.
The filter wheel turns at 15 RPM and has 2nm half-power-band-width light filters with nominal center wavelengths from 290nm to 324nm. Every four seconds, a new scan begins. The PMT current output (which is proportional to the radiation intensity through each filter) is converted to a voltage and measured by a precision 20-bit A/D converter.
Data from all light filter channels, and the two dark readings are averaged for one minute intervals, and along with internal temperature and control information, transmitted over three wire RS-232 data cable to the data acquisition and control unit.
A second unit houses the external power supply for the sensor head system. This power supply provides electrical power both to the head electronics and to the thermoelectric PMT temperature regulator. This unit is contained in a weatherproof housing.
The primary monitoring site at SERC is a 120 ft meteorological tower which provides a stable platform and unobstructed sky view. Instruments on the tower are connected to data acquisition computers and the SERC computer network so that outputs can be viewed remotely. The current UV conditions at SERC can be viewed on the web page (link under construction).
Other SR-18s are operated in Maryland and across the US. Some instruments are sited in connection with our effects research program, e.g. near other locations around the Chesapeake Bay. One instrument is routinely deployed at NOAA's Table Mountain Test Facility, where it participates in ongoing UV instrument intercomparisons (link). Other instruments were operated from 1996-2012 in collaboration with the National Institutes of Standards and Technology (NIST) Polymeric Materials Group to monitor UV weathering. (Future content: table with a list of current locations).
SERC collects weather data to complement its collection of ultraviolet data. The above weather data are the real time readings from various instruments that are located on top of a 120 foot tower, located at 38.89 N 76.56 W ( NAD27 ), adjacent to the Mathias laboratory. The data are stored in computer files for long term reference. The archived weather data are available for query and download.
Global ( total hemispherical ) solar flux is measured with a pyranometer. SERC-SRL uses an Eppley model PSP precision spectral pyranometer to measure radiation from 285 to 2800 nanometers. This thermopile-type instrument produces a voltage linearly proportional to the irradiance reaching the detector. The PSP is connected to a data logger by shielded twisted pair copper wires. The reported one minute data is the average of six readings each 10 seconds apart.
Temperature & Relative Humidity are measured with an active probe, a Vaisala model HMP45AC. The probe utilizes a 1000 ohm platinum resistance thermometer as the sensor for ambient temperature. A capacitive polymer sensor is used to sense relative humidity. The reported one minute data value is the average of six readings each 10 seconds apart.
Barometric pressure is measured using a Vaisala PTB 101B series barometric pressure transducer. Data collection is with a CSI CR10X.
Wind speed / wind direction is measured by a R. M. Young 05103 Wind Monitor. Data collection is with a CSI CR10X.
Rainfall is measured minute by minute using a TE525 "Tipping Bucket" rain gauge with a resolution of 0.01 inches. Data collection is with a CSI CR10X.