Central California Ozone Study (CCOS) - Desert Research Institute

CCOS Field Operations Plan Version 2: 5/31/00
2.3.4 Solar Radiation Measurements
The US Department of Agriculture’s UVB Radiation Monitoring Program was initiated
in 1992, through a grant to Colorado State University, to provide information on the
geographical distribution and temporal trends of UVB radiation in the United States.The
climatology network of the USDA-CSREES monitoring program is designed to provide an
adequate density of measurement sites to establish the spatial and temporal characteristics of
UVB irradiance. The United States is divided into 26 regions of about equal area. The
measurement sites are located primarily in rural areas near agricultural and forested regions. The
US Department of Agriculture operates a UVB radiation measurement network with the
following two sites in California.
Station Name Closest town Longitude Latitude Elevation
UC Davis Climate (Davis) 121.8 38.5 18
Station
Desert Research and
Extension Center
(Holtville) 115.4 32.8 -18
The US Department of Agriculture’s UVB Radiation Monitoring Program maintains one
site, near Davis, California (Latitude: 38.529, Longitude: 121.761). The site is operated by the
University of California at Davis. Co-location of one spectral radiometer for the measurement of
actinic flux to this site will provide significant advantage to CCOS. There are two existing
Multifilter Rotating Shadowband Radiometers (MFRSR) at the USDA, Davis site. A
shadowband radiometer has a rotating metal band that periodically blocks the direct solar beam
from entering the radiometer. This feature allows a measurement of the total horizontal, diffuse
and direct normal components of the irradiance. One MFRS radiometer operates in the visible
region and makes filter measurements at 415, 500, 610, 665, 862 and 940 nm and for the total
visible radiation. The second MFRS operates in the UV and makes measurements at 300, 305.5,
311.4, 317.6, 325.4, 332.4, 368 nm. The sampling time for the visible MFRSR is 15 seconds and
the sampling time for the ultraviolet MFRSR is 20 seconds however data from both MFRSR’s
are stored as 3-minute averages. There are several other measurements that support the UVB
radiation measurements at this site including sensors for UVA, UVB-1 and surface reflection and
measurements of relative humidity, temperature and barometric pressure. Sample data from this
site may be obtained from:
http://uvb.nrel.colostate.edu/UVB/cgi_scripts/uvb_site_info_page.cgi?CA01&C.
Actinic flux in the troposphere depends most critically on the total ozone column, the
aerosol loading, size and optical properties and the presence and characteristics of clouds. The
UV and visible MFRSR will allow the total ozone column and the aerosol characteristics to be
estimated. The total ozone column measured by an UV-MFRSR was compared with standard
Brewer and Dobson spectrophotometers. The average ratio of ozone retrieved by the UV-
MFRSR to that of the Brewer was found to be 1.02 and to the Dobson 1.03. The USDA’s visible
and UV- MFRSRs data provides an estimate optical depths as a measure of the total aerosol
loading. These data together with the actinic flux measurements will allow a direct evaluation of
the photolysis rate parameters calculated for use in air quality models.
Chapter 2: FIELD MEASUREMENTS 2-11