Research Activities

Research at the USDA UV-B Monitoring and Research Program centers on quantifying the atmospheric effects that influence UV irradiances and on improving calibration accuracy.  These efforts fall into 3 broad categories: column ozone retrievals, the Langley method of irradiance calibration, and investigation into the nature of tropospheric aerosols using optical depths at a number of wavelengths.

• Ozone retrievals - At Boulder, CO we have made an extensive comparison of column ozone retrieved using the UV-MFRSR with retrievals from collocated Brewer and Dobson spectrophotometers.  Using the Stamnes et al. (1988) radiative transfer algorithm, a lookup table is constructed that simulates the ratio of total horizontal irradiances at 332 and 305 nm.  The method is similar to those of Stamnes et al. (1991) and Dahlback (1997).  Results from a 5 month study (Slusser et al., 1999) show that the average ratio of ozone retrieved by the UV-MFRSR to that of the Brewer is 1.02 and to the Dobson 1.03.  These results are summarized in Table 1.  Work is proceeding to make daily ozone retrievals for all of the USDA sites. Work is also underway to retrieve ozone using the direct beam utilizing the 305/325 and 311/332 channels.

Table 1:  Summary of ozone retrieval comparison results

 

Ratio	UV-MFRSR/Dobson	UV-MFRSR/Brewer	Brewer/Dobson
Mean Oct-Dec '96	1.0214	1.010	1.011
Standard Deviation	0.0325	0.0189	0.0224
Mean May-June'97	1.0278	1.0372	0.990
Standard Deviation	0.0321	0.0219	0.0298

• Langley method of calibrations - The USDA is studying the practicality of UV Langley calibrations at Mauna Loa, HI (Slusser et al., 2000) as well as CPER, CO.  These high altitude sites have low turbidity and stable atmospheres which result in reproducible Langley plot voltage intercepts. 
  

  Figure 1.  Plot of the natural log of the voltage at 300 nm versus the air mass for a typical morning at Mauna Loa Observatory.

  Figure 2.  Time series of total UV optical depths measured at Big Bend, TX during 1997 and 1998.

  
  This method has several advantages over the more traditional National Institute of Standards and Technology (NIST) standard lamp calibrations such as a universally available and very stable source (to within ~0.5 % over the 11 year solar cycle) and the ability to return regular daily checks of calibration stability. 

  Difficulties arise due to changing ozone optical depth during the Langley event and the breakdown of the Beer-Lambert law over a finite bandpass with rapidly changing optical depth with wavelength.  The calibration  of the radiometers using the Langley method depends critically upon the spectral characterization (filter function) of each channel and on the wavelength and absolute calibration of the extraterrestrial spectrum chosen. Results of more than 80 Langley events using the normal shadowband as well as a direct sun tracker show agreement to within 3% of standard lamp calibrations.  Figure 1 shows a typical Langley plot.

• Variations in amount and average size of tropospheric aerosols - Aerosols are suspended atmospheric particles in the solid or liquid phase excluding cloud droplets or precipitation.  These particles are of critical importance to the hydrological cycle because they provide condensation sites upon which cloud droplets form in slightly supersaturated air. 
  In addition aerosols scatter and absorb solar radiation, changing the amount of UV reaching the earth's surface as well as modifying the heating of the atmosphere.  The USDA UV-B Monitoring and Research Program has the capability to report optical depths, a measure of the total aerosol loading, at 30 sites across the continental U.S.

This constitutes the largest network of ground-based aerosol optical depths in North America and thus provides atmospheric scientists with a unique data set with which to constrain their models which quantify precipitation processes, cloud formation, and global warming.  Figure 2 shows a time series of optical depths from Big Bend, Texas.

Each of the sites of the UVMRP Network is equipped with both a UV-MFRSR and a Visible-MFRSR which by measuring the direct beam return the total optical depths on clear days at a total of 13 wavelengths from 300 nm to 940 nm.

More about aerosol properties
 

• Bibliography

Dahlback A., Measurements of biologically effective UV doses, total ozone abundances, and cloud effects with multi-channel, moderate width filter instruments, Appl. Opt., 35, 6514-6521, 1996.

Slusser, J.R., D. Kolinski, W.F. Mou, G. Koenig, J.H. Gibson, D.S. Bigelow, Comparison of column ozone retrievals from 3 different ground-based spectral measurements, Appl. Opt., 38, 1543-1551, 1999.

Slusser, J.R., J.H. Gibson, D.S. Bigelow, D. Kolinski, P. Disterhoft, K. Lantz and A. Beaubien, Langley method of calibrating UV filter radiometers, J. Geophys. Res. 105, 4841-4849, 2000.

Stamnes K., S.C. Tsay, W. Wiscombe and K. Jayaweera, Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media, Appl. Opt., 27, 2502-2509, 1988.

Stamnes K., J.R. Slusser, and M. Bowen, Derivation of total ozone abundance and cloud effects from spectral irradiance measurements,   Appl. Opt., 30, 4418-4426, 1991.