Derived Products

By applying the techniques and algorithms developed in recent years to the measurements, various useful products can be derived. Examples of the derived products included in our database are optical depth, daily column ozone, and UV-B

irradiance with 1 nm spectral resolution. Using the synthetic spectrum algorithm discussed below, various agriculturally significant indices are derived, such as the Caldwell and Flint biologically weighted irradiances. An estimate of the vitamin D dosage is available, along with erythemally weighted irradiance and the closely related UV index. In addition to these products routinely available on the UVMRP website, numerous products are processed on demand for users in the agricultural, medical, and industrial materials communities.

8.5.1 Optical Depth

Optical depths are regularly acquired using measured spectral irradiances. These measurements are useful in developing regional aerosol climatology, validating satellite aerosol observations, and offering atmospheric corrections for satellite retrievals. Aerosol optical depth may also serve as a good indicator of surface visibility (Hand et al., 2004).

8.5.1.1 Instantaneous Optical Depth

Total and aerosol plus cloud optical depths are retrieved at 3-min intervals from the measurements of spectral irradiance. The total optical depths are derived using the Beer-Lambert law, ln Vax- ln V.

m where tx is the total optical depth and m denotes the air mass, which is a function of SZA. Vx is the measured raw voltage value from the channel centered at wavelength A. The value of V0,x for each channel and the requested date are determined from a time series of Langley-generated voltage intercepts for morning periods of that day as discussed above. The Langley analysis is performed based on the objective algorithms developed by Harrison and Michalsky (1994), as discussed in Section 8.4.3.

For clear sky data, Rayleigh and ozone optical depths are subtracted from total optical depths to obtain the aerosol optical depths. Methods to accurately estimate Rayleigh optical depths in the atmosphere have been well documented in Fröhlich and Shaw (1980), Young (1981), Teillet (1990), Bucholtz (1995), and Bodhaine et al. (1999). We use the following formula to compute the Rayleigh optical depth for simplicity (Marggraf and Griggs, 1969; Stephens, 1994):

z,, = 0.00882(-415+0-)e<-01188z-°.°0116 z2), (8.7)

where r^y,z,x represents the Rayleigh optical depth at the altitude z (in km) and the wavelength is in the units of microns (^ m). This formula is valid under the assumption that the variation of air density with altitude follows the variation of pressure with altitude (Stephens, 1994). Ozone optical depth is calculated by:

where Q is total column ozone in Dobson units, is the ozone cross section, and cO = 0.001 is a conversion constant. For UV wavelengths, the effective ozone cross sections are used (Bigelow et al., 1998). For visible wavelengths, the ozone absorption coefficients of Shettle and Anderson (1995) are used. Aerosol optical depth for clear days at 3-min intervals is then calculated by subtracting the Rayleigh optical depth and ozone optical depth from the total optical depth. These calculations are included in the UVMRP database. Note that aerosol optical depths are not available on cloudy days.

8.5.1.2 Average Optical Depth

The averaged optical depths for the mornings and/or afternoons are calculated from the instantaneous results. To ensure the accuracy of the results, time periods included in the averaging process are limited to specific air mass ranges which vary by wavelength: 1.5 -3.0 for 332 nm and 368 nm, and 2.0 -6.0 for 415 nm- 860 nm.

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