Multi-Filter Rotating Shadowband Radiometer (MFRSR)

As of April 2000, a Yankee Environmental Systems (YES) multi-filter rotating shadowband radiometer (or MFRSR) has been making measurements of sunlight on the roof of Reines Hall on the University of California at Irvine campus. These measurements are used to derive (vertically integrated column) ozone and aerosol above UCI.

Background

MFRSR Background

Photographs of UCI MFRSR

RSR Measurements and Analysis

ASRS Solar Group's MFRSR Page

Daily plot of direct beam irradiance

These are currently uncalibrated.

Ozone time series from May 12, 2000

This shows ozone column in Dobson Units, DU (1 DU=2.7x10¹⁶molecules/cm²), as a function of local time (about 8:30am to 4:30pm). The measurement indicate roughly 315 DU (compared with 307 DU from the TOMS satellite)

The Multi-Filter Rotating Shadowband Radiometer

The MFRSR has seven diodes which measure downwelling visible and near-IR solar irradiance. Six of these diodes are filtered such that transmission is limited to a 10 nm window; the seventh measures the entire spectrum over which silicon is sensitive. The six interference filters are centered at (nominal wavelengths only):

415 nm (aerosol and Rayleigh)
499 nm (ozone, aerosol, and Rayleigh)
614 nm (ozone, aerosol, and Rayleigh)
670 nm (ozone, aerosol, and Rayleigh)
868 nm (aerosol and Rayleigh)
936 nm (water vapour, aerosol, and Rayleigh)
broadband

This set of wavelengths are sensitive to ozone absorption, water vapour absorption, aerosol scattering, and Rayleigh scattering, as indiated above.
An initial measurement of both the diffuse (or scattered) and the direct solar beam is made. The shadowband is then rotated to block out the direct solar beam so that a diffuse measurement can be made. A measurment of the direct solar beam is obtained by subtracting these two.

MFRSR Photographs

The MFRSR is located on the roof of Physcial Sciences II on the University of California, Irvine campus (33.68^oN, 117.75^oW) Official Homepage

Analysis and Retrieval of Aerosol and Ozone

The means by which the vertical column of ozone and aerosol is retrieved is Langley analysis. The direct solar beam as it passes through our atmosphere is attenuated according to the Beer-Bouguer-Lambert law (a.k.a. Beer's law),

I = I_o exp(-OD^.AMF)

where AMF is the airmass factor which describes the enhancement of the slant path over the vertical, and OD is the vertical optical depth of the atmosphere. Taking the natural logarithm of both sides,

ln(I) = ln(I_o) - OD^.AMF

which is of the form

y = a + bx

such that a=ln(I_o) represents the y-intercept and b=OD represents the slope. Thus, if the AMF is plotted versus the logrithm of the irradiance (at a given wavelength) for many data points throughout a morning or afternoon, a straight line should result. Linear regression performed on this data will give the slope and intercept, which amount to the logrithm of the extra-terrestrial irradiance and the vertical optical depth. The vertical optical depth of the atmopshere includes components from molecular and aerosol scattering and, depending on the channel, may also include absorption by ozone or water vapour.

The airmass factor can be determined from the solar zenith angle (which is a funciton of latitude, season, and time of day). If SZA represents the solar zenith angle, then the airmass factor is,

AMF = [cos(SZA)]^-1,

as long as the sun is fairly high in the sky (i.e., SZA<70^o; AMF<3). Otherwise, a modified expression should be used which accounts for the sphericity of the Earth,

AMF = [cos(SZA)+0.50572(1.46468-SZA)^-1.6364]^-1,

where SZA is in radians (Kasten and Young, 1989).

Ozone & Aerosol Optical Depth

The primary purpose of sunphotometers is measurement of the aerosol optical depth, which is calculated through,

OD_aer=OD_total - (OD_ray+OD_O₃+OD_H₂O)

We first use a channel (or wavelength) in which there is no significant absorption by O₃ and H₂O (i.e., OD_O₃=OD_H₂O=0). The two channels are 415 and 868 nm. The Rayleigh optical depth can be calculated based solely on the pressure of the measurement site, P, using,

OD_ray= 0.008569w^-4(1+0.0113w^-2+0.00013w^-4) P/P_o,

where P_o=1013.24 mbar is standard surface pressure and wavelength is in nm (Hansen and Travis, 1974).

The method used to analysis these data is that of Alexandrov et al. which solves for aerosol optical depth, ozone column, NO₂ column,

References

1. Bigelow, D. S., J. R. Slusser, A. F. Beaubien, and J. H. Gibson, The USDA ultraviolet radiation monitoring program, Bulletin of the American Meterological Society, 79, 601-615, 1998.
2. Harrison, L., J. Michalsky, and J. Berndt, Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements, Applied Optics, 33, 5118-5125, 1994.
3. Harrison, L. and J. Michalsky, Objective algorithms for the retrieval of optical depths from ground-based measurements, Applied Optics, 33, 5126-5132, 1994.
4. Hansen J. E. and L. D. Travis, Light scattering in planetary atmospheres, Space Science Reviews, 16, 527-610, 1974.
5. Kasten and Young, Revised optical air mass tables and approximation formula, Applied Optics, 28, 4735-4738, 1989.
6. Alexandrov, M. D., A. A. Lacis, B. E. Carlson, and B. Cairns, Remote sensing of atmospheric aerosols, nitrogen dioxide, and ozone by means of multi-filter rotating shadow-band radiometer, in Remote Sensing of Clouds and Atmosphere, J. Russell and C. Serio, Editors, Proc. SPIE 3867, 156-170, 1999.

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This page was last updated 19-May-2000.