FED/MAC Data

Aerosol Optical Thickness

Measurement of atmospheric attenuation and hence estimate of the aerosol optical thickness were made in the Northern Experimental Forest in Howland, Maine, with sunphotometers. This parameter is useful in calibration and correction of other measurements made with remote sensing instruments at FED sites.

Data Source

The eight channel sun-photometer named SXM-2 (440, 522, 613, 672, 781, 871 and 1030 nm with 10 nm FWHM) was used on the ground. It follows the sun automatically using a 4 quadrant detector.

Spatial Resolution

Spatial resolution is a function of the solar zenith angle and as such is not pertinent to measurements by a sunphotometer. For very accurate measurements however, it can be estimated to be approximately 10 * sin(zenith angle) in km. Thus for 30 degrees, it is 5 km. Meteorological parameters determine the effective spatial resolution.

The time of optical thickness data acquisition was in September, 1990. SXM-2 instrument cycles through a set of 8 filters in about 20 seconds and the frequency of sampling can be varied from once every 4 minutes to one-half minute.

Instrument Description

Mission Objectives - To measure the aerosol optical thickness using a multi-channel sunphotometer. This is part of the Correction and Calibration effort for FED, and will allow the estimation of the atmospheric effects on the transmitted and reflected radiation so that the appropriate correction schemes can be employed to infer reflectivity of the ground from aircraft radiometric data.

Principles of Operation - The SXM-2 automatically tracks the sun. The detector is a silicon photodiode detector which is kept at a constant tempera- ture. The instrument has 1.5 degree field-of-view. Data can be acquired once every 1/2 min or 4 min onto data cassettes or a computer. 4.4 Instrument Measurement Geometry. The field-of-view (FOV) for SXM-2 is 1.5 degrees.

Data Processing Sequence

Processing Steps and Data Sets. To compute the total optical thickness, use the formulae given in the next section to compute the Rayleigh opt. thk. and ozone opt. thk (if any) and add them to the aerosol optical thickness. For channels including the water absorption band at 940 nm, optical thickness due to water absorption must be taken into account.

Derivation Techniques/Algorithms - There are many references for formulae to reduce solar intensity measurements to obtain the aerosol optical thickness. One such reference is by King and Byrne, 1976.

        From pressure values obtain Rayleigh optical thickness:
        (Hansen and Travis, 1974)

        tau_R = (p / p0) (0.008569 / L^4) (1 - 0.0113 / L^2 - 0.00013 / L^4)

        where, p is the measured surface pressure, p0 is pressure
        at sea level and L is the wavelength in micro meters.
        The ozone optical thickness is obtained by multiplying the
        column abundance in Dobson units (matm - cm) with the
        absorption coefficient given in the following table
        and dividing by 1000.

                     Ozone Absorption Coefficients
         _____________________________________________________
          Wavelength                  Absorption Coefficient
             nm                           atm^(-1) cm^(-1)
         _____________________________________________________
             368                          6.18 E-4
             380                          3.22 E-4
             441                          3.36 E-3
             500                          3.28 E-2
             522                          4.80 E-2
             551                          9.73 E-2
             613                          1.19 E-1
             640                          2.89 E-2
             671                          4.55 E-2
             781                          4.61 E-3
             872                          6.17 E-4
             945                          0.00 E-0
            1030                          0.00 E-0
         _____________________________________________________

The precision of SXM-2 is quite at 4 x 10-4 in some channels. The accuracy is determined by the accuracy in calibration which is estimated to be 1% transl- ating to +/- 0.01 in aerosol optical thickness at 1 airmass.

Errors can arise (i) while calibrating and (ii) while measuring. During calibration, a requirement of the Langley plot method is that the atmospheric optical thickness remains constant during the period of maximum airmass change. At mountain sites this requirement is usually met on clear days but not always. The effect of varying atmospheric conditions is reflected in the quality of the Langley plots (see Halthore and Markham, 1992). The calibration coefficient which is the y-intercept in Langley plots can be obtained to a consistency better than 1% and we take this as the uncertainty in our measure- ments. A 1% uncertainty in Vo translates to an uncertainty in the aerosol optical thickness of 0.01 at airmass of 1.

The equation is:

          Delta(tau_aerosol) = (1 / m)(Delta Vo / Vo).

Uncertainties in the Rayleigh optical thickness and ozone optical thickness are negligible for the conditions encountered in FED.

For sunphotometers that do not employ constant temperature detectors, a major source of variation and hence uncertainty arises due to inadequate temperature compensation for the response. Furthermore, for sunphotometer that do not employ auto-tracking or peak hold features, another major source of uncertainty arises due to imperfect pointing. For measurements reported here, these are not expected to contribute to absolute errors in the measurements of aerosol optical thickness.

Notes

Aerosol optical thickness is highly variable in space and time and an accurate estimate is made by measuring the direct solar radiance at the ground by the use of a calibrated sunphotometer. Solar radiation reaching the ground is attenuated by absorption and scattering by atmospheric molecules as well as particles. Molecular scattering (Rayleigh scattering) is estimated by measuring the pressure. Molecular absorption is avoided by an appropriate choice of band passes.
Calibration is important and is performed at high altitudes where the sunphotometer is above most of the atmospheric water vapor. Since the aerosol growth rate is tied to the amount of water in the atmosphere, the aerosol optical thickness at the mountain sites especially in the early mornings can be expected to be a constant. Under such conditions the response of the sunphotometer during a rapid change in airmass yields a fictitious exoatmospheric response which is the calibration coefficient. The calibration procedure is known as the Langley plot method.
Calibration can also be performed by comparing responses with a better calibrated instrument. Alternatively, responses can be monitored continuously by comparing responses with a number of sunphotometers.

Data Available:

Data File