Sunphotometry at the High Altitude Research Station Jungfraujoch, 3580 m a.s.l.

Wouter Knap, Ed Worrell, Piet Stammes
Royal Netherlands Meteorological Institute, KNMI

1. Introduction

The aerosol optical thickness (AOT), which is directly related to the atmospheric aerosol load, is the principal variable describing the effect of aerosols on radiative transfer in the earth’s atmosphere. The level of understanding of this effect is rather low, which is one of the reasons why the current state of the climate is not well understood and why predictions of the future climate are uncertain.

Traditionally, the AOT is derived from direct solar irradiance measurements with a sunphotometer. Figure 1 shows our six-channel sunphotometer (model SPUV-6, YES, inc), as it is operational in Cabauw, The Netherlands. The central wavelengths are: 368, 501, 675, 780, 871, and 940 nm (the latter is not used for aerosol research). The aims of our project at Jungfraujoch are: (1) outdoor calibration of our sunphotometer, (2) AOT comparisons with values derived from the Precision Filter Radiometers of the Swiss Atmospheric Radiation Monitoring program (CHARM). The experiment will take place in August-October 2003.

Figure 1. The six-channel SPUV sunphotometer,
here shown in Cabauw, The Netherlands.

2. Principle of calibration

The solar irradiance incident at the top of the earth’s atmosphere, Io, is attenuated by absorption and scattering by various molecules and aerosols (and clouds, but these are not considered here). The result is a reduced irradiance at the surface, Is. On the basis of Bouguer’s Law, the transmissivity T of the atmosphere can be written as

where t is the total atmospheric optical thickness and m is the (relative) air mass which, in approximation, equals 1/cosqo (qo is the solar zenith angle). This equation implies that when the sunphotometer signal is plotted on a logarithmic scale as function of the relative air mass, a straight line fit is produced. This graph is referred to as a Langley plot. The slope of the line equals the value of the total atmospheric optical thickness, t. For the SPUV wavelengths (apart from 940 nm) t is almost entirely due to Rayleigh scattering, ozone absorption, and aerosol scattering/absorption. So we can write

where tR, tO3 and taer are the optical thicknesses due to the three processes mentioned. If a Langley plot has been constructed from direct solar radiation measurements, the aerosol optical thickness can be derived by substracting model calculations of tR and tO3 from t.

Figure 2 shows an example of measurements made with our sunphotometer in The Netherlands. The Langley plot in the right panel shows that if the cloudy measurements are filtered out (the red data points), the measurements obey to the expected exponential behaviour.

Figure 2. Example of direct solar irradiance measurements, made in The Netherlands,
as a function of time (left panel) and air mass (right panel; the Langley plot). The
shaded area in the left plot indicates the time interval used for the Langley analysis.
The red data points correspond to cloudy measurements or other irregularities that
are left out in the Langley analysis. Co is the Extraterrestrial Constant. The black line
in the Langley plot shows how a Langley regression may look like at Jungfraujoch. The
small slope is due to the small optical thickness at Jungfraujoch, which adds to the
precision of Co.

No matter the value of the optical thickness, there is one invariant in the Langley plot: the value of Co (interception with the y-axis). Since it is the uncalibrated value of the sun’s irradiance at the top of the atmosphere, Io, it is also referred to as Extraterrestrial Constant. Its value is unique for each sunphotometer and channel. If it is accurately known, we need only a single measurement of the direct solar irradiance to fix the slope of the Langley plot and to derive the aerosol optical thickness. The determination of the Extraterrestrial Constants for all SPUV channels is the primary objective of our project at Jungfraujoch.

3. Why calibrating at the Jungfraujoch?

The precision of the Extraterrestrial Constant Co can be increased by reducing (1) the slope of the Langley regression, (2) the scatter in the Langley plot. These reductions are accomplished when the direct solar irradiance measurements are made in a place where the optical thickness is small (reducing the slope) and invariable (reducing the scatter). Precisely these conditions can be met at a high-altitude site such as Jungfraujoch, in particular when the site is situated in the free troposphere and only background aerosol concentrations are found. Under these conditions we expect that we can increase the precision of our sunphotometer calibration by a factor of 10, as compared to a Langley calibration in The Netherlands.

4. Collaboration with other institutes

MeteoSwiss in Payerne operates at the Jungfraujoch the Precision Filter Radiometers of the CHARM network. These instruments were designed at PMOD/WRC and make measurements for the full set of 12 wavelengths recommended by the WMO (368 to 1024 nm). The 6 wavelengths of the SPUV coincide with the PFR wavelengths, so we will make a direct comparison of SPUV- and PFR-derived aerosol optical thicknesses. Simultaneous to the SPUV measurements, Kipp & Zonen will measure with their POM-01 solar radiometer, which allows for an extra independent comparison.

Several other research groups use the unique location of the Jungfraujoch for aerosol research. The Paul Scherrer Institute and the Swiss Federal Laboratory for Material Research and Testing take a leading role in measuring aerosol properties for the GAW Aerosol Programme. Many parameters are measured on an automated basis, such as the light absorption and scattering coefficient, total aerosol mass, number concentration, and ionic concentration. Although our experiment focuses primarily on instrument calibration and comparison, these parameters are extremely useful for a more scientific interpretation of the measurements, with the help of radiative transfer calculations.

Acknowledgements

We greatly ackowledge the hospitality we experienced at the Research Station, and we thank Prof. Erwin Flueckiger, Mrs. Louise Wilson, Mr. and Mrs. Martin Fischer, and Mr. and Mrs. Hemut.

Contact

If you have questions please contact Wouter Knap.

Results and pictures

Preliminary results: click here

Pictures: click here