Australia Telescope National Facility

Infrared and Sub-millimetre Observing Conditions on the Antarctic Plateau

Marton G. Hidas, Michael G. Burton, Matthew A. Chamberlain, John W.V. Storey, PASA, 17 (3), 260.

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The effect of aerosols

Chamberlain et al. (2000) found that the modelled flux in the dark windows of the mid-IR (8.2 - 9.2 $\mu$ m and 10.2 - 12.2 $\mu$ m) was strongly dependent on the amount of aerosols included in the model. One of MODTRAN's standard aerosol types, ``radiative fog'', was used in these models.

To investigate the effect of aerosols on the transmission and emission, models with different visibilities were calculated. Starting with that used above for 164 $\mu$ m of precipitable water vapour (see Fig. 1), increasing amounts of ``radiative fog'' were added, corresponding to visibilities of ``clear'' (no aerosols), then 100, 50, 10, 5 and 1km. Figures 7-10 show the resulting spectra in the same wavelength ranges as before. Table 2 and Table 3 give some numerical values for the sky flux and transmission for visibilities of 1, 10, 100km and ``clear'', averaged over several wavelength regions of interest.

**Figure 7:** Emission and transmission spectra for models with varying visibility of aerosols, as labelled (no aerosols, 100, 50, 10, 5 & 1km), in the near-IR from 2-6 $\mu$ m. The model has 164 $\mu$ m of precipitable water vapour, as in Fig. 1.
$\begin{figure} \begin{center} \psfig{file=2to6aero.eps,width=16cm}\end{center}\end{figure}$

**Figure 8:** As for Fig. 7, emission and transmission for models with varying visibilities, in the mid-IR from 5-15 $\mu$ m (note the difference in flux range).
$\begin{figure} \begin{center} \psfig{file=5to15aero.eps,width=16cm}\end{center}\end{figure}$

**Figure 9:** As for Fig. 7, emission and transmission for models with varying visibilities, in the mid-IR from 15-60 $\mu$ m (note the difference in flux range).
$\begin{figure} \begin{center} \psfig{file=15to60aero.eps,width=16cm}\end{center}\end{figure}$

**Figure 10:** As for Fig. 7, emission and transmission for models with varying visibilities, in the far-IR and sub-mm from 50-500 $\mu$ m (note the difference in flux range and transmission range).
$\begin{figure} \begin{center} \psfig{file=50to500aero.eps,width=16cm}\end{center}\end{figure}$

For a visibility of 1km, the sky is almost a perfect greybody at all wavelengths, with a temperature of approximately $-43^{\circ}$ C (which is the temperature at the top of the inversion layer). In some cases where the sky is completely optically thick the flux is actually less than this level, but this corresponds to path lengths so short they remain close to ice level, where the temperature is some 30 $^{\circ}$ C lower.

For wavelengths generally longer than 50 $\mu$ m, aerosols have negligible effect both on the transmission and the emission. Since the sky is always rather optically thick here, the emission spectrum is already close to that of a blackbody at the temperature of the lower atmosphere. Therefore the aerosols, which are additional emitters at the same temperature, cannot increase the flux significantly. This is also the case in the optically thick parts of the mid-IR spectrum (in the water, ozone and carbon dioxide bands, at 7.8, 9.3 and 14.0 $\mu$ m, respectively).

However, in windows at wavelengths shorter than 50 $\mu$ m, aerosols have a considerable effect on the emission and transmission. In the near-IR, visibilities greater than 10km are needed for good observing conditions. The transmission is no better than 60% for a visibility of 10km, with the flux levels close to their black body value. In the mid-IR windows, while flux levels increase significantly from their lowest values for a visibility of 10km, the transmission itself is not seriously degraded unless the visibility falls below 5km. When the visibility is 50km or better then the mid-IR windows at 8-9, 10-12 and around 20 $\mu$ m are particularly clear.

The aerosols have been modeled as ``radiative fog''. We note, however, that they do not necessarily correspond to any physical aerosol observed in the Antarctic atmosphere. The aerosols affecting observing conditions in Antarctica are suspected to consist of ice crystals (``diamond dust'') within the inversion layer, or possibly within cirrus clouds. However, very little is currently known about the exact properties, origin and location within the atmosphere of these particles.

Nevertheless, it is clear from the above discussion that the effect of aerosols in the sub-mm will be insignificant, regardless of their exact nature. In addition, the ice crystals (if indeed that is what the aerosols are) have physical dimensions of order 10 $\mu$ m, so at sub-mm wavelengths will behave as simple scatterers. Smith & Harper (1998) reported a small increase in flux levels at 10 $\mu$ m during a short period of visible diamond dust during daytime measurements. Van Allen et al. (1996) also note ever-present levels of blowing snow from measurements they made at the South Pole with a Fourier transform spectrometer. If these are the main contributors to the aerosols then on the summit of the plateau, where wind levels are lower, the visibility would generally be expected to be higher than at the Pole and therefore the site would be superior for near-IR and mid-IR observations. At the present time, no IR measurements have been made on the higher plateau, although Valenziano & Dall'Oglio (1999) report on good sub-mm conditions at Dome C.

Next Section: Conclusions
Title/Abstract Page: Infrared and Sub-millimetre Observing
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Contents Page: Volume 17, Number 3

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