In two consecutive studies, the suitability of different experimental set-ups for detecting and measuring the emission of infrared-active gases is investigated, as this is of particular importance for understanding the atmospheric greenhouse effect.
The first part presents a horizontally arranged Styrofoam box, as described occasionally in the literature for such experiments. The gases are slightly heated by a heater at the bottom of the container, and the radiation—emitted by the infrared-active gases—is detected through an infrared-transparent window at the side. However, this arrangement is only suitable for demonstrating the positive or negative greenhouse effect with very strong greenhouse gases such as Freon. For weaker greenhouse gases like CO2 or CH4, the gas radiation can only be inadequately distinguished from the broadband background radiation of the container walls. In addition, the supplied heating power creates a circular air flow, which further complicates detection. Nevertheless, these investigations provide important clues for a significantly improved setup in the form of a vertical aluminum cylinder, which can be used to detect infrared emission under conditions such as those observed in the lower troposphere.
For a gas layer above a cooler surface, as this occurs during inversion weather conditions or can be observed in the Arctica and Antarctica during winter months, the radiation emitted by the surface and partially absorbed by the warmer gas layer is lower than the radiation emitted by the gas upward. This is known as negative greenhouse effect, which, despite the limited detection sensitivity, can be simulated for the first time in the laboratory with the presented set-up.