Introducing our newest addition to PVL, Dr. Paul Godin who comes to us from the University of Toronto! Paul specialized in laboratory-based research in the atmospheric sciences, and will now apply that strong base to the atmospheres of other worlds. The image above is from the Intergovernmental Panel on Climate Change and depicts IR absorptions both of the atmosphere (top) and of select molecules (bottom).
by Paul Godin
Hello World! My name is Paul and I’m the newest member of the PVL, so I should probably introduce myself, eh? I just completed my PhD in physics at the University of Toronto studying the radiative impacts of several chemicals on the atmosphere, using a metric known as a global warming potential (GWP). A GWP is the measure of the radiative forcing of a pulse emission of one kilogram of gas over a defined period of time (commonly taken to be 100 years), relative to an identical pulse emission of carbon dioxide. Radiative forcing is defined as the net change of radiation at the tropopause; positive radiative forcing means more radiation directed towards the surface (leading to higher surface temperatures), whereas negative radiative forcing corresponds to a net cooling effect.
The radiative forcing of a molecule depends largely on two main factors, the absorption spectrum of the molecule and the absorption profile of the atmosphere. The absorption spectrum of a molecule is a result of the quantum mechanical interactions within the molecule, thus the structure and composition of a molecule will dictate at what wavelengths of light the molecule can absorb. The atmospheric absorption spectrum is the sum of the absorption spectra of all the species present in the atmosphere (largely made up of water, carbon dioxide, ozone, nitrogen, etc.). The atmospheric absorption spectrum for the infrared (wavelengths associated with outgoing radiation) is shown in the top half of the figure at the start of this article. As can be seen, the atmosphere normally absorbs a significant fraction of outgoing radiation, but also has a region where it doesn’t naturally absorb radiation (8-13 μm), which is known as the atmospheric window. This is great for life on Earth; we need to trap some of the radiation to keep the planet from being frozen, but also allows enough heat escape that we don’t turn in to a furnace (i.e. Venus).
Problems arise if we emit gases into the atmosphere that absorb in the atmospheric window. Now what was once an open path to outer space for radiation no longer exist, instead the radiation is trapped and contributes to global warming. The lower half of the Figure shows the absorption spectra of a few man-made fluorine containing molecules. The stretching of the carbon-fluorine bonds in these molecules is what causes them to absorb strongly in the atmospheric window. Therefore, halogenated species are often considered greenhouse gases, since their absorption patterns efficiently block outgoing radiation, trapping in on Earth, leading to positive radiative forcing and increased global warming.
The last part of the GWP that hasn’t been discussed yet is atmospheric lifetime. Emissions into the atmosphere don’t stay there forever, eventually they are removed via a variety of mechanisms from chemical reactions to simple transportation back to the ground. Thus, a molecule with a high radiative forcing may not have a large GWP if it’s lifetime is short; conversely a weakly absorbing molecule could have a large GWP if it has a long lifetime.
GWP are useful for policy makers as they can help them decide which emissions should be controlled to prevent global warming in the future. A GWP doesn’t say much about how damaging a species is currently, but rather helps predict what can happen if emissions aren’t regulated.
In order to contribute to the study of GWP, I made measurements of absorption spectra of select molecules using laboratory absorption spectroscopy. Laboratory absorption spectroscopy is preformed by filling a temperature controlled vacuum chamber with a single species of gas, then passing a laser beam through and looking at the absorption pattern of the light. These absorption cross-sections can then be used along with literature values of the lifetime and the atmospheric spectrum to dervie new GWP values.
My experience with laboratory spectroscopy is what lead me to joining the PVL. My project with the PVL is to develop a camera system that can detect frost on the moon in permanently shadowed regions (PSR) using reflected starlight. Since PSR don’t receive sunlight, simulations have shown that water molecules can become trapped in them and should form frost deposits. Unfortunately, due to being permanentely shadowed, it’s difficult to view into them and confirm that there are indeed deposits of water ice. While PSRs don’t receive sunlight, they do receive light from other stars. The type of starlight we plan on using is the Lyman-α line in the UV (121.6nm).
The project in the lab is to use a vacuum chamber to simulate the atmosphere of the moon. Inside this simulation chamber, frost will be grown on simulated lunar regolith. A UV lamp will supply the “starlight”, and it’s reflection off of the frost will be detected by a CCD camera. With the exception of the CCD camera, all other components have been obtained, and hopefully a suitable camera will be found soon.