Many of us probably remember learning about greenhouse gases in middle school. The one that first comes to mind is likely carbon dioxide. But it is only one of several primary greenhouse gases. Water vapor, methane, nitrous oxide, and ozone are all on the list as well.

But what is the determining factor of whether a gas is a greenhouse gas? A greenhouse gas is a gas in the atmosphere that absorbs and emits radiation within the thermal infrared range. In other words, first it must be able to absorb radiation. Traditionally, to absorb radiation a molecule must have a dipole moment; this means that the positive and negative charges are not equally balanced in the molecule. For example, water consists of a negatively charged oxygen and two positively charged hydrogens, and like a magnet, it has a negative end and a positive end. As a counter example, hydrogen gas, H₂, has no dipole moment because the charge distribution is symmetric, with an equal number of positive and negative charges between the two identical atoms.

However, it is possible to create a temporary dipole moment in a molecule, which normally would have a symmetric charge distribution. In the atmosphere, gaseous molecules are constantly moving around, which can cause them to collide. When two molecules collide, such as water and oxygen (O₂), the dipole moment in the water molecule induces a dipole in oxygen. In other words, the dipole moment creates a field which polarizes the oxygen molecule. This is like when you place a magnet in a pile of iron filings; the magnet induces a field which affects the iron filings. This overall phenomenon leads to an effect known as collision-induced absorption — the nontraditional absorber can now absorb radiation due to its induced dipole.

Collision-induced absorption is very weak compared to traditional absorption. Still, given the high concentration of oxygen in the atmosphere (21%), even this weak greenhouse gas can have a significant effect on the earth’s energy budget.

To investigate these collision complexes, the C. C. Jarrold Group in the Chemistry Department at Indiana University is studying complexes that involve oxygen and volatile organic compounds. In their most recent publication, the volatile organic compounds: hexane, benzene, and isoprene were specifically studied. Their results showed what the collision complexes may look like and quantified the effect the volatile organic compound has on the electronic structure of oxygen. The sum of their work helps to inform our understanding of the role these collision complexes have in the balance of the energy budget.

Collision-induced absorption is not limited to oxygen-containing complexes. Nitrogen, hydrogen, and methane have also been determined to be collision-induced absorbers which affect the earth’s energy budget. Although it is not plausible for us to eliminate all collision partners from the atmosphere, such as water, many partners, such as volatile organic compounds, stem from anthropogenic sources such as paints, cleaning products and fossil fuels. If we can reduce their release, we should be able to decrease the contribution of collision-induced absorption toward the greenhouse effect.

Edited by: Elizabeth Rosdeitcher and Taylor Nicholas