Is ozone a reliable proxy for molecular oxygen?

In the search for life in the Universe, molecular oxygen (O2) combined with a reducing species, such as methane (CH4), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O2 (such as in the mid-IR or low O2 levels), it has been suggested that ozone (O3), the photochemical product of O2, could be used as a proxy for determining the abundance of O2. As the O2–O3 relationship is known to be nonlinear, the goal of this series of papers is to explore how it would change for different host stars and atmospheric compositions and learning how to use O3 to infer O2. We used photochemistry and climate modeling to further explore the O2–O3 relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O2 between 0.01% and 150%, along with high and low CH4 abundances of 1000% and 10% PAL, respectively. Methane is of interest not only because it is a biosignature, but it is also the source of hydrogen atoms for hydrogen oxide (HOx), which destroys O3 through catalytic cycles, and acts as a catalyst for the smog mechanism of O3 formation in the lower atmosphere. We find that varying CH4 causes changes to the O2–O3 relationship in ways that are highly dependent on both the host star and O2 abundance. A striking result for high CH4 models in high O2 atmospheres around hotter hosts is that enough CH4 is efficiently converted into H2O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O3. Changes in HOx have also been shown to influence both the HOx catalytic cycle and production of smog O3, causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O3 feature in emission spectra. This study further demonstrates the need to explore the O2–O3 relationship in different atmospheric compositions in order to use O3 as a reliable proxy for O2 in future observations.