Is ozone a reliable proxy for molecular oxygen?

In the search for life in the Universe, molecular oxygen (O 2 ) combined with a reducing species, such as methane (CH 4 ), is considered a promising disequilibrium biosignature. In cases where it would be difficult or impossible to detect O 2 (such as in the mid-IR or low O 2 levels), it has been suggested that ozone (O 3 ), the photochemical product of O 2 , could be used as a proxy for determining the abundance of O 2 . As the O 2 –O 3 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 O 3 to infer O 2 . We used photochemistry and climate modeling to further explore the O 2 –O 3 relationship by modeling Earth-like planets with the present atmospheric level (PAL) of O 2 between 0.01% and 150%, along with high and low CH 4 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 (HO x ), which destroys O 3 through catalytic cycles, and acts as a catalyst for the smog mechanism of O 3 formation in the lower atmosphere. We find that varying CH 4 causes changes to the O 2 –O 3 relationship in ways that are highly dependent on both the host star and O 2 abundance. A striking result for high CH 4 models in high O 2 atmospheres around hotter hosts is that enough CH 4 is efficiently converted into H 2 O to significantly impact stratospheric temperatures, and therefore the formation and destruction rates of O 3 . Changes in HO x have also been shown to influence both the HO x catalytic cycle and production of smog O 3 , causing variations in harmful UV reaching the surface, as well as changes in the 9.6 μm O 3 feature in emission spectra. This study further demonstrates the need to explore the O 2 –O 3 relationship in different atmospheric compositions in order to use O 3 as a reliable proxy for O 2 in future observations.