The phrase "ozone concentrations" refers to the amount or level of ozone gas present in a specific area or environment. It indicates how much ozone is there in the air we breathe. Full definition
That increase will have a disproportionately large impact on vegetation because ozone concentrations in many locations will rise above the critical level where adverse effects are observed in plants and ecosystems. (news.mit.edu)
High ozone concentrations typically occur outside of the cities and represent a risk factor for the health of rural population as well as for agricultural production. (sciencedaily.com)
Note that increased methane also leads to increased ozone concentrations, for added radiative forcing bang for your methane buck. (realclimate.org)