Effect of Co 2 Inhibition on Biogenic Isoprene Emission: Implications for Air Quality 1 under 2000-to-2050 Changes in Climate, Vegetation and Land Use 2 3

• Effect of CO 2-isoprene interaction on air quality by year 2050 is simulated 17 • CO 2-isoprene interaction reduces ozone and aerosol sensitivity to climate 18 • Human land use change will become a key driver for future air quality 19 20 Abstract 21 The inhibition of biogenic isoprene emission by elevated CO 2 as observed in 22 many plant taxa may significantly alter the sensitivity of air quality to global changes. 23 We use a coupled modeling framework to perform simulations under various 24 combinations of 2000-to-2050 changes in climate, natural vegetation, anthropogenic 25 emissions and land use to examine the effect of the CO 2-isoprene interaction on 26 atmospheric composition. We find that consideration of CO 2 inhibition substantially 27 reduces the sensitivity of surface ozone and secondary organic aerosol (SOA) to climate 28 and natural vegetation, resulting in much smaller ozone and SOA increases in major 29 populated regions than are projected by previous studies. The impact of land use on air 30 quality is relatively insensitive to CO 2 inhibition, rendering land use change the key 31 factor that can offset or enhance the effects of anthropogenic emissions and shape air 32 quality and climate-relevant species in the mid-21 st century.