E.3 Atmosphere: Air Quality

• The range in projections of air quality (ozone and PM2.5 17 in near-surface air) is driven primarily by emissions (including CH4), rather than by physical climate change (medium confidence). There is high confidence that globally, warming decreases background surface ozone. High CH4 levels (as in RCP8.5) can offset this decrease, raising background surface ozone by year 2100 on average by about 8 ppb (25% of current levels) relative to scenarios with small CH4 changes (as in RCP4.5 and RCP6.0) (high confidence). {11.3}

• Observational and modelling evidence indicates that, all else being equal, locally higher surface temperatures in polluted regions will trigger regional feedbacks in chemistry and local emissions that will increase peak levels of ozone and PM2.5 (medium confidence). For PM2.5, climate change may alter natural aerosol sources as well as removal by precipitation, but no confidence level is attached to the overall impact of climate change on PM2.5 distributions. {11.3}