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The Community Atmosphere Model (CAM), version 5, is now coupled to extensivetropospheric and stratospheric chemistry, called CAM5-chem, and is availablein addition to CAM4-chem in the Community Earth System Model (CESM) version1.2. The main focus of this paper is to compare the performance ofconfigurations with internally derived "free running" (FR) meteorology and"specified dynamics" (SD) against observations from surface, aircraft, andsatellite, as well as understand the origin of the identified differences. Wefocus on the representation of aerosols and chemistry. All modelconfigurations reproduce tropospheric ozone for most regions based on in situand satellite observations. However, shortcomings exist in the representationof ozone precursors and aerosols. Tropospheric ozone in all modelconfigurations agrees for the most part with ozonesondes and satelliteobservations in the tropics and the Northern Hemisphere within thevariability of the observations. Southern hemispheric tropospheric ozone isconsistently underestimated by up to 25%. Differences in convection andstratosphere to troposphere exchange processes are mostly responsible fordifferences in ozone in the different model configurations. Carbon monoxide(CO) and other volatile organic compounds are largely underestimated inNorthern Hemisphere mid-latitudes based on satellite and aircraftobservations. Nitrogen oxides (NOx) are biased low in the free tropicaltroposphere, whereas peroxyacetyl nitrate (PAN) is overestimated inparticular in high northern latitudes. The present-day methane lifetimeestimates are compared among the different model configurations. These rangebetween 7.8 years in the SD configuration of CAM5-chem and 8.8 years in theFR configuration of CAM4-chem and are therefore underestimated compared toobservational estimations. We find that differences in tropospheric aerosolsurface area between CAM4 and CAM5 play an important role in controlling theburden of the tropical tropospheric hydroxyl radical (OH), which causesdifferences in tropical methane lifetime of about half a year betweenCAM4-chem and CAM5-chem. In addition, different distributions of NOx fromlightning explain about half of the difference between SD and FR modelversions in both CAM4-chem and CAM5-chem. Remaining differences in thetropical OH burden are due to enhanced tropical ozone burden in SDconfigurations compared to the FR versions, which are not only caused bydifferences in chemical production or loss but also by transport and mixing.For future studies, we recommend the use of CAM5-chem configurations, due toimproved aerosol description and inclusion of aerosol–cloud interactions.However, smaller tropospheric surface area density in the current version ofCAM5-chem compared to CAM4-chem results in larger oxidizing capacity in thetroposphere and therefore a shorter methane lifetime

Description and evaluation of tropospheric chemistry and aerosols in the Community Earth System Model (CESM1.2)