Why are there large differences between models in global budgets of tropospheric ozone?

Global 3-D tropospheric chemistry models in the literature show large differences in global budget terms for tropospheric ozone. The ozone production rate in the troposphere, P(O-x), varies from 2300 to 5300 Tg yr(-1) across models describing the present-day atmosphere. The ensemble mean of P(O-x) in models from the post-2000 literature is 35% higher than that compiled in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). Simulations conducted with the GEOS-Chem model using two different assimilated meteorological data sets for 2001 (GEOS-3 and GEOS-4), as well as 3 years of GISS GCM meteorology, show P(O-x) values in the range 4250-4700 Tg yr(-1); the differences appear mostly because of clouds. Examination of the evolution of P(O-x) over the GEOS-Chem model history shows major effects from changes in heterogeneous chemistry, the lightning NOx source, and the yield of organic nitrates from isoprene oxidation. Multivariate statistical analysis of model budgets in the literature indicates that 74% of the variance in P(O-x) across models can be explained by differences in NOx emissions, inclusion of nonmethane volatile organic compounds (NMVOCs, mostly biogenic isoprene), and ozone influx from stratosphere-troposphere exchange (STE). Higher NOx emissions, more widespread inclusion of NMVOC chemistry, and weaker STE in the more recent models increase ozone production; however, the effect of NMVOCs does not appear generally sensitive to the magnitude of emissions within the range typically used in models (500-900 Tg C yr(-1)). We find in GEOS-Chem that P(O-x) saturates when NMVOC emissions exceed 200 Tg C yr(-1) because of formation of organic nitrates from isoprene oxidation, providing an important sink for NOx.