MAY 2006: 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(Ox), varies from 2300 to 5300 Tg yr-1 across models describing the present-day atmosphere. The ensemble mean of P(Ox) in models from the post-2000 literature is 35% higher than that compiled in the IPCC [2001] assessment. Multivariate statistical analysis of model budgets in the literature indicates that 68% of the variance in P(Ox) across models can be explained by differences in NOx emissions, stratosphere-troposphere exchange (STE), and inclusion of non-methane volatile organic compounds (NMVOCs). Weaker STE and inclusion of NMVOC chemistry in the more recent models increase ozone production, though the effect of NMVOCs is insensitive to the magnitude of emissions within the range typically used in models (500-1000 Tg C yr-1). We find in GEOS- Chem that P(Ox) saturates when NMVOC emissions exceed 200 Tg C yr-1 due to formation of organic nitrates from isoprene oxidation, which provides an important sink for NOx.

The upper figure shows that the global ozone production rates reported in litertature can be reasonably approximated by the regression equation incorporating STE and NOx emissions as well as a dummy variable indicating whether NMVOC chemistry is considered, which implies that the above 3 factors can largely explain the difference between global models in P(Ox). The lower figure shows that P(Ox) tend to be higher when NMVOC chemistry is considered in the global model, however P(Ox) is insensitive to the magnitude of NMVOC with NMVOC emissions in the range typically used in global models. A full description is given in Wu et al. [2006].