In the SAR, the chapter on atmospheric chemistry included two modelling studies: PhotoComp (comparison of ozone photochemistry in box models) and Delta-CH₄ (methane feedbacks in 2-D and 3-D tropospheric chemistry models). These model studies established standard model tests for participation in IPCC. They resulted in a consensus regarding the CH₄ feedback and identified the importance (and lack of uniform treatment) of NMHC chemistry on tropospheric O₃ production. This synthesis allowed for the SAR to use the CH₄-lifetime feedback and a simple estimate of tropospheric O₃ increase due solely to CH₄. The SAR noted that individual CTMs had calculated an impact of changing NO_x and CO emissions on global OH and CH₄ abundances, but that a consensus on predicting future changes in O₃ and OH did not exist.

Since 1995, considerable research has gone into the development and validation of tropospheric CTMs. The IPCC Special Report on Aviation and the Global Atmosphere (Derwent and Friedl, 1999) used a wide range of global CTMs to predict the enhancement of tropospheric O₃ due to aircraft NO_x emissions. The results were surprisingly robust, not only for the hemispheric mean O₃ increase, but also for the increase in global mean OH reported as a decrease in the CH₄ lifetime. The current state-of-modelling in global tropospheric chemistry has advanced since PhotoComp and Delta-CH₄ in the SAR and now includes as standard a three-dimensional synoptic meteorology and treatment of non-methane hydrocarbon chemistry. A survey of recent CTM-based publications on the tropospheric O₃ budget, collected for this report, is discussed in Section 4.5.

This assessment, building on these developments, organised a workshop to compare CTM results for a few, well-constrained atmospheric simulations. An open invitation, sent out in March 1999 to research groups involved in 3-D global tropospheric chemistry modelling, invited participation in this report’s assessment of change in tropospheric oxidative state through a model intercomparison and workshop (OxComp). This workshop is an IPCC-focused follow-on to the Global Integration and Modelling (GIM) study (Kanakidou et al., 1999). The infrastructure for OxComp (ftp site, database, graphics, and scientific support) was provided by the University of Oslo group, and the workshop meeting in July 1999 was hosted by the Max Planck Institute for Meteorology (MPI) Hamburg. Participating models are described by publications in peer-reviewed literature as summarised in Table 4.10; all include 3-D global tropospheric chemistry including NMHC; and assessment results are based on models returning a sufficient number of OxComp cases. The two goals of OxComp are (i) to build a consensus on current modelling capability to predict changes in tropospheric OH and O₃ and (ii) to develop a useful parametrization to calculate the greenhouse gases (including tropospheric O₃ but not CO₂) using the IPCC emissions scenarios.

Table 4.10: Chemistry-Transport Models (CTM) contributing to the OxComp evaluation of predicting tropospheric O3 and OH.
CTM	Institute	Contributing authors	References
GISS	GISS	Shindell /Grenfell	Hansen et al. (1997b)
HGEO	Harvard U.	Bey / Jacob	Bey et al. (1999)
HGIS	Harvard U.	Mickley / Jacob	Mickley et al. (1999)
IASB	IAS/Belg.	Mülller	Müller and Brasseur (1995, 1999)
KNMI	KNMI/Utrecht	van Weele	Jeuken et al. (1999), Houweling et al. (2000)
MOZ1	NCAR/CNRS	Hauglustaine / Brasseur	Brasseur et al. (1998b), Hauglustaine et al. (1998)
MOZ2	NCAR	Horowitz/ Brasseur	Brasseur et al. (1998b), Hauglustaine et al. (1998)
MPIC	MPI/Chem	Kuhlmann / Lawrence	Crutzen et al. (1999), Lawrence et al. (1999)
UCI	UC Irvine	Wild	Hannegan et al. (1998), Wild and Prather (2000)
UIO	U. Oslo	Berntsen	Berntsen and Isaksen (1997), Fuglestvedt et al. (1999)
UIO2	U. Oslo	Sundet	Sundet (1997)
UKMO	UK Met Office	Stevenson	Collins et al. (1997), Johnson et al. (1999)
ULAQ	U. L. Aquila	Pitari	Pitari et al. (1997)
UCAM	U. Cambridge	Plantevin /Johnson	Law et al. (1998, 2000) (TOMCAT)

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