Air Pollution as a Climate Forcing: A Workshop

Day 1 Presentations

Projections for Future Ozone Change Driven by Changes in Climate and Precursor Emissions

R.G. Derwent*, W.J. Collins*, C.E. Johnson* and D.S. Stevenson+
* Climate Research Division, Met Office, Bracknell, UK
+ Department of Meteorology, University of Edinburgh, Edinburgh, UK

A global 3-D Lagrangian chemistry-transport model (STOCHEM) has been coupled to the Hadley Centre atmosphere-ocean global climate model (HadCM3). Every three hours fields of wind, temperature, cloud amounts and positions, precipitation, boundary layer depth, surface heat flux and pressure are passed to the chemistry model. Two experiments were carried out each covering the period from 1990 through to the year 2100: a climate change (A2 scenario IPCC SRES) experiment and a control experiment with the same trace gas emissions but with pre-industrial atmosphere and ocean conditions.

In the control experiment, the globally-averaged methane concentrations increased from 1670 ppb in 1990 to 3650 ppb in 2100 but only to 3230 ppb in the climate change experiment. Most of the differences in the methane build-up occur after the year 2040. Global mean surface ozone concentrations increased from 23.3 ppb in the year 2000 to 35.4 ppb in 2100 in the control experiment and to 29.4 ppb in the climate change experiment. In the middle troposphere, the increases were from 31.3 ppb in 2000 to 48.9 ppb and 39.5 ppb in 2100, respectively, for the control and climate change experiments.

The build-up in the global burdens of methane and ozone in the IPCC SRES A2 scenario over the period 1990 — 2100 are dramatically reduced by climate change. The mechanism of the feedback from climate change involves changes in stratosphere-troposphere exchange, atmospheric circulations, convection, temperatures and humidities. However, by far the greatest influence of climate change resulted from the increased humidities in the climate change experiment and the increased flux through the O1D + H2O reaction. This increased the tropospheric hydroxyl OH radical concentrations and hence the sinks for methane and ozone.

A methodology has been developed to aid the quantification of the likely impacts of trace gas emission reductions on the build-up of tropospheric ozone. Emission pulses of major tropospheric ozone precursor gases: methane, carbon monoxide, oxides of nitrogen and organic compounds have been studied in the STOCHEM model driven by 6-hourly meteorological analysis fields taken from the Met Office numerical weather prediction model for the period from October 1994 through to December 1998. Pulses have been emitted during the month of January and July, from each major continent for each trace gas and the subsequent methane and ozone responses followed over a four-year period.

The impacts of a wide range of precursor gases on the global distributions of methane and ozone have been quantified in this way. The responses to emission pulses have then been used to determine the responses to step-changes in emissions from particular continents and emission sources and these are summarised in Table 1.

Table 1: Maximum global ozone responses in Tg to 1 Tg emission pulses of each trace gas from particular geographical regions and in particular seasons of the year determined using the STOCHEM 3-D CTM.
Precursor Continent Season Maximum global ozone response in Tg for a 1 Tg pulse
methaneN hemisphereJanuary0.009
carbon monoxideN AmericaJanuary0.021
NOx (as N) surfaceN AmericaJanuary2.3
N hemisphereJanuary2.6
S hemisphereJanuary5.1
NOx (as N) aircraftAircraftJanuary50
ethaneN hemisphereJanuary0.13

Step-change responses were then estimated from pulse responses using the respective adjustment times. It is estimated that, by curtailing European man-made sources of methane, carbon monoxide, NOx and organic compounds, a potential reduction in the global ozone burden of about 4.5 Tg would be achieved in steady state, that is about 1.5%. The corresponding reduction from curtailing USA emissions should be somewhat similar in magnitude. However, the balance between the contributions from the main contributors: methane, surface NOx, aircraft NOx and carbon monoxide emission reductions are different. Such estimates are highly uncertain and this study is just the beginning of those needed for a detailed policy assessment of the impact of trace gas emissions on the future build-up of the global burdens of methane and ozone.

Acknowledgments. This work was supported by the GMR R&D programme of the Met Office and by the UK Department for Environment, Food and Rural Affairs under contracts PECD 7/12/37 and EPG 1/3/164.

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