Results from the Intergovernmental Panel on Climatic Change Photochemical Model Intercomparison (PhotoComp) (original) (raw)

Abstract

Results from the Intergovernmental Panel on Climatic Change (IPCC) tropospheric photochemical model intercomparison (PhotoComp) are presented with a brief discussion of the factors that may contribute to differences in the modeled behaviors of HO x cycling and the accompanying 03 tendencies. PhotoComp was a tightly controlled model experiment in which the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an important greenhouse gas. Calculated tropospheric photodissociation rates displayed significant differences, with a root-mean-square (nns) error of the reported model results ranging from about _+6-9% of the mean (for 03 and NO2) to up to +15% (H202 and CH20). Models using multistream methods in radiative transfer calculations showed distinctly higher rates for photodissociation of NO 2 and CH20 compared to models using two-stream methods, and this difference accounted for up to one third of the rms error for these two rates. In general, some small but systematic differences between models were noted for the predicted chemical tendencies in cases that did not include reactions of nonmethane hydrocarbons (NMHC). These differences in modeled 03 tendencies in some cases could be identified, for example, as being due to differences in photodissociation rates, but in others they could not and must be ascribed to unidentified errors. 03 tendencies showed nns errors of about___10% in the moist, surface level cases with NO x concentrations equal to a few tens of parts per trillion by volume. Most of these model to model differences can be traced to differences in the destruction of 03 due to reaction with HO 2. Differences in HO 2, in turn, are likely due to (1) inconsistent reaction rates used by the models for the conversion of HO 2 to H202 and (2) differences in the model-calculated photolysis of H202 and CH20. In the middle tropospheric "polluted" scenario with NO x concentrations larger than a few parts per billion by volume, 03 tendencies showed nns errors of _+10-30%. These model to model differences most likely stem from differences in the calculated rates of 03 photolysis to O(•D), which provides about 80% of the HO x source under these conditions. The introduction of hydrocarbons dramatically increased both the rate of NO x loss and its model to model differences, which, in turn, are reflected in an increased spread of predicted 03. Including NMHC in the simulation approximately doubled the rms error for 03 concentration. Kathy Law: kathy@atm. ch.cam.ac.uk Mafia Kanakidou: mafiak@obelix.saclay.cea.fr Prasad Kasibhatla: psk@hpcc.epa. gov Loft Perliski: lmp@gfdl.gov Larry Horowitz: lwh@europa.harvard.edu Michael Kuhn: kuhn@ifu.fhg.de Peter Cormell: cormell2 @11nl.gov Joyce Penner: joyce_penner@umich.edu Robert Chatfield: chatfield@clio.arc.nasa. gov Anne Thompson: thompson @gator 1.gsfc. nasa. gov Jennifer Olson: j.r. olson@larc.nasa. gov Frode Stordal: frode@zardoz.nilu.no Shengxin Jin: sjin@gw.dot.state.ny.us Michael Prather: prather@halo.ps.uci.edu Gregory Carmichael: gcarmich@icaen.uiowa.edu Richard Derwent: rgderwent@email.meto. govt. uk Sanford Sillman: sillman@ kudzu.sprl.umich. edu Terje Bernmen: terje.berntsen@geofysikk.uio.no

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