A quantitative analysis of grid-related systematic errors in oxidising capacity and ozone production rates in chemistry transport models J. G. Esler1, G. J. Roelofs2, M. O. Köhler3, and F. M. O'Connor3 1Department of Mathematics, University College London, UK 2Institute for Marine and Atmospheric research, Utrecht – IMAU, The Netherlands 3Centre for Atmospheric Science, University of Cambridge, Cambridge, UK
Abstract. Limited resolution in chemistry transport models (CTMs) is necessarily
associated with systematic errors in the calculated chemistry, due to the
artificial mixing of species
on the scale of the model grid (grid-averaging). Here, the errors in
calculated hydroxyl radical
(OH) concentrations and ozone production rates 3 are investigated
quantitatively using both direct observations and model results. Photochemical
steady-state models of radical chemistry are exploited in each case to examine
the effect on both OH and 3
of averaging relatively long-lived precursor species, such as O3, NOx,
CO, H2O, etc. over different spatial scales. Changes in modelled 3 are
estimated, independently of other model errors, by calculating
the systematic effect of spatial averaging on the ozone production efficiency
1, defined as the ratio of ozone molecules produced per
NOx molecule destroyed. Firstly, an investigation of in-flight measurements
suggests that, at least in the northern midlatitude
upper-troposphere/lower stratosphere, averaging precursor species on the
scale of a T42 grid (2.75° x 2.75°) leads to a
15-20% increase in OH concentrations and a 5-10% increase in 1.
Secondly, results from CTM model experiments
are compared at different horizontal resolutions.
Low resolution experiments are found to have significantly higher [OH] and
3 compared with high resolution experiments. The extent to which these
be explained by the systematic error in the model chemistry associated with
grid size is
estimated by degrading the high resolution data onto a low resolution grid
and then recalculating 1 and [OH]. The change in calculated 1 is found to
be significant and can account for much of the difference in 3 between
the high and low resolution experiments.
The calculated change in [OH] is less than the difference in [OH] found
experiments, although the shortfall is likely to be due to the
indirect effect of the change in modelled NOx, which is not accounted for
in the calculation. It is argued that
systematic errors caused by limited resolution
need to be considered when evaluating the relative impacts of
different pollutant sources on tropospheric ozone.
Citation: Esler, J. G., Roelofs, G. J., Köhler, M. O., and O'Connor, F. M.: A quantitative analysis of grid-related systematic errors in oxidising capacity and ozone production rates in chemistry transport models, Atmos. Chem. Phys., 4, 1781-1795, doi:10.5194/acp-4-1781-2004, 2004.