A quantitative analysis of grid-related systematic errors in oxidising capacity and ozone production rates in chemistry transport models

[摘要] Limited resolution in chemistry transport models (CTMs) is necessarilyassociated with systematic errors in the calculated chemistry, due to theartificial mixing of specieson the scale of the model grid (grid-averaging). Here, the errors incalculated hydroxyl radical(OH) concentrationsand ozone production rates 3 are investigatedquantitatively using both direct observations and model results. Photochemicalsteady-state models of radical chemistry are exploited in each case to examinethe effect on both OH and 3of averaging relatively long-lived precursor species, such as O₃, NO_x,CO, H₂O, etc. over different spatial scales. Changes in modelled 3 areestimated, independently of other model errors, by calculatingthe systematic effect of spatial averaging on the ozone production efficiency1, defined as the ratio of ozone molecules produced perNO_x molecule destroyed. Firstly, an investigation of in-flight measurementssuggests that, at leastin the northern midlatitudeupper-troposphere/lower stratosphere, averaging precursor species on thescale of a T42 grid (2.75° x 2.75°) leads to a15-20% increase in OH concentrations and a 5-10% increase in 1.Secondly, results from CTM model experimentsare compared at different horizontal resolutions.Low resolution experiments are found to have significantly higher [OH] and3 compared with high resolution experiments. The extent to which thesedifferences maybe explained by the systematic error in the model chemistry associated withgrid size isestimated by degrading the high resolution data onto a low resolution gridand then recalculating 1 and [OH]. The change in calculated 1 is found tobe significant and can account for much of the difference in 3 betweenthe high and low resolution experiments.The calculated change in [OH] is less than the difference in [OH] foundbetween theexperiments, although the shortfall is likely to be due to theindirect effect of the change in modelled NO_x, which is not accounted forin the calculation. It is argued thatsystematic errors caused by limited resolutionneed to be considered when evaluating the relative impacts ofdifferent pollutant sources on tropospheric ozone.

[学科分类] 大气科学