Does the resolution of megacity emissions impact large scale ozone?

The importance of using high resolution when modelling the impact of megacity emission on large scale tropospheric ozone has been investigated using the regional WRF-Chem model. Two 3-day summer periods in July and August 2003 have been simulated for three megacities; London, Ruhr, and Cairo. Simulations have been performed with constant model resolution (9 × 9 km2) and with megacity emission resolutions at 9 × 9 km2, 27 × 27 km2, and 81 × 81 km2, by using detailed emission inventories averaged to the appropriate resolution. In order to quantify the megacities’ contributions to regional ozone, simulations without megacity emissions were also performed.In general, results from the six case studies show that the inaccuracies that arise on a large scale when using megacity emission resolution at a typical scale of a global Chemistry-Transport Model (81 × 81 km2) were relatively small (12% or less). However, the impact of resolution on ozone changes caused by megacity emissions depends strongly on meteorological conditions. The case with the largest impact (London, July 2003) showed that net ozone formed from the megacity were 12% higher when using 81 × 81 km2 rather than 9 × 9 km2 emission resolution, due to stronger ozone production since the NOx emissions were more rapidly diluted to regions where the ozone production was NOx limited in the coarse resolution case. Our study suggests that high resolution is more important for local air pollution studies than for large scale ozone changes relevant for climate studies, since a change in megacity emission resolution induces small-scale spatial changes in ozone fields, but relatively small changes when integrating over a large volume. These results indicate that parameterization of megacity emissions in large scale models may be unnecessary. However, the impact of a finer resolution of the meteorology is not studied here and could possibly give larger effects seen from a climate perspective.

► Scale interactions in megacity ozone formation studied with WRF-Chem.
► Relatively small ozone changes (<12 %) when changing emission resolution.
► Ozone impact strongly dependent on meteorological conditions.
► High resolution more important for local air pollution than for climate studies.