Abstract. Hemispheric transport of air pollutants can have a significant impact on regional air quality, as well as on the effect of air pollutants on regional climate. An accurate representation of hemispheric transport in regional chemical transport models (CTMs) depends on the specification of the lateral boundary conditions (LBCs). This study investigates the use of new LBCs of two moderately long-lived trace gases, CO and O3, for the European model domain. The LBCs are generated by use of the global EMEP MSC-W model; they are evaluated at the lateral boundaries by comparison with satellite observations of the Terra/MOPITT sensor (CO) and the Aura/OMI sensor (O3) for use with European domain calculations with the Swedish Multi-scale Atmospheric Transport and CHemistry (MATCH) model. The LBCs from the global EMEP model lie well within the satellite uncertainties for both CO and O3. The biases increase below 700 hPa for both species, although it should be noted that satellite data below this height are more influenced by a priori data and hence less reliable than at e.g. 500 hPa. CO is, on average, underestimated by the global model, while O3 tends to be overestimated during winter, and underestimated during summer. Next, the validated LBCs are applied in a dynamical and climatological setup, respectively, to the MATCH model, set up over the European domain. The MATCH results obtained with climatological and dynamic LBCs are then validated against independent satellite retrievals from the Aqua/AIRS sensor at 500 hPa, and against in situ ground observations from the Global Atmospheric Watch (GAW) network. The application of the EMEP LBCs in the regional MATCH model greatly impacted the model results. The direct impact on ground-level concentrations strongly depends on the distance from the inflow boundary. The improvements of dynamic over climatological LBCs become most apparent when using AOT40 as a metric. Also, when focusing at ground observations taken near the inflow boundary of the model domain, one finds that the use of dynamical LBCs yields a more accurate representation of the seasonal variation, as well as of the variability of the trace gas concentrations on shorter time scales. The greatest impact from the new LBCs, was seen aloft in the free troposphere. Taking AIRS retrievals as a reference, the use of LBCs substantially improves spatial pattern correlations in the free troposphere as compared to results obtained with the LBCs that were originally used in MATCH. Also, the magnitude of the bias is reduced by the new LBCs for both trace gases.
Evaluation of lateral boundary conditions in a regional chemical transport model
