Local vs. external contribution to the budget of pollutants in the Po Valley (Italy) hot spot

• Model simulation of pollutants in the Po Valley hot spot was performed.
• External and internal source contributions were evaluated.
• Half of the year, external contribution on the total column exceeds local one.
• External contribution at the ground exceeds local one 15% of the time.
• Due to the specific orography, vertical mixing is found to play a fundamental role.

Numerical experiments using both passive tracers and full chemistry were performed to investigate the budget of pollutants in the Po Valley hot spot, using the BOLCHEM integrated model of atmospheric dynamics and composition. A one-year run was undertaken over the European domain, with a resolution of 50 × 50 km2. Sources internal and external to the Po Valley were treated in separated runs.For the tracer experiment, the ratio between total masses of locally and externally emitted tracers showed that for half of the year the contribution from remote sources exceeds the local one. Although local sources prevailed at the ground level, external contribution exceedance was observed 15% of the time. Since the Po Valley is mostly surrounded by high mountains, vertical mixing and entrainment at the boundary layer top were found to be more effective than the advection at low levels.As the concentration of reacting species is affected by the nonlinearity of the transport-reaction mechanism, the results of the full chemistry runs with internal and external sources can be compared only in cases where this effect is weak. In these cases, the results for tracers were broadly confirmed for the less reacting species (CO, PM10) and, to some extent, also for more reactive ones (NO2, O3) for which more pronounced seasonal and diurnal cycles were found due to photochemical reactivity.