Measurements and modelling of ozone in the Mediterranean MBL: an investigation of the importance of ship emissions to local ozone production

Abstract. Elevated concentrations of ground level ozone are both hazardous to human health and detrimental to agricultural production. The Mediterranean Basin, due to its position under the descending branch of the Hadley Cell circulation during the summer months, enjoys periods of stable, sunny and warm weather which provide ideal conditions for the production of ozone. The presence of major population centres and numerous industrialised areas in the coastal zone result in both a continual supply of ozone precursor compounds and also a significant number of people to suffer the consequences of high ozone concentrations. Using the WRF/Chem model validated with data obtained from seven oceanographic measurement campaigns, performed between 2000 and 2010, aboard the Italian Research Council's R. V. Urania, and also from a number of EMEP monitoring stations located around the Mediterranean Basin, the importance of emissions from maritime traffic in the region has been investigated. The model results indicate that over large areas of the Mediterranean emissions from shipping contribute between 5 and 10 ppb to the ground level O₃ daily average concentration during the summer. The contribution to the hourly average O₃ is up to 40 ppb in some particularly busy shipping lanes. Importantly the results suggest that in a number of coastal areas the contribution from ship emissions to the local O₃ concentration can make the difference between complying with the EU Air Quality standard of a maximum 8 h mean of 120 μg m⁻³ and exceeding it.