An assessment of the sea/valley breeze and its impact on ozone behaviour

Ozone (O-3) is the secondary pollutant formed in the atmosphere through photochemical reactions involving nitrogen oxides and volatile organic compounds in the presence of sunlight. In addition to the chemical and emission variables, a variety of meteorological variables also influence ozone concentrations. Understanding the meteorological parameters (namely temperature, vertical temperature structure, surface winds, aloft winds, local flow patterns, etc) that influence ozone levels is important for selecting variables to help ozone prediction. The selected study area is a typical coastal area located in the middle of Italy. Topography in this area is fairly complex and the proximity to the coast implies a significant interaction between the sea and valley breezes. The climate in this area, in fact, is classified under sub-coastal where there is an all year round sea breeze. The sea/land breezes play a significant role in the distribution of ozone and transport of ozone from the coast to the sea and valley areas. The sea breeze is a weak system, extending vertically to a height of less than I km with the low wind speed. The land breeze can transport the photochemically produced ozone and its precursors over the sea. The accumulated ozone on the sea can return to the land in the daytime with the sea breeze. This kind of transport tends to contribute significantly to high-ozone episodes in coastal areas. For this purpose RAMS simulations have been implemented by assimilating data from provincial monitoring networks and from the European Center for Medium-range Weather Forecast elaborations. Four regular nested grids represent the simulation domain: the first grid is an 18x18 mesh with a 50000m side width; the finest grid is a 3707; the four nesting ratios are respectively 1,5,5,5. Simulations show that the sea/valley breezes in complex coastal areas play a significant role in the distribution and transport of ozone. It is also very important to consider the dispersion effect that breeze-driven "air bouncing" may have on ozone-precursors concentrations ratio in certain hours. We missed upper air sounding in order to have a complete and clearer idea of boundary layer dynamics, first of all mixed layer height and its evolution. These data would be very advantageous for this kind of study. Understanding breeze cycle in our study area is an inalienable starting point for any meteorological study. However, the most significant result comes from the comparison between the shown simulations with those related to some particular periods. In fact, we could find out two days highly characterized by weak insolation (probably cloudy days) and by no breeze development. In correspondence to these two periods we could clearly recognize a consequent strong ozone depletion. Another interesting aspect is related to experimental evidence: an about 12-hours-delay between the breeze ceasing and the ozone abatement. These last observations and consideration strongly encouraged us to sustain the hypothesis of considering breezes as mayor forcing during coastal ozone episodes.