A five-year study of coastal recirculation and its effect on air pollutants over the East Mediterranean region

Many studies have shown that air pollutants concentrations in coastal cities may be gravely affected by coastal recirculation. In this study an attempt is made to examine the properties of coastal recirculation over a long period (5 yrs) at multiple sites along the East Mediterranean Sea (EMS). For this purpose, a single station quantitative measure of horizontal recirculation is used based on wind field measurements over periods of 1-96 hrs. The horizontal recirculation is examined with respect to the integration time period, synoptic flow, seasonality, coastline variations, elevation, and air pollutants concentrations. The interaction between synoptic and mesoscales is shown to be a governing factor by allowing or overruling the land sea breeze winds. Favorite conditions for coastal recirculation are shown to be light or variable winds such as under a Cole or a High-Pressure system. The monthly distribution of the recirculation potential has a bimodal behavior with two peaks during the transitional seasons and October in particular. This is as a result of the annual cycle of night-time land-sea temperature difference driving the land breeze and the more frequent passage of synoptic scale flows with an easterly wind component at the EMS. Two factors leading to variations along the coastline are the urban heat island, weakening the breeze winds and reducing recirculation potential, and the concaved shape of the southern shoreline that causes a convergence and strengthening of the land breeze, thus supporting recirculation. The primary pollutants NO_x and SO₂ have the highest concentrations during weak daily mean wind speeds. O₃ levels depict an almost opposite image of NO_x, with higher values for both high and low recirculation, possibly resulting from either long range transport or coastal recirculation.