Recirculation of polluted air masses over the East Mediterranean coast

Abstract

An air quality observatory was operated at a rural site on the Mediterranean coast of Israel near the ancient city of Caesarea between May 1993 and October 1995. The objective of the study was to monitor transport of air pollutants from remote sources that arrive at the Israeli coast. Normally, under onshore westerly winds, which come from the Mediterranean sea, the levels of NO, NO y and SO 2 at the site dropped to below 0.5 ppbv, CO to below 150 ppbv, and the O 3 levels ranged between 30 and 60 ppbv. During the last week of October 1993, an unusual series of pollution episodes occurred, with elevated values for all the pollutants being recorded during onshore flows. The SO 2 concentration reached 30 ppbv, NO and NO y more than 100 ppbv and O 3 levels rose above 200 ppbv. On 26 October the O 3 levels were the highest observed with an abnormally high concentration of 230 ppbv being recorded. The episode that occurred on that day was investigated in detail in order to understand the cause for this high ozone value. During the above day, night and early morning, easterly winds swept the air parcels containing the locally emitted pollution westwards over the sea. The intense photochemical activity that occurred while the air mass was over the sea for a relatively extensive time combined with the low mixing height on that day and the fact that the returning air mass reached the measuring site at peak ozone formation time, give rise to record ozone levels. The elevated concentrations were measured when the polluted air parcels returned to the coast during the afternoon onshore flows. An excellent linear correlation between O 3 and NO y was obtained for the time period between 1400 and 1700. The linear fit between O 3 and NO y indicates that approximately 11 O 3 molecules were formed for each NO y molecule present. Similar ratios have been reported in other studies dealing with aged air masses under NO y limited conditions.