RECIRCULATION OF POLLUTED AIR MASSES OVER THE EAST MEDITERRANEAN COAST

An air quality observatory was operated at a rural site on the Mediter ranean coast of Israel near the ancient city of Caesarea between May 1 993 and October 1995. The objective of the study was to monitor transp ort of air pollutants from remote sources that arrive at the Israeli c oast. Normally, under onshore westerly winds, which come from the Medi terranean sea, the levels of NO, NOy and SO2 at the site dropped to be low 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 serie s of pollution episodes occurred, with elevated values for all the pol lutants being recorded during onshore hows. The SO2 concentration reac hed 30 ppbv, NO and NOy more than 100 ppbv and O-3 levels rose above 2 00 ppbv. On 26 October the O-3 levels were the highest observed with a n abnormally high concentration of 230 ppbv being recorded. The episod e that occurred on that day was investigated in detail in order to und erstand the cause for this high ozone value. During the above day, nig ht and early morning, easterly winds swept the air parcels containing the locally emitted pollution westwards over the sea. The intense phot ochemical activity that occurred while the air mass was over the sea f or a relatively extensive time combined with the low mixing height on that day and the fact that the returning air mass reached the measurin g site at peak ozone formation time, give rise to record ozone levels. The elevated concentrations were measured when the polluted air parce ls returned to the coast during the afternoon onshore flows. An excell ent linear correlation between O-3 and NOy was obtained for the time p eriod between 1400 and 1700. The linear fit between O-3 and NOy indica tes that approximately 11 O-3 molecules were formed for each NOy molec ule present. Similar ratios have been reported in other studies dealin g with aged air masses under NOy limited conditions. (C) 1997 Elsevier Science Ltd.