OZONE LEVELS IN CENTRAL ISRAEL

Measurements performed during the early summer months of 1988-1991 at a rural site in central Israel, some 50 km east of the highly urbanize d coastal region, have shown that during the afternoon hours the area was often under the influence of ozone mixing ratios above the Israel ambient standard (117 ppbv) and occasionally even above 150 ppbv. Anal ysis of air mass back trajectories has shown that only those air masse s passing over the Tel Aviv metropolitan area cause elevated ozone mix ing ratios at the rural site. This highly urbanized region emits large amounts of precursors which are entrapped in the air parcels entering Israel under the predominantly westerly wind flows. As these air mass es travel inland, sufficient time is available (3-5 h) to allow the ph otochemical reactions to generate ozone before reaching the rural site . The above hypothesis is further supported by the fact that parallel to the increase of ozone at the rural site, elevated carbon monoxide ( up to 0.8 ppmv) and other trace gases were also observed. A significan t correlation (R(2) > 0.8) was found to exist between the ozone mixing ratio and the NOx concentrations in photochemically aged air masses. In several cases an excess of up to 12 ozone molecules was formed for each NOx molecule present.