Atmospheric conditions leading to buildup of benzene concentrations in urban areas in Israel

Benzene is a carcinogenic air pollutant and its concentration in the atmosphere is derived from the rate of its emission and the atmospheric conditions that cause it to be dispersed and transported. An analysis of the daily benzene concentrations for a period of six years measured in the three largest population centers in Israel (Tel-Aviv, Jerusalem and Haifa) revealed that practically all values were lower than the Israeli 24-h air quality standard. Even though, daily elevated benzene occurrences (here defined as being higher than 30 percent of the 24-h standard) were found to be associated with four out of the 22 synoptic weather types prevailing over the Eastern Mediterranean, more than half of them occurring during the winter and the transitional seasons. These weather types are characterized by a persistent south-easterly dry flow over Israel. During nighttime the very high frequency of the radiative inversion characterizing these 4 synoptic types trap the benzene near the surface, resulting in elevated concentrations. At daytime, a significantly higher relative frequency of weak winds was found for the four relevant synoptic types, as compared to their overall relative frequency. These calm conditions contribute to the observed buildup in the benzene concentrations. The very high prevalence of both radiative inversion during the nights and weak winds in daytime for the four synoptic circulation types did not alter the strength of the synoptic classification in predicting the prevalence of these elevated benzene occurrences. The annual frequency of benzene elevated concentrations is about 30 percent, mostly associated with the presence of the two modes of the Red Sea Trough (eastern and central axis). This points at the strength of the synoptic classification to serve as a valuable predictor of elevated concentrations.