A detailed study of the 26-year series of global radiation K↓, measurements at Bet Dagan, the Israel Meteorological Service's pyranometer station in the central coastal plain of Israel, confirmed earlier findings of a significant reduction in insolation which were based on a small sample of this data set (Stanhill and Moreshet, 1992). Between 1956 and 1987 the annual reduction averaged 45.2 ± 4.3 MJ m−2, equivalent to −0.63% yr−1. Relatively the reduction was greater in midwinter (−0.91% yr−1), than midsummer (−0.56% yr−1), and under average (−0.63% yr−1), than cloudless (−0.48% yr−1) sky conditions. No changes were found in the degree of cloud cover observed at Bet Dagan.
The annual decrease of K↓ at Bet Dagan was highly correlated (r = −0.78) with the increase in the number of motor vehicles using the major roads passing within 1 km of the site: each additional vehicle passing was associated with a 21.5 J m−2 decrease in K↓ The causal nature of this correlation was confirmed by the difference of 18% found in daily values of K↓ measured at Bet Dagan under traffic-free and extremely congested road conditions.
The reduction in the K↓ at Bet Dagan could not, however, be attributed exclusively to the increase in motor traffic in the immediate vicinity of the site, as no significant difference was found in values measured at a relatively traffic-free site 2 km downwind of the pyranometer station. The effect of aerosol pollutants originating in Tel Aviv - the major urban and industrial connurbation upwind of Bet Dagan - was confirmed by the changes recorded in the relative size of the direct and diffuse components of K↓ measured at this site.
The importance of pollution from Tel Aviv would also explain the absence of any significant changes in the annual values of K↓ measured at Jerusalem, a smaller and less industrialized urban center 46 km downwind of Bet Dagan, or at Qidron, an uninhabited, isolated site on the NW coast of the Dead Sea, 25 km further downwind.
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