Last update - 00:00 05/11/2007
Study: Dry winter days mean more urban air pollution
By Zafrir Rinat, Haaretz Correspondent
A winter without rain is bad news not only for farmers, but also for urban dwellers in the Dan region. Scientists see a direct link between dry winters and an increase in the number of days with excessive air pollution in the greater Tel Aviv area. The connection stands out in the fall and early winter seasons, which are characterized by a climate phenomenon termed "Red Sea trough" - a system of surface atmospheric circulation patterns whose influence is recognizable at sea level.
A study dealing with the impact of synoptic conditions on nitrogen oxide concentrations in the Dan region was presented last week at a conference on air pollution organized by Tel Aviv University and the Environment Ministry.
The researchers presented the results of a study dealing with nitrogen oxides. The compounds are a major cause of air pollution, and a large share of these are emitted by motor vehicles. Researchers studied the link that existed in the years 1998 to 2004 between various synoptic scenarios, such as sharav, easterly hot airflow or strong westerly winds, and days on which nitrogen oxide concentrations exceeded allowable levels.
Conditions for high-level air pollution usually develop when winds are weak, and the air pollution is prevented from dispersing and dissipating. These conditions ordinarily exist at night and under the impact of situations termed "barometric levels." In the case of the Dan region, the research found that the greatest number of days with excessive air pollution was during periods of Red Sea trough (RST). This is a synoptic condition characterized by hot southeasterly winds and it occurs in the transition seasons and early winter.
More than half of the days with excessive air pollution in the period studied (477 days in total) coincided with the RST. More than a third of the days on which the RST was felt registered excessive air pollution. Summertime had almost no days of excessive air pollution. That season is dominated by the Persian trough, which is characterized by strong westerly winds and mixing of air layers that helps disperse the pollution.
According to the study, the contrast between the easterly winds brought by the RST and the sea breeze typical of the coast weakens the winds, reducing the ability to disperse pollutants. In addition, the cool air coming in from the sea increases stability. These conditions predominate in winter and typify conditions of pollution from transportation.
Pollution dispersal, however, can adversely affect other areas in Israel. Winds bring air pollution from the Dan region to areas distant from the coast, and there solar radiation causes chemical reactions that turn the pollutants into ozone at a lower atmospheric layer. Exposure to ozone greatly increases health risks. According to the Environment Ministry, last year there were 260 days on which the Etzion region registered ozone levels in excess of that recommended by the World Health Organization.
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