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ON THE RATIO OF SULFUR DIOXIDE TO NITROGEN OXIDES AS AN INDICATOR OF AIR POLLUTION SOURCES

Ronit Nirel(1) and Uri Dayan(2)

(1) Department of Statistics (2) Department of Geography The Hebrew University of Jerusalem, Jerusalem, Israel

March 2000

ABSTRACT

The ratio of sulfur dioxide to nitrogen oxides (RSN=SO2/NOx) is one indicator of air pollution sources. In this paper the factors which affect this ratio are studied using a model based on binary regression trees. The model is nonparametric and has an intuitive interpretation. A comparison with a linear regression model indicates similar performance of the tree-based model and the parametric model, with an added robustness of the nonparametric model for predictions based on test datasets. The method is illustrated for the Ashdod area, located in the southern coastal plain of Israel. The main sources of pollution in this area are the tall stacks of the Eshkol power plant; stacks of oil refineries; and areal sources (stationary and mobile). A combination of two trees, in original and logarithmic scales, highlights the sources of variability in RSN: (a) very low values (interquartile range of [0.12, 0.48]) are associated with mobile sources; (b) low values ([0.43,1.00]), are attributed to the power plant and to daytime emissions of local industry; (c) medium values ([0.74, 1.90]) are associated with local industry emissions during cooler hours of the day and refineries emissions mainly on slow wind episodes; and (d) high values ([1.07, 4.30]) are attributed to refineries emissions during moderate to fast wind episodes. Analysis of the number of spells of elevated concentrations indicates that during 1996 and 1997 about 44% of SO2 spells are attributable to the power plant and 34% to the refineries. Elevated NOx spells are mainly contributed by traffic (91%) and power plant (4.5%) emissions.

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