On the ratio of sulfur dioxide to nitrogen oxides as an indicator of air pollution sources

The ratio of sulfur dioxide to nitrogen oxides (RSN = SO2/NOx) is one indic ator of air pollution sources. The role of this ratio in source attribution is illustrated here for the Ashdod area, located in the southern coastal p lain of Israel. The main sources of pollution in the area are the tall stac ks of the Eshkol power plant, the stacks of oil refineries, and areal sourc es (stationary and mobile). The factors that affect RSN are studied using f our regression models: a binary regression tree in original scale, a tree i n logarithmic scale, a data partition produced by a combination of the two trees, and a linear regression model. All models have similar relative pred iction error, with the combined partition best highlighting the sources of variability in RSN: (a) very low values (interquartile range of [0.12, 0.48 ]) are associated with traffic, (b) low values ([0.43, 1.00]) are attribute d 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 refinery emissions mainly on slow wind episodes , and (d) high values ([1.07, 4.30]) are attributed to refinery emissions d uring moderate to fast wind episodes. Analysis of the number of episodes of increased concentrations indicates that, during 1996 and 1997, about 42% o f SO2 episodes are attributable to the power plant and 33% to the refinerie s. Increased-NOx episodes are mainly contributed by traffic (91%) and power plant (4.5%) emissions.