The spatial and temporal variation of the sulphate to nitrate ratio in precipitation in eastern North America

Four years of precipitation chemistry data for eastern North America were used to investigate seasonal and geographical variations in \({\text{SO}}_{\text{4}}^{\text{ = }} {\text{/NO}}_{\text{3}}^{\text{ - }} \) ratio. Several distinct regimes occur. One, in the region of heaviest acidic deposition extending from the states south of the Great Lakes across New England and southeastern Canada, has a very strong seasonal variation in the \({\text{SO}}_{\text{4}}^{\text{ = }} {\text{/NO}}_{\text{3}}^{\text{ - }} \) molar ratio in deposition. The ratio ranges from about 1.5 in summer to about 0.5 in winter. Another, in the smaller area of Texas and surrounding states, shows the reverse seasonal pattern. Yet another, in the high plains states, has a double maximum in the ratio in Spring and Fall. The remainder of the region has an irregular seasonal pattern. Insight into the cause of \({\text{SO}}_{\text{4}}^{\text{ = }} {\text{/NO}}_{\text{3}}^{\text{ - }} \) variations was obtained using a simple chemical transport box model. It showed that the chemical transformation of S0₂ and NO_x in the atmosphere is a major factor. A comparison of model predictions and observations indicate that in the vicinity of mid-western American sources the molar ratio of amount of S0₂ oxidized in-cloud to that of N0₂ is O.5 in winter and 1.5 in summer.