Relationships between concentration and deposition of nitrate and sulphate in precipitation

It has been shown that annual NO₃⁻ concentrations in Swedish precipitation rose from the late 1950s to the early 1970s and then remained constant at around the early 1970s level. The mean concentration in precipitation of a chemical species is one measure of pollution but of possible greater relevance for potential ecological damage is deposition, the total amount of the species input to the surface. We demonstrate here, using data from Eskdalemuir, Scotland (55°19′ N, 3°12′ W), that time trends in sulphate and nitrate deposition may differ in response to the influence of changes in precipitation amount. The consequence is that a change in precipitation amount alone will produce changes in the relative sulphate and nitrate deposition levels. Moreover, such changes could occur even if other parameters such as relative emission rates remained constant. Consequently, any assessment of nitrate/sulphate deposition should consider climatological as well as emission factors.