Sulphur and oxygen isotope ratios in spruce needles as a tracer of atmospheric pollution

Abstract

The biospheric part of the anthropogenic sulphur cycle was investigated in a heavily polluted area at the northern slope of the Karkonosze Mountains, SW Poland. Norway spruce (Picea abies) needles, from a 900 m vertical transect (400 to 1300 m above sea level), were collected on two consecutive days of spring 1998 and one day of winter 1999. Concentrations of sulphate sulphur (SO42−), and organic sulphur (Sorg)n and isotope ratios of organic sulphur and sulphate from spruce needles have been analyzed. (SO42−)n and (Sorg)n were rather constant and δ34S(SO42−)n and δ34S(Sorg)n values increased with higher altitude. This is attributed to an increase in atmospheric SO2 concentration and light intensity, which enhance emission of 34S‐depleted hydrogen sulphide (H2S) from needles. The δ18O(SO42−)n decreases with altitude due to the altitudinal 18O‐depletion of atmospheric precipitation and increased formation of needle sulphate from atmospheric SO2. The isotope effect related to a reduction process during spring can also be seen at higher altitudes by a negative correlation between δ18O(SO42−)n versus (SO42−)n and δ34S(SO42−)n versus (SO42−)n of spring needles. Winter needles show an opposite trend of potential oxidation of the SO2 assimilated. The Δ34S(SO42− − Sorg)n value shows a very good correlation to the abundance of dust on needles. Higher abundances of dust may limit foliar gas exchange and thus higher Δ34S(SO42− − Sorg)n values reflect conditions which are closer to sulphur isotope equilibrium in the sulphate‐organic sulphur system, whereas lower Δ34S(SO42− − Sorg)n values characterize a higher gas exchange rate and more dynamic conditions for the sulphur system in needles. Hydrogen sulphide emission is the most likely mechanism to control variations in the observed δ‐values, and dust abundance may control variations in the Δ‐values.