Sulphur isotope tests of seasalt correction factors in precipitation: Nova Scotia, Canada

Sulphate in precipitation is an index of atmospheric pollution but natural sources also contribute to its concentration. One of the dominant natural sources of sulphate is seasalt and reseachers have routinely calculated its proportion using reference species such as Cl−, Na+ and Mg2+. Such a method of estimation assumes conservatism of, and a purely marine origin for, the reference species. The stable isotopes of sulphur were utilized to assess the validity of these assumptions for a coastal area in Canada by taking advantage of the sulphur isotopic signature of seasalt sulphate (+20‰). The seaspray component (PSS) was calculated for 39 coastal rain events using Cl−, Na+ and Mg2+ as reference species and these parameters were plotted against the sulphur isotopic composition of sulphate in the sample. All three regression lines yielded a value of ≈ +4‰ for a sample containing no seasalt. The regression lines extrapolated to δ34S≈+20, +17 and +16‰ using Cl−, Na+ and Mg2+ respectively for a theoretical sample whose sulphate was derived totally from seasalt. This observation confirms that for this locality, the marine component of precipitation sulphate utilizing Cl− is isotopically equivalent to sea spray sulphate, suggesting that Cl− is a suitable reference species. It also supports the use of δ34SSO4 as a constraint on the selection of the correct reference species for estimation of the seasalt component in a particular study area.