Fluorescence of road salt additiives: potential applications for residual salt monitoring

Residual salt monitoring on roads is an ongoing challenge for winter maintenance engineers. Traditionally, estimates are made by measuring the electrical conductivity of the surface water, but this approach is often unreliable and ultimately site specific. In this paper, fluorescence techniques are used to identify the fluorescence signal emitted from a molasses based de-icer mixed with rock salt. Tests have shown that molasses based de-icing products have a unique fluorescence signal, the peak intensity of which excites and emits at 340 and 420 nm respectively and is unaffected by changes in temperature. The fluorescence signal is easily identifiable at temperatures as low as -8 degrees C, and the intensity of the signal has been found to be reasonably constant over the temperature range 0 to 5 degrees C, which is most associated with the marginal nights in the UK where there is a chance a road surface may freeze. Total organic carbon analysis has revealed a relationship between molasses and salt concentrations, meaning the fluorescence signal from the molasses could be used to quantify salt concentrations. Remote sensing tests of the fluorescence signal using a fibre optic probe have revealed difficulties in applying such a technique in a road environment. The potential applications of this research to road weather and other scientific fields are discussed.