Controls on the Stable Isotope Composition of Atmospheric Water Vapour, Southwestern Ontario, Canada

The stable isotopic composition of near-ground level atmospheric water vapour on the southern shore of Lake Huron (southwestern Ontario) from July 2004 to May 2005 was controlled mostly by the range of different air masses present in this region at various times during the sampling period. A linear relationship exists between the isotopic composition of water vapour and surface air temperature, with slopes of 0.41 (oxygen-18 versus T; r2=0.68) and 3.2 (hydrogen-2 versus T; r2=0.72). A polynomial relationship was observed between the isotopic composition of water vapour and absolute humidity (r2=0.64 for oxygen-18 versus absolute humidity; r2=0.63 for hydrogen-2 versus absolute humidity). Deuterium-excess (d-excess) values were particularly useful in distinguishing between air masses with short versus long transport histories across land. For example, d-excess values were typically less than 15 for maritime air masses. Continental air masses, by comparison, commonly had d-excess values greater than 15 during the winter months. Local effects such as lake- and soil-water evaporation, transpiration and snow sublimation had minimal effects on the stable isotopic composition of the water vapour, even though the samples were collected very close to the ground surface (0.5 and 3m). Gradients in the isotopic composition of water vapour between these elevations can be used to partition the contribution of local evaporation versus transpiration to the atmospheric water vapour during the summer months, when these processes were most active.