Wintertime decoupling of urban valley and rural ridge hydrological processes revealed through stable water isotopes

Water from fossil fuel combustion can represent > 10% of urban specific humidity, but this fraction is difficult to constrain from meteorological measurements alone. Stable water vapor isotopes can be used to estimate the fraction of combustion-derived vapor (CDV) and characterize the contrast between anthropogenically-altered and natural systems due to CDV's distinctive isotope composition. However, accurate estimates of the CDV fraction of urban humidity requires information on vapor isotope ratios in the absence of anthropogenic emissions. We present data from an urban site in Salt Lake City, UT and from a high-elevation site in the adjacent Wasatch Mountains. Urban vapor d-excess values (d-excess = delta H-2-8 delta O-18) closely track CO2 concentrations on diurnal to weekly timescales, but high-elevation vapor values do not. Instead, the high-elevation site captures large-scale atmospheric variability, with d-excess changes largely following changes in delta O-18 and humidity. Isotope ratios at the two sites remain distinct throughout most of the winter, indicating that these sites are regularly decoupled from each other and that high-elevation winter measurements may rarely be representative of valley conditions in the absence of urban emissions. Furthermore, high-elevation d-excess may be higher than emissions-free values at the same elevation as the urban area, as vapor d-excess changes non-linearly during condensation at low specific humidity, and would result in an overestimate of urban humidity amounts. Therefore, the high-elevation site may not help place additional constraints on the amount of CDV in urban systems, yet the paired sites show changes in vapor isotope ratios, particularly in d-excess, that capture differences in urban and natural impacts on water vapor cycling.