Abstract
Abstract. Water emitted during combustion may comprise a significant portion of ambient humidity (> 10 %) in urban areas, where combustion emissions are strongly focused in space and time. Stable water vapor isotopes can be used to apportion measured humidity values between atmospherically transported and combustion-derived water vapor, as combustion-derived vapor possesses an unusually negative deuterium excess value (d-excess, d = δ2H − 8δ18O). We investigated the relationship between the d-excess of atmospheric vapor, ambient CO2 concentrations, and atmospheric stability across four winters in Salt Lake City, Utah. We found a robust inverse relationship between CO2 excess above background and d-excess on sub-diurnal to seasonal timescales, which was most prominent during periods of strong atmospheric stability that occur during Salt Lake City winter. Using a Keeling-style mixing model approach, and assuming a molar ratio of H2O to CO2 in emissions of 1.5, we estimated the d-excess of combustion-derived vapor in Salt Lake City to be −179 ± 17 ‰, consistent with the upper limit of theoretical estimates. Based on this estimate, we calculate that vapor from fossil fuel combustion often represents 5–10 % of total urban humidity, with a maximum estimate of 16.7 %, consistent with prior estimates for Salt Lake City. Moreover, our analysis highlights that changes in the observed d-excess during periods of high atmospheric stability cannot be explained without a vapor source possessing a strongly negative d-excess value. Further refinements in this humidity apportionment method, most notably empirical validation of the d-excess of combustion vapor or improvements in the estimation of the background d-excess value in the absence of combustion, can yield more certain estimates of the impacts of fossil fuel combustion on urban humidity and meteorology.
Highlights
Fossil fuel combustion releases carbon dioxide and water to the atmosphere
Paired d-excess and CO2 − min (CO2) measurements are available for 76.8 % of the period of record, including for 22 of the 26 persistent cold air pools (PCAPs) events
Measurements of ambient vapor d-excess were paired with CO2 observations across four winters in Salt Lake City, Utah
Summary
Fossil fuel combustion releases carbon dioxide and water to the atmosphere. Water vapor from fossil fuel combustion may impact urban air quality and meteorology, for example, through direct changes in radiative balance by increased water vapor concentrations (Holmer and Eliasson, 1999; McCarthy et al, 2010), impacts on aerosols and cloud properties (Pruppacher and Klett, 2010; Mölders and Olson, 2004; Kourtidis et al, 2015; Twohy et al, 2009; Carlton and Turpin, 2013; Kaufman and Koren, 2006), and altered local or downwind precipitation amounts (Rosenfeld et al, 2008). Using standard meteorological measurements it remains difficult to isolate combustionderived vapor (CDV) from “naturally occurring” water vapor, or vapor from other anthropogenically influenced fluxes (e.g., snow sublimation from buildings), making the impact of CDV on the urban atmosphere difficult to assess
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