Urban Emissions of Water Vapor in Winter

Elevated water vapor (H₂O_v) mole fractions were occassionally observed downwind of Indianapolis, IN, and the Washington, D.C.-Baltimore, MD, area during airborne mass balance experiments conducted during winter months between 2012 and 2015. On days when an urban H₂O_v excess signal was observed, H₂O_v emissions estimates range between 1.6 × 10⁴ and 1.7 × 10⁵ kg s^-1, and account for up to 8.4% of the total (background + urban excess) advected flow of atmospheric boundary layer H₂O_v from the urban study sites. Estimates of H₂O_v emissions from combustion sources and electricity generation facility cooling towers are 1-2 orders of magnitude smaller than the urban H₂O_v emission rates estimated from observations. Instances of urban H₂O_v enhancement could be a result of differences in snowmelt and evaporation rates within the urban area, due in part to larger wintertime anthropogenic heat flux and land cover differences, relative to surrounding rural areas. More study is needed to understand why the urban H₂O_v excess signal is observed on some days, and not others. Radiative transfer modeling indicates that the observed urban enhancements in H₂O_v and other greenhouse gas mole fractions contribute only 0.1°C day^-1 to the urban heat island at the surface. This integrated warming through the boundary layer is offset by longwave cooling by H₂O_v at the top of the boundary layer. While the radiative impacts of urban H₂O_v emissions do not meaningfully influence urban heat island intensity, urban H₂O_v emissions may have the potential to alter downwind aerosol and cloud properties.