Elevated H2Ov mole fractions were occasionally observed downwind of Indianapolis, IN and the Washington, D.C.-Baltimore, MD, (D.C.-Balt.) area during airborne mass balance experiments conducted during winter months for 2012 and 2015, respectively. Past studies have reported elevated urban H2Ov concentrations from tower site or surface-mobile measurements, however, this study represents the first reported observations and quantification of citywide enhancements in H2Ov mole fractions during daytime. Simultaneous measurements of CO2 and CH4 mole fractions during the airborne experiments allow us to evaluate impacts of elevated urban CO2, CH4, and H2Ov concentrations on the intensity of the urban heat island.
The wintertime airborne case studies around D.C.-Balt. and Indianapolis show significant urban emissions of H2Ov that result in elevated H2Ov mole fractions downwind of the urban area relative to nearby rural H2Ov mole fractions. On flight days where an elevated H2Ov signal was observed, the emission rate of excess urban H2Ov ranged between 1.6 (±0.66)x104-1.7 (±0.81)x105 and 2.1(±1.2)x104-3.5(±1.4)x104 kg/s for D.C.-Balt. and Indianapolis, respectively. The emissions of excess urban H2Ov contributed between 1.5% and 8.4% to the total flow of atmospheric boundary layer H2Ov out of the urban areas. Radiative transfer modeling indicates elevated urban H2Ov and other greenhouse gas concentrations do not have a significant impact on urban heat island intensity, but urban H2Ov emissions could have other local-scale impacts, which will be discussed.