Emissions of Water Vapor from Indianapolis and Washington, D.C. in Winter

For decades, urban areas have been shown to modulate humidity levels relative to the natural surrounding landscapes. Urban-rural humidity gradients have been linked to differences between urban and rural moisture sources and energy balance. Cities are generally expected to be drier than rural areas, however, our study focuses on instances when cities are relatively more humid. Anthropogenic emissions of heat and moisture via combustion and industry have been implicated as contributors to instances of elevated urban atmospheric moisture levels compared to rural areas. Understanding the magnitude and impact of urban emissions of water vapor (H₂O_v) is important because H₂O_v is responsible for approximately 2/3 of the greenhouse effect, and urban emissions of heat, aerosol, and H₂O_v have been shown to influence regional cloud cover, water cycling, and the liquid water content of aerosols, a chemistry and climate-relevant property.

Elevated H₂O_v 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 H₂O_v concentrations from tower site or surface-mobile measurements, however, this study represents the first reported observations and quantification of citywide enhancements in H₂O_v 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, CH_4, and H₂O_v 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 H₂O_v that result in elevated H₂O_v mole fractions downwind of the urban area relative to nearby rural H₂O_v mole fractions. On flight days where an elevated H₂O_v signal was observed, the emission rate of excess urban H₂O_v 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 H₂O_v contributed between 1.5% and 8.4% to the total flow of atmospheric boundary layer H₂O_v out of the urban areas. Radiative transfer modeling indicates elevated urban H₂O_v and other greenhouse gas concentrations do not have a significant impact on urban heat island intensity, but urban H₂O_v emissions could have other local-scale impacts, which will be discussed.