Urban Atmospheric Water Vapor and Its Stable Isotopologues

Olivia E Salmon, Purdue University


Water vapor (H2Ov) is known to be a key agent in global scale processes, including: atmospheric circulation, weather, radiative forcing, and energy transfer. Anthropogenic modification of atmospheric H 2Ov, and the processes it influences are not as thoroughly studied on the urban or regional scale, in comparison. Airborne measurements made aboard the Purdue University Airborne Laboratory for Atmospheric Research (ALAR) investigated a series of science questions tied to atmospheric H 2Ov concentrations in Indianapolis, IN, and the near megacity-sized urban area connecting the Washingon, D.C.-Baltimore, MD area. The first study described herein focuses on the intermittent urban H 2Ov excess signal that was investigated using multi-year airborne measurements in Indianapolis, IN, and the Washington, D.C.-Baltimore, MD area. On days when an urban H2Ov excess signal was observed, H2Ov 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 H2Ov from the urban study sites. Estimates of H2Ov emissions from combustion sources and electricity generation facility cooling towers are 1–2 orders of magnitude smaller than the urban H2Ov emission rates estimated from observations. Instances of urban H 2Ov 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. The second study describes airborne measurements of H2O v stable isotopologues made around two continental U.S. cities in February – March 2016. Research flights were designed to characterize the δD, δ 18O, and deuterium-excess (d-excess = δD – 8×δ 18O) vertical profiles extending from the surface to ≤2 km. Three case studies indicate that anomalies in the d-excess signature, along with complementary meteorological observations, can be used to identify the type of boundary layer, cloud, and mixing processes present. The results of the study indicate that in situ H2Ov stable isotope measurements, and d-excess in particular, could be useful for improving the community’s understanding of moisture processing, transport, andmixing between the boundary layer, inversion layer, and free troposphere. The third study describes airborne mass balance experiments conducted around the Washington, D.C.-Baltimore area using several research aircraft to quantify emissions of nitrogen oxides (NOx = NO + NO2) and carbon monoxide (CO). The airborne measurements supported the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign, an intensive airborne study of anthropogenic emissions along the Northeastern United States in February–March 2015, and the Fluxes of Atmospheric Greenhouse Gases in Maryland (FLAGG-MD) project which seeks to provide best estimates of anthropogenic emissions from the Washington, D.C.-Baltimore area. Inventory and observations-derived NOx emission rates are consistent within the measurement uncertainty. However, observed CO emission rates are a factor of 2 lower than reported by the NEI. A strong influence of CO seasonal trends, nationwide CO reductions, or misrepresentation of CO emissions in models is likely responsible for the discrepancy. There is a need for reliable observation-based criteria pollutant emission rate measurements independent of the NEI. Such determinations could be supplied by the community’s reporting of sector-specific criteria pollutant/CO2 enhancement ratios, and subsequent multiplication with currently available and forthcoming high-resolution CO2 inventories.




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Subject Area

Atmospheric Chemistry|Analytical chemistry

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