Date of Award
Doctor of Philosophy (PhD)
Civil and Environmental Engineering
Aerosol, Dry Deposition, Green Roof, Leaf Area Index, Surrogate Surface
Dry deposition refers to the set of processes by which particles and gases are removed from the atmosphere in the absence of precipitation. Predicting particle dry deposition onto surfaces in populated areas is important for estimating the depletion of concentrations of airborne particles moving downwind and for calculating the accumulation of particles onto surfaces. However, there are no standard criteria for estimating dry deposition to surfaces in cities. The equivalent of the throughfall method for urban areas (urban throughfall) involves collecting fresh rain above building roofs while simultaneously sampling rain washoff draining from the roofs. It has not yet been used to estimate dry deposition. This dissertation aimed to test the concept of urban throughfall to estimate dry deposition rates of several chemical species in airborne particles to surfaces. For each experiment, two flat disks and two disks of similar size with rims were exposed to the atmosphere on the roof of Hinds Hall at Syracuse University to collect dry deposited material. The disks represented surrogates for flat horizontal building roofs. Airborne concentrations were simultaneously measured using filter packs to estimate dry deposition velocities. Ratios of the mean dry deposition flux onto the rimmed disks to the mean dry deposition flux onto the flat disks had an overall average of 0.89 for the measured chemical species. Dry deposition velocities estimated to the flat disks had mean values in cm s-1 of 1.4 (fluoride), 0.33 (sulfate), 0.99 (potassium), and 3.1 (calcium), and values obtained by collecting rain washoff were generally comparable. In the future, green roofs may comprise an appreciable fraction of urban surface area. Understanding the surface characteristics and vertical transport of contaminant particles to green roofs may aid in reliably predicting dry deposition to cities. Therefore, this dissertation also aimed to estimate the leaf area index and vegetation cover of a green roof 5550 m2 in surface area in downtown Syracuse, NY by measuring the characteristic dimensions of leaves and by taking photographs on the roof in 2020 and 2021. It was hypothesized that the two predominant species of vegetation were Phedimus takesimensis and Sedum floriferum, which was confirmed by calculating fractional cover percentages of 0.23 and 0.38, respectively. Using a weighted average, the leaf area index of the entire area of this green roof was estimated as 2.15 m2 leaf area m-2 ground area (4.0 if moss is included). Finally, dry deposition rates of sulfate were modeled to the green roof using input parameters such as friction velocity u* and roughness length zo obtained from a meteorological tower on the roof. Values of friction velocity u* had a mean and standard deviation of 0.2 +/- 0.06 m s-1 while values of the roughness length zo had a mean and standard deviation of 0.24 +/- 0.47 cm, with the large standard deviation showing the sensitivity of zo to slight changes in the slope of the windspeed versus ln z graph. Both u* and zo were small, suggesting that the green roof was not as complex compared to other canopies such as tall grasses and forests. Deposition velocities obtained by modeling were consistent with the overall small size of sulfate particles. Preliminary experimental estimates were also obtained by washing the vegetation. Overall, this dissertation provides results that the urban throughfall method is worth additional investigation as a new way of estimating dry deposition.
Johnson, Alexander James, "Estimating Dry Deposition of Atmospheric Aerosols by Rain Washoff from Urban Surfaces" (2023). Dissertations - ALL. 1689.
Available for download on Thursday, June 12, 2025