2022-23 AEESP Distinguished Lecturer

Professor Davidson will present two lectures in the 2022-2023 Tour:

Lecture 1: "The Green Roof as a Complex System"

Abstract: "As cities grow and modify the natural environment, many municipal governments have recognized the benefits of installing green roofs and other types of green infrastructure. Green roofs can reduce combined sewer overflow, minimize flooding, decrease the intensity of the urban heat island, and provide habitat for urban wildlife. In this talk, we consider how the performance of a green roof can be modeled and measured in an effort to understand its benefits in built-up urban areas, using the instrumented extensive green roof on the Onondaga County Convention Center in Syracuse, NY. We examine two general categories of performance, energy flow and storage, and water flow and storage. In the first category, the soil and vegetation that make up the green roof can dissipate heat much more quickly than concrete and other building materials, reducing high temperatures that can impact people’s health and damage urban ecosystems. By installing temperature sensors in the various layers of a green roof during its construction, we can produce thermal profiles as a function of time that can aid in estimating the heat flow through the roof. This is examined in mid-summer when the plants are fully grown and the building is air conditioned, and in cold temperatures in winter when the vegetation is dormant, both with and without a blanket of snow. Other factors such as soil moisture can also impact heat flow through the roof. Regarding water flow and storage, the soil of the green roof can hold stormwater until evaporation as well as transpiration by the plants can remove it. The total rate of water loss, known as evapotranspiration, influences the time it takes for the roof to recharge its water-storing capacity after a rainstorm. By conducting a water balance on the roof, we can understand the complexity of factors that influence how much incoming rainwater flows down the roof drains and how much is retained by the soil and vegetation to evapotranspire slowly over time. Finally, we attempt to explain why some cities have moved forward with many green infrastructure projects, while other cities have preferred traditional gray infrastructure such as regional treatment facilities and storage tanks to reduce flooding and combined sewer overflow. Factors that underlie decisions on green infrastructure by municipal officials are discussed. A separate tutorial will be available on the capabilities of a new website showing real-time data and archived data from the Onondaga County green roof. The website is intended for use in the classroom to help students understand the physical processes taking place on a green roof and the functions of a green roof."

Lecture 2: "The Interactions of Airborne Particles with Surfaces"

Abstract: "Airborne particles exist in a wide variety of shapes, sizes, and chemical compositions. Some are natural, some are emitted from human activities, and others are formed in the atmosphere from gases. The gases can also be natural or anthropogenic. Once airborne, particles can be carried hundreds or even thousands of kilometers by wind before interacting with surfaces and depositing. In this talk, we examine the many ways in which atmospheric particles interact with surfaces of all kinds – natural vegetation, agriculture crops, landscaping, bare soil, water, snowfields, and urban hardscape surfaces. Such understanding is important when predicting the ultimate fate of particulate matter, whether the particles are inhaled and reach the human respiratory system, or whether they deposit on surfaces and cause damage. In all cases of deposition from the atmosphere, particles carried in the mainstream of the airflow must somehow be delivered to the quasi-laminar boundary layer adjacent to the surface, and must then traverse the boundary layer to rest on the surface. These two steps, as well as a third step in which particles rebound off the surface back into airflow, define the deposition process. For a large field of uniform vegetation less than a few meters in height, the wind field and boundary layer characteristics are well known, and deposition onto the vegetation can be predicted for a range of particle sizes and wind speeds. For more complex vegetation, such as a forest canopy, we usually resort to empirical methods to estimate deposition. For water surfaces, the hygroscopicity of the particles may need to be taken into account. Deposition on large lakes and the oceans must also account for wave action. Deposition to snow is complicated by the porous nature of the surface, and the fact that the surface area of individual snow crystals may influence the motions of very small particles. Finally, estimating deposition to buildings, roads, and other urban surfaces can be a challenge due to the changes in geometry of the surface over short distance scales. We discuss the special case of estimating particle deposition onto urban surfaces, include a large extensive green roof. Both modeling and measurement of particle interaction with surfaces is presented, and use of well-controlled experimental surfaces in wind tunnels as well as in the ambient atmosphere is discussed as a means of improving our understanding of the deposition process. A separate tutorial covering the airflow and rain impinging on a green roof in Syracuse, NY will be presented. The tutorial will explain the capabilities of a new website showing real-time data and archived data from the green roof. The website is intended for use in the classroom to help students understand the physical processes taking place on a green roof and the functions of a green roof."