Date of Award
Doctor of Philosophy (PhD)
Airborne lead (Pb) has been regulated as a criteria pollutant by the United States Environmental Protection Agency (EPA) since the Clean Air Act and its amendments in the 1970s. During the 1970s, atmospheric Pb emissions were dominated by the combustion of leaded automobile fuel and metals manufacturing. Over time, those emissions have decreased greatly and according to the EPA the largest emitter of Pb today is piston-engine aircraft. Additionally, in 2008 the EPA reduced the National Ambient Air Quality Standard (NAAQS) for Pb by an order of magnitude. These combined factors served as the impetus for further study of general aviation Pb emissions by the EPA, local and regional air planning agencies, and airports.
This dissertation is focused on characterizing Pb impacts at and around general aviation airports because of piston-engine aircraft activity. It includes a detailed analysis of Pb emissions and concentrations by both measurement and modeling. On-site particulate matter (PM) sampling was conducted at three general aviation airports across the United States with varying size, meteorological, and layout characteristics. Those airports were Richard Lloyd Jones Jr. Airport (RVS) in Tulsa, OK, Centennial Airport (APA) in Denver, CO, and Santa Monica Airport (SMO) in Santa Monica, CA. Airborne PM samples collected at these airports were digested and analyzed for Pb by inductively coupled plasma-mass spectrometry (ICP-MS) and a subset of samples were analyzed by X-ray fluorescence (XRF) to examine both Pb and Bromine (Br). Measurement data were used to characterize Pb at airports by examining differences in Pb concentrations at sampling locations upwind and downwind of piston-engine aircraft activity on the airport footprints. Specific analyses included upwind-downwind differences in total Pb concentrations, differences in Pb-Br correlations for samples with predicted high and low aircraft emissions impacts, and differences in Pb isotope ratios measured in the high and low impact samples. The analysis showed that Pb-Br correlation and especially Pb isotope ratios, could serve as markers for identifying Pb impacts from aircraft. Measured Pb concentrations were also used to validate the modeling performed as part of this work.
Further analysis of Pb impacts was conducted by performing air dispersion modeling of Pb emissions at airports. Modeling of Pb impacts is critical because Pb measurements are usually only collected at a single or limited number of locations at or near an airport. Initially, Pb emissions at APA were modeled using the Federal Aviation Administration’s (FAA) Emission and Dispersion Modeling System (EDMS), without having detailed information about aircraft activity at the airport. Subsequently, field campaigns were conducted at RVS, APA, and SMO to collect detailed on-site activity data and to characterize the aircraft fleet. These data were collected concurrently with the on-site Pb sampling. The airport-specific data collection was used to generate a spatially and temporally resolved emission inventory which was used as input to the EPA’s AERMOD air dispersion model to estimate Pb concentration fields at and around each of the three airports. Modeled concentrations agreed well with measured values at RVS and SMO, while comparisons at APA were inferior but still acceptable by conventional air quality modeling permeance metrics. The modeling was also used to determine the aircraft operations most significantly contributing to Pb hotspots.
The on-site data collection and air quality modeling framework was then applied to a fourth airport, Palo Alto Airport (PAO) in Palo Alto, CA. Data collection was conducted over a shorter period of time than the other airports. The modeled results at PAO showed excellent comparison to on-site concentrations measured by the local air agency, even though the data collection, other than total daily activity, did not occur at the same time as the modeled period. The model setups for RVS, SMO, and PAO were then used to evaluate two mitigation strategies: moving some activity areas away from others to reduce converging emissions; and replacing leaded aviation gasoline with motor vehicle gasoline in planes that are certified to use it. Moving activity areas significantly reduced maximum Pb concentrations at RVS and SMO with a smaller reduction at PAO, while using motor vehicle gasoline significantly reduced concentrations across the full airport footprints at all three airports.
Richard Axelbaum, Pratim Biswas, Rex Couture, Zorimar Rivera-Núñez, Brent Williams