Airport Air Quality Management 101 (2018)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

12 Air quality analysis is essential for understanding the impacts of airport air emissions on people as well as the environment. It requires specialized knowledge and tools because of the complexity of the processes taking place. This section discusses the need for air quality analysis, the various types of analyses, and when and why different analyses and methodologies are needed. Air quality computer models are required to determine the specific impacts of airport emissions. FAA uses an approved suite of tools that allows for a thorough assessment of the environmental effects of aviation. The primary or central air quality tool is Aviation Environmental Design Tool (AEDT). AEDT is used for planning, environmental compliance, and research analysis (see Section 5: Tools for Airport Air Quality Analysis). 4.1 Emissions Inventory An air quality analysis begins by quantifying the mass (e.g., lb or kg) of pollutant emissions from all sources on the airport to produce an airport emissions inventory. Major emissions sources at airports are aircraft engines, APUs, GSE, stationary equipment, and ground access vehicles (see Section 3: Airport Emissions and Sources). They are generally described as mobile sources (e.g., aircraft and passenger automobiles where discrete vehicles are emitting pollutants) or stationary sources (e.g., emergency generator or a boiler used to heat water for terminal heat- ing). Emissions inventories are frequently organized by source type. Emissions contributions from individual sources are computed based on two components: (1) activity levels for a predetermined period of time and (2) source-specific emission factors (expressed as emission mass produced according to some measure of activity like period of operation, fuel consumption, or operating cycles). AEDT includes an extensive database of emission factors for equipment commonly found at airports (e.g., aircraft, GSE, and emergency generators). These two components are then used for all sources to calculate the total source- related emissions at the airport. The airport’s inventory of emissions can then be expressed by period of time (e.g., hour, day, week, month, or year) or by pollutant (see Section 6: Air Envi- ronmental Regulations Applicable to Airports). An emissions inventory is useful in comparing the emissions from individual sources to better understand their relative contribution to airport air quality or tracking emissions over time and may be an essential element of regulatory reports, planning studies, or sustainability programs. For most airports, aircraft are the largest source of emissions and because of the complexity of their operations, aircraft activity is broken out into several segments to simplify computation and highlight the significance of each segment. The aircraft operating segments used by AEDT, referred to as the LTO, are • Approach: The airborne segment of an aircraft’s arrival extending from the start of the flight profile (i.e., the mixing height) to touchdown on the runway. S E C T I O N 4 Air Quality Analysis

Air Quality Analysis 13 • Taxi in: The landing ground roll segment of an arriving aircraft from touchdown to the runway exit, including reverse thrust braking and taxiing from the runway exit to a gate. • Taxi out: Aircraft main engine startup at the gate and taxiing from the gate to a runway end. • Takeoff: From the start of the ground roll on the runway, through wheels off, and the airborne ascent up to engine cutback during which the aircraft operates at maximum thrust. • Climb out: The portion from engine cutback to the end of the flight profile. The activity level used in computing aircraft emissions is “time-in-mode,” which is the time an aircraft spends in each mode of the LTO cycle. Automobiles, trucks, and other vehicles are commonly the second largest source of emis- sions. Motor vehicle emissions inventories at airports typically focus on three distinct activities within the airport boundary: (1) vehicles operating on airport roadways, (2) vehicles accessing parking facilities, and (3) vehicles operating along the terminal curbside areas for passenger pickup and drop-off. EPA’s Motor Vehicle Emission Simulator (MOVES) is the federally approved model for computing motor vehicle emissions rates (see Section 5: Tools for Airport Air Quality Analysis). 4.2 Dispersion Modeling To understand the actual impact of the emissions inventory, it is necessary to understand how airport emissions are transported and dispersed, how they mix with background pollutants, and how they react and/or transform in the atmosphere to result in a pollutant concentration [e.g., mass per unit volume measured in part per million (ppm) or microgram per cubic meter (µg/m3)] at specified locations surrounding the airport. In addition, it is important to know the average concentration over a specified averaging time period since that is how U.S. air quality regulations are defined (see Section 6: Air Environmental Regulations Applicable to Airports). Calculating concentrations is very complex and requires special computer models to quan- tify and simulate the movement of pollutants from the emission source as they spread out in the atmosphere due to weather, nearby structures, and local terrain. Emission sources release

14 Airport Air Quality Management 101 pollutants to the air, creating a plume. Atmospheric motion determines the overall speed and direction the plume travels and is primarily responsible for the mixing, or dispersion, that takes place within the ambient atmosphere. An important factor in how emissions disperse has to do with plume dynamics, or the physical condition of the emission plume. The temperature of the plume affects its buoyancy. High-temperature emissions, such as those from aircraft engines, will rise once released into the surroundings as a result of temperature difference; the greater the temperature differ- ence, the greater the buoyancy. Another factor in how emissions disperse is the plume velocity. High-velocity emissions, again like those from aircraft engines, lead to shear with the local wind causing turbulence, which causes entrainment of and mixing with the surrounding air. Low-velocity emissions will have much less shear and lower entrainment. The direction of the plume is determined by large-scale impacts, such as wind, while mixing is related to small- scale effects like turbulence. Likewise, terrain characteristics and local building structures can have an effect on local pollutant concentrations due to changes in the wind patterns and the generation of turbulence. Other emission-specific processes also may have an effect such as dry and wet deposition. All of these parameters affect atmospheric dispersion and lead to a three-dimensional, time-dependent concentration distribution of the pollutants. Air dispersion models are used to simulate the transport and dispersion of pollutants over time in the atmosphere. These models are composed of a sequence of mathematical equa- tions that require information about the physical setting, emission sources and their emission rate, temperature, velocity, direction, spatial location, background pollutant concentrations, weather conditions, and surface characteristics of the airport. The dispersion model predicts pollutant concentrations at user-specified receptors that adequately surround the airport and nearby vicinity to capture maximum impacts. Even though air dispersion models are simplified repre- sentations of reality, they are very complex and based on the current best scientific understanding of the factors that influence the movement, dispersion, and transformation of pollutants in the atmosphere. After quantifying emissions from sources over a common time period, the models compute the impact of local meteorology, or how weather and other atmospheric conditions cause the emissions to migrate or disperse into the environment. Wind speed and direction, ambient temperature stratification, and surface heating and cooling are the most significant meteorological factors causing emissions to disperse. Local geographical features can disrupt the effects of meteorology, and these are accounted for in dispersion models. These features could include airport buildings and other structures on the airfield as well as nearby hills that make for complex terrain. Downwash, the effect of the turbulent wake in the lee of a building, is a term used to represent the potential effects of a building on the dispersion of emissions from a source. In dispersion modeling, downwash is considered for structures that are typically modeled as buildings and affects only point sources. The height and proximity of a point source to a structure (building) along with the structure characteristics (size and location) can be used to determine the significance of the downwash effect. Downwash does not apply to area or volume sources, which release emissions near ground level. The pollutants released at the airport (i.e., source emissions) mix with the pollutants that are already in the atmosphere (i.e., background emissions) forming a more complex mixture.

Air Quality Analysis 15 Adding further complexity, many pollutants undergo a chemical reaction and transform in the atmosphere. For example, at the aircraft engine exit, hot combustion gases mix with ambient air to quickly cool the gas stream. Some gases, like heavy hydrocarbons, can condense under these conditions to form aerosol particles. In the exhaust plume, as emissions continue to cool, some molecules undergo chemical reactions and produce other molecules that can also condense into particles. Likewise, gaseous and particle emissions from cars, trucks, and ground vehicles that have exhaust pipes, catalytic converters or particle traps, and mufflers transform in the exhaust plume after mixing with the ambient atmosphere. Therefore, to truly understand the health and envi- ronmental impacts of airport emissions, it is necessary to understand and track all of these factors. Once the air quality models have computed an emissions inventory and evaluated the effects of meteorology, plume dynamics, and emissions chemistry, they determine the pollutant con- centration at defined receptor sites, including locations of expected maximum concentration, locations where employees are present, and locations where the general public is commonly present. This shows the degree to which passengers, citizens living nearby, and the local com- munity are subject to airport emissions impacts. As noted above, AEDT was developed specifically for airport air quality analysis. Its default dispersion model is AERMOD, a steady-state dispersion model developed and maintained by EPA. It models air dispersion based on boundary layer turbulence structure and scaling con- cepts, includes treatment of both surface and elevated sources, and can be applied in both simple and complex terrain. AERMOD originally was designed to model stationary sources and has since been adapted for use at airports, simulating aircraft as a series of area sources. There are no EPA-approved dispersion models developed specifically for airports. AERMOD is non-proprietary and is EPA’s preferred regulatory dispersion model for near field (< 50 km) applications. Emissions computed by AEDT are dispersed as predicted by AERMOD to produce pollutant concentrations. The AEDT/AERMOD combination is used for the vast majority of airport air quality analyses performed in the United States. Essential References for Section 4: Air Quality Analysis • Aviation Emissions, Impacts & Mitigation: A Primer, U.S. FAA Office of Environment & Energy, 2015 • Aviation Emissions and Air Quality Handbook, Version 3, Update 1, FAA Office of Environment & Energy, January 2015 • Airport Air Quality Manual, International Civil Aviation Organization, 2011 • ACRP Report 11: Guidebook on Preparing Airport Greenhouse Gas Emissions Inventories, Transportation Research Board, 2009 • ACRP Report 84: Guidebook for Preparing Airport Emissions Inventories for State Implementation Plans, Transportation Research Board, 2013 • ACRP Report 102: Guidance for Estimating Airport Construction Emissions, Transportation Research Board, 2014 (includes ACEIT and instructional video) • ACRP Report 149: Improving Ground Support Equipment Operational Data for Airport Emissions Modeling, Transportation Research Board, 2015 • ACRP Report 179: Dispersion Modeling Guidance for Airports Addressing Local Air Quality Health Concerns, Transportation Research Board, 2017 • User’s Guide for the AMS/EPA Regulatory Model—AERMOD, EPA-454/B-03-001, U.S. EPA Office of Air Quality Planning and Standards, September 2004, Addendum March 2011