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26 emission factors for aircraft. A commercial jet engine and a These values fall in line with a EUROCONTROL study (Kugele military jet engine were tested under varying thrust conditions, et al. 2005) that estimated the average tire and brake emission and the studies identified that formaldehyde and acetaldehyde rates per LTO as 130,000 mg and 30 mg, respectively. are the two predominant HAPs contained in jet exhaust. These The percentage of emissions from tire and brake wear that studies represent a thorough analysis of two jet engines, but become suspended and are classified as PM10 (or PM2.5) is an also highlight the need for expanded measurements on a area of current research. Little data are available for vehicles; broader and more modern range of engine types. none is available for aircraft. For tires, it is believed that less than 10% of emissions become PM10, but studies have shown it can be as high as 30% (Boulter 2005). Nearly all tire wear 6.2 Literature Reports emissions are larger than PM2.5. For brakes, a study conducted on Aircraft PM by Sanders et al. (2003) observed that between 50% and 90% Particulate matter emitted within airport boundaries comes of brake emissions become airborne particles (mass mean di- from many sources, such as aircraft engines, aircraft APU, tire ameter is 6 m and the number-weighted mean is between and brake wear, GSE, vehicles that travel to and from the 1 to 2 m). The measurement brackets the United Nations airport (ground access vehicles), dust from construction, Economic Commission for Europe (UNECE) estimate of 70% boilers, and training fires for firefighters. The relative contri- of brake lining becoming suspended matter (UK DfT 2007). butions of all of these sources are not well characterized, as Since aircraft experience more extreme braking conditions there are limited data for some PM sources. than vehicles do, the PSDH study uses the upper limits of 10% for tire wear and 100% for brake wear for estimates of PM10 emissions for aircraft. 6.2.1 Relative Contributions from GSE and Aircraft Brakes/Tires 6.3 Modeling PM Using EDMS The relative contribution of GSE to total airport emissions depends on many factors, including the size of aircraft served Researchers use an FAA-developed, EPA-approved tool and the length of flight. In addition, the fuel type of the GSE known as EDMS to estimate PM emissions from aircraft directly affects the PM emissions, especially in the case of main engines, GSE, on-road vehicles, and stationary sources. electric GSE where the emissions are generated off-site and The required tool for assessing the changes to local air quality are therefore not included in the airport inventory. As a resulting from airport projects is EDMS. result of these factors, the relative contribution of GSE emis- The EDMS tool estimates primary PM emissions for ICAO- sions is small (less than 20% of the airport total) at some certified aircraft main engines with a smoke number using airports and large (greater than 50% of the airport total) at FOA 3.0a for U.S. airports and FOA 3.0 for airports outside others. An analysis of the mix of GSE equipment and its uti- the United States. The FOA 3.0 method is accepted by the lization is needed to properly quantify its contribution to Committee on Aviation Environmental Protection (CAEP), emissions at a specific airport. and FOA 3.0a has been approved by the EPA. Together, they Tire wear rates are calculated both by experiment and by represent the latest methods approved by these groups to estimation from statistical information. For vehicles, wear rates approximate primary PM emissions from aircraft. The estimate are typically reported as milligrams per vehicle-kilometer (vkm) of non-volatile PM emissions is based on smoke number, where traveled (vkm takes into account the four tires on a typical the estimates of volatile PM are based on UHC and fuel sulfur vehicle). Wear rates vary depending on numerous factors, in- content, and--in the case of FOA3a--lubricating oil. For jet cluding the weight of the vehicle, tire composition, and driving and turboprop aircraft without smoke numbers, only the conditions. Brake and tire wear rates typically are reported volatile contribution to primary PM is computed. EDMS does as mg/vkm and can vary greatly depending on braking con- not estimate any PM emissions for piston aircraft (CSSI 2008). ditions. For light-weight vehicles, the brake wear rates range EDMS uses a standard, single fuel, sulfur level for each aircraft; from 8.8 mg/vkm to 20 mg/vkm (Legret and Pagotto 1999). the level of sulfur can be adjusted for scenarios and aircraft. Aircraft tire and brake emissions are reported on a per LTO EDMS models PM from ground support equipment using basis. Much like vehicles, aircraft tire and break emissions EPA's NONROAD model and PM from on-road vehicles estimates contain large uncertainties and vary depending on the using EPA's MOBILE model. The EPA's NONROAD model type of aircraft and the landing conditions. For six different air- can also be used outside of EDMS to estimate the PM emis- craft listed in the Project for the Sustainable Development of sions from construction equipment engines, but not from Heathrow (PSDH) study (UK DfT 2007), the range of emission other PM sources, such as fugitive dust, that can result from rates (tire and brake) was measured to be between 110,000 mg earthmoving activities. Airport modelers must account for per landing (A321) and 780,000 mg per landing (B747-400). these emissions separately.