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3 Focus on Shortâterm NO2 Concentrations The focus of the ACRP 02â43 Research is primarily on modeling shortâterm (i.e., one hour) NO2 concentrations â commensurate with the promulgation of the new NAAQS for this pollutant.Â Â Â ï§ Reasonable Accuracy - The Preferred Method should be as accurate as possible and produce results that are comparable to other similar modeling applications. For dispersion modeling, reasonable accuracy is generally considered to be able to accurately predict concentrations to within a factor of two, as it is very difficult for dispersion models to show close agreement when comparing âmodeled-to- measuredâ results. ï§ Compatible with EDMS/AEDT/AERMOD - Because the FAA follows the U.S. EPAâs strategy of using AERMOD for most dispersion modeling needs, the Preferred Method should be either adoptable within, or work in conjunction with, AERMOD. This is particularly relevant to use of AERMOD in the EDMS/AEDT models. These âKeyâ targets are outlined here to help ACRP 02-43 users focus their attention on how and why the Preferred Method was selected and developed. It is also important to note that the Research is aimed primarily at improving the accuracy for predicting short-term (i.e., one hour) NO2 concentrations for comparison to the new National Ambient Air Quality Standard (NAAQS) for this pollutant. Recent modeling efforts to predict levels of this pollutant near airports and over this short timeframe have revealed that the resultant values can be significantly overestimated. This is in variance to modeling of annual NO2 values which are thus far shown to be in compliance with the NAAQS. 2.0 Background Information This section provides summary information on what is known about NO2/NOx emission characteristics and their formation mechanisms as it applies to this Research Project. This information is greatly expanded upon in the Interim Report (published separately) and is further supplemented with information pertaining to emission standards, human health effects and other relevant materials. 2.1 Emission Indices Table 1 provides a listing of the ICAO NOx emission indices (i.e., rates) for aircraft engines currently within the EDMS/AEDT models. As shown, the NOx emission indices are higher for takeoff than for climbout, approach, and idle. Secondly, the NOx emission indices tend to be higher for turbine engines than for turboprop and piston engines. Table 1.  EDMS/AEDT Emission Indices for NOx (grams per kilogram of fuel) Engine Type  Thrust Settings Maximum Minimum Average Turbine Takeoff (100%) 65.8 2.09 26.8 Climbout (85%) 46.3 2.30 21.2 Approach (30%) 28.0 1.15 9.40 Idle (7%) 8.53 0.80 3.89 Turboprop Takeoff (100%) 20.4 0.01 10.2 Climbout (85%) 17.9 0.01 9.53 Approach (30%) 10.7 0.89 6.99 Idle (7%) 7.47 0.45 3.03 Piston Takeoff (100%) 5.88 0.36 2.69 Climbout (85%) 5.60 0.24 3.89 Approach (30%) 10.2 0.95 2.00 Idle (7%) 1.90 0.39 1.01
4  NO, NO2, and total NOx emissions testing reveals that these NOx emission rates for aircraft engines agree reasonably well with the ICAO values, although the actual thrust values used can greatly affect the NO2/NOx ratios. For example, seven percent thrust usually overestimates actual ground-idle thrust and 100 percent thrust usually overestimates actual takeoff thrust. 2.2 Emission Ratios The following summarizes important information about NO2/NOx ratios as they pertain to aircraft engine emissions: ï§ Thrust Settings â In contrast to most other airport-related NOx emission sources (e.g., motor vehicles, ground support equipment), NO2 emissions from aircraft engines account for a widely-variable and, at times, a large portion of total NOx emissions. For example, the NO2/NOx emission ratios range from over 0.95 at the lowest power setting (four percent rated thrust or taxi/idle) to under 0.10 at higher power settings (65 to 100 percent rated thrust or climbout and takeoff). ï§ Idle Versus Takeoff â Largely as a result of the thrust dependence of the NO2/NOx emission ratio described above, the climbout and takeoff portions account for the biggest portion of total NO (and NOx) emissions from the LTO cycle, but the idle mode accounts for the greatest portion of NO2 emissions. ï§ Vertical Variation â Again, because of the dependence of the NO2/NOx emission ratio on thrust setting, the factor is also altitude-dependent. In other words, NO2 is emitted mainly at ground level, whereas NO emissions occur over a wider range of altitudes (i.e., both at surface and aloft). Presently, the extent to which the vertical variation in the NO2/NOx emission ratio affects downwind NO2 concentrations is not well understood. ï§ NonâAircraft Sources â NO2/NOx emission ratios from on-road vehicles are typically less than 0.05 for gasoline-fueled vehicles, 0.05 to 0.2 for diesel vehicles not equipped with particulate filters or oxidation catalysts, and 0.15 to 0.8 for diesel-fueled vehicles equipped with particulate filters/oxidation catalysts. Table 2 provides a summary listing of the emission ratios for most airport-related sources of NO2/NOx emissions. Table 2.  NO2/NOx Emission Ratios for Airport Emission Sources Source NO2/NOx Ratio Aircraft - Approach ~0.16 - Idle 0.6 to 0.98 - Takeoff ~0.08 - Climbout ~0.09 APUs 0.25 to 0.55 Gasoline Vehicles 0.01 to 0.05 Diesel Vehicles 0.05 to 0.20 (with DPM) 0.15 to 0.80 GSE (Gasoline) 0.05 (Diesel) 0.05 to 0.90 Stationary Engines (Diesel) 0.06 (CNG) 0.13   Â
5 2.3 Plume Chemistry  In basic terms, NOx (i.e., NO and NO2) is formed during fuel combustion mainly as a result of the thermal oxidation of atmospheric nitrogen (N2). From most combustion sources, NOx is emitted primarily in the form of NO and in the atmosphere this NO can be converted to NO2 by reaction with ozone (simply represented as follows): ï NO + O3 â NO2 + O2 NO can also be converted to NO2 by reaction with hydroperoxy radicals (HO2) and organic peroxy radicals (RO2, where âRâ represents a carbon-based radical like CH3). During the day (when sunlight is available), NO2 undergoes photolysis, reforming as NO and O3 (represented as follows): ï NO2 + sunlight + O2 â NO + O3 Both of these reactions (i.e., NO to NO2 and NO2 to NO) require quantification to accurately predict downwind NO2 concentrations. Notably, when the rates of these two reactions are equal, NO, NO2, and O3 are said to be in a âPhotostationary Stateâ â one of the principal areas of the Research Project (see Section 1.0). The time required for the photostationary state to form is typically at least a few minutes, or significantly longer if mixing of the exhaust with ambient air is the limiting factor in attaining a photostationary state. Mixing of different exhaust plumes (e.g., aircraft exhaust from the runway and exhaust from on-road vehicles at the terminal) can also disrupt the photostationary state. Meteorological conditions (e.g., wind direction and speed) are also important factors because of their role in atmospheric mixing and plume evolution and also because of the dependence of the reaction NO + O3 on temperature and that of the NO2 photolysis rate on solar radiation. Compounding this process, emissions of NO can rapidly lead to reductions in O3 concentrations and the associated increases in NO2 concentrations. In this way, the maximum concentration of NO2 that can be formed (i.e., âsecondary NO2â) is limited by the ambient O3 concentration and, to a smaller extent, peroxy radicals. Further downwind, O3 levels can recover and even exceed background values via additional photochemical reactions involving volatile organic compounds (VOCs). The impact of NOx emissions on NO2 and O3 values is fairly well known for numerous types of NOx sources (e.g., power plants, on-road and off-road vehicles, all of which emit NOx mainly in the form of NO [i.e., when NO2/NOx emission ratios are less than five percent]). Modeling of NO2/NOx formation mechanisms at airports is more challenging than at single, stationary sources, since airports comprise numerous and varied emission sources, levels of activity, locations, etc. when compared to sources such as power plants. By comparison, aircraft NOx emissions are markedly different since at low engine thrust (i.e., during idle/taxi), the NO2/NOx emission ratio can exceed 90 percent, as discussed above. As a result, the maximum possible NO2 concentrations are not âlimitedâ by the ambient O3 concentration since there is so much âprimaryâ NO2 emitted. This renders the chemical conversion of NO to NO2 by O3 more difficult to simulate for airport-related sources in comparison to point sources. Chemical reactions that convert NO to NO2 are also different for some airport-related sources. For example, there is an additional mechanism by which NO emitted from aircraft engines at low power is directly converted to NO2. Although not well understood, this conversion is not from O3 chemistry (as discussed above) but Photostationary State  Among the main objectives of this Research Project is to assess at least one NO2/NOx conversion method that involves the NO, NO2, and O3 âphotostationary stateâ.  NO2 Formation & Modeling The various and complex formation mechanisms of NO2/NOx make the modeling of these compounds challenging, particularly when attributable to airportârelated emission sources.  Â