less). PM$_{10 }$ includes solid and liquid material suspended in the atmosphere for med as a result of incomplete combustion. 

PM$_{2.5 }$ - a criteria pollutant for which there are NAAQS and represents the category particulates categorized as “fine” PM (i.e., with an aerodynamic diameter of 2.5 microns or less). These particles are consider ed a health risk because of their ability to penetrate deep into the human respiratory system. Aircraft are the primary source of PM$_{2.5 }$emissions at airport. 

Point Source - In air quality modeling a pollutant source that is fixed to the ground and that releases pollutants through a relatively small area. Common stationary sources at airport include boilers, heaters, incinerators and fuel storage tanks. 

ppb - Parts per billion (10$^{9}$) by volume 

ppm - Parts per million (10$^{6}$) by volume. 

Precursor - A chemical compound that leads to the formation of a pollutant. HC and NO $_{x}$, are precursors of photochemical oxidants. 

Preferred Model - A refined model that is recommended for a specific type of regulatory application. 

Prevention of Significant Deterioration (PSD) Area - A geographic area that contains air which is relatively clean and not in violation of NAAQS. The emissions in these area regulated to prevent degradation of its air quality. 

Primary Pollutant - Chemical contaminants which are released directly to the atmosphere by a source. 

Primary Standard - A NAAQS set to protect human health. 

Receptor - A location at which ambient air quality is measured or estimated. 

Record of Decision (ROD) - The decision document, prepared after the EIS, that states what the decision is, identifies all alternative considered by the lead agency in reaching its decision, and states whether all practicable means to avoid or minimize environmental harm have adopted, and if not, why not. 

Scoping - An early and open process (that invites the participation of affected Federal, state and local agencies, any affected Indian tribe, the proponent of the action and other interested persons) that determines the issues to be addressed in an environmental document and ide ntifies relevant and/or significant issues related to a proposed action. 

Screening Technique - A relatively simple analysis technique to determine if a given source is likely to pose a threat to air quality. Concentration estimates from screening technique s are conservative. 

Secondary Pollutant - Atmospheric contaminants formed in the atmosphere as a result of such chemical reactions as hydrolysis, oxidation and photochemistry. 

Secondary Standard - A NAAQS set to protect human welfare. 

Semi-volatile Organic Compounds (SVOCs) - an organic compound which has a boiling point higher than water and which may vaporize when exposed to temperatures above temperature. Semi-volatile organic compounds include phenols and polynuclear aromatic hydrocarbons (PAH). 

