Pollutant  General Characteristics  Ozone  (O$_{3}$)  Ozone occurs both in the earth's upper atmosphere and at ground level. O $_{3 }$occurs naturally in the  upper atmosphere, where it forms a protective layer that shields the earth from the sun's ultraviolet rays. Tropospheric, or ground level O$_{3}$, is not emitted directly into the air, but is a result  of VOCs and NO$_{x }$reacting in the presence of sunlight in the atmosphere.  Typically, O$_{3 }$ levels are  highest in warm-weather months. VOCs and NO$_{x }$are termed "ozone precursors" and their emissions  are regulated in order to control the creation of O$_{3. }$VOCs, which are a subset of hydrocarbons (HC),  are released in industrial processes, mobile sources and from the evaporation of gasoline, solvents  and other hydrocarbon-based compounds.  In humans, O$_{3 }$ is a pulmonary irritant that affects the respiratory mucous membranes, other tissues, and respiratory functions. Exposure to O$_{3 }$at high concentrations can result in symptoms such  as tightness in the chest, coughing, and wheezing, and can trigger an attack or symptoms of asthma, bronchitis, and emphysema.  Particulate  Matter  (PM)  Particulate matter is made up of small solid particles and liquid droplets suspended or settling out of  the atmosphere. PM consists of a number of components, including acids (such as nitrates sulfates), organic chemicals, metals, and soil or dust particles. PM can be formed from both natural  and man-made sources including forest fires and wind erosion over exposed soils (i.e., fugitive dust);  the incineration of solid wastes; and as an exhaust product from the internal combustion engine . Of  growing concerns are the effects of PM on visibility and the potential impairment to human health  by small PM (i.e., ultrafine particle  emissions or PM$_{0.1}$)$_{. }$ The regulatory standards for PM segregated by sizes: less than or equal to 10 micrometers (denoted PM $_{10}$) and less than or equal to  2.5 micrometers (denoted PM$_{2.5}$).  • PM$_{10 }$represents the category of particulates categorized as “inhalable coarse” PM (i.e., with  an aerodynamic diameter of 10 microns or less). PM 10  includes solid and liquid material suspended in the atmosphere. Formed as a result of incomplete fuel combustion, industrial  processes, or wind erosion, examples of PM 10  include dust, fog, and fu mes. The level of PM$_{10 }$in the atmosphere is largely affected by wind and rainfall conditions.  • PM$_{2.5 }$ represents the category of particulates categorized as “fine ” PM (i.e., with a n aerodynamic diameter of 2.5 microns or less).  These particles are more formed from the combustion of fuel, other industrial processes and various processes.  PM$_{10 }$and PM$_{2.5 }$are considered a health risk in humans because of their ability to penetrate into the  human respiratory system.  Sulfur  Dioxide  (SO$_{2}$)  Sulfur dioxide is a colorless gas  also with a strong characteristic odor. SO$_{2 }$ is emitted into atmosphere by both natural processes and by man-made sources such as the combustion of sulfur - containing fuels and sulfuric acid manufacturing. When combined with other substances in the air,  SO$_{2 }$ can precipitate out as rain, fog, snow, or dry particles (commonly referred to as “acid rain”).  Sulfate particles are a major cause of reduced visibility in many areas of the U.S.  In humans, the i nhalation of elevated concentrations  of SO 2  can cause irritation of the membranes, bronchial damage, and can exacerbate pre-existing respiratory diseases such as asthma,  bronchitis, and emphysema.  Source: EPA,  Six Common Air Pollutants , 2013, http://www.epa.gov/airquality/urbanair/.  Hazardous air pollutants (HAPs) are pollutants for which there are no NAAQS, but are regulated under the federal CAA because of their potentially adverse effects on human health  and the environment. Also known as “air toxics”, these pollutants are comprised of a wide array  3.2.2.  Hazardous Air Pollutants  Table 3-1. EPA Criteria Air Pollutants  Aviation Emissions and Air Quality Handbook  14$_{ }$