Methane (CH$_{4}$) - Methane is the second most prevalent GHG, emitted from industry, agriculture, and waste management activities. 

Microscale - Small scale analysis involving distances up to approximately one kilometer and averaging times up to several tens of minutes. 

Mitigation - This term is defined in 40 CFR 1508.20. It includes: (i) avoiding the impact altogether by n ot taking a certain action or parts of an action or finding a new site; (ii) minimizing impacts by limiting the degree or magnitude of the action and its implementation; (iii) rectifying the impact by repairing, rehabilitating, or restoring the affected environment; (iv) reducing or eliminating the impact over time by preservation and maintenance operations during the life of the action; and (v) compensating for the impact by replacing or providing substitute resources or environments. 

Mixing Height - The height of the completely mixed portion of atmosphere that begins at the earth's surface and extends to a few thousand feet overhead where the atmosphere becomes fairly stable. See also "inversion''. 

Mobile Source - A moving vehicle that emits pollutants. Such sources include airplanes, automobiles, trucks and ground support equipment. 

Mobile Source Air Toxics (MSATs) - Mobile source air toxics are compounds emitted from highway vehicles and nonroad equipment which are known or suspected to cause cancer or other serious health and environmental effects. Mobile sources are responsible for direct emissions of air toxics and contribute to precursor emissions which react to form secondary pollutants. Examples of mobile source air toxics include benzene, 1,3- butadiene, formaldehyde, acetaldehyde, acrolein, polycyclic organic matter (POM), naphthalene, and diesel PM. 

Modal Emissions Factors - Vehicular emissions factors for individual modes of operation. For aircraft, these modes are takeoff, climbout, approach and taxi. 

Model - A quantitative or mathematical representation or simulation which attempts to describe the characteristics or relationships of physical events. 

Monitoring Site - A location of a measurement device in a monitoring network. 

NAAQS Assessment - For the purpose of this Handbook , this refers to the use of an air quality model to predict ambient concentrations of air pollutants and the comparison of these results to the NAAQS. 

National Ambient Air Quality Standard (NAAQS) - Air Quality standards established by the EPA to protect human health (primary standards) and to protect property and aesthetics (secondary standards). 

National Environmental Policy Act (NEPA) - An Act established “…to declare a national policy which will encourage productive and enjoyable harmony between man and environment; to promote efforts which will prevent or eliminate damage to the environment and biosphere and stimulate the health and welfare of man; to enrich the understanding of ecological systems and natural resources important to the Nation; and to establish a Council on Environmental Quality.” [42 USC § 4321 - Congressional declaration of purpose ] 

