Forest Trail


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The gaseous phase of liquids or solids at atmospheric temperature and pressure.

Vapor Density

The vapor density is expressed in grams per liter (g/L) and is compared to the density of air (air=1).

Vapor Pressure

The pressure, often expressed in millimeters of mercury (mm Hg) or pounds per square inch (PSI), that is characteristic at any given temperature of a vapor in equilibrium with its liquid or solid form.

Vapor Recovery Systems

Mechanical systems that collect and recover chemical vapors resulting from transfer of gasoline from operations such as tank-to-truck systems at refineries, tanker-to-pipeline systems at offshore oil operations and pump-to-vehicle systems at gasoline stations. For more information, visit our vapor recovery website.


Permission granted for a limited time (under stated conditions) for a person or company to operate outside the limits prescribed in a regulation.

Vehicle Miles Traveled (VMT)

The miles traveled by motor vehicles over a specified length of time (e.g., daily, monthly or yearly) or over a specified road or transportation corridor.

Vessel Speed Reduction (VSR)

A way to reduce emissions of NOx, SOx, diesel PM and CO2 from oceangoing vessels. Emissions are decreased when vessels slow their speeds, thereby reducing the energy requirements of the main engine. The Ports of Los Angeles (POLA), Long Beach (POLB), and San Diego (POSD) currently have a voluntary VSR program in place which requests that vessels slow to 12 knots from certain distances from port (20 nautical miles (nm) for POSD and 20 or 40 nm for POLA and POLB). Ports typically offer incentives for complying.


The degree to which a fluid resists flow under an applied force.


A measurement of the ability to see and identify objects at different distances. Visibility reduction from air pollution is often due to the presence of sulfur and nitrogen oxides, as well as particulate matter.

Visibility Reducing Particles (VRP)

Any particles in the atmosphere that obstruct the range of visibility.


Any substance that evaporates readily.

Volatile Organic Compounds (VOCs)

Carbon-containing compounds that evaporate into the air (with a few exceptions). VOCs contribute to the formation of smog and/or may themselves be toxic. VOCs often have an odor and some examples include gasoline, alcohol and the solvents used in paints.