Most of the compounds in ambient air are due to emissions into a mixing volume while other compounds found in ambient air are due to chemical reactions of the emitted species. The first type are called primary species and the second type are called secondary species. Examples of primary species are nitrogen (N2), oxygen (O2), carbon dioxide (CO2), water vapor (H2O), nitric oxide (NO), volatile organic carbon (VOC), and carbon monoxide (CO). Examples of secondary species are nitrogen dioxide (NO2), ozone (O3), and organic oxygenated species like aldehydes and ketones.
Some of the emissions are due entirely to natural processes in the earth (called geogenic emissions) and to biospheric processes (called biogenic emissions) from plants and soils. Another set of emissions are due to man's activities (called anthropogenic emissions) and are mostly caused by energy production and use.
Emissions are quantified in a data base called an emissions inventory, which includes lists of emissions sources and their intensities. Inventories are usually sub-divided in to source sector types: area, highway mobile, non-road mobile, point, and biogenic. Emissions intensities are usually stated as a mass per area per unit time (i.e., tons in NC per average weekday, moles/gird-cell-hour).
The air near the earth's surface is heated or cooled from below by contact with the surface. On clear nights, the surface cools and cools the air near the surface, while the air aloft will remains near its daytime, warmer temperatures.. Cooler air has a higher density than warmer air and, in the absence of significant winds, this more dense air on the bottom and warmer, lighter air on top is a stable system that subpresses vertical mixing. During the daytime, the sun heats the earth's surface, which heats the nearby air, causes "bubbles" of lower density air that tend to move upward, bring aloft air down toward the surface to be heated. The more surface heating, the more intense and the higher surface air is mixed.
Emissions into the nighttime stable condition are not very diluted due to the subpressed vertical mixing and build up a higher concentration of emitted compounds. Emissions into the higher daytime mixing height result in much lower primary concentrations.
Winds move emitted mass from the source to other locatons. air near the earth's surface is heated or cooled from below by
NO, VOC, CO are primary species
O3, aldehydes, CO2 are secondary species
Regionally, biogenic VOC exceeds anthropogenic VOC by a factor of 5, and anthropogenic NOx exceeds biogenic NOx by a factor of 100.
Maximum summer mixing heights in NC are 1200--2000 meters.