Aerosol Optical Depth
Aerosol Optical Depth & Fire
In addition to gases and clouds, Earth's atmosphere contains tiny liquid and solid particles called aerosols. Aerosols influence air quality and public health, and they can influence climate by reflecting or absorbing sunlight and by changing where and when clouds form. Aerosols include sea salt, dust, and volcanic ash, as well as soot, sulfates, and other particles produced by people burning fossil fuels. Natural and human-caused fires are also significant sources of aerosols.
The aerosol maps show average monthly aerosol amounts around the world based on observations from the MODIS sensor on NASA's Terra satellite. Satellite measurements of aerosols, called aerosol optical thickness, are based on the fact that the particles change the way the atmosphere reflects and absorbs visible and infrared light. An optical thickness of less than 0.1 (palest yellow) indicates a crystal clear sky with maximum visibility, whereas a value of 1 (reddish brown) indicates very hazy conditions.
The fire maps show the locations of actively burning fires around the world on a monthly basis, based on observations from the MODIS sensors on NASA's Terra and Aqua satellites. The colors are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count — as many as 100 fires in a 1,000-square-kilometer area per day. Yellow pixels show as many as 10 fires, orange shows as many as 5 fires, and red areas as few as 1 fire per day.
The comparison shows the places and times of year in which fires play a major role in aerosols. For example, fire counts and aerosols increase in tandem across South America from July through September, and taper off in tandem in October. This pattern is due to land clearing and agricultural fires that are widespread across the Amazon Basin and Cerrado regions during the dry season. A similar relationship is apparent in Central America (March-May), central and southern Africa (June-September, and Southeast Asia (January-April).
In other cases, however, aerosol concentrations rise in the absence of significant fire activity. For example, from May through August each year, aerosol amounts rise dramatically around the Arabian Peninsula and nearby oceans, even though there is no significant fire activity in the vicinity. Here, dust storms are the likely source of the aerosols. An arc of elevated aerosol amounts at the foothills of the Himalaya Mountains in northern India in some months when fire activity is minimal (for example, November 2006) is an indication that urban or industrial air pollution is playing a role.