Suggested Units for Quantities Related to Visibility in the Atmosphere

Abstract

When interpreting data for the scattering, absorption, or extinction of light in the atmosphere by aerosols, it is often useful to report data for the effectiveness of the aerosol in causing these phenomena. Depending on whether the mass or volume concentration of the aerosol is known, this can be done by reporting values for the cross section per unit aerosol mass or cross section per unit aerosol volume. There is great variability among authors for the units used in reporting this information, and cumbersome units are frequently used. The units recommended in this note are: aerosol mass concentration, ..mu..g/cm/sup 3/; aerosol volume concentration, ..mu..g/cm/sup 3/; aerosol particle density, ..mu..g/cm/sup 3/; extinction cross section per mass, m/sup 2//g; extinction cross section per volume, ..mu..m/sup -1/; extinction coefficient, Mm/sup -1/ = 10/sup -6/m/sup -1/. The major advantage of the recommended units is that no unit conversion factor appears in any of them. The main disadvantage is that distances are typically measured in kilometers rather than megameters, so that calculations in which the extinction coefficient is multiplied by a distance to obtain an optical depth typically require a unit conversion factor of 1000. The most common prior practice was to use units ofmore » 10/sup -4/ m/sup -1/, which also require a conversion factor inoptical depth calculations. Therefore, the units recommended here introduce no new inconveniences. In the recommended units, fine particle aerosol scattering and extinction cross sections per unit mass typically have a value near 3 m/sup 2//g. If the aerosol particle density is 1.8g/cm/sup 3/, this corresponds to a scattering or extinction cross section per unit aerosol volume of 5 or 6 ..mu..m/sup -1/. These are convenient numbers to remember.« less