Concentration vs. Deposition
The concentration and deposition of compounds are linked, but are not the same. The National Atmospheric Deposition Program (NADP) at the Wisconsin State Laboratory of Hygiene uses both measurements to determine where and how much acids and other pollutants are deposited, at what rate, and by which route.
Concentration is a measure of how much of a given substance (e.g., nitric acid) is mixed with another substance (e.g., water). The greater the amount of chemicals in the substance, the greater the concentration. For example, for health reasons it is important to determine how much of a quantity of nitrates is in a lake, or how much arsenic is in groundwater.
Concentration is expressed as a quantity or mass per unit volume of water collected. At the NADP laboratory, scientists measure the concentration of various compounds found in samples of rainwater, typically how many milligrams of a substance are in a liter of water (mg/L).
Deposition is a measure of the amount of a chemical that is transferred to a certain amount of ground, typically by rainfall, snow, hail, or sleet. The NADP network of sites samples precipitation weekly, so deposition is measured by the amount or mass of chemical transferred to a square meter of ground in one week (kilograms/m²).
The concentration of certain substances is measured in the lab, and this result, multiplied by the amount of rainfall, provides an estimate of the deposition of that substance within a specific area. Scientists use this information to detect short- and long-term trends in acid and mercury deposition.