Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| Acid Deposition | The transfer of acidic substances from the atmosphere to the Earth’s surface via either wet or dry deposition. |
| Acid Precipitation | Water from the atmosphere in the form of rain, sleet, snow, hail, etc., with a pH less than 5.6. |
| Acid Rain | A popular term generally used to describe precipitation with a pH less than 5.6. It is sometimes used more broadly to include all forms of acid deposition, but this usage is confusing and should be avoided. |
| Anions | Negatively charged ions. In precipitation, the major anions are sulfate (SO₄²⁻), nitrate (NO₃⁻), chloride (Cl⁻), and bicarbonate (HCO₃⁻). |
| Atmospheric (or Total) Deposition | The total transfer of gases, particles, and precipitation from the atmosphere to the ground; it is the sum of wet and dry deposition. |
| Below-cloud scavenging (Washout) | The process by which chemicals (gases, liquids, solids) are incorporated into falling water (liquid or ice) below the base of a cloud. |
| Bulk Sampling | A collection method where a container is continuously exposed to the atmosphere, thus collecting a mixture of wet and dry deposition. |
| Cations | Positively charged ions. In precipitation, the major cations are hydrogen (H⁺), ammonium (NH₄⁺), calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺), and sodium (Na⁺). |
| Dry Deposition | The process and material deposited from the atmosphere onto surfaces in the absence of precipitation. |
| In-cloud scavenging (Rainout) | The process by which chemicals (gases, liquids, solids) become incorporated into cloud water (liquid or ice) during cloud formation. |
| Ion Balance | The principle that in a water solution, the total quantity of positive charge from cations must equal the total quantity of negative charge from anions, ensuring the solution remains electrically neutral. |
| Isopleths | Lines on a map connecting points of equal value. In this context, they are used to show geographic areas with the same median ion concentration or pH. |
| NADP/NTN | The National Atmospheric Deposition Program/National Trends Network, a long-term U.S. network designed to monitor the chemistry of precipitation. |
| NAPAP | The National Acid Precipitation Assessment Program, a massive U.S. federal research and assessment program conducted in the 1980s to summarize the causes and effects of acidic deposition. |
| Net Ions | A calculated quantity representing the difference between the sum of major acid-forming anions (sulfate, nitrate, chloride) and the sum of major base-forming cations (calcium, magnesium, sodium, potassium, ammonium). This value is used to calculate the theoretical pH of a sample. |
| pH | A measure of acidity defined as the negative logarithm of the hydrogen ion activity (or concentration). Each unit decrease on the scale represents a 10-fold increase in acidity. |
| Precipitation | Aqueous material reaching the Earth’s surface from the atmosphere in liquid or solid form, such as rain, snow, sleet, and hail. |
| Quality Assurance/Quality Control (QA/QC) | A set of procedures used to ensure that data produced by a monitoring program are of a specified quality. This includes following documented methods, measuring “check” solutions, and submitting blind reference samples for analysis. |
| Wet Deposition | The process of transferring gases, liquids, and solids from the atmosphere to the ground during a precipitation event; also refers to the amount of material removed by this process. |
| Wet-Only Sampling | A collection method using an automated sampler that is open only during precipitation events to collect wet deposition exclusively. |