GLOSSARY

Agricultural Burning: the intentional burning of areas for agricultural control. Smoke from agricultural burning can cause increases in local and downwind PM2.5 and O3.

Air Quality Index (AQI): a color-coded, unitless numerical index developed by U.S. EPA to communicate observed and forecasted air pollution levels to the public. There are different AQI scales corresponding to the six criteria pollutants. AQI categories include Code Green (Good; 0-50), Code Yellow (Moderate; 51-100); Code Orange (Unhealthy for Sensitive Groups or USG; 101-150), Code Red (Unhealthy; 151-200), Code Purple (Very Unhealthy; 201-300), and Code Maroon (Hazardous; 301-500). Each color code is tied to health effects that an exposed person may experience within a few hours or days after breathing polluted air.

Anthropogenic: man-made

Back Door Cold Front (BDCF): a cold front the moves from north to south or northeast to southwest into the Mid-Atlantic.

Bay Breeze: a coastal mesoscale wind that blows from the water surface of a bay onto the surrounding shores during the afternoon. It occurs in the spring and summer in the Mid-Atlantic and is caused by the difference in surface temperature between land (warmer) and water (cooler). A bay breeze can act as a line of convergence inland, leading to a buildup of pollutants. Example: Chesapeake Bay breeze.

Canadian Maritimes: collective name for the Canadian provinces of New Brunswick, Nova Scotia, and Prince Edward Island.

Central Mid-Atlantic (CMA): West Virginia, Maryland, Delaware, and northern Virginia.

Coarse Particulate Matter (PM10): particulate matter with aerodynamic diameters less than or equal to 10 microns. PM10 is mainly a primary pollutant with natural and anthropogenic sources. The main chemical components of PM10 are Earth’s crustal materials (dust).

CONUS: Continental United States

Convection: transport of heat and moisture by the movement of a fluid.  Meteorologists often use the term “convection” as a synonym for “thunderstorms,” although technically, thunderstorms are only one form of convection.  The term refers to the vertical transport of heat and moisture in the atmosphere, particularly by updrafts and downdrafts in an unstable atmosphere.

Convergence (low-level): in a horizontal wind field, an area where more wind is entering than leaving at that level; on a weather map, an area where horizontal wind fields meet (wind vectors “point” at each other). Upward forcing from low-level convergence increases the potential for thunderstorm development when other factors, such as instability, are favorable. For air quality, assuming sufficient subsidence to prevent thunderstorm development, areas of convergence can act as locations for buildup of pollutants. Examples: bay/sea breeze fronts, weak stationary or cold fronts.

Criteria Air Pollutants: six atmospheric pollutants that U.S. EPA considers harmful to public health and the environment, including ozone (O3), fine particulate matter (PM2.5), coarse particulate matter (PM10), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2).

Delmarva: the peninsula that encompasses Delaware and portions of Maryland and Virginia and is surrounded by the Chesapeake Bay, Delaware Bay, and the Atlantic Ocean.

Dust: solid particulate matter, typically PM10, that can become airborne and irritate individuals who suffer from respiratory problems. Usually, dust is picked up by strong winds and transported from desert areas. Saharan dust transported across the Atlantic Ocean from the Saharan Desert in northern Africa impacts the Mid-Atlantic periodically, usually in the summer months.

Emissions: pollutants or pollutant precursors released into the ambient air. Industrial plants, power plants, and vehicles are major sources of anthropogenic pollutant emissions in the Mid-Atlantic, while trees, croplands, and forests are natural sources of pollutant emissions.

Energy Generating Units (EGUs): a term that loosely refers to electrical power plants, which are a source of NOx and, in the case of coal-burning plants, SO2.  Specifically, EGUs include all of the equipment necessary for production of electricity.  Most power plants consist of several independent EGUs that can be brought online or taken offline based on demand for electricity.

Exceedance: when observed levels of a criteria pollutant are higher than the corresponding National Ambient Air Quality Standard. For example, an O3 exceedance occurs when observed 8-hour average O3 is ≥ 71 ppbv.

Fine Particulate Matter (PM2.5): particulate matter with aerodynamic diameters less than or equal to 2.5 microns. PM2.5 is both a primary and secondary pollutant, with natural and anthropogenic sources. The chemical components of PM2.5 vary widely, but most commonly include nitrate, sulfate, ammonium, organic carbon, and elemental carbon.

Hydrocarbons: organic chemicals that contain hydrogen and carbon. Hydrocarbons are precursors for both PM2.5 and O3 production.

I-95 Corridor: the urbanized line of major metropolitan areas that runs along Interstate 95 in the Mid-Atlantic region, including Washington, DC; Baltimore, MD; Wilmington, DE; Philadelphia, PA; Trenton, NJ; and New York City, NY.

Inversion: a section of the atmosphere where temperature increases with increasing altitude, in contrast to the typical temperature profile, where temperature decreases with increasing altitude. Vertical mixing is limited when inversions develop, which causes pollutants to be trapped in a shallow layer near the surface. In the Mid-Atlantic, inversions occur most often in the winter months, often when there is a layer of snow on the ground, and can result in high concentrations of PM2.5.

Lee Trough: a pressure trough formed on the lee or downwind side of a mountain range when the wind is blowing roughly perpendicular to the mountain ridge. Lee troughs are often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachian Mountains. A weak lee trough can act as a line of convergence, leading to a buildup of pollutants.

Mason-Dixon Line (MDL): the latitudinal boundary that geographically separates Pennsylvania and Maryland.

Mesoscale: scale for weather phenomena ranging from approximately 1-2 km to 500-1,000 km on the spatial scale and 1-12 hours on the time scale. Examples: bay/sea breeze fronts, squall lines, and mesoscale convective systems (MCSs).

Micrograms per cubic meter (μg/m3): a unit of measure for ambient particulate matter.

Mid-Atlantic Region: a geographic region that encompasses North Carolina, Virginia, Maryland, Delaware, West Virginia, Pennsylvania, and New Jersey.

Mid-Level: refers to analysis at 850 mb.

Mississippi River Valley (MRV): a geographic region that encompasses land bordering the Mississippi River, including parts of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana.

Mobile Sources: emitters of pollutants or pollutant precursors that are not fixed in a specific location, such as vehicles, trucks, and airplanes.

National Ambient Air Quality Standard (NAAQS): limits established by U.S. EPA under the Clean Air Act for the six criteria air pollutants. Primary standards protect public health, and secondary standards protect public welfare, including visibility and damage to animals, crops, vegetation, and buildings.

Northern Mid-Atlantic (NMA): Pennsylvania and New Jersey.

NOx: a term that collectively refers to nitric oxide (NO) and nitrogen dioxide (NO2). NOx is a pollutant precursor for both PM2.5 and O3. The main sources of NOx in the Mid-Atlantic are vehicles, industrial processes, and EGUs.

Ohio River Valley (ORV): a geographic region that encompasses land bordering the Ohio River, including parts of Illinois, Kentucky, Indiana, Ohio, and West Virginia. The ORV is historically an upwind source of O3 and PM2.5 precursors for the Mid-Atlantic, due to the many EGUs in the region.

Ozone (O3): a colorless gas formed from photochemical reactions involving NOx and hydrocarbons. O3 is a criteria air pollutant.

Particulate Matter: any material, except for pure water, that exists as a solid or liquid state in the atmosphere. PM10 and PM2.5 are criteria air pollutants.

Parts Per Billion by Volume (ppbv): a unit of measure for ambient O3; equal to one part in a volume of 10xE9.

Photochemistry: general term to describe chemical reactions that require light to occur.  Example: O3 formation in the atmosphere involves photochemical reactions.

Planetary Boundary Layer (PBL): the lowest part of the atmosphere, adjacent to the Earth’s surface, where overturning, or mixing, occurs. The PBL height (or vertical mixing depth) varies throughout the day.  On a sunny day, it is typically 1-2 km above the ground by mid-afternoon.  During the overnight hours, the depth of the PBL is limited to 100-200 m.  In general, the PBL is lower on cloudy days and higher on sunny days.

Point Sources: emitters of pollutants or pollutant precursors that are in fixed locations, such as EGUs.

Precursors: chemicals required for the formation of secondary pollutants. For example, NOx and hydrocarbons are precursors for O3 production in the atmosphere.

Pre-Frontal Trough: an elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction and sometimes with a decrease in dew points.

Primary Pollutant: a pollutant that is directly emitted into the atmosphere. PM2.5 emitted from vehicles is an example of a primary pollutant.

Ring of Fire: the northern edge of an upper level (500 mb) ridge. In the warm season, smaller scale disturbances (shortwaves) often develop west of the axis of the ridge, in the Great Plains and Midwest, and then ride over the crest of the ridge into the Mid-Atlantic region. These shortwaves trigger convection and cloud cover that can limit local photochemistry.

Sea Breeze: a coastal mesoscale wind that blows from the water surface of the ocean onto the surrounding shores during the afternoon. It occurs in the spring and summer in the Mid-Atlantic and is caused by the difference in surface temperature between land (warmer) and water (cooler). A sea breeze can act as a line of convergence inland, leading to a buildup of pollutants. Example: sea breeze along the coasts of New Jersey and southern Delaware.

Secondary Pollutant: a pollutant that is formed from chemical reactions in the atmosphere; it is not directly emitted into the atmosphere. O3 is an example of a secondary pollutant.

Sensitive Groups: people who are especially susceptible to the hazardous health impacts of criteria pollutants, including children, the elderly, and individuals with existing cardiovascular and respiratory diseases.

Southern Mid-Atlantic (SMA): southern Virginia and North Carolina.

Subsidence: the sinking or downward motion of air in a given area, causing surface high pressure as the air builds up near the ground. Strong subsidence is associated with calm surface winds and clear skies. For air quality, areas of strong subsidence are favorable for the buildup of pollutants at the surface.

Sulfur Dioxide (SO2): a gas released from the combustion of fossil fuels, such as coal.  SO2 is the predominant anthropogenic sulfur-containing air pollutant in the U.S. and is a criteria air pollutant.  Historically, coal-burning EGUs in the ORV were a major source of SO2 for the Mid-Atlantic.  SO2 is a precursor for PM2.5 formation.

Synoptic: scale for weather phenomena ranging from approximately 1,000-3,000 km on the spatial scale and 12 hours to 2 weeks on the time scale. Examples: hurricanes, extra-tropical cyclones.

Tennessee River Valley (TRV): a geographic region that encompasses land bordering the Tennessee River, including parts of Tennessee, Alabama, Georgia, South Carolina, North Carolina, Virginia, and Kentucky.

Upper Level: refers to analysis at 500 mb or higher.

Vertical Mixing: overturning of the planetary boundary layer (PBL) in the vertical direction. For air quality, limited vertical mixing is associated with a low PBL and can be conducive for the buildup of pollutants. In contract, vigorous vertical mixing ventilates the atmosphere, is associated with a high PBL, and is typically conducive for the dispersion of pollutants.

Volatile Organic Compounds (VOCs)hydrocarbons that exist preferentially in the gas phase in the atmosphere. They have natural and anthropogenic sources. Examples include isoprene (natural) and gasoline (anthropogenic).