Glossary

Terms with an asterisk* can be found elsewhere in this glossary.

1-Hz Data: The frequency or speed that air pollution* and environmental sensors* obtain measurements in the Aclima Mobile Platform*. 1 Hertz (Hz) means 1 measurement performed every second.

µg/m³: Amount of an air pollutant in air. 1 microgram (1 millionth of a gram) per cubic meter of air. Particulate matter is often expressed in these units or nanogram (ng/m³), where ng is 1/1000 of a µg. Other similar units can be used. Gases are typically expressed in ppb* or ppm* but also can be expressed in µg/m³ or ng/m³.

AB 617: Assembly Bill (AB) 617 is a California Assembly Bill that requires agencies such as CARB* and local air districts* of California to work with communities to develop and implement reporting, monitoring, reduction plans, and measures in an effort to reduce air pollution* in communities most impacted by air pollution*.

AB 617 Community: A community in California identified by a the local air district* and CARB* as disproportionately impacted by air pollution*, (typically due to nearness to sources), in response to California Assembly Bill (AB) 617*.

Accuracy: How close a measured value is to the true value.

Aclima Mobile Node (AMN or Node): A proprietary temperature-controlled device, located inside the Aclima Mobile Platform*, containing several small air pollution sensors* that measure air quality* and GHGs* every second as well as electronics that control and transmit data to Aclima.

Aclima Mobile Platform: A zero- or low-emission vehicle outfitted with an Aclima Mobile Node* that collects air quality* and greenhouse gas* measurements block-by-block* throughout a specified region.

Air District: Develops rules to improve air quality and protect the climate. All states have air districts that typically cover a large region that includes major cities, rural areas impacted by cities, and human-influenced* sources of air pollution*.

Air Pollution: Consists of a wide range of chemical compounds in the gas and particle phases that are emitted by natural and human-influenced* sources.

Air Pollution Indicators: Provide information on the impact of source types (e.g., heavy-duty diesel trucks, or gasoline vehicle traffic) in an area. These insights may assist communities with identifying specific stationary* and mobile* sources impacting them. Aclima has developed four Air Pollution Indicators:

• Diesel Pollution Indicator (DPI)*
• Particulate Matter Indicator (PMI)*
• Traffic Pollution Indicator (TPI)*
• Methane Indicator (MI)*

Air Quality: Describes the condition of ambient air based in part on the concentration of air pollutants* present. Good air quality is when the air is clear and contains only small amounts of solid particles and gases above natural levels. Poor air quality is when high levels of air pollutants exist in air causing the sky to appear hazy and inhaling the air may be dangerous to health and the environment. EPA’s* Air Quality Index (AQI)* is a relative measure of air quality.

Air Quality Index (AQI): A relative measure of air quality used by USEPA* to inform individuals of the daily status of the air around them. The higher the AQI* value, the greater the level of air pollution* and the greater the health concern.

Ambient Pollutants: Refers to outdoor air pollutants potentially harmful to human health measured by Aclima, including:

• Carbon monoxide (CO)*
• Carbon dioxide (CO₂)*
• Nitrogen oxide (NO)
• Nitrogen dioxide (NO₂)*
• Ozone (O₃)*
• Methane (CH₄)*
• Ethane (C₂H₆)*
• Volatile organic compounds (VOCs)*
• Fine particulate matter (PM_2.5)*
• Black carbon (BC)*

Annual Baseline: Data collected in a project area over a 12-month period.

AQMIS (Air Quality and Meteorological Information System): An accessible database at CARB*, which contains air quality and supporting data from a single web-based source.

Artifact (statistical and methodological): An unintended consequence of measurement error that can bias* the measurement. The implication is that the findings do not reflect the real world. Also see MSA.

Average: See mean*.

BAAQMD (Bay Area Air Quality Management District): Regulatory agency in the San Francisco Bay Area of California, responsible for helping ensure that federal and state air quality standards are achieved.

Baseline: A specified period, either three months (quarterly) or 12 months (annual) during which hyperlocal data is collected by the Aclima Mobile Platform*.

BC (Black Carbon): Results from incomplete combustion and is a component of soot*, the black smoke emitted, for example by diesel engines and burning candles. BC may cause lung, heart, and other severe health problems when inhaled. BC contributes to global warming* and causes glaciers to melt faster.

Bias: A consistent difference between the true value and measured value. Calculation of bias is based on USEPA*  guidance.

Biogenic Methane: Methane generated and emitted as a by-product of biological processes, like vegetative decomposition and livestock waste while under anaerobic (low-oxygen) conditions.

Block-by-block: Aclima air pollution* measurements on publicly accessible roads in a study area, reported approximately every 100 meters, and providing air pollution* information to communities and regulators (see Hyperlocal*).

BNMed (Background Normalized Median): Uses regulatory site* measurements to account for variations in the regional background variability that may occur as a result of unusual emissions or meteorological impacts, such as wildfire, or other factors.

Calibrate, Calibration: When an instrument, like an air pollution sensor*, is compared to a known standard (e.g. reference instrument* - FRM*, FEM*) to develop calibration factors that include environmental factors (e.g., temperature, relative humidity, pressure) to convert the direct sensor output to ambient concentrations.

CARB (California Air Resources Board): Regulatory agency in the State of California responsible for helping set state air quality standards and ensure applicable federal and state air quality standards are met.

C₂H₆ (Ethane): see CH₄.

CH₄ (Methane) and C₂H₆ (Ethane): Methane is the major part (~85-95%) of natural gas along with ethane (C₂H₆; ~2-5%). Methane and ethane may be emitted into the air because of leaks associated with natural gas use. Methane is also naturally emitted by livestock (e.g., cows) and organic waste decomposition, e.g. landfills; ethane is not emitted from livestock or landfills, allowing natural gas leaks to be separated and identified from these other sources of methane. There are no direct health effects from methane and ethane.

Chemical Processing: In air, many pollutants are changed by reacting with other pollutants. Sunlight can also be involved. For example, NOx* and VOCs*, and other gases react in sunlight to form O₃*. Components formed in air are referred to as secondary pollutants*. Those emitted directly are primary pollutants*.

Climate and Economic Justice Indicators (CEJI): Data used to help identify sensitive, underserved, overburdened, and at-risk populations, considers these inequalities in evaluating climate and economic issues, and helps to ensure climate and economic investments advance equity.

Climate Change: Warming of the atmosphere due to GHGs* (greenhouse gas) emissions and other pollutants like BC* (black carbon). Currently, CO₂* (carbon dioxide), CH₄* (methane), and BC* are the three pollutants most responsible for climate change*.

CO (Carbon Monoxide): Results from incomplete combustion; is emitted from vehicles, industry, and other sources; and may be harmful to the health of sensitive groups, such as those with preexisting heart disease .

CO₂ (Carbon Dioxide): The fourth most abundant gas in the earth's atmosphere, results from incomplete combustion of something that contains carbon and is emitted from human-influenced* sources like vehicles, industry, and others and natural sources like volcanoes and wildfires. At ambient levels, CO₂ does not impact human health but is the most prevalent GHG* affecting climate change*.

Cold Spot: A road segment* or other geographically defined area (i.e. multiple adjacent road segments), where pollutant levels are consistently lower than those nearby, as opposed to a hot spot*.

Completeness: The number of verified data* points collected divided by the number of data points expected.

Criteria Air Pollutants: Six ambient air pollutants regulated by the USEPA* that have been shown to impact human health and the environment. They are measured by Federal Reference Methods (FRM*) or Federal Equivalent Methods (FEM*), 40 CFR part 53, as approved by the USEPA. They include:

• Carbon monoxide (CO)*
• Nitrogen dioxide (NO₂)*
• Ground-level ozone (O₃)*
• Sulfur dioxide (SO₂)*
• Particulate matter, initially total suspended particulate matter (TSP), which was revised to PM_2.5* and PM₁₀*
• Lead (Pb)*

Data Verification: During and after collection, measurements collected by the Aclima Mobile Platform* undergo an extensive, multistep evaluation to ensure data of the highest quality are reported. The end result is verified data*.

Deployment: Consider the time period an individual device spends in a vehicle.

Diagnostic and Platform Variables: Parameters collected by Aclima to ensure the quality of pollutant measurements. These include diagnostic variables, such as temperature, relative humidity, and pressure, and platform variables, such as position (longitude, latitude), and speed. All are measured at 1-second intervals.

Diesel Pollution Indicator(DPI): Allows block-by-block* identification of higher levels of diesel emissions on Aclima’s maps.

Disadvantaged Communities: Communities that are marginalized, underserved, and overburdened by pollution.

Distribution of Measurements: When many measurements are obtained they fall over a range of values from low to high. Most measurements fall near the mean* with values extending out in either direction - consider an upside down bell as the shape of curve (normal or Gaussian distribution) where the ends of the bell are the upper and lower ends of the measurement distribution and the center of the bell, highest point, would be the mean* value. The curve is symmetric about the mean* and the median* and mean* are equal. Other measurement distribution shapes can be seen. For example, in ambient air quality data one side of the bell shape is longer than the other, the distribution* is often spread (skewed) to higher values, and this distribution is referred to as a log-normal distribution.

DQIs (Data Quality Indicators): Qualitative terms that characterize the uncertainty* in or quality of Aclima’s measurements. DQI are quantified by the project's Measurement Quality Objective* (MQO) and include terms like precision*, bias*, completeness*, limit of detection*, and others.

DQOs (Data Quality Objectives): Qualitative or quantitative statements that clarify project objectives, define the appropriate type of data, and specify tolerable error levels for decisions.

Dust: A general term for solid particles in air; see also PM_2.5* and PM₁₀*.

Emissions Reductions: The result of regulatory policies to lower source emissions and obtain ambient air pollutant* levels that are believed to be less harmful to humans and the environment.

Excluded Data: Single pass mean* or 1-Hz* data that does not meet Measurement Quality Objectives* (MQO) or were not successfully collected (e.g. outside environmental conditions for the sensor; sensor zero, calibration, or other), are maintained in the original 1-Hz* data archive but are excluded from further data analysis.

FEM (Federal Equivalent Method): USEPA*-designated monitoring method for Criteria Air Pollutants* that is different from the FRM* but can obtain similar measurement performance; e.g. time-integrated measurement (FRM*) vs. continuous measurement (FEM*). These ambient air sampling methods are defined in the Federal Register and are used to determine if an area is in compliance with NAAQS*.

FRM (Federal Reference Method): USEPA*-designated monitoring methods for Criteria Air Pollutants* that must meet specific performance requirements. These methods are designed for ambient air sampling methods as defined in the Federal Register and are used to determine if an area is in compliance with NAAQS*.

Fugitive Emissions: Many small nonpoint sources that are often not regulated, often spread over a large area or emitted from a large industrial site, and therefore cannot be easily measured individually. Also referred to as area sources. For example, wind blown dust or leaks from flanges and values in an industrial complex.

GHG (Greenhouse Gases): Chemical components found in the atmosphere that absorb infrared radiation emitted by the earth after the earth is warmed by sunlight. Carbon dioxide (CO₂) and CH₄* are the two most important GHGs* in air that contribute to global warming*.

Global Warming: See climate change.

Hexagon Grid: Average of road segment* data medians over a small six-sided area (hexagon) and placed side by side to cover the monitoring area in a grid pattern. Each hexagon has an area of ~0.28 mi². Hexagons allow for a quick visual analysis of region wide patterns and development in insights useful for communities to evaluate pollutant levels and sources that may be impacting their health and the environment.

Hot Spot: A road segment* or other geographically defined area (i.e. multiple adjacent road segments), where pollutant levels are consistently higher than those nearby, as opposed to a cold spot*.

Hub: During deployment, locations where ore or more vehicles are maintained and stored within a geographic monitoring area that may be part of a much larger project area consisting of multiple hubs.

Human-influenced Air Pollution: Consists of a wide range of chemical compounds in the gas and particle phases (liquid, solid, solid with liquid coating) that exist in the atmosphere as a result of human activities, for example, from industry, transportation, and farming. Pollutants may be toxic, cause adverse health and welfare effects, cause combustion risks, and result in global warming*.

Hyperlocal: Block-by-block* air pollution* measurements approximating the lived environment, where each block (road segment*) is approximately 100 meters in length.

LOD (Limit of Detection): In general, the lowest level of a pollutant that can be consistently detected above measurement variability (noise) when low levels of a pollutant are passed through the monitor. The USEPA* definition is used to calculate LOD for air pollution sensors* used by Aclima.

m/s (mps): meters per second, a measurement of speed.

Max (Maximum): The highest value in a specified set of data. For example, 25 is the maximum value in 2, 4, 6, 7, 9, 11, and 25.

Mean: Adding a group of numbers together and dividing by the total number of data points. For example, add four values 2 + 5 + 8 + 5 and divide by 4, where 4 is, the total number of data points. The mean is 5. The mean can be influenced in one direction or the other by values that are at one end or the other of the distribution* of measurements observed.

Measurement Period: A specified period, typically 3- to 12-months in duration, when Aclima’s measurements are collected. Also referred to as the baseline period*.

MQOs (Measurement Quality Objectives): Quantitative criteria defined to assess data quality, including specific values for precision*, bias*, LOD*, and completeness*.

Median: The middle value in a series of numbers when the numbers are ordered from least to greatest (or vice versa). For example, the median or middle value when ordered from smallest to largest of 2, 4, 6, 7, 9, 11, 25 is 7. The mean* is 9.1, showing how higher values can influence the mean*. The median tends to minimize the influence of values that are at one end or the other of distribution* of measurements observed.

Methane* Algorithms: Approach used by Aclima to identify areas where CH₄* (methane) is persistently higher than nearby areas.

Methane* Indicator (MI): Allows Aclima’s block-by-block* maps to identify where methane emissions may be present.

Microscale: Includes an area that is approximately less than ~25 feet up to about ~330 feet in diameter around a Regulatory Site* (or other) with relatively uniform land use and ambient concentrations, and is a category of USEPA’s* Spatial Scales of Representativeness*.

Middle Scale: Includes an area that is approximately 330 feet to 1,640 feet in diameter around a Regulatory Site* (or other) with relatively uniform land use and ambient concentrations, and is a category of USEPA’s* Spatial Scales of Representativeness.

Minimum: The smallest value in a specified set of data. For example, 2 is the minimum value in 2, 4, 6, 7, 9, and 11.

Mobile Monitoring: Using a zero- or low-emissions vehicle, or other moving platform, to collect air pollution* data.

Mobile Source: An emissions source that moves within, and in and out of a monitoring area and may be emitted at ground level (motor vehicles, ships, and trains that use gasoline, diesel, or CNG) or at elevation (aircraft).

Monitoring Platform: Where instruments are placed to obtain measurements of air pollution* and related parameters. Aclima using mobile* and stationary monitoring platforms.

mph: Miles per hour, a measurement of speed.

MSA (Mobile Sampling Artifacts): An unintended consequence of measurement error that may arise when sampling one or a few polygons* on any given day over a monitoring period that is due to the influences of synoptic-scale variability (normal weather), anomalous emissions events (e.g., Western US wildfires in 2020), day-to-day meteorology, and outliers — even when temporal balance* has been achieved. Statistical approaches are used to minimize MSAs.

NAAQS (National Ambient Air Quality Standards): USEPA* defined ambient air quality regulatory standards for six key pollutants, referred to as Criteria Air Pollutants*; primary standards protect public health, secondary standards protect the environment.

National and Global Scales: Geographic measurement scales representing ambient pollutant levels that characterize the nation and the globe as a whole, and are categories of USEPA’s Spatial Scales of Representativeness*.

Natural Air Pollution: Consists of a wide range of chemical compounds in the gas and particle phases (liquid, solid, solid with liquid coating) that exist in the atmosphere naturally without influence from major human-influenced* sources. Nitrogen (N₂), oxygen (O₂), and carbon dioxide (CO₂) being a few of the major gases in air. Wind blown dust, volcanic emissions (gases and particles), pollen, detritus, even bacteria are particle components.

Natural Gas: Consists primarily of CH₄* (methane) (~85-95%) and C₂H₆* (ethane) (2-5%). The largest source of natural gas in air is from leaks found across the nation's natural gas infrastructure, from extraction to individual homes. Natural gas is a key contributor to global warming* since a large fraction of natural gas contains methane*, a potent greenhouse gas*.

Neighborhood Scale: Includes an area that is approximately 0.3 to 2.5 miles in diameter around a Regulatory Site* (or other) with relatively uniform land use and is a category of USEPA’s* Spatial Scales of Representativeness.*

NO (Nitric Oxide): Comes primarily from combustion of fuels like diesel and gas in vehicles and power plants but also from natural sources such as wildfires and lightning. Breathing NO* can cause lung irritation and long term respiratory issues (USEPA*).

NO₂ (Nitrogen Dioxide): Mostly formed in air from the conversion (oxidation) of nitrogen oxide (NO)* to NO₂*. Nitrogen dioxide* and NO* are important chemicals involved in making a portion of fine particulate matter and ozone (O₃*). NO₂* is a Criteria Air Pollutant and has been associated with a range of health effects.

NOx (Nitrogen Oxides): The sum of NO* and NO₂*.

O₃ (Ozone): A gas, found in the earth’s upper atmosphere (stratosphere), which protects lifeforms from the harmful rays of the sun. When found at the earth’s surface, O₃* is a main ingredient in smog* and can degrade human health, vegetation, and buildings (USEPA*).

Oxidation: Chemicals in air often react with compounds in air that contain oxygen (e.g. ozone) and the result is the addition of oxygen to that compound and loss to the oxygen containing compound. NO* reacting with O₃* forming NO₂* + O₂*.

PMI (Particulate Matter Indicator): Allows block-by-block* identification on Aclima’s maps where higher levels of fine particulate matter may be present.

Particulate Matter: See PM_2.5 and PM₁₀.

Pass: One or more 1-second measurements of air pollutant* and GHGs*, collected during a single drive of an Aclima Mobile Platform* over an approximately 100-meter length of road, .

Percent (%) of Segments: Fraction of road segments* observed within a specified distance around a sensitive receptor or specific air pollution* source, identified on an Aclima map, divided by the total number of road segments in the circle. The center of the road segment* is counted if it falls inside the circle.

PM_2.5: Airborne particles approximately 25 times smaller than the diameter of a human hair and smaller. As defined by EPA, particles equal to and less than 2.5 micrometers in diameter. PM_2.5 is harmful to human health once inhaled (USEPA; WHO) causing a range of heart, lung, and other adverse health issues.

PM₁₀: Airborne particles approximately 10 times smaller than the diameter of a human hair and smaller. As defined by EPA, particles equal to and less than 10 micrometers in diameter. PM₁₀ is harmful to human health once inhaled. Particles larger than PM_2.5* are more likely to cause asthma and upper respiratory issues.

Pollutant Maps: Aclima’s segment and aggregated mobile monitoring pollutant measurements visualized on community maps showing areas of higher and lower pollutant levels at ~100 m road segments* or an average of road segments* over a small area, like a hexagon*.

Pollutant Transport: Movement of pollutants from one area to another, typically by wind.

Polygon: A spatially-defined geographic area represented as a multi-sided shape on a map.

ppb (parts per billion): Unit of measurement indicating the amount of a pollutant in air; 1 part in one billion parts, e.g. 1 molecule of O₃ per 1 billion molecules of air.

ppm (parts per million): Unit of measurement expressing the amount of a pollutant in air; 1 part per million parts, i.e. 1 molecule of O₃ per 1 million molecules of air.

Precision: A measure of how close a series of measurements of the same property, usually under prescribed similar conditions, are to one another. Precise measurements are highly reproducible, even if the measurements are not near the correct value. This is the random component of error and can be positive or negative. Precision is estimated by various statistical techniques using some derivation of the standard deviation*. Calculation of precision is based on USEPA* guidance.

Primary Pollutant: A pollutant directly emitted into the air from natural or human-influenced* sources (e.g. CO*, NO*, BC*).

QA (Quality Assurance): Is the process followed to obtain and ensure data of known quality, and forms the basis for a project or program’s quality assurance*.

QAPP (Quality Assurance Project Plan): Forms the basis for a project’s quality assurance*. Provides a project-specific “blueprint” for obtaining the type and quality of environmental data needed for a specific decision or use, that is, how QA* and QC* procedures are applied to collection of environmental; identifies project DQOs*, defines project DQIs*, and provides quantitative values for MQOs* as well as outlines project management structure, standard operating procedures (SOP), and other aspects of QA*. The USEPA* approved Aclima’s initial QAPP in 2016 and an updated QAPP in April 2022.

QC (Quality Control): Any process that monitors and maintains laboratory and field standards of measurement. The processes of quality control can include detecting, reducing, and correcting any problems. QC is a part of the overall QA process.

Quarterly Baseline: Data collected in a project area over a 3-month period.

Reference Instruments or Reference Method: Laboratory-grade or historically-used methods that have been thoroughly tested over time and have well-known performance, e.g. USEPA*-designated monitoring methods for Criteria Air Pollutants* must meet specific requirements for performance (Federal Reference Methods (FRM)* or Federal Equivalent Methods (FEM)* (40 CFR part 53). All reference methods used by Aclima fall into one of these categories.

Regional Scale: Includes an area that is approximately tens to hundreds of miles in diameter around a Regulatory Site* (or other) with relatively uniform geography (e.g. usually a rural area) and ambient concentrations, without large sources, and is a category of USEPA’s* Spatial Scales of Representativeness*.

Regulatory Air Monitoring Station or Site: A stationary monitoring site, with a specified Spatial Scale of Representativeness*, operated by the local air district or state air pollution agency, which measures Criteria Air Pollutants* with USEPA* approved monitoring methods (FRM* or FEM*; see Reference Instruments*), typically including meteorological measurements and sometimes non-criteria air pollutants and used to evaluate compliance with NAAQS*.

Representativeness of Sampling: Measurements collected over different times of day and days of week (see Temporal Balance*) throughout a specified measurement period to minimize the influence of unusual emissions events (e.g. major wildfire) or meteorology. A minimum of 20 passes over each road segment has been achieved.

Road Segment: An approximately 300-foot (100-meter) length of a publicly accessible road monitored repeatedly over the monitoring period. Road segment mean (SPM*) values are aggregated over the monitoring period and the median* value of these measurements are visualized on Aclima’s maps, allowing pollution levels to be observed on each block.

Road Segment Aggregate: Various statistical approaches are used to combine multiple road segments* in an area, like a hexagon*, into a single number with an associated uncertainty* (statistically defined range of possible values around the single number).

Road Type: Four road types are considered - highway, major, residential, and other. Roads are identified and mapped using data from OpenStreetMap (OSM).

Sampling or Measurement Frequency: The rate data are collected by air pollution* and environmental sensors in the Aclima Mobile Platform. Most data are collected every second except ozone, which is every 2 seconds.

SCAQMD (South Coast Air Quality Management District): Regulatory agency in the South Coast Air Basin (greater Los Angeles, CA) that is responsible for helping to ensure Federal and state air quality standards are achieved.

Secondary Pollutant: A pollutant formed in air from gases emitted into the air by natural or human-influenced* sources.

Segment: see Road Segment.

Self Pollution: When a vehicle is stationary for an extended period of time or if the vehicle passes through its own plume due to circumstances encountered while driving. Identified as repeated sampling of a segment within 30 seconds that might occur within a cul de sac or idling for greater than 75 seconds (not stop lights or signs) and might occur due to a prolonged stay in a single location due to a driver stopping for one reason or another.

Sensitive Populations: Younger and older people, people with chronic medical conditions, unhoused populations, and anyone else that may experience greater health impacts from air pollution* than the average population, even if pollution levels are below regulatory thresholds.

Sensitive Receptors: Locations or areas where sensitive populations* live, work, and play, like schools, childcare centers, libraries, healthcare and convalescent facilities, and subsidized housing.

Sensor, Air Pollution: Small, portable, typically easy-to-use, low-power, and low-cost (<$2500) instruments designed to measure atmospheric particles and gases.

Single Pass Mean: The mean* of 1-second measurements taken during a single pass* by an Aclima Mobile Platform* across a road segment*.

Smog: Term originally derived from the words smoke and fog, although today considered air pollution* as a result of sunlight reacting with NOx* and VOCs* in ambient air.

Snap, Snapped, or Snapping: Assignment of each 1-Hz* measurement to the nearest ~100-m road segment* on the basis of measured location (latitude and longitude).

Soot: Dark smoke emitted during incomplete combustion (e.g. diesel engines or candles), an important component of soot is BC*, which ranks in the top 3 global warming* pollutants in air.

Spatial Scales of Representativeness: The geographic area around a monitoring site where the air pollutant levels are similar and not strongly influenced by one or more emissions sources. Spatial scales of representativeness range in size from <330 feet to global. Spatial scales are microscale*, middle*, neighborhood*, urban, regional*, national*, and global*.

Standard Deviation: A statistic that indicates the spread of data about the mean * of a distribution of measurements over the range of values measured. The standard deviation (SD or σ) is small when the data are close to the mean and large when the distribution is spread over a large range of values.

Stationary Measurement: Monitoring for one or more air pollutants at a single location over time.

Stationary Source: A major pollution source, at a fixed location, and often regulated, e.g. various industries, power plants.

Strata: Defined periods in time associated with morning (A.M. = midnight to noon), afternoon/night (P.M. = noon to midnight), and each quarter (three-month period) within a year (Q1, Q2, Q3, Q4) for a total of eight (2 X 4) time periods.

Temporal Balance: When road segments are evenly sampled across times of day, days of week, and months of the year and the influence of atypical source emissions and meteorological conditions that extend over several or more days has been minimized. Under these conditions the baseline* is representative of the area and time period.

Thermogenic Methane: Methane (CH₄)* resulting from breakdown of organic material by heat deep in the earth's crust, with or without a catalyst, and typically associated with natural gas.

Title V: USEPA* regulations [Title 40 of the Code of Federal Regulations, Chapter 1, Part 70 (Part 70)] that identify regulated emissions sources required to obtain operating permits, which define maximum allowable emissions limits.

Toxic Air Pollutants: The USEPA* has classified 188 pollutants as hazardous, meaning known or suspected to cause cancer or other serious health effects.

TPI (Traffic Pollution Indicator): Allows block-by-block* identification of higher levels of traffic emissions on Aclima maps.

USEPA ( U.S. Environmental Protection Agency ): The Federal regulatory agency, works with state and local regulatory agencies, and is responsible for ensuring healthy air, water, and soil.

Uncertainty: Level of variation around a measured value or a statistically calculated value based on many measurements. Uncertainty includes both accuracy* and precision* in its estimate.

Urban Scale: Includes an area that is approximately 2.5 to ~31 miles in diameter around a Regulatory Site* (or other) with relatively uniform land use and ambient concentrations, and is a category of USEPA’s* Spatial Scales of Representativeness*.

Verified Data: The final step in data verification* where irregularities in data have been treated or flagged, the overall uncertainty* defined, and no further changes are expected in the data.

VOCs  (Volatile Organic Compounds): A large set of organic gases found in air. All contain carbon and hydrogen, and may include oxygen, nitrogen, and other elements. Some are considered toxic and may result in adverse health effects, some participate in the formation of ozone* along with NOx* and sunlight, some have foul odors, and some are regulated by USEPA*. VOCs have both natural (biogenic) and human-influenced* (primarily industrial and transportation) sources.

WS & WD (Wind speed and direction): Wind speed refers to how fast wind travels and is typically stated in mph* or m/s*. Wind direction refers to where wind is traveling to and is typically provided in degrees around the compass (0 - 360, where 0 degrees is north and each 90 degrees clockwise is east, south, and west, ending at north again at 360 degrees).

WHO (World Health Organization ): The United Nations agency that connects nations, partners, and people to promote health, keep the world safe and serve the vulnerable – so everyone, everywhere can attain the highest level of health. WHO provides global air quality guidelines that are proposed to reduce health risks from air pollution*.