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Terms starting with L
Definition
A clean and controlled environment is necessary for efficient calibration output. Gas cylinders with known parameters within recommended values should be available. To ensure correct results, nearby sources of ambient contamination need to be identified and eliminated. Furthermore, multiple readings help determine the consistency of sensor operation. Laboratory Evaluations – are typically short-term evaluations (e.g., hours) conducted in an environmentally controlled chamber. One or more sensors and the inlet(s) of a reference instrument(s) are placed in the chamber and exposed to different test conditions (e.g., high or low temperatures, high or low humidity, target and interferent pollutant concentrations and types) and the measurements are compared.
Latitude lines, also known as parallels, run horizontally around the Earth and measure the distance north or south of the equator. The equator, which circles the Earth midway between the North and South Poles, is designated as 0 degrees latitude. Latitude values north of the equator are positive, while those south of the equator are negative. The range of latitude extends from -90 degrees (the South Pole) to +90 degrees (the North Pole). Longitude lines, also called meridians, run vertically from the North Pole to the South Pole. The Prime Meridian, which passes through Greenwich, London, is designated as 0 degrees longitude. Longitude values to the east of the Prime Meridian are positive, while those to the west are negative. The range of longitude extends from -180 degrees (180 degrees west) to +180 degrees (180 degrees east).
Lower Explosive Limit (LEL): The Lower Explosive Limit is the minimum concentration of a flammable or combustible gas or vapor in the air below which it will not ignite or explode when exposed to a source of ignition. It represents the lower boundary for the presence of a potentially explosive atmosphere. Below the LEL, there is insufficient fuel to sustain combustion. The LEL is a critical parameter for assessing the safety of potentially hazardous environments, such as those in industrial settings.
Sunlight and high-temperature trigger chemical reactions among primary air pollutants which includes nitrogen oxides and oxygen, causing a chemical response that generates Ozone. The hotter the day and the greater the intensity of the solar, the more ozone is formed.
Light scattering is a phenomenon that occurs when light interacts with particles or irregularities in a medium and is redirected in various directions. It is responsible for making substances visible by scattering light in different directions, allowing our eyes to perceive objects. Light scattering plays a crucial role in a wide range of scientific and everyday phenomena, from the blue color of the sky to the sparkle of diamonds and the appearance of fog. The principle of light scattering is based on the scattering of photons (particles of light) as they interact with small particles or irregularities in a medium. This interaction causes the incoming light to change direction and scatter in different ways. There are two main types of light scattering: Reyleigh and Mie scattering. Mie scattering principle is used in optical particle counter to measure concentration of PM. Mie – Mie scattering occurs when the particles or irregularities in the medium are on the same order of magnitude as the wavelength of the incident light or when the medium contains a mixture of particle sizes. Mie scattering is responsible for the white appearance of clouds, the visibility of dust particles in the air, and the sparkle of diamonds. Unlike Rayleigh scattering, Mie scattering does not strongly favor shorter wavelengths, so it scatters light of all colors more evenly.
A “line source” is a term used in environmental science and pollution monitoring to describe a source of pollution that releases contaminants, such as gases or particulate matter, in a continuous or linear fashion along a specific path or line. This line could represent a roadway, an industrial process, a vent, or any other emission source that emits pollutants along its length.
Linear regression is a statistical method used in data analysis and machine learning to model the relationship between a dependent variable (also known as the target or response variable) and one or more independent variables (predictors or features) by fitting a linear equation to the observed data. The primary goal of linear regression is to find the best-fitting linear relationship that describes the association between variables. This linear equation can then be used for prediction, understanding relationships, or making inferences about the data. Linear Equation: In simple linear regression, which involves one independent variable, the linear equation is represented as: Y = a + bX, Y represents the dependent variable, X represents the independent variable, “a” is the intercept, which represents the value of Y when X is 0, “b” is the slope, which represents the change in Y for a one-unit change in X.
A measure of the extent to which the measurements reported by a sensor can explain the concentrations reported by the reference instrument. It is often quantified by the coefficient of determination (R2) obtained from the simple linear regression fitting sensor measurements (y-axis) to reference instrument measurements (xaxis) with values closer to 1 generally indicating better linearity. In some cases, sensor measurements can be linear with a near perfect R2 but may differ significantly from the reference instrument measurements. For example, a linear regression can result in an R2 of 0.99 and slope of 5. This indicates that the reported sensor measurement is always 5 times higher than the reference instrument measurements.
Air sensors are a class of non-regulatory technology that are lower in cost, portable, and generally easier to operate than monitors used for regulatory monitoring purposes. Air sensors and regulatory monitors differ in that regulatory monitors are the gold standard and are designed to meet strict performance requirements for use in regulatory monitoring.
