PM Coarse internal cabinet temperature. Battery probe is a measure of the battery voltage (in volts) at a monitoring site. Some sites (especially water monitoring sites) operate on a 12-volt battery that is recharged by either a solar panel or a hydrogen fuel cell. Water temperature at real time surface water quality monitoring stations is measured in degrees Celsius (°C).The temperature of water is an important factor in an aquatic ecosystem because it controls biological activities and chemical processes. Stream systems exhibit diel (daily) temperature variations. Most aquatic organisms depend upon the environment to regulate metabolic rates and have adapted to temperature ranges that occur in their habitat. However, alteration of habitat, especially by human activities, can cause temperatures to exceed these ranges.Removal of riparian (riverside) vegetation decreases shading and may allow water temperatures to exceed the typical range (as well as destroy microhabitat) due to increased exposure to solar radiation. Increased solar radiation warms water causing metabolic processes to break down.Runoff from previously natural, undeveloped areas which are now covered by impermeable surfaces (buildings and pavement) may enter a stream system through storm drains at velocities that can scour and widen a stream channel. Water flowing in wide, shallow streams tends to warm up more quickly than water flowing in natural deep, narrow channels, especially during hot, dry seasons when water volume is low.Temperature also affects the amount of dissolved oxygen that is available to aquatic organisms. As a general rule, cool water is capable of retaining more dissolved oxygen than warm water. As water temperatures increase, metabolic activities such as respiration also increase, but less dissolved oxygen is available for consumption. Industrial discharges and wastewater outflows may also release warm water into a stream system increasing the oxygen demand of organisms due to increased metabolic activities. In addition, the affect of oxygen demanding wastes on the aquatic environment is enhanced by warm water temperatures. Specific conductance is a measure of the ability of water to carry an electric current. Specific conductance at real time surface water quality monitoring stations is measured in micro-Siemens per centimeter (µS/cm).Generally, rainwater has a very low conductivity. However, as runoff flows through a watershed, the slightly acidic nature of rainwater can release ions (negatively or positively charged particles) from soil and rock which dissolve in water and increase specific conductance. These ions are commonly referred to as inorganic salts. The concentration of these salts or salinity can differ from one watershed to another depending upon the parent rock of the drainage basin. The most common inorganic salts include calcium, magnesium, sodium, potassium, bicarbonate, carbonate, sulfate, and chloride. Specific conductance is often used as a way to indirectly measure the salinity of water.Runoff from agricultural and mining operations, as well as industrial and wastewater discharge can affect specific conductance. Chlorophyll refers to a group of photosynthetic pigments that enable plants to convert solar energy into chemical energy. Plants can utilize this chemical energy to change carbon dioxide and water into carbohydrates to be used for growth and maintenance. By measuring the concentration of chlorophyll in water, an estimate of the amount of aquatic plant biomass (living material) can be made, which in turn, may be used to estimate biological productivity. Elevated chlorophyll levels indicate an increase in productivity. However, large quantities of algae and phytoplankton in the water can lower the water clarity which limits sunlight penetration, as well as cause extreme fluctuations in dissolved oxygen levels and ultimately lower productivity. High chlorophyll levels may also indicate increased nutrient loading into an aquatic system. Chlorophyll at real time surface water quality monitoring stations is measured in micrograms per liter (µg/L). Turbidity is a measure of water clarity (cloudiness) and is affected by the concentration of suspended particles in the water column. Suspended matter may include clay, silt, organic matter, and plankton (microscopic plants and animals). The cloudier the water, the greater the turbidity. Turbidity is measured in NTUs (Nephelometric Turbidity Units).The amount of suspended particles affects the dissolved oxygen (DO) concentration in water. As turbidity increases, the ability of sunlight to penetrate the water decreases. Sunlight is scattered and/or absorbed by the particles suspended in the water column. Reduction in the availability of light decreases the amount of phytoplankton, algae and other aquatic plants, thereby limiting photosynthetic activity (DO production is decreased). In addition to the reduction in DO levels, organisms (zooplankton and fish) dependent on plants for food are also affected. Soil erosion from agricultural lands and construction sites, as well as urban runoff, can increase the turbidity in streams and rivers. Discharge from wastewater treatment plants may contain organic matter and plant nutrients (nitrogen and phosphorus) and may provide attachment places for other pollutants, such as metals and bacteria. Algae blooms can also cause an increase in turbidity.Suspended matter remains in the water column as long as there is sufficient current to carry it. The deposition of suspended matter creates problems for aquatic organisms by covering up habitat and filling in lakes and slow moving areas of streams. By covering up habitat the amount of invertebrate food for fish is reduced and predators feed less efficiently in turbid water. If the sediment load is too high fish gills can become clogged. Turbidity also affects the human perception of water quality. Increased turbidity causes people to regard water as "dirty" or polluted. Dissolved oxygen (DO) is a measure of the amount of oxygen in the form of microscopic bubbles dissolved in water. DO is measured in milligrams per liter (mg/L). The concentration of DO is a single, easy-to-measure characteristic of water that correlates with the occurrence and diversity of aquatic life. A water system that can support diverse, abundant life is a good indication of excellent water quality.Two major sources of DO are absorption from the atmosphere and the release of oxygen during photosynthesis by aquatic plants and algae. Over a 24-hour period, DO fluctuation occurs naturally in most water bodies. Normally, as the sun rises, photosynthetic organisms begin to produce oxygen and the DO concentration increases through the day (production is greater than consumption). As the sun sets, photosynthesis essentially ceases and DO levels drop due to respiration by plants and animals (consumption is greater than production). The lowest DO levels usually occur just before dawn. In general, oxygen consumption (respiration) can be considered a constant drain on an aquatic system, day and night. The introduction of nutrients and organic waste also increases the drain on an aquatic system. Nutrients, mainly nitrogen and phosphorus, are essential for plant growth. However, in large quantities (primarily from wastewater discharge, urban and agricultural runoff, and leaking septic systems), nutrients can cause algae to grow at an accelerated rate and create algae blooms. During algae blooms, DO levels in the daylight hours can be extremely high. However, during the night when oxygen production (photosynthesis) ceases and respiration (consumption) begins, the DO levels can drop to levels that lead to fish kills. The effect of an algal bloom can be compounded on overcast days. Decreased sunlight causes a reduction in oxygen production which results in a net loss of DO. As algae die, decomposition by oxygen consuming bacteria occurs. Inflow of organic waste (grass clippings, animal waste, raw sewage) can increase the amount of oxygen consumed by bacteria. pH is a measurement of hydrogen ion concentration and is used to describe the acidity or alkalinity of a solution. Measurements range from 0 to 14 on an exponential scale. A pH value less than 7 is acidic, while a pH value greater than 7 is basic (alkaline). A pH value of 7 is neutral. The pH scale is negatively logarithmic which means that each whole number down the scale is 10 times more acidic than the number before it (Example: A pH of 6 is 100 times more acidic than a pH of 8).The pH of surface waters in Texas is generally between 5 and 9.The pH of an aquatic system is determined by a number of factors. Carbon dioxide (CO2) from the respiration of aquatic organisms and from the atmosphere reacts with water to form a weak acid. Because plants release CO2 during the night and take in CO2 during the day, pH levels can vary from day to night.The rock and soil of the watershed also affect the pH of an aquatic system. A drainage basin composed primarily of limestone tends to be alkaline because the slightly acidic nature of rain rapidly weathers the soft rock releasing minerals that buffer the water and slightly raise the pH. Mineral-rich water like this is commonly called hard-water. Watersheds containing sandy soils from harder igneous bedrock weather more slowly and there is little chemical breakdown. These soft-water aquatic systems are low in minerals that buffer the slightly acidic rainfall and tend to have a pH less than 7. Human activities also influence pH. Water flowing through mine tailings (waste rock from mining operations) can become acidic because of the presence of minerals containing sulfide (which may form sulfuric acid). Drainage from these areas into soft-water streams or streams with little buffering minerals lower the pH. Also, emissions from power plants and car exhausts containing nitrogen oxides and sulphur dioxide can react with moisture in the atmosphere to form nitric acid and sulphuric acid. This is more commonly known as acid rain. Water pH in millivolts is a read out of the raw measurement from the instrument. This measurement is converted to engineering units (pH) by the datalogger. Negative values are caused by the configuration of the datalogger and can ignored since we are really only interested in the magnitude of the measurement. The pressure difference between the inside of a sampling manifold and the ambient atmosphere. Pressures below atmospheric are negative and above atmospheric are positive. This parameter can be used as an indicator that the manifold fan is working. There are three conductivity measurements included in the computed soil parameters: soil conductivity, temperature corrected soil conductivity, and temperature corrected soil water conductivity. The unit of measurement is the MKS unit µS/cm. Temperature corrected soil water conductivity is covered under Soil Salinity and will not be discussed here. The soil conductivity and temperature corrected soil conductivity are both based directly on, respectively, the imaginary dielectric constant and the temperature corrected imaginary dielectric constant. The accuracy of these two conductivity values is thus directly tied to the accuracy of the respective dielectric constants. Typical accuracy in the soil conductivity (as measured at the prevailing soil temperature) is +/- 0.0014 S/m or +/- 1%, whichever is greatest. However, in cases where the imaginary dielectric constant is small compared to the real dielectric constant, the accuracy will be less. The accuracy of the temperature corrected soil conductivity is +/- 0.0014 S/m or +/- 5% over the temperature range of 0° C to 35° C. Soil salinity is expressed in units of total Sodium Chloride (NaCl) burden (grams) per unit volume (liter). The salt content of the soil water can be calculated by multiplying the soil salinity by the water fraction by volume (wfv). For example, a soil that has a soil salinity of 0.1 g NaCl/liter and a wfv of 0.20 will have a (1/0.2) X 0.1 g NaCl/liter=0.5 g NaCl/liter dissolved salt in the soil water. It is important to keep the distinction clear. Barring any salt migration in a soil, the soil salinity (or total salt burden) stays fixed. As the soil wets and dries the soil water salinity will, respectively, drop and rise, as the fixed salt content is diluted or concentrated.It is also important to note that salinity is expressed in terms of a NaCl burden. In actual soils, a wide variety of different natural and man made salts may be present in soil in addition to NaCl. The total burden (by weight) will then in general be different from the value calculated based on NaCl. For example, the conductivity of Potassium Nitrate (KNO3) by weight in water is approximately 67% of the conductivity of an equal weight of NaCl. Thus, a NaCl burden of 0.1 g/liter corresponds to a burden of (0.1 g KNO3/0.67 g NaCl) x 0.1 g NaCl/liter=0.15 g KN03/liter. In order to be consistent with other soil salinity measuring techniques, the temperature corrected soil water conductivity is also calculated. This is a measure of the conductivity of the water in the soil pores as influenced by the dissolved salts. It should be noted again that for a soil with no salt migration, as the soil water content rises, the salts present will be diluted resulting in a falling soil water conductivity. The temperature correction amounts to calculating the effective conductivity at 25° C. As with all other conductivity measurements, the unit is the standard MKS unit S/m or mhos/m. The output of the data Conversion program is water fraction by volume (wfv). For example a water content of 0.20 wfv means that a one liter soil sample contains 200 ml of water. Full saturation (all the soil pore spaces filled with water) occurs typically between 0.3-0.45 wfv and is quite soil dependent.For example to use the unit common in tensiometer measurements, a one Bar sand and a one Bar clay will have vastly different water contents. The wfv unit can also be readily used to estimate the effects of precipitation or irrigation. For example consider a soil that is initially 0.20 wfv, and assume a 5 cm rainfall that is distributed uniformly through the upper one meter of soil. What will the resultant soil moisture in the upper one meter of soil be? 5 cm is 0.05 of one meter so the rainfall will increase the soil moisture by 0.05 wfv to result in a 0.25 wfv soil. For other units, this calculation can be much less straightforward, particularly when soil moisture is measured as a tension. Groundwater is the water that has percolated through surface soils into underlying layers of rock, gravel, sand, and clay. Depth to groundwater - how deep you have to go in a well to get a reliable supply of water - varies on seasonal and yearly time scales.Groundwater levels are extremely important in providing information about the overall groundwater flow regime and the water budget for an aquifer. Generally, water level data is recorded as X, Y (global coordinates) and Z (elevation) format and provides the key data set for contouring and mapping water tables. Advanced software applications, such as HydroGeo Analyst are extremely useful in managing and reporting years of groundwater level data. Groundwater levels help to develop regional water table to establish sources of water for industry, agriculture, and human consumption, calculate groundwater flow direction and velocities for risk assessment, analyze pumping test data to determine the hydraulic properties of the aquifer, highlight areas of concern for source water protection initiatives, determine potential impacts to wetlands, ecology, and freshwater streams, identify groundwater divides to be used as boundary conditions for modeling, provide a source of real data for model calibration, and compare historical data to establish trends in water table elevations resulting from overuse or climatic changes. Specific Conductance (SC) is a measure of how well water can conduct an electrical current. Conductivity increases with increasing amount and mobility of ions. These ions, which come from the breakdown of compounds, conduct electricity because they are negatively or positively charged when dissolved in water. Therefore, SC is an indirect measure of the presence of dissolved solids such as chloride, nitrate, sulfate, phosphate, sodium, magnesium, calcium, and iron, and can be used as an indicator of water pollution.Specific Conductance measures how well water can conduct an electrical current for a unit length and unit cross-section at a certain temperature. More specifically, it is defined as the reciprocal (opposite) of the resistance in ohms measured between opposite faces of a centimeter cube of an aqueous solution at a specified temperature. That is, conductance = 1 / resistance.Specific conductance is measured using a sensor which measures resistance. Resistance means how well something can resist an electrical current, and is reported in ohms. The unit of conductance was originally ohm spelled backwards - mho. More recently, however, the name siemen has been used to match the term used by the International System of Units. So both mho and siemen are sometimes seen in water quality reports. One siemen is equal to one mho. Because SC in natural waters is usually much less than 1 siemen/cm, SC is usually reported in microsiemens (1/1,000,000 siemen) per centimeter, or µS/cm. SC is affected by temperature, so for consistency SC values are converted to what they would be at room temperature (25° C). pH is a measure of the acidity or alkalinity of a solution. Groundwater with a pH less than seven is considered acidic, while that with a pH greater than seven is considered basic (alkaline). pH 7 is considered neutral because it is the accepted pH of pure water at 25° C, although, strictly, pure water cannot be assigned a pH value. pH is formally dependent upon the activity of hydrogen ions (H+), but for very pure dilute solutions, the molarity may be used as a substitute with some sacrifice of accuracy; because pH is dependent on activity, a property which cannot be measured easily or predicted theoretically, it is difficult to determine an accurate value for the pH of groundwater. Temperature influences many important parameters in groundwater and surface water systems. These parameters include density, specific weight, viscosity, surface tension, and thermodynamic properties such as equilibrium and kinetic constants. In addition, temperature plays an important role in risk assessments, as the temperature can impact the amount of contaminant available via some exposure pathways. Water temperature at real time surface water quality monitoring stations is measured in degrees Celsius (°C).The temperature of water is an important factor in an aquatic ecosystem because it controls biological activities and chemical processes. Stream systems exhibit diel (daily) temperature variations. Most aquatic organisms depend upon the environment to regulate metabolic rates and have adapted to temperature ranges that occur in their habitat. However, alteration of habitat, especially by human activities, can cause temperatures to exceed these ranges.Removal of riparian (riverside) vegetation decreases shading and may allow water temperatures to exceed the typical range (as well as destroy microhabitat) due to increased exposure to solar radiation. Increased solar radiation warms water causing metabolic processes to break down.Runoff from previously natural, undeveloped areas which are now covered by impermeable surfaces (buildings and pavement) may enter a stream system through storm drains at velocities that can scour and widen a stream channel. Water flowing in wide, shallow streams tends to warm up more quickly than water flowing in natural deep, narrow channels, especially during hot, dry seasons when water volume is low.Temperature also affects the amount of dissolved oxygen that is available to aquatic organisms. As a general rule, cool water is capable of retaining more dissolved oxygen than warm water. As water temperatures increase, metabolic activities such as respiration also increase, but less dissolved oxygen is available for consumption. Industrial discharges and wastewater outflows may also release warm water into a stream system increasing the oxygen demand of organisms due to increased metabolic activities. In addition, the affect of oxygen demanding wastes on the aquatic environment is enhanced by warm water temperatures. Specific conductance is a measure of the ability of water to carry an electric current. Specific conductance at real time surface water quality monitoring stations is measured in micro-Siemens per centimeter (µS/cm).Generally, rainwater has a very low conductivity. However, as runoff flows through a watershed, the slightly acidic nature of rainwater can release ions (negatively or positively charged particles) from soil and rock which dissolve in water and increase specific conductance. These ions are commonly referred to as inorganic salts. The concentration of these salts or salinity can differ from one watershed to another depending upon the parent rock of the drainage basin. The most common inorganic salts include calcium, magnesium, sodium, potassium, bicarbonate, carbonate, sulfate, and chloride. Specific conductance is often used as a way to indirectly measure the salinity of water.Runoff from agricultural and mining operations, as well as industrial and wastewater discharge can affect specific conductance. Dissolved oxygen (DO) is a measure of the amount of oxygen in the form of microscopic bubbles dissolved in water. DO is measured in milligrams per liter (mg/L). The concentration of DO is a single, easy-to-measure characteristic of water that correlates with the occurrence and diversity of aquatic life. A water system that can support diverse, abundant life is a good indication of excellent water quality.Two major sources of DO are absorption from the atmosphere and the release of oxygen during photosynthesis by aquatic plants and algae. Over a 24-hour period, DO fluctuation occurs naturally in most water bodies. Normally, as the sun rises, photosynthetic organisms begin to produce oxygen and the DO concentration increases through the day (production is greater than consumption). As the sun sets, photosynthesis essentially ceases and DO levels drop due to respiration by plants and animals (consumption is greater than production). The lowest DO levels usually occur just before dawn. In general, oxygen consumption (respiration) can be considered a constant drain on an aquatic system, day and night. The introduction of nutrients and organic waste also increases the drain on an aquatic system. Nutrients, mainly nitrogen and phosphorus, are essential for plant growth. However, in large quantities (primarily from wastewater discharge, urban and agricultural runoff, and leaking septic systems), nutrients can cause algae to grow at an accelerated rate and create algae blooms. During algae blooms, DO levels in the daylight hours can be extremely high. However, during the night when oxygen production (photosynthesis) ceases and respiration (consumption) begins, the DO levels can drop to levels that lead to fish kills. The effect of an algal bloom can be compounded on overcast days. Decreased sunlight causes a reduction in oxygen production which results in a net loss of DO. As algae die, decomposition by oxygen consuming bacteria occurs. Inflow of organic waste (grass clippings, animal waste, raw sewage) can increase the amount of oxygen consumed by bacteria. pH is a measurement of hydrogen ion concentration and is used to describe the acidity or alkalinity of a solution. Measurements range from 0 to 14 on an exponential scale. A pH value less than 7 is acidic, while a pH value greater than 7 is basic (alkaline). A pH value of 7 is neutral. The pH scale is negatively logarithmic which means that each whole number down the scale is 10 times more acidic than the number before it (Example: A pH of 6 is 100 times more acidic than a pH of 8).The pH of surface waters in Texas is generally between 5 and 9.The pH of an aquatic system is determined by a number of factors. Carbon dioxide (CO2) from the respiration of aquatic organisms and from the atmosphere reacts with water to form a weak acid. Because plants release CO2 during the night and take in CO2 during the day, pH levels can vary from day to night.The rock and soil of the watershed also affect the pH of an aquatic system. A drainage basin composed primarily of limestone tends to be alkaline because the slightly acidic nature of rain rapidly weathers the soft rock releasing minerals that buffer the water and slightly raise the pH. Mineral-rich water like this is commonly called hard-water. Watersheds containing sandy soils from harder igneous bedrock weather more slowly and there is little chemical breakdown. These soft-water aquatic systems are low in minerals that buffer the slightly acidic rainfall and tend to have a pH less than 7. Human activities also influence pH. Water flowing through mine tailings (waste rock from mining operations) can become acidic because of the presence of minerals containing sulfide (which may form sulfuric acid). Drainage from these areas into soft-water streams or streams with little buffering minerals lower the pH. Also, emissions from power plants and car exhausts containing nitrogen oxides and sulphur dioxide can react with moisture in the atmosphere to form nitric acid and sulphuric acid. This is more commonly known as acid rain. Extinction coefficient is a fundamental measure of atmosphere clarity. The extinction coefficient measures the attenuation of light by scattering and absorption in the mid-visible light wavelength of 550 nanometers. Visibility measurements are derived from the extinction coefficient. A rough indicator of whether it is day or night at the site. The algorithms used to derive visibility from the extinction coefficient are different for daytime and night. Present weather is a qualitative assessment of the amount and type of precipitation observed. This assessment if automated and uses the output of several different sensors. Present weather values can include: <ul><li>NP = no precipitation<li>?0 = unknown precipitation<li>?1 = unknown precipitation<li>?2 = unknown precipitation<li>?3 = unknown precipitation<li>P = light precipitation<li>R- = light rain<li>R = rain<li>R+ = heavy rain<li>S- = light snow<li>S = snow<li>S+ = heavy snow</ul> Present weather obscuration is an automated qualitative assessment of atmospheric obstruction that is inferred from the measurement of visibility, temperature, dew point, and present weather (precipitation). There are four types of obstructions that can be determined: <ul><li>Fog<li>Mist<li>Freezing Fog<li>Haze</ul> Carbon monoxide is a colorless, odorless, very toxic gas produced by the incomplete combustion of carbon-containing fuels, most notably by gasoline powered engines, power plants, and wood fires.The <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>eight-hour standard</a> has been exceeded occasionally in El Paso during the winter months when very stable atmospheric conditions exist. The <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>one-hour standard</a> for carbon monoxide has never been exceeded in Texas. Sulfur dioxide is produced by burning sulfur-containing fuels (such as coal), smelting metallic ores containing sulfur, and removing sulfur from fuels.The Houston-Galveston-Brazoria, Beaumont-Port Arthur, and El Paso areas have historically shown the highest measured sulfur dioxide levels in the state. There are three <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>sulfur dioxide standards</a> which include a 24-hour average, an annual average, and a three-hour average. Hydrogen sulfide gas carries the foul smell of rotten eggs. It is a by-product of petroleum refining, kraft pulping for paper production, and other various chemical processes. The maximum 5-minute sample observed during an hour. This parameter is only used for reporting to the AQS database. Nitric oxide is one of several oxides of nitrogen produced by high-temperature combustion; however, there is no national standard for nitric oxide. The only <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>standard</a> is for the annual average of nitrogen dioxide. Although there are several oxides of nitrogen produced by high-temperature combustion, the only <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>standard</a> is for the annual average of nitrogen dioxide. This annual standard has never been exceeded in Texas. Many oxides of nitrogen are produced by high-temperature combustion. All higher oxides of nitrogen are grouped together in this parameter; however, there is no national standard for these oxides of nitrogen. The only <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>standard</a> is for the annual average of nitrogen dioxide. Methane is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a mixture containing about 75% methane, 15% ethane, and 5% other hydrocarbons, such as propane and butane. The "firedamp" of coal mines is chiefly methane. Anaerobic bacterial decomposition of plant and animal matter, such as occurs under water, produces marsh gas, which is also methane.Methane is not toxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled. Methane is a greenhouse gas and as a contributor to climate change is second only to carbon monoxide. Ethylene is an odorless, colorless gas that exists in nature and is also created by man-made sources. Not easily detectable, it exists where produce is stored. In nature, the largest producers are plant and plant products (ie. fruits, vegetables and floral products) which produce ethylene within their tissues and release it into the surrounding atmosphere. It is also a by-product of man-made processes, such as combustion. Propylene is a colorless, almost odorless gas. Propylene is produced primarily as a by-product of petroleum refining and of ethylene production by steam cracking of hydrocarbon feedstocks. Propylene is a major chemical intermediate. The most important derivatives of chemical and polymer grade propylene are polypropylene, acrylonitrile, propylene oxide, isopropanol and cumene. Use of polypropylene in plastics (injection moulding) and fibers (carpets) accounts for over one-third of U.S. consumption. It is also used in the production of synthetic rubber and as a propellant or component in aerosols. In the environment, propylene occurs as a natural product from vegetation. It is also a product of combustion of organic matter (biomass burning, motor vehicle exhausts and tobacco smoke) and is released during production and use. Trans-2-Butene is a colorless, highly flammable gas. Trans-2-butene is a member of the alkene group of hydrocarbons and is very similar chemically to Cis-2-butene. Alkenes serve as intermediates in the preparation of a variety of organic compounds. In the industrially important Oxo process, alkenes react catalytically with carbon monoxide and hydrogen to give aldehydes. Alkenes are polymerized by heating with catalysts to give high-octane gasolines, plastics, and synthetic rubber. Butenes are formed during the cracking (breaking down of large molecules) of petroleum to produce gasoline; they can also be prepared commercially by the catalytic dehydrogenation (elimination of hydrogen atoms from the molecule) of butanes. The major part of the butenes is utilized for the production of octanes, which are important constituents of gasoline. This is done either by causing the butenes to react with isobutane or by dimerizing (combining two molecules of) butenes to form octenes, which, on hydrogenation (addition of hydrogen atoms to molecules), yield octanes. On treatment with water in the presence of catalysts, the butenes are transformed into secondary and tertiary butyl alcohols, which are used as commercial solvents. On dehydrogenation, normal butenes form butadiene, the principal starting material for synthetic rubber. Cis-2-Butene is a colorless, highly flammable gas. Cis-2-butene is a member of the alkene group of hydrocarbons and is very similar chemically to Trans-2-butene. Alkenes serve as intermediates in the preparation of a variety of organic compounds. In the industrially important Oxo process, alkenes react catalytically with carbon monoxide and hydrogen to give aldehydes. Alkenes are polymerized by heating with catalysts to give high-octane gasolines, plastics, and synthetic rubber. Butenes are formed during the cracking (breaking down of large molecules) of petroleum to produce gasoline; they can also be prepared commercially by the catalytic dehydrogenation (elimination of hydrogen atoms from the molecule) of butanes. The major part of the butenes is utilized for the production of octanes, which are important constituents of gasoline. This is done either by causing the butenes to react with isobutane or by dimerizing (combining two molecules of) butenes to form octenes, which, on hydrogenation (addition of hydrogen atoms to molecules), yield octanes. On treatment with water in the presence of catalysts, the butenes are transformed into secondary and tertiary butyl alcohols, which are used as commercial solvents. On dehydrogenation, normal butenes form butadiene, the principal starting material for synthetic rubber. 1,3-Butadiene is a colorless, highly flammable gas that has a slight gasoline-like smell. 1,3-Butadiene ranks 36th in the most produced chemicals in the United States. Three billion pounds per year are produced in the United States and 12 billion globally. 1,3-Butadiene is produced through the processing of petroleum and is mainly used in the production of synthetic rubber, but is also found in smaller amounts in plastics and fuel. About 75% of the manufactured 1,3-butadiene is used to make synthetic rubber. Synthetic rubber is widely used for tires on cars and trucks. 1,3-Butadiene is also used to make plastics including acrylics. Small amounts are found in gasoline. Formaldehyde is a gas with a pungent smell. Formaldehyde readily results from the incomplete combustion of carbon-containing materials. It may be found in the smoke from forest fires, in automobile exhaust, and in tobacco smoke. In the atmosphere, formaldehyde is produced by the action of sunlight and oxygen on atmospheric methane and other hydrocarbons. Small amounts of formaldehyde are produced as a metabolic byproduct in most organisms, including humans. Formaldehyde is an important chemical used widely by industry to manufacture building materials and numerous household products. Thus, it may be present in substantial concentrations both indoors and outdoors. Sources of formaldehyde in the home include building materials, smoking, household products, and the use of un-vented, fuel-burning appliances, like gas stoves or kerosene space heaters. Formaldehyde, by itself or in combination with other chemicals, serves a number of purposes in manufactured products. For example, it is used to add permanent-press qualities to clothing and draperies, as a component of glues and adhesives, and as a preservative in some paints and coating products. Acetaldehyde is a highly reactive flammable liquid with a strong fruity smell. Acetaldehyde occurs naturally in ripe fruit, coffee, and fresh bread and is produced by plants as part of their normal metabolism. The largest use of acetaldehyde is as a chemical intermediate for the production of acetic acid. The production of esters, principally ethyl acetate and isobutyl acetate, is the second most significant use. It is also used as an intermediate for a number of other chemicals, including pyridine and pyridine bases, pentaerythritol, peracetic acid, 1,3-butylene glycol, and chloral. Acetaldehyde is also used in silvering mirrors; hardening gelatin fibers; denaturing alcohol; and in the manufacture of disinfectants, dyes, drugs, explosives, flavorings, phenolic and urea resins, rubber accelerators and antioxidants, varnishes, and yeast. Dichloromethane or methylene chloride is the chemical compound with the formula CH2Cl2. It is a colorless, volatile liquid with a moderately sweet aroma. It is widely used as a solvent, the general view being that it is one of the less harmful of the chlorocarbons. Ozone is a form of oxygen with three atoms instead of the usual two atoms. It is a photochemical oxidant and, at ground level, is the main component of smog. Unlike other gaseous pollutants, ozone is not emitted directly into the atmosphere. Instead, it is created in the atmosphere by the action of sunlight on volatile organic compounds and nitrogen oxides. In Texas, higher levels of ozone usually occur on sunny days with light winds, primarily from March through October. An ozone exceedance day is counted if the measured eight-hour average ozone concentration exceeds the <a href='http://www.tceq.state.tx.us/compliance/monitoring/air/monops/naaqs.html'>standards</a>. Benzene is a colorless and flammable liquid with a sweet smell and a relatively high melting point. It is carcinogenic and is no longer used as an additive in gasoline, but it is an important industrial solvent and precursor in the production of drugs, plastics, synthetic rubber, and dyes. Benzene is a natural constituent of crude oil, but it is usually synthesized from other compounds present in petroleum. Its most widely-produced derivatives include styrene, which is used to make polymers and plastics, phenol for resins and adhesives (via cumene), and cyclohexane, which is used in the manufacture of Nylon. Smaller amounts of benzene are used to make some types of rubbers, lubricants, dyes, detergents, drugs, explosives and pesticides. Styrene is colourless, oily, toxic, flammable, and occurs in very small amounts in some plants, but is produced in industrial quantities from petroleum. Styrene is used as a monomer to make plastics such as polystyrene, ABS, styrene-butadiene (SBS) rubber, styrene-butadiene latex, SIS (styrene-isoprene-styrene), S-EB-S (styrene-ethylene/butylene-styrene), styrene-divinylbenzene (S-DVB), and unsaturated polyesters. At room temperature, Styrene is a liquid, but it evaporates easily and has a sweet smell. It often contains other chemicals that can result in a sharp, unpleasant smell. Styrene dissolves in many organic liquids but does not dissolve significantly in water. Millions of tonnes (metric tons or 1,000 kilograms) per day are produced to make products such as rubber, plastic, insulation, fiberglass, pipes, automobile parts, food containers, and carpet backing. Wind speed values are the average of the 5-minute wind speeds for each hour. This average is calculated by adding all the 5-minute wind speed averages measured in one hour and dividing by the number of measurements. Resultant wind speed values are obtained by converting the 5-minute wind speeds and directions for the hour into a single hourly vector. Resultant wind speed is the magnitude of this vector. Resultant wind direction values are obtained by converting the 5-minute wind speeds and directions for the hour into a single hourly vector. Resultant wind direction is the direction of this vector. Wind direction, the direction from which the wind is blowing, is measured to the nearest degree based on a 360 degree compass with 0 or 360 degrees being from the North and 180 degrees being from the South. The maximum wind gust is the peak wind speed observed during the hour. Standard deviation of horizontal wind direction is a measure of the variability of the direction from which the wind is blowing. Zero standard deviation means that there is no variability in the wind direction, thus no deviation. A high standard deviation indicates a large variation in the direction from which the wind is blowing. This is the temperature of the air outside the monitoring site. Dew point temperature values represent the temperature at which the ambient air would become saturated; i.e., dew will form. The dew point can be used to predict the formation of dew, frost, fog, and even the minimum temperature. This is the temperature of the station interior as measured by the thermocouple located inside the datalogger. Diurnal (daily) temperature fluctuations between daytime highs and nighttime lows may be observed with the Hydra Probe's temperature data. These fluctuations will become less pronounced with depth. Vegetation, tree canopy, and soil moisture are factors that will effect the diurnal soil temperature fluctuations. For example, in the American Southwest, a Hydra Probe buried at a five inch depth will have very pronounced temperature fluctuations between the nighttime lows and the daytime highs if there is no vegetation insulating the soil. Seasonal trends can also be observed in soil temperature data. Relative humidity is a measure of the moisture present in the air expressed in percent. Warmer air has the ability to hold more water than colder air. One-hundred percent relative humidity is totally saturated air (the air cannot hold any more moisture). Visibility is a measure of how far the human eye can see through the atmosphere in the absence of obstructions such as nearby buildings and trees. Visibility is not measured directly but is derived from a measured extinction coefficient. Solar radiation is the total electromagnetic radiation emitted by the sun which is received at our monitoring site. These values are expressed in Langleys per minute. A Langley is a unit of energy per unit area (1 gram-calorie/cm2) commonly employed in radiation measurements. Barometric pressure is the force per unit area (pressure) exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above a specific point. These values are expressed in millibars. The millibar is a metric unit of pressure commonly used in aviation and meteorology. One millibar is equal to 100 Newtons/meter2 (100 pascals). Precipitation is any form of water (liquid or solid) that falls from a cloud and reaches the ground. Coarse particulates (PM-10) come from sources such as windblown dust from the desert or agricultural fields (sand storms) and dust kicked up on unpaved roads by vehicle traffic. PM-10 data is the near real-time measurement of particulate matter 10 microns or less in size from the surrounding air. This measurement is made at standard conditions, meaning it is corrected for local temperature and pressure. Coarse particulates (PM-10) come from sources such as windblown dust from the desert or agricultural fields (sand storms) and dust kicked up on unpaved roads by vehicle traffic. PM-10 data is the near real-time measurement of particulate matter 10 microns or less in size from the surrounding air. This measurement is made at local conditions, meaning it is not corrected for temperature or pressure. Fine particulates (PM-2.5) are generally emitted from activities such as industrial and residential combustion and from vehicle exhaust. Fine particles are also formed in the atmosphere when gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds, emitted by combustion activities, are transformed by chemical reactions in the air. Large-scale agricultural burning or sand storms can produce huge volumes of fine particulates. PM-2.5 data is the near real-time measurement of particulate matter 2.5 microns or less in size from the surrounding air. This measurement is made at local conditions, and is not corrected for temperature or pressure. Fine particulates (PM-2.5) are generally emitted from activities such as industrial and residential combustion and from vehicle exhaust. Fine particles are also formed in the atmosphere when gases such as sulfur dioxide, nitrogen oxides, and volatile organic compounds, emitted by combustion activities, are transformed by chemical reactions in the air. Large-scale agricultural burning or sand storms can produce huge volumes of fine particulates. PM-2.5 data is the near real-time measurement of particulate matter 2.5 microns or less in size from the surrounding air. This measurement is made at local conditions, and is not corrected for temperature or pressure.