HOW DO WE KNOW WHAT'S IN THE AIR??
There are several ways to determine the quantity of air pollutants being emitted from sources and their concentrations in the ambient air.
Continuous Emissions Monitoring Systems (CEMS) are installed on some point sources (stacks, etc.). These monitors sample gases just before they exit the stack into the atmosphere and can determine the concentration (generally in parts per million) of the pollutants being monitored. The monitors take essentially continuous readings and the results can be routed to process computers and some can be accessed via the Internet. These monitors are generally operated by individual facilities, but must pass EPA and/or TCEQ accuracy and on-stream time requirements.
Ambient Air Monitors are utilized to determine how much of certain constituents are found in the air over long periods of time. These Ambient Air Monitoring Stations are generally operated by governmental entities. The sampling can vary from once every six or twelve days (which gather the air sample in a canister that must be sent to a laboratory for analysis), to continuous type monitors; but, the continuous monitoring systems can only monitor a few constituents, compared to the fifty or more that a six or twelve day canister sample can yield in a laboratory analysis. These stations often have meteorological stations located with them, in order to gather an array of weather data to better understand how pollutants move, how they react with each other, and how the weather affects those processes. Though operated by governmental entities, these systems must pass quality control requirements for the data to be useable.
Ambient Air Monitoring Station
(With Meteorological Tower)
Episodic Air Monitoring is used to determine airborne concentrations of specific pollutants during short-term release events. If an Ambient Air Monitoring Station is in the path of the release, then it may detect those components it is set up to monitor, but such circumstances are rare. Generally, hand-held, portable instruments are used because the “plume” can be tracked, even if the wind direction shifts during the episode. The portable instruments are not as sensitive as laboratory equipment, but are certainly sensitive enough to be protective of human health during episodes lasting less than eight hours. If very low levels of detection are required, “bag” samples can be taken and sent to a laboratory for analysis, but the results won’t be available for several days.
CEC taking “Bag” air sample
Stack Emissions Testing is used to determine the rate that specific pollutants are leaving a point source (stack, etc.) during the timeframe of the test. The tests utilize flow measurements, and chemical reaction methods for each pollutant tested. The tests must follow strict procedures issued by the EPA and/or TCEQ. The stack tests cannot be conducted until notice has been give to the appropriate regulatory agencies, in order to provide the agency an opportunity to witness the test. Generally these tests are conducted on newly constructed equipment that is being put into service for the first time. The test results are used by regulatory agencies to determine if the equipment meets the necessary emissions limits in the regulations or air permit. The performance tests are conducted under operating conditions that maximize emissions, so that normal operating variations will not result in emissions exceeding any established regulatory limit. While stack tests are performed during the start-up phase of new equipment, regulatory agencies can ask for a facility to test its stack at any time.
Leak Detection and Repair (LDAR) Monitoring is utilized to determine the concentration of volatile organic compounds (VOC’s) leaking from equipment in manufacturing facilities. Technicians must monitor (at specified time intervals – quarterly, monthly, etc.) valves, flanges, pumps, compressors, and similar equipment with portable equipment, calibrated to detect VOC’s. When a leak (generally defined as 500 ppm) is found, it must be repaired within specified timeframes, and re-monitored to assure it has been fixed. Facilities must keep scrupulous records of components monitored, leaks found, dates of repairs, and several other parameters so regulatory agencies can verify how well the LDAR program is working.
Fugitive Emissions Technician taking readings
Special
methods of air monitoring are sometimes used during scientific studies
of air quality. Everything from monitoring equipment aboard weather
balloons to sophisticated equipment in airplanes is used to gather
data. Sometimes, new technology is introduced promising quicker,
easier, and more accurate results. However, regulatory agencies require
a proven record, over time, achieving those promises before they will
consider using the equipment for monitoring. That is why industrial
facilities and agency field offices do not use the latest “gee whiz”
technology when it is first marketed. It is not a matter of cost or
fear of results, but the simple fact that regulatory agencies will not
accept data from systems that have not had an extensive peer review and
proven sustained accuracy and reliability in monitoring the air under
real conditions in the field. However, regulatory agencies, industry,
and equipment manufacturers do often participate in field studies to
determine the feasibility of using new technology and finding out its
capability to produce accurate results under field conditions, not just
in the laboratory. By utilizing the types of systems described above,
we are in a better position to know what types of controls, if any, are
needed, and where they will do the most to improve the air we all
breathe.
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