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General Information
Gas concentration levels in ambient air are often expressed in Parts per Million (ppm) or Parts per Billion (ppb). These two
common ways of expressing readings sound similar but in reality they are very different, by a factor of 1000 to be exact.
1 ppm = 1,000 ppb
Therefore as an example, a gas concentration reading of 5 ppm is the same as 5,000 ppb or a reading of 5 ppb is the same as
0.005 ppm
Readings from a sensor may drift by as much as +/- 100 ppb. This means that one should not place too much emphasis
on any specific reading. For the most part, we are better served by viewing averages and trends when using sensor based monitors.
Sensor units have the advantage of being small, portable and low cost.
Air Pollution
Polluted air is defined as air containing harmful substances that are persistent, in high concentrations and can cause health
problems to humans, plants or animals or result in damage to property.
Polluted air may contain particulate matters or gases. The oil and gas industry is generally concerned with gases such as
Hydrogen Sulfide (H2S), Sulfur Dioxide (SO2), Nitrogen Dioxide (NO2) and flammable gases (LEL).
Hydrogen Sulfide (H2S)
Hydrogen Sulfide or sour gas is a flammable, colorless gas that is toxic at low concentrations. It is classified as a chemical
asphyxiant, which would be the same classification as carbon monoxide for example. It is heavier than air and may accumulate in
low-lying areas. It smells like rotten eggs at low concentrations and quickly deadens a person's sense of smell at higher
concentrations. H2S is soluble in water and oil. It burns with a clear blue flame and produces Sulfur Dioxide (SO2) as one
product of combustion.
Sulfur Dioxide (SO2)
Sulfur Dioxide is a colorless, non-flammable gas with a sharp, pungent odor similar to a struck match. Because SO2 is less toxic
than H2S, it is a common safety precaution to flare gases containing H2S rather than vent them to atmosphere. The flaring
process produces SO2 (among other gases). Operators are required to ensure that SO2 emissions during flaring are kept within
government guidelines. SO2 can cause respiratory problems. It can also displace breathing air if it accumulates in low-lying
areas and it can be detrimental to vegetation.
Nitrogen Dioxide (NO2)
Nitrides of Oxygen (NOx) are the total of Nitrogen Dioxide (NO2) and Nitric Oxide (NO). NO is produced during flaring and will
generally combine with Ozone (O3) to form NO2. Nitrogen Dioxide is a reddish-brown gas with a pungent odor and is partially
responsible for the brown haze associated with smog. NO2 is classified as toxic and can cause respiratory problems - it can also
be detrimental to vegetation in some circumstances.
ALBERTA'S AMBIENT AIR QUALITY OBJECTIVES
The Alberta Environmental Protection and Enhancement Act allows Alberta Environment to develop ambient air quality objectives.
They have consulted with stakeholders, other government departments, industry, environmental organizations and the scientific
community to develop the following standards.
H2S
- 1 hour average 10 ppb
- 24 hour average 3 ppb
The above noted recommended maximum concentrations for H2S are based on "odor perception". In other words, the intention is to
minimize objectionable odor, not necessarily to avoid health problems. For example, when the objective is to avoid an unhealthy
work environment, workers in Alberta are allowed exposure to the following concentrations:
- up to 8 hours exposure 10 ppm
- up to 15 minute exposure 15 ppm
- never more than 20 ppm
One might then conclude that the Alberta Ambient Air Quality Objectives have a factor of safety of more than 1000 if health
concerns are the issue as opposed to avoidance of nuisance odors.
SO2
- 1 hour average 172 ppb
- 24 hour average 57 ppb
The SO2 recommendations are based on avoiding respiratory problems and/or damage to vegetation.
NO2
- 1 hour average 212 ppb
- 24 hour average 106 ppb
NO2 recommendations are based on odor perception, i.e. minimizing objectionable smells - see H2S notes above.
Gas Sensors
The sensors used by Eagle Technologies are electrochemical cells that generate an electrical current proportional to the
fractional volume of gas being measured.
They use 3 electrodes and an electrolyte reservoir to generate and balance the current being measured. The composition of the
electrodes and electrolyte are specific to the gas being monitored. The working electrode either oxidizes or reduces the target
gas and the counter electrode tries to balance the current generated by this reaction. The reference electrode keeps the current
within a limited range to enable accurate measurement.
Interfering Gases
Gas sensors are built for optimum response to a specific target gas - they may however be affected by other gases that might be
present. For example, NO2 is an interfering gas for H2S sensors. NO2 has the effect of lowering the signal from H2S gas and will
result in a lower than actual reading of H2S concentrations. This is one of the reasons that Eagle EYE monitors have NO2 sensors -
they allow for analysis of situations when high NO2 concentrations might have resulted in lower than actual H2S readings.
Sensors have a shelf life of about 2 years. Their useful lifespan can also be shortened by overexposure to target or interfering
gases. Eagle Technologies technicians test and replace sensors on a scheduled basis to maximize accuracy and consistency when
using Eagle EYE stand-alone air monitors.
