The pictures above show FT-IR spectra
of smokestack plumes being collected passively. In these photos,
the smokestacks being monitored are from coal-burning power
plants and the background scene behind the plumes is a
cloud-filled sky. In theory, almost any gaseous cloud or
plume, often called the "target", can be passively
monitored in this way but sensitivity is best when the target is
at a much different temperature than the background against which
it is viewed, as is the case with many industrial stack plumes,
jet engine exhausts, flares, etc..
In the smokestack monitoring shown
above, the plumes are exiting a combustion process and are
therefore much warmer than the clouds in the background. If
no clouds were present, the apparent temperature contrast between
the plume and the sky would probably be even greater because
clear, blue skies appear very cold over most of the infrared
region.
The spectrum below was generated from
the work shown in the photos above. This spectrum is a difference
spectrum, created by subtracting a spectrum of the clouds alone
from a spectrum of the plume in front of a cloud
background. Since the plume was warmer than the background,
the difference spectrum shows the infrared signatures of the
plume constituents as emission bands. An emission band of
sulfur dioxide appears as two humps centered around 1150
wavenumbers (approximately) and two emission bands of carbon
dioxide are seen, one centered around 1064 wavenumbers and the
other centered around 961 wavenumbers (approximately).
To generate quantitative information
from the emission bands seen in the spectrum above, some
additional information would have to be known including the FT-IR
instrument's spectral response to infrared energy and the
temperature of the plume. These things can be determined
with advanced spectroscopic techniques and calibrations performed
in the field at the time of the data collection.
In many air monitoring applications,
the FT-IR instrument can be located so that a warm object fills
the sensor's field of view. In these cases, high-quality
absorption-based spectra of the intervening atmosphere can be
passively collected and analyzed in much the same way as would be
done in active open-path FT-IR air monitoring over the same beam
path. This approach to passive air monitoring is especially
useful for the analysis of air quality around industrial sites,
where warm objects are usually abundant.
The top spectrum in the figure below
is an absorbance spectrum created from single-beam spectra
collected passively at an agri-chemical plant, using the warm
wall of a cooling tower as an on-site source of infrared
energy. The signatures of three pollutants from the
industrial site - nitrous oxide, methane, and ammonia can be
readily identified in this field spectrum. Reference
spectra for the three pollutants are also shown in the figure for
comparison. The methane signature is largely obscured by
the nitrous oxide signature in the raw field spectrum but becomes
much more apparent after the nitrous oxide signature is stripped
out of the field spectrum with spectral analysis software.
This data illustrates the possiblity
of generating quantitative information concerning air quality
from passively-collected FT-IR spectra, without the need for
elaborate instrument calibrations in the field. It should
also be noted that each single-beam spectrum was collected over a
period of approximately one minute, and the field spectrum shown
in the figure above was just one of more than a hundred collected
over a period of a few hours. FT-IR air monitoring offers
excellent time-resolution in the information produced, making it
possible to characterize the emissions from transient processes
occurring on a site being monitored.
A general
discussion of passive FT-IR air monitoring.
References
and Suggested Reading
AeroSurvey's Web Site / AeroSurvey, Inc. / www.aerosurvey.com