Lee Harrison, a senior research associate at the Atmospheric Sciences Research Center of SUNY Albany, sent this comment as part of an exchange following this recent post on Dot Earth: “Can Public Leak Patrols Stem Gas Emissions at a Profit?” Link: http://nyti.ms/TI96uv The post explored how infrared imagery might offer the public a role in tracking emissions of methane and other gases from gas and oil facilities. The comment is posted in full here because of the space constraints in the comment section of the blog.

1. Lee Harrison, a senior research associate at the Atmospheric Sciences Research Center
of SUNY Albany, sent this comment as part of an exchange following this recent post on
Dot Earth: “Can Public Leak Patrols Stem Gas Emissions at a Profit?”
The post explored how infrared imagery might offer the public a role in tracking
emissions of methane and other gases from gas and oil facilities. The comment is posted
in full here because of the space constraints in the comment section of the blog.
From Lee Harrison:
Andy -- this isn't any sort of grudge match on my part, nor I assume on yours. However I
think there is a pretty serious list of issues to discuss and get right here:
First, a significant part of whatever issue is before us is about the word "detect" ... and it
is on that score that I have a pretty serious problem as a scientist in my field with what
"somebody" is claiming here; at best this is an extremely "non-specific indicator," not a
detector. Indications ("signs" in medical parlance) have great utility in diagnostic work
of any kind, but they don't represent scientific or legal "detection."
A second issue of practical importance here is the distinction between "Thermal Imaging"
and "IR camera." These are not the same thing at all. I will return to some details of this
below ... but had your article talked about citizens with "thermal imagers" I would not
have been so immediately critical in terms of physics of detection ... but would have
quickly remarked that few citizens have decent thermal imagers, they are expensive
(though the technologies are getting better).
I'll discuss these in backward order:
*****
With regard to "Thermal Imaging" please note that the patent you cite, Benson et al.
US2008/0231719 states (capitalizations other than acronyms are mine for emphasis):
"the focal plane array comprises a COOLED two-dimensonal array of quantum-well
infrared photodetectors (QWIP) having a peak responsivity in the wavelength range 10.4
to 10.8 micrometers. The camera has a COOLED band pass optical filter having a peak
transmittance approximately centered at 10.57 micrometers and a full width half
maximum spectral transmittance bandwidth of approximately 10.3 to 10.7
micrometers. ..."
much later in the document at [0010] it states "the example thermography cameras each
include a cyrocooler refrigeration device or container of liquid nitrogen, for cooling the
focal plane array and the optical filter (cold filter) to 77 to 100 Kelvin during
operation ...."
Joe Average sure as heck does not have or operate one of these, and they are not a simple
"IR camera." I'll discuss the significance of lines [0011] and [0012] below, about lower-
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2. cost systems operating at more reasonable temperatures, but before going onto that please
note that the fact that a patent has been granted is not proof that something works,
certainly not any statement of how well it works. This patent claims a physical
embodiment and "a use."
Low performance thermal imaging systems in the 10 micron domain (the maximum of
the Planck function at reasonable ambient temperatures, and also by quantum-mechanical
circumstance a "window region" for CO2 and relatively low H2O absorption ... which is
why this window is so important to climate) are getting "reasonably cheap" .... see e.g.
http://www.opticsplanet.com/flir-scout-ps-24-thermal-vision-digital-camera.html?
gclid=CJmeksjX2LMCFYuZ4AodDRMAWg&ef_id=UFza4wAADivJ5mXu:201211181
40401:s
But this device, which is good enough to see hot bodies at night, is not likely to have the
sensitivity or the dynamic range to do the differential absorption problem, particularly
during the day.
*****
There is also the problem/issue of the intended use of this detection legally. If the goal is
to actually make a citizen's army of leak detectors self funding via a bounty mechanism,
the method of detection must pass the legal standard for "preponderance of evidence." In
this regard there is a large body of legal history and precedent about specificity (lack of
interferences) for forensic methods ... for very good reason. I strongly doubt that anyone
could claim such a bounty on the basis of an IR camera photograph alone.
As a practical matter I think the attraction of the IR camera in this application is that one
can see something without having access; no trespass is intrinsically necessary. However
the law on IR camera and Thermal Imaging doesn't favor warrantless use: see Kyllo vs
US, 2001 and
http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=us&vol=000&invol=99-8508
One always wants to double-check anything you find in Wikipedia (I haven't!) but its
article on this is "a first indication" of interpretation and suggests that there has not been
a major change in court position since Kyllo vs US.
http://en.wikipedia.org/wiki/Kyllo_v._United_States
Now, going back to the physics and fundamentals of the issues here ... which was, and
remains, my principal objection here:
That "EPA background sheet" you provide me doesn't have ANY description of the
actual physics of the detection or discussion of interferences .. none. It contains no
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3. references to reviewed work which demonstrate the claims. It simply asserts "a variety of
components including methane .... pentane (the alkanes in increasing carbon order to 5)
can be detected by an IR camera."
Offering me a document which simply repeats the claim and nothing more is
embarrassingly circular, other than it attempts to argue from the authority of "the EPA."
But I see no signature, nobody taking responsibility for this at all. I'm not accusing you
of dummying this up ... but it's ridiculous to bring something like this to a discussion as
"evidence" ... whether that discussion is being conducted by scientific, legal, or
journalism standards of evidence.
Going back to the Benson patent -- look at lines [0011, 0012]. These lines claim that far
simpler IR systems operating in the 3.0 - 5.0 micron domain are "suitable for detecting
methane and other hydrocarbons" ... no other proof or reference is supplied, nor is any
required for this patent, which is not about that.
***
OK, let's talk about some real physics here. Casting around the web for a convenient
graph of the absorption spectrum of Methane I came across this:
http://icb.u-bourgogne.fr/omr/SMA/methane/
from the Molecular Spectroscopy and Applications (Department of Optics and Matter
Radiation Interactions), Institut Carnot de Bourgogne. The spectroscopy is well
explained and there is a nice graph of the low-pressure absorption spectrum of methane.
The unit of "wavenumber" may not be familiar to you, it's effectively a unit of optical
frequency, but has the curious dimension of inverse centimeters. It's a spectroscopist's
convenience to avoid very big numbers for frequencies of optical wavelengths.
Wavelength = speed of light / frequency ... and the speed of light being very large.
1000 wavenumbers = 10 micron wavelength
10000 wavenumber = 1 micron wavelength
The abscissa of this graph is inverse to wavelength. And what you see is four Methane
absorption bands, the leftmost one on this graph is at wavelengths just below 10 microns,
and the rightmost one is at wavelengths around 1.6 microns.
Ok, on the face of it ... the band(s) to the right are "good" for cheap camera detection.
This is why all of these folks are claiming "detection detection detection"
And if one had a spectrometer capable of the resolution shown AND the earth's
atmosphere was at 1 Torr pressure (1/760 th of sea level) then one could see a spectral
fingerprint like this ... and presentation of such data would lead almost any scientist or
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4. forensic witness would accept "yep, that's methane."
But here's the big problem -- the earth's atmosphere isn't 1 Torr, and there is quantum-
mechanical pressure broadening, aka Lorenz broadening. And the data shown here with
large opacities is taken over a pathlength of 100 m of pure methane at that pressure, and a
temperature well below ambient anywhere except the earth's poles (there is also
temperature broadening, less important here though). These data are looking through a
lot of methane. You won't get columns of that much methane unless the leaking tank is
pressurized and spewing a huge plume of methane (in which case OMFG is it a fire
hazard, huge one)
Under real atmospheric conditions the spectrum is "smeared" by the pressure broadening.
That's "ok"as far as a broad spectral "camera" is concerned, but the consequence is that
the camera doesn't see very much contrast ... precisely because it has a heck of a large
optical bandpass. And in that bandpass there can be all sorts of other absorbers ... starting
with H2O, which has good absorption bands in these domain(s).
So what is going on is that these folks are taking images, seeing something wispy or
contrasty, and arguing "it must be a methane leak." Possibly so, but jeez, a great many
possibilities that there is not. And "image interpretation" is not something with a good
history of evidence period.
If the claim is that this is suggestive of a leak, or might be an "indicator" of a leak...
grudgingly maybe ... but there's no way I can see that any court would accept this
measurement forensically, even under "preponderance of evidence," given a half-way
competent defense. And under Kryllo the evidence likely wouldn't be admissible
anyway. Perhaps your argument is that that a court might accept this as "probable cause"
... for a REAL search warrant /measurement? I'm not a lawyer, wouldn't argue that one,
but again Kryllo comes into play. (And "corporations are people too, my friend.")
On the otherhand if the evidence is "I marched up to the tank, lit a match ... and
WHOOSH!" Not so much argument, eh? That requires trespass ... and has other
consequences though.
And finally, back to the point I started with -- why bother with the IR camera, with all of
it's interferences and poor sensitivity and specificity for the job? There are many ways
to measure Methane, relatively cheap laser-diode absorption cell devices do it using the
absorption lines shown and are quantitative and specific, and very sensitive. Presuming
one can go downwind of the tank without trespass, or even fly through the downwind
plume ... bingo. And there are no trespass arguments wrt the aircraft measurement,
although there are FAA restrictions on low flight in densely populated areas.
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