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the interview below, in-cites correspondent Gary Taubes talks
with Dr. Laurence Rothman of the Harvard-Smithsonian Center
for Astrophysics about his highly cited paper, "The
HITRAN molecular spectroscopic database and HAWKS (HITRAN
Atmospheric Workstation): 1996 edition" (J. Quant.
Spectrosc. Radiat. 60[5]: 665-710, November 1998).
According to the ISI
Essential Science Indicators
Web product, this paper has been cited 688 times to date, and
is currently the most-cited paper of the past decade in the
field of Engineering. Dr. Rothman has 12 highly cited papers
in this field, with a total of 863 citations to date.
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Would you please tell us about how HITRAN got started and what
it is?
HITRAN stands for HIgh-resolution
TRANsmission molecular absorption database, and it goes all the way
back
to 1961, before I did my Ph.D. on an aspect of molecular
spectroscopy. Through some prior research I had done with an Air
Force lab nearby in Bedford, Massachusetts, I got a job out there.
This Air Force lab was interested in detecting jet aircraft from a
distance, with detectors, say, from another aircraft. But you have
the Earth’s atmosphere in between, obviously, with a lot of
absorption at different frequencies. Hot sources give out a lot of
energy in the infrared region of the spectrum, but if you happen to
tune your detector, for instance, to where there’s major
water-vapor absorption, you’re not going to see anything. What the
Air Force needed was a database, a whole compendium of the major
absorbers in the Earth’s atmosphere, where they are and how strong
they are. The thing you have to realize is that the molecules in
gasses have discrete frequencies of absorption; all these guys—water
vapor, carbon dioxide—absorb electro-magnetic radiation at
discrete frequencies, but they’re smeared out a little because
they’re colliding all the time. If you had a database of all this
information, you’d have a fingerprint of what’s going on in the
atmosphere. It can be an incredible amount of information.
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“HITRAN is like the human genome of gasses, if you will.”
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I should also mention that these absorbers in the infrared range
are what some people call the minor constituents of the atmosphere.
These are the trace constituents, like water vapor, methane, and
carbon dioxide, which happen to be incredibly efficient absorbers.
This was over 40 years ago. How did it develop once you got
this assignment?
Well, we assembled a kind of committee from around the States,
key people to work on this. Not just from our lab. We had
sufficient funding at the time. We could have contracts with them.
Someone from Ohio State, from the University of Maryland, from a
major company in California. We had the research department at
Hanscom Air Force Base in Bedford Massachusetts. Together, we
developed this big database and then made it public domain. The
first edition of this came out on a magnetic tape in 1973, and we
freely distributed it.
Who were the original users?
Certainly the military. It started out a military project. But
then NASA immediately realized they could use it for remote
sensing of the Earth’s atmosphere. And very early on the
Department of Transportation (DOT) got involved. They had this
very special problem. Nixon wanted to build a supersonic transport
(SST), and the critics worried that it would destroy the ozone
layer by giving out nitric oxide and some other gasses. So we got
funding from the DOT and that enabled me to expand on this HITRAN
database from the original seven gasses we had. Meteorologists
started getting into it, so that pushed it to spectral regions
beyond the infrared. This was over the course of time. HITRAN is
like the human genome of gasses, if you will. And over the years,
an unbelievable number of applications were developed to make use
of it.
It sounds like it was driven as much by the applications as
anything else?
Applications did tend to drive it. In 1973 we came out with a
database that involved only seven molecules in the infrared—water
vapor, carbon dioxide, ozone, nitrous oxide, carbon monoxide,
methane, and oxygen, which has a couple of weak bands. I jokingly
call these the original seven sins. They are the major absorbers
in the infrared. That application was driven by the Air Force. In
1978 we had this input from the DOT because of the SST. In the
early 1980s, the Air Force started thinking maybe we should be
looking at things in the microwave, so we extended the spectral
coverage to microwave. In the 1980s NASA came in with this whole
idea of remote sensing. They have been the biggest driver to me;
they also have taken on the major funding support for this
program. Using my database, they can calculate what the
constituent quantities are in the Earth’s atmosphere. That meant
I had to expand beyond the original seven gasses to the real
trace gasses—the anthropogenic things in the atmosphere. We want
to see if they’re increasing or decreasing or have odd
geographical or altitude distributions. Now the project has become
really big. We also have more parameters than we started with.
Originally, we tried to catalog the exact wavelength, the
intensity of the line and its width, and the lower state energy.
That was four parameters. For the sophisticated remote sensing
NASA is doing, they needed more. So in addition to having to add
many, many gasses, we had to expand the parameter set and get
better accuracy.
How accurate are the HITRAN measurements?
We require these things, because of the instrumental
capabilities of NASA satellites, to have incredible accuracy. That
means we want to know line positions to one part in 10 million or
even better. That’s a tough demand. We want to know the
intensities to better than two percent. That’s tough, too. There
are so many sources of error. For example, the pressure and the
temperature in the cells that we’re using to measure these
quantities. We’re pushing the envelope.
You’re working at an astrophysical laboratory but you’re
highly cited in engineering. How does that happen?
Well, the engineering citations are simple. There are so many
applications and a lot of them are industrial, and so this is
where it shows up. The Environmental Protection Agency is now
using it. They’re worried about what’s coming out of various
smoke stacks, for example. It’s everything about gasses and so
everybody is using it. Now why is it here? A place called the
Harvard-Smithsonian Center for Astrophysics. We have a division
here called atomic and molecular physics. A lot of people here do
spectroscopy and, in fact, most of the people on my floor don’t
do traditional astrophysics; we do atmospheric physics. We have
experts in the ultraviolet, microwave, infrared. Also this
facility has a tradition in building databases. The archiving of
mankind’s knowledge and achievements fits very well with the
Smithsonian mission in this joint facility.
How do you see HITRAN evolving over the next five years?
One thing, which some people say is just crossing t’s
and dotting i’s,
is replacing a lot of our stuff with much more accurate data. That’s
needed for upcoming satellites. I also see adding a lot of
spectroscopic features that are important but that I’ve never
gotten into—effects due to the collisions of molecules, for
instance. These have been seen, but they haven’t been very well
characterized up to now. I’d also like to push this toward
higher temperature parameters. I recently had some calls from a
Nobel Prize winner studying red giant stars, and he needs
something like my database, but pushed to higher temperatures.
What’s been the biggest surprise over the last 40 years?
Well, one of them is how many things man is putting up there.
It makes my job never-ending. I just added a new molecule to the
database that I can’t even pronounce. It gets a little strange.
We have all these new gasses that replace CFCs, which attack the
ozone, but the new gasses are long-lived and have other effects.
They become greenhouse gasses. The only solution is to have fewer
humans.
Is there a global message about HITRAN that you’d like to
leave for the public?
Well, I’ve been managing HITRAN for more than 30 years almost
on a shoestring. I get support. I now have one or two post-docs. I
rely a lot on goodwill from various countries. But one thing is
that all of us involved are getting older, and so I need to make
sure this carries on. Ideally it should be run internationally
with an institute. But that needs to be pursued. Everyone
recognizes it has incredible applications. We need to put more
molecules in. We need to cover more of the spectral region. We
need even more accuracy. But we have to keep it alive. And many
people are getting a free ride on this. I have 6,000 people on my
mailing list from academia to industry. They’re all getting a
free ride, which is okay but it doesn’t keep things going. NASA
has been great, but you can’t expect them to take on the entire
burden. In recent years they have taken the lion’s share. They
get a lot, and they need it. But they shouldn’t be the only
ones. They know it, and so they ask me, "Who else is funding
you?" And so I go ask the other users, and they say,
"You don’t need more money. NASA is supporting you. Just
give us the data."
Dr. Laurence S. Rothman
Harvard-Smithsonian Center for Astrophysics
Atomic and Molecular Physics Division
Cambridge, MA, USA
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