Adding a new gas -- Overview
Currently the scripts allow for a collection of strong and some weak absorbers.
Considerations that went into this list were suggestions from relevant literature,
experiences of colleagues, expressed aims of the TACTS campaign, and limited time
available to develop the gas fit scripts.
It might be that later we decide we want to include more gases. Unfortunately, you can’t
just tell the script that you want it to look at a brand new gas. It is not magic. Instead,
there is a process that must be done to include a new script.
Before we add a new file, there are some things you should understand about this
process. While it does allow us to extract more information, that information comes at a
cost.
Disadvantages of adding a new gas:
 Decreased strength of fit – adding more free parameters gives a greater
possibility of over-constraining the fit. DOASIS might return an almost “perfect”
fit, which actually contains less usable information then a “less perfect” fit.
o Every gas you add contributes a degree of freedom. For post-campaign
processing, it might be useful to limit the number of gases in each fit. You
can still test for all the gases, but you might want to only include the
strong absorbers + any cross correlated gases.
Research statistics break down when there are too many degrees of
freedom.
 Increased runtime. Every wavelength-intensity data point you add, is one more
point that DOASIS must try to fit. This naturally increases runtime, which can be
problematic when processing time-sensitive results.
o Simply adding a high resolution HITRAN H2O spectrum already slows
down the evaluation to impractically slow speeds. This can be temporarily
avoided by fitting using a lower resolution spectrum. For precision
measurements this is to be avoided.
 Cost of scientist’s time. It takes time to add a new gas. There might be
something more important to do. Be aware that this process takes at least 1-2
hours/gas.
But, it gives us useful information. So you might still want to add a gas.
(For post-campaign processing, you might want custom fit scenarios for each gas – only
including that gas + strong absorbers. You do not need all the weak absorbers in every fit
scenario)
Step-by-step – Adding a Gas
1. Find possible evaluation wavelength ranges
a. These are “first guesses” – you do not need to know exactly which one yet
b. How to do this:
i. search recent literature. Some papers will tell you the evaluation
wavelength range, but many will not. (It might be nice for us to
publish our evaluation ranges, for future researchers).
ii. Often you will find there are a few ranges that are common
iii. Check that our spectrometers can see the range
2. Obtain reference cross section.
a. Good resource for finding cross sections:
http://www.atmosphere.mpg.de/enid/2295/
b. Decide what temperature + resolution. Higher resolution is often better.
c. Check that they have the correct wavelength range (step 1)
d. Save relevant information (temperature, author) in the filename, or inside
the file
e. Place cross section in the correct campaign cross section folder
i. Eg: “TACTS_FitScenarios_CrossSections”
3. Open relevant GasFit script (UV or VIS)
a. Usually, if the script uses a cross section, it will do so with all the cross
sections. Add a new line each time, formatted like the others
b. Example places:
i. Creating spectrum object
ii. Opening file
iii. Convolution
iv. Linking the right fit scenario index
v. And more.
4. Open the relevant Fit Scenario in DOASIS (eg: “UV_master.fs”)
a. Convolve the new cross section
b. Add the convolved spectrum to a new line in the fit scenario
c. Make sure that the other spectra are convolved.
d. Save fit scenario
e. Keep DOASIS open for next step
5. Perform a fit by hand.
a. Test each of the possible wavelength ranges (step 1)
i. Remember “fit range” is measured in measurement spectrum
channel numbers, not nanometers
ii. If “Log Spectrum” is checked, do NOT take a logarithm of the
measurement spectra
iii. Remember to take a ring spectrum, AND THEN the log of the
reference spectrum and the ring spectrum
iv. At this point, re-add the ring + reference spectrum to the fit
scenario list of spectra. It might already say “RefSpec”,
“RingSpec” but you need to add the spectra again, manually.
b. Observe the fit results + error plots
6. Optional: test an entire set of spectrum with wavelength ranges
a. Do any ranges look “better”?
i. Features that you would expect
ii. Smaller error bars
b. Do any ranges look too similar to other gases?
i. You might have a cross-correlation (eg: O4, H2O at about 450nm)
7. Check with someone else that your decision makes sense.
Note: you can also play around with the use of DOAS/Offset polynomials. It is possible
to add this to the GasFit codes, but I did not originally include that feature. (Due to time
constraints, I left that functionality for post-campaign processing.) At time this document
was originally written (27 Aug 2012) all UV fits are done with the same polynomial
settings, and all VIS fits are done with the same polynomial settings (but UV and VIS
settings need not be the same)