[isftir_users] MALT concentration a priori [SEC=UNCLASSIFIED]

  • From: "PARKES, Stephen" <ses@xxxxxxxxxxxx>
  • To: <isftir_users@xxxxxxxxxxxxx>
  • Date: Mon, 9 Aug 2010 08:31:34 +1000

Hi everyone,


I just came across something last week that it is worth being aware of for 
MALT. It isn’t really problem but is something to take note of.


To give you some background we run an OOOFTI that is set up to measure water 
vapour isotopes (18O and D), so we don’t dry the air. According to Dan this is 
an unhappy OOOFTI. 


Last week I was running some experiments on the OOOFTI at low water 
concentrations. These concentrations were much lower than what we normally look 
at so I changed the a priori water concentration for each of the water isotopes 
in the “prm file” used by MALT. When I changed them back for a typical 
measurement routine I changed them back to a slightly lower concentration (0.4% 
H2O, normally 0.6%). I then ran a calibration experiment and noticed a huge 
offset in delta18O (15‰) and a relatively small one for deltaD (0.5‰). Took me 
a long time to figure out why. But when I figured it out I reanalysed the 
spectra using the old a priori concentration and the offset disappeared.


I then ran a number of reanalyses where I used different a priori H2O 
concentrations. Each time I got a different delta18O and deltaD value.


Has anyone else come across this before?


I didn’t look at the trace gas concentrations, so I don’t know if these are 
affected, but I guess it’s something to be aware of. 


Speculating here, but I’m guessing that it probably isn’t so much an issue for 
trace gas concentrations, but for isotopes and delta calculations we are 
looking at very small changes in concentration so it may be important. Might be 
worth looking at 13C in CO2 and see if the a priori CO2 concentration affects 
the delta13CO2 value?







Stephen Parkes                 
Atmospheric Isotope Scientist                       
Institute for Environmental Research,
Australian Nuclear Science and
Technology Organisation (ANSTO)

Locked Bag 2001, Kirrawee DC NSW 2232 Australia
Tel: +61  2  9717 3001
email: stephen.parkes@xxxxxxxxxxxx


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  • » [isftir_users] MALT concentration a priori [SEC=UNCLASSIFIED] - PARKES, Stephen