Tuesday, 3 May 2011

CIMS instrument confirmed for BORTAS

CIMS stands for Chemical Ionisation Mass Spectrometry and this technique has been shown to be brilliantly versatile with the potential to measure a vast array of important atmospheric species. The system that will be used on the BORTAS flights (operated by the University of Manchester) this summer uses polonium-210, one of the 33 radioactive isotopes of polonium. The polonium-210 emits alpha particles which can knock electrons out of the nitrogen molecules that are present. These 'free' electrons are then captured by neutral molecules which become negatively charged. In this case it is the reagent gas CH3I (methyl iodide) that captures the electron and dissociates giving negatively charged iodine atoms (I-). This I- then reacts with the species we want to detect and transfers the negative charge as shown below or attaches to the other species forming a negatively charged adduct such as HCN.I-.

Because our compounds of interest are now negatively charged they can be separated according to their mass to charge ratio and then detected. Chemical ionisation is used because it is gentle giving little fragmentation (where the ion splits up creating many more ions with different mass to charge ratios which complicate the spectrum). I- is used because it is fairly selective; it is unreactive with most compounds present in the atmosphere but reacts with the compounds we want to detect. So this set up will allow us to detect the sum of NO3 and N2O5 (they rapidly interconvert), nitric acid, formic acid, propanoic acid, butanoic acid and HCN. HCN has been shown to be an important tracer for biomass burning (eg. see http://bit.ly/laf97J) so this measurement particularly is useful for us.

The CIMS instrument on board the UK Atmospheric Research Aircraft

Like the LIF instrument (see blog posts from 11th October 2010 and 11th June 2010), the CIMS was flown on the RONOCO flights in August and September last year and January this year. It was during these flights that a correlation between the NO3 and N2O5 measured by broadband cavity enhanced absorption spectroscopy (BBCEAS, operated by the University of Cambridge) and a peak in the CIMS spectrum was noticed. Then through a comparison exercise carried out at the Facility for Airborne Atmospheric Measurements the University of Leicester's NO3/N2O5 calibration unit was used to confirm that the suspected NO3 peak was indeed due to the sum of NO3 and N2O5.

 The top plot shows the time series from the CIMS suspected NO3 peak and the BBCEAS NO3 and the bottom plot shows the strength of the correlation between the two.

At the beginning of the post I mentioned the versatility of the CIMS technique so as an example here is a list of the species I haven't mentioned that can be measured using different reagent ions: SO2, HO2NO2, PAN, PPN, MPAN, HOOH, CH3C(O)OOH, HCl, ClONO2, NH3, DMSO, C5H8, HO2 HO2+RO2, H2SO4, amines, various volatile organic compounds.

If you want to learn more about the CIMS or what the guys at University of Manchester do see this page and pages linked from it. Thanks to Jennifer Muller from the University of Manchester for the information on CIMS and the results from the RONOCO campaign.

No comments:

Post a Comment