The PICAB campaign

By Rupert Holzinger

In September my research group at IMAU organized the Proton-transfer-reaction mass-spectrometer (PTR-MS) Intercomparison campaign in CABauw (PICAB) as part of the European infrastructure program Actris-2. Eleven PTR-MS instruments operated by European and US groups measured for two weeks the ambient air composition at the CESAR observatory near Cabauw. All instruments were subjected to new calibration procedures, developed at IMAU, using a gas standard custom manufactured by the National Physics Laboratory (NPL), UK. PTR-MS technology recently underwent mayor innovations that boosted the sensitivity by 2-3 orders of magnitude. These new generation instruments detect organic species at extremely low mixing ratios of 100 parts per quadrillion (fmol/mol) with a really great performance: 10 seconds time resolution and a precision of ~10% ! We were fortunate to attract this latest technology (only 2 instruments exist worldwide) as well as instruments that have served already for more than 10 years.

Fig 1. The surroundings of the PICAB campaign. At first sight rural site, but the four major Dutch cities (Amsterdam, Rotterdam, The Hague, and Utrecht) are all within 30 km of the site.

The first results clearly indicate which operating conditions ensure the highest stability and intercomparability between instruments. The new calibration approach holds the potential of fully automated and fast (<1 minute) in-field calibrations, and we are extending our software tools for user friendly and fast data processing. With this campaign we aim at enhancing PTR-MS technology to become a potent instrument for long-term monitoring of atmospheric composition. Apart from the calibration exercise, some interesting discoveries were made. For example, during daytime we saw background levels of a few parts per trillion (pmol/mol) of Decamethylcyclopentasiloxaan (D5), which is a chemical used in various body care products. At night the levels dropped by at least one order of magnitude, proving that D5 is deposited on the ground.

Last but not least, it was a great pleasure and experience to work together with colleagues from so many different countries. Yes, science can be hectic, sometimes it is a struggle, but also a lot of fun!

Fig 2. Busy activities inside the building.