Wetenschap - 1 juni 2011

Measuring particulates on the edge of Wageningen

Trees capture particulate matter from the air. The question is, though, whether this helps combat pollution. Annette Pronk of PRI is doing tests in the Binnenveld to find out.

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GA--20110525-_ND71645.JPG

Foto: .

The test site for these experiments is a field on the Plassteeg, north of the campus, where Pronk and her colleagues from the ASG are measuring how much (homemade) particulate matter is captured by a stand of trees. The experiment looks deceptively simple. On six stepladders stand as many 'nebulizers': ventilators which blow droplets of a particulate solution out of a jerrycan in the direction of about thirty trees a small distance away. In front and behind the trees are six masts full of measuring gear.

Yard vegetation
Project leader Pronk's test is being done at the behest of the ministry of EL&I, and concentrates on the potential for capturing particulates from barns using garden or farmyard vegetation. Intensive livestock farming is a major producer of particulate matter, and there are many possible measures for reducing emissions. One option is to use air filters to absorb some of the emissions. But trees are much cheaper. In fact, Pronk's test is a follow-up to an experiment conducted two years ago, when Alterra and the ASG were involved in testing whether trees along the A50 at Valburg captured particulates emitted by road traffic. This test was a partial failure. A positive effect for the capture of nitrogen oxides was observed, but the tests turned out not to be reliable enough to draw conclusions about particulates.

Wind
The tests at the Binnenveld should provide more conclusive results. The experiment boils down to an attempt to determine where the particulate matter blown into the air ends up. How much of it is captured by the trees, and what is the influence of the weather on this? Wind, air humidity and upward air currents are all carefully monitored.
Pronk is testing how well pine and hornbeam trees purify the air. The vegetation is sprayed for about an hour at a time, after which samples are taken from various parts of the plants, the soil and strips of paper that have been hung up around at the 'scene of the crime'.

Fluorescence
Measurements of the particulates are done using fluorescence. The particulate used by Pronk is a fluorescence tracer (BSF). The particles (5-10 micrometres in size) from the samples are dissolved again and measured. This ensures that only the experimenter's 'own' particles are visible and not others from the surroundings. Pronk still needs at least ten readings. The results will only be announced after the summer vacation.

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