Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 6 2007 10.5194/acpd-7-18179-2007 http://www.atmos-chem-phys-discuss.net/7/18179/2007/ http://www.atmos-chem-phys-discuss.net/7/18179/2007/acpd-7-18179-2007.html http://www.atmos-chem-phys-discuss.net/7/18179/2007/acpd-7-18179-2007.pdf 18179 18220 2007-12-19 The Comparative Reactivity Method – a new tool to measure total OH reactivity in ambient air V. Sinha J. Williams williams@mpch-mainz.mpg.de J. N. Crowley J. Lelieveld Max Planck Institute for Chemistry, J. J. Becher Weg 27, 55128 Mainz, Germany Hydroxyl (OH) radicals play a vital role in maintaining the oxidizing capacity of the atmosphere. To understand variations in OH radicals both source and sink terms must be understood. Currently the overall sink term, or the total atmospheric reactivity to OH, is poorly constrained. Here, we present a new on-line method to directly measure the total OH reactivity (i.e.~total loss rate of OH radicals) in a sampled air mass. In this method, a reactive molecule (X), not normally present in air, is passed through a glass reactor and its concentration is monitored with a suitable detector. OH radicals are then introduced in the glass reactor at a constant rate to react with X, first in the presence of zero air and then in the presence of ambient air containing VOCs and other OH reactive species. Comparing the amount of X exiting the reactor with and without the ambient air allows the air reactivity to be determined. In our existing set up, X is pyrrole and the detector used is a proton transfer reaction mass spectrometer. The present dynamic range for ambient air reactivity is about 6 to 300 s^−1. 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