OH reactivity measurements within a boreal forest: evidence for unknown reactive emissions.

Boreal forests emit large amounts of volatile organic compounds (VOCs) which react with the hydroxyl radical (OH) to influence regional ozone levels and form secondary organic aerosol. Using OH reactivity measurements within a boreal forest in Finland, we investigated the budget of reactive VOCs. OH reactivity was measured using the comparative reactivity method, whereas 30 individual VOCs were measured using proton transfer reaction mass spectrometry, thermal-desorption gas chromatography mass spectrometry, and liquid chromatography mass spectrometry, in August 2008. The measured OH reactivity ranged from below detection limit (3.5 s−1), to ∼60 s−1 in a single pollution event. The average OH reactivity was ∼9 s−1 and no diel variation was observed in the profiles. The measured OH sinks (∼30 species) accounted for only 50% of the total measured OH reactivity, implying unknown reactive VOCs within the forest. The five highest measured OH sinks were: monoterpenes (1 s−1), CO (0.7 s−1), isoprene (0.5 s−1), p...