On-line field measurements of monoterpene emissions from Scots pine by proton-transfer-reaction mass spectrometry

We measured the daily patterns of monoterpene emissions from Scots pine (Pinus sylvestris) trees in a boreal coniferous forest in August and September 2004, using an on-line chamber method combined with a proton-transfer-reaction mass spectrometry (PTR-MS) analyzer. The on-line measurements were made in two chambers with a one-year old shoot inside. Simultaneous measurements were performed for carbon dioxide (CO2) exchange, transpiration (H2O), exchange of trace gases (NOx, O3), photosynthetic photon flux density (PPFD), air temperature and relative humidity (RH). The composition of the monoterpene emission did not change during the measurement period, with [delta]3-carene and [alpha]-pinene being the dominant species. The total mono-terpene emission rate (per needle area) was on average 0.5 ng m–2 s–1, varied from non detectable to 2.1 ng m–2 s–1, and showed a typical diurnal pattern with afternoon maximum and nighttime minimum. The emission rates determined with this on-line chamber method were in agreement with results from a simultaneously used established adsorbent sampling technique with offline GC-MS analysis. The monoterpene emissions from the chamber walls were correlated with the chamber temperature and this measurement artifact was dominating at night. Emission rates normalized to 30 °C, using temperature regression coefficient of 0.09 °C–1, ranged from 2.1 µg g(dw)–1 h–1 to 4.4 µg g(dw)–1 h–1. Measurements of emission dynamics of biogenic volatile organic compounds (BVOCs) together with plant physiological activity are urgently needed for the development of mechanistic BVOC emission models in order to assess their regional and global influence.