We present a novel analytical approach to measure the chemical composition of organic aerosol. The new instrument combines proton-transfer-reaction mass-spectrometry (PTR-MS) with a collection-thermal-desorption aerosol sampling technique. For secondary organic aerosol produced from the reaction of ozone with isoprenoids in a laboratory reactor, the TD-PTR-MS instrument detected typically 80% of the mass that was measured with a scanning mobility particle sizer (SMPS). The first field deployment of the instrument was the EUCAARI-IOP campaign at the CESAR tall tower site in the Netherlands. For masses with low background values (∼30% of all masses) the detection limit of aerosol compounds was below 0.2 ng/m3 which corresponds to a sampled compound mass of 35 pg. Comparison of thermograms from ambient samples and from chamber-derived secondary organic aerosol shows that, in general, organic compounds from ambient aerosol samples desorb at much higher temperatures than chamber samples. This suggests that chamber aerosol is not a good surrogate for ambient aerosol and therefore caution is advised when extrapolating results from chamber experiments to ambient conditions

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), propanal and acetone (0.3 s−1), and methane (0.3 s−1). We suggest that models be constrained by direct OH reactivity measurements to accurately assess the impact of boreal forest emissions on regional atmospheric chemistry and climate.

An analysis of acetone (CH3COCH3) and acetonitrile (CH3CN) measurements, performed during the Indian Ocean Experiment (INDOEX), using a chemistry general circulation model is presented. A comparison with measurements indicates that the model simulates realistic CO and acetone distributions, except toward the Indian west coast near the surface. The latter may be related to a sea breeze circulation at the Indian west coast, which is not resolved by the model. A comparison of the measured and modeled correlation between CO and acetone indicates the presence of a background marine acetone source. A model sensitivity study suggests a global marine emission strength of 15-20 Tg acetone yr-1, which is a significant contribution to the estimated global acetone source of 56 (37-80) Tg acetone yr-1. The comparison of measured and modeled CO-acetonitrile correlation from surface measurements indicates that a model sink of acetonitrile in the marine boundary layer is missing. A model sensitivity study suggests that this could be dry deposition (deposition velocity estimate: 0.01-0.05 cms-1) on the ocean surface. A comparison of measured and modeled tropospheric acetonitrile indicates that the model overestimates the free tropospheric acetonitrile mixing ratios up to a factor of 3, which points to a missing free tropospheric sink of acetonitrile in the model. A possible explanation may be stratospheric loss and subsequent stratosphere-troposphere exchange, which was not included in the model.