<p>Herein, we report on the development of a compact proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) for airborne measurements of volatile organic compounds (VOCs). The new instrument resolves isobaric ions with a mass resolving power (m/Δm) of &nbsp;1000, provides accurate m/z measurements (Δm &lt; 3 mDa), records full mass spectra at 1 Hz and thus overcomes some of the major analytical deficiencies of quadrupole-MS-based airborne instruments. 1 Hz detection limits for biogenic VOCs (isoprene, α total monoterpenes), aromatic VOCs (benzene, toluene, xylenes) and ketones (acetone, methyl ethyl ketone) range from 0.05 to 0.12 ppbV, making the instrument well-suited for fast measurements of abundant VOCs in the continental boundary layer. The instrument detects and quantifies VOCs in locally confined plumes (&lt; 1 km), which improves our capability of characterizing emission sources and atmospheric processing within plumes. A deployment during the NASA 2013 DISCOVER-AQ mission generated high vertical- and horizontal-resolution in situ data of VOCs and ammonia for the validation of satellite retrievals and chemistry transport models.</p>

A series of 1,3,5-trimethylbenzene (TMB) photo-oxidation experiments was performed in the 27-m3 Paul Scherrer Institute environmental chamber under various NOx conditions. A University of Innsbruck prototype high resolution Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-TOF) was used for measurements of gas and particulate phase organics. The gas phase mass spectrum displayed  200 ion signals during the TMB photo-oxidation experiments. Molecular formulas CmHnNoOp were determined and ion signals were separated and grouped according to their C, O and N numbers. This allowed to determine the time evolution of the O:C ratio and of the average carbon oxidation state OSC of the reaction mixture. Both quantities were compared with master chemical mechanism (MCMv3.1) simulations. The O:C ratio in the particle phase was about twice the O:C ratio in the gas phase. Average carbon oxidation states of secondary organic aerosol (SOA) samples OSCSOA were in the range of −0.34 to −0.31, in agreement with expected average carbon oxidation states of fresh SOA (OSC = −0.5–0).

Biogenic volatile organic compounds (BVOCs) are important precursors for secondary organic aerosol (SOA) formation (Hallquist et al., 2009). In addition reactive BVOCs play a crucial role in local tropospheric ozone production (Atkinson, 2000). According to the present scientific understanding vegetation is recognized as a major VOC emission source rather than a deposition sink. Our recent observations however demonstrate that an uptake of terpene compounds to mountain grassland can be significant - at least under certain atmospheric conditions. After a severe hailstorm volume mixing ratios (VMR) of locally emitted terpene compounds originating from conifers located at the mountain slopes were strongly enhanced, even during daytime hours. Weeks after the hailstorm our PTR-MS and PTR-time-of-flight (PTR-TOF) instruments still measured deposition fluxes of monoterpenes (m/z 137.133), sesquiterpenes (m/z 205.195), and oxygenated terpenes (m/z 153.128) to the grassland. The total amount of terpenoids (on a carbon basis) deposited to the grassland during the weeks after the hailstorm is comparable to the total methanol emission of the entire growing season (Bamberger et al., 2011). These findings pose the question whether the terminology should be adjusted from VOC emission to VOC exchange.

Eddy covariance flux measurements were carried out for two subsequent vegetation periods above a temperate mountain grassland in an alpine valley using a proton-transfer-reaction-mass spectrometer (PTR-MS) and a PTR time-of-flight-mass spectrometer (PTR-TOF). In 2008 and during the first half of the vegetation period 2009 the volume mixing ratios (VMRs) for the sum of monoterpenes (MTs) were typically well below 1 ppbv and neither MT emission nor deposition was observed. After a hailstorm in July 2009 an order of magnitude higher amount of terpenes was transported to the site from nearby coniferous forests causing elevated VMRs. As a consequence, deposition fluxes of terpenes to the grassland, which continued over a time period of several weeks without significant reemission, were observed. For days without precipitation the deposition occurred at velocities close to the aerodynamic limit. In addition to monoterpene uptake, deposition fluxes of the sum of sesquiterpenes (SQTs) and the sum of oxygenated terpenes (OTs) were detected. Considering an entire growing season for the grassland (i.e., 1 April to 1 November 2009), the cumulative carbon deposition of monoterpenes reached 276 mg C m−2. This is comparable to the net carbon emission of methanol (329 mg C m−2), which is the dominant nonmethane volatile organic compound (VOC) emitted from this site, during the same time period. It is suggested that deposition of monoterpenes to terrestrial ecosystems could play a more significant role in the reactive carbon budget than previously assumed.

Eddy covariance (EC) is the preferable technique for flux measurements since it is the only direct flux determination method. It requires a continuum of high time resolution measurements (e.g. 5–20 Hz). For volatile organic compounds (VOC) soft ionization via proton transfer reaction has proven to be a quantitative method for real time mass spectrometry; here we use a proton transfer reaction time of flight mass spectrometer (PTR-TOF) for 10 Hz EC measurements of full mass spectra up to m/z 315. The mass resolution of the PTR-TOF enabled the identification of chemical formulas and separation of oxygenated and hydrocarbon species exhibiting the same nominal mass. We determined 481 ion mass peaks from ambient air concentration above a managed, temperate mountain grassland in Neustift, Stubai Valley, Austria. During harvesting we found significant fluxes of 18 compounds distributed over 43 ions, including protonated parent compounds, as well as their isotopes and fragments and VOC-H+ – water clusters. The dominant BVOC fluxes were methanol, acetaldehyde, ethanol, hexenal and other C6 leaf wound compounds, acetone, acetic acid, monoterpenes and sequiterpenes.

We have developed a High-Temperature Proton-Transfer-Reaction Mass Spectrometer (HT-PTR-MS) in which both the ion source and the ion drift tube can be continuously operated at temperatures up to 250 °C. The instrument was characterized in a high E/N-mode (130 Td) and in a low E/N-mode (87 Td) at an operating temperature of 200 °C. Instrumental sensitivities and 2σ-detection limits were on the order of 50–110 cps/ppb and 100 ppt (1 s signal integration time), respectively. The HT-PTR-MS is primarily intended for measuring "sticky" or semi-volatile trace gases. Alternatively, it may be coupled to a particle collection/thermal desorption apparatus to measure particle-bound organics in near real-time. In view of these applications, we have measured instrumental response times for a series of reference compounds. 1/e2-response times for dimethyl sulfoxide, ammonia and monoethanolamine were in the sub-second to second regime. 1/e2-response times for levoglucosan, oxalic acid and cis-pinonic acid ranged from 8 to 370 s.

Eddy covariance (EC) is the preferable technique for flux measurements since it is the only direct flux determination method. It requires a continuum of high time resolution measurements (e.g. 5-20 Hz). For volatile organic compounds (VOC) soft ionization via proton transfer reaction has proven to be a quantitative method for real time mass spectrometry; here we use a proton transfer reaction time of flight mass spectrometer (PTR-TOF) for 10 Hz EC measurements of full mass spectra up to m/z 315. The mass resolution of the PTR-TOF enabled the identification of chemical formulas and separation of oxygenated and hydrocarbon species exhibiting the same nominal mass. We determined 481 ion mass peaks from ambient air concentration above a managed, temperate mountain grassland in Neustift, Stubai Valley, Austria. During harvesting we found significant fluxes of 18 compounds distributed over 43 ions, including protonated parent compounds, as well as their isotopes and fragments and VOC-H+ - water clusters. The dominant BVOC fluxes were methanol, acetaldehyde, ethanol, hexenal and other C6 leaf wound compounds, acetone, acetic acid, monoterpenes and sequiterpenes. The smallest reliable fluxes we determined were less than 0.1 nmol m-2 s-1, as in the case of sesquiterpene emissions from freshly cut grass. Terpenoids, including mono- and sesquiterpenes, were also deposited to the grassland before and after the harvesting. During cutting, total VOC emission fluxes up to 200 nmolC m-2 s-1 were measured. Methanol emissions accounted for half of the emissions of oxygenated VOCs and a third of the carbon of all measured VOC emissions during harvesting.

Over the last one and a half decades Proton Transfer Reaction Mass Spectrometry (PTR-MS) [1, 2] has gained recognition as fast on-line sensor for monitoring volatile organic compounds (VOC) in the atmosphere. Sample collection is very straight forward and the fact that no pre-concentration is needed is of particular advantage for compounds that are notoriously difficult to pre-concentrate and/or analyze by gas chromatographic (GC) methods. Its ionization method is very versatile, i.e. all compounds that perform exothermic proton transfer with hydronium ions - and most VOCs do so - are readily ionized, producing quasi-molecular ions VOC.H+. In the quasi-molecular ion the elemental composition of the analyte compound is conserved and allows, in combination with some background knowledge of the sample, conclusions about the identity of that compound. De Gouw and Warneke (2007) [3] summarized the applicability of PTR-MS in atmospheric chemistry but they also pointed out shortcomings in the identification capabilities. Goldstein and Galbally (2007) [4] addressed the multitude of VOCs potentially present in the atmosphere and they emphasized the gasphase-to-aerosol partitioning of organic compounds (volatile and semi-volatile) in dependence of carbon-chain length and oxygen containing functional groups. In collaboration with Ionicon and assisted by TOFWERK we developed a PTR time-of-flight (PTR-TOF) instrument that allows for the identification of the atomic composition of oxygenated hydrocarbons by exact-mass determination. A detection limit in the low pptv range was achieved at a time resolution of one minute, one-second detection limit is in the sub-ppbv range. In 2008 the Innsbruck PTR-TOF was field deployed in the icebreaker- and helicopter based Arctic Summer Cloud Ocean Study (ASCOS) to characterize the organic trace gas composition of the High Arctic atmosphere. During the six-week field campaign the PTR-TOF was run without problems even under harsh conditions in the open water and during ice breaking. Continuous time-series of full mass spectra with a one minute time resolution were recorded throughout the campaign between August 2nd and September 7th 2008 running up to a net VOC data set of 745 hours. Over 370 mass peaks have been separated, about 340 show signal intensities above the 30 minute detection limit of  3pptv. Additionally we analyzed samples from nine helicopter based soundings providing vertical VOC profiles up to 3000 m.a.s.l. The performance of the newly developed instrument will be discussed and ASCOS data will be shown. Acknowledgment: The ASCOS expedition was arranged by the Swedish Polar Research Secretariat (SPRS) and was an effort within the framework of SWEDARCTIC 2008. For more information on ASCOS see http://ascos.se/. We thank the ASCOS organizers - Caroline Leck and Michael Tjernström - all ASCOS participants, the SPRS and the Oden crew for the excellent team work and Armin Wisthaler for his assistance in planning and preparations. The TOF-MS system was funded by the University of Innsbruck (Uni Infrastruktur Programm). The development project was financially supported by the Austrian Research Funding Association (FFG). [1] Hansel, A.; Jordan, A.; Holzinger, R.; Prazeller, P.; Vogel, W.; Lindinger, W. International Journal of Mass Spectrometry and Ion Processes 1995, 149-150, 609-619. [2] Lindinger, W.; Hansel, A.; Jordan, A. Chemical Society Review 1998, 27, 347-375. [3] De Gouw, J. A.; Warneke, C. Mass Spectrometry Reviews 2007, 26, 223-257. [4] Goldstein, A. H.; Galbally, I. E. Environmental Science and Technology 2007, 41, 154-1521.

We performed MS/MS investigations of biogenic volatile organic compounds (BVOC) using a triple quadrupole tandem mass spectrometer (QqQ-MS) equipped with a Townsend Discharge ion source and a Proton Transfer Reaction Linear Ion Trap (PTR-LIT) mass spectrometer. Both instruments use H2O chemical ionization to produce protonated molecular ions. Here we report a study of the application of these instruments to determine methyl vinyl ketone (MVK) and methacrolein (MACR) and a series of monoterpenes (α-pinene, β-pinene, 3-carene, limonene, myrcene, ocimene) and sesquiterpenes (humulene and farnesene). Both instruments achieved sub-ppb detection limits in the single MS mode and in the MS/MS mode for differentiating MVK and MACR. Collision induced dissociation (CID) of protonated monoterpenes and sesquiterpenes was studied under the high-energy, single-to-few collision conditions of the QqQ-MS instrument and under the low-energy, multiple collision conditions of the PTR-LIT. Differences and similarities in the breakdown curves obtained are discussed. In addition, we performed MS4 of protonated limonene to illustrate the analytical power of the PTR-LIT. In spite of the progress we have made, the selective on-line mass-spectrometric detection of individual monoterpenes or sesquiterpenes in complex mixtures currently does not yet seem to be possible.