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.

Biogenic VOC emissions are often dominated by 2-methyl-1,3-butadiene (isoprene) and 2-methyl-3-buten-2-ol (232 MBO). Here we explore the possibility to selectively distinguish these species using NO+ as a primary ion in a conventional PTR-MS equipped with an SRI unit. High purity of NO+ (>90%) as a primary ion was utilized in laboratory and field experiments using a conventional PTR-TOF-MS. Isoprene is ionized via charge transfer leading to the major product ion C5H8+ (>99%) (e.g. Spanel and Smith, 1998). 232 MBO undergoes a hydroxide ion transfer reaction resulting in the major product ion channel C5H9+ (>95%) (e.g. Amelynck et al., 2005). We show that both compounds are ionized with little fragmentation (>5%) under standard operating conditions. Typical sensitivities of 11.1 ± 0.1 (isoprene) and 12.9 ± 0.1 (232 MBO) ncps ppbv−1 were achieved, which correspond to limit of detections of 18 and 15 pptv respectively for a 10 s integration time. Sensitivities decreased at higher collisional energies. Calibration experiments showed little humidity dependence. We tested the setup at a field site in Colorado dominated by ponderosa pine, a 232 MBO emitting plant species. Our measurements confirm 232 MBO as the dominant biogenic VOC at this site, exhibiting typical average daytime concentrations between 0.2–1.4 ppbv. The method is able to detect the presence of trace levels of isoprene at this field site (90–250 ppt) without any interference from 232 MBO, which would not be feasible using H3O+ ionization chemistry, and which currently also remains a challenge for other analytical techniques (e.g. gas chromatographic methods).

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.

Recently, multiple research have highlighted important roles of BVOCs to understand regional air qualities of megacities in East Asian. The findings strongly urge multi-faceted research on emission and oxidation processes of BVOCs and potential impacts on regional air quality from the interactions between biosphere and atmosphere. To address these emerging research topics in a longer term, National Institute of Environmental Research of South Korea established a research site in the Taehwa Research Forest, located at the edge of the Seoul Metropolitan Area (population   25 million). A 40 meter high tower was built in a pine tree plantation (Pinus Koraiensis), surrounded by a natural broad leaf ecosystem. Three inlets were configured to characterize trace gas gradients, above canopy(40 m), top of the canopy (23 m) and inside of the canopy (5 m) and multi-level meteorological sensors include PAR sensors were set up. In the laboratory space, high-sensitivity Ionicon PTR-MS, and CO, NOx, SO2, and ozone analyzers are operational for continuous gradient measurements. We will present preliminary gradient measurements results of both antropogenic and biogenic VOCs to quantify emission and deposition potential of the compounds.In addition, ozone and their precursors such as CO, NOx measurement results at the site will be presented to discuss biosphere-atmosphere interactions and their impacts towards regional air quality.

The biosphere is the major source and sink of nonmethane volatile organic compounds (VOCs) in the atmosphere. Gas-phase chemical reactions initiate the removal of these compounds from the atmosphere, which ultimately proceeds via deposition at the surface or direct oxidation to carbon monoxide or carbon dioxide. We performed ecosystem-scale flux measurements that show that the removal of oxygenated VOC via dry deposition is substantially larger than is currently assumed for deciduous ecosystems. Laboratory experiments indicate efficient enzymatic conversion and potential up-regulation of various stress-related genes, leading to enhanced uptake rates as a response to ozone and methyl vinyl ketone exposure or mechanical wounding. A revised scheme for the uptake of oxygenated VOCs, incorporated into a global chemistry-transport model, predicts appreciable regional changes in annual dry deposition fluxes.

Two proton-transfer-reaction mass spectrometry systems were deployed at the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen-Southern Rocky Mountain 2008 field campaign (BEACHON-SRM08; July to September, 2008) at the Manitou Forest Observatory in a ponderosa pine woodland near Woodland Park, Colorado USA. The two PTR-MS systems simultaneously measured BVOC emissions and ambient distributions of their oxidation products. Here, we present mass spectral analysis in a wide range of masses (m/z 40+ to 210+) to assess our understanding of BVOC emissions and their photochemical processing inside of the forest canopy. The biogenic terpenoids, 2-methyl-3-butene-2-ol (MBO, 50.2%) and several monoterpenes (MT, 33.5%) were identified as the dominant BVOC emissions from a transmission corrected mass spectrum (PTR-MS), averaged over the daytime (11 a.m. to 3 p.m., local time) of three days. To assess contributions of oxidation products of local BVOC, we calculate an oxidation product spectrum with the OH- and ozone-initiated oxidation product distribution mass spectra of two major BVOC emissions at the ecosystem (MBO and β-pinene) that were observed from laboratory oxidation experiments. The majority ( 76%) of the total signal in the transmission corrected PTR-MS spectra could be explained by identified compounds. The remainder are attributed to oxidation products of BVOC emitted from nearby ecosystems and transported to the site, and oxidation products of unidentified BVOC emitted from the ponderosa pine ecosystem.

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.

Toluene and benzene are used for assessing the ability to measure disjunct eddy covariance (DEC) fluxes of Volatile Organic Compounds (VOC) using Proton Transfer Reaction Mass Spectrometry (PTR-MS) on aircraft. Statistically significant correlation between vertical wind speed and mixing ratios suggests that airborne VOC eddy covariance (EC) flux measurements using PTR-MS are feasible. City-median midday toluene and benzene fluxes are calculated to be on the order of 14.1±4.0 mg/m2/h and 4.7±2.3 mg/m2/h, respectively. For comparison the adjusted CAM2004 emission inventory estimates toluene fluxes of 10 mg/m2/h along the footprint of the flight-track. Wavelet analysis of instantaneous toluene and benzene measurements during city overpasses is tested as a tool to assess surface emission heterogeneity. High toluene to benzene flux ratios above an industrial district (e.g. 10–15 g/g) including the International airport (e.g. 3–5 g/g) and a mean flux (concentration) ratio of 3.2±0.5 g/g (3.9±0.3 g/g) across Mexico City indicate that evaporative fuel and industrial emissions play an important role for the prevalence of aromatic compounds. Based on a tracer model, which was constrained by BTEX (BTEX– Benzene/Toluene/Ethylbenzene/m, p, o-Xylenes) compound concentration ratios, the fuel marker methyl-tertiary-butyl-ether (MTBE) and the biomass burning marker acetonitrile (CH3CN), we show that a combination of industrial, evaporative fuel, and exhaust emissions account for >87% of all BTEX sources. Our observations suggest that biomass burning emissions play a minor role for the abundance of BTEX compounds in the MCMA (2–13%).

The ability to measure sesquiterpenes (SQT; C15H24) by a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was investigated. SQT calibration standards were prepared by a capillary diffusion method and the PTR-MS-estimated mixing ratios were derived from the counts of product ions and proton transfer reaction constants. These values were compared with mixing ratios determined by a calibrated Gas Chromatograph (GC) coupled to a Flame Ionization Detector (GC-FID). Product ion distributions from soft-ionization occurring in a selected ion drift tube via proton transfer were measured as a function of collision energies. Results after the consideration of the mass discrimination of the PTR-MS system suggest that quantitative SQT measurements within 20% accuracy can be achieved with PTR-MS if two major product ions (m/z 149+ and 205+), out of seven major product ions (m/z 81+, 95+, 109+, 123+, 135+, 149+ and 205+), are accounted for. Considerable fragmentation of bicyclic sesquiterpenes, i.e. β-caryophyllene and α-humulene, cause the accuracy to be reduced to 50% if only the parent ion (m/z 205+) is considered. These findings were applied to a field dataset collected above a deciduous forest at the PROPHET (Program for Research on Oxidants: Photochemistry, Emissions, and Transport) research station in 2005. Inferred average daytime ecosystem scale mixing ratios (fluxes) of isoprene, sum of monoterpenes (MT), and sum of SQT exhibited values of 15 μg m−3 (4.5 mg m−2 h−1), 1.2 μg m−3 (0.21 mg m−2 h−1), and 0.0016 μg m−3 (0.10 mg m−2 h−1), respectively. A range of MT and SQT reactivities with respect to the OH radical was calculated and compared to an earlier study inferring significantly underestimated OH reactivities due to unknown terpenes above this deciduous forest. The results indicate that incorporating these MT and SQT results can resolve  30% of missing OH reactivity reported for this site.

Significant ecosystem-scale emissions of methylsalicylate (MeSA), a semivolatile plant hormone thought to act as the mobile signal for systemic acquired resistance (SAR) (Park et al., 2006), were observed in an agroforest. Our measurements show that plant internal defence mechanisms can be activated in response to temperature stress and are modulated by water availability on large scales. Highest MeSA fluxes (up to 0.25 mg/m2/h) were observed after plants experienced ambient night-time temperatures of  7.5°C followed by a large daytime temperature increase (e.g. up to 22°C). Under these conditions estimated night-time leaf temperatures were as low as  4.6°C, likely inducing a response to prevent chilling injury (Ding et al., 2002). Our observations imply that plant hormones can be a significant component of ecosystem scale volatile organic compound (VOC) fluxes (e.g. as high as the total monoterpene (MT) flux) and therefore contribute to the missing VOC budget (de Carlo et al., 2004; Goldstein and Galbally, 2007). If generalized to other ecosystems and different types of stresses these findings suggest that semivolatile plant hormones have been overlooked by investigations of the impact of biogenic VOCs on aerosol formation events in forested regions (Kulmala et al., 2001; Boy et al., 2000). Our observations show that the presence of MeSA in canopy air serves as an early chemical warning signal indicating ecosystem-scale stresses before visible damage becomes apparent. As a chemical metric, ecosystem emission measurements of MeSA in ambient air could therefore support field studies investigating factors that adversely affect plant growth.

We deployed two proton-transfer-reaction mass spectrometry instruments (PTR-MS, IONICON ANALYTIK) for ambient and branch enclosure measurements at the Manitou Experimental Forest, located in the Southern Rocky Mountain area as a part of the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen (BEACHON) field campaign in 2008. Vegetation at the field site is dominated by Ponderosa Pine. BVOC emissions from Ponderosa Pine along with temperature, photosynthetic photon flux density (ppfd), relative humidity, and CO2 uptake were measured from two branch-enclosures (shade and sun). Diurnal variations and the emission response to environmental conditions are described and compared to existing models. In addition, we analyzed the speciation of BVOCs from enclosures by GC-MS. We will present quantitative and qualitative characteristics of BVOC emissions from Ponderosa Pine and analytical characteristics of PTR-MS such as fragmentation patterns of semi-volatile compounds (sesquiterpene, bornyl acetate etc) that we identified as major emissions from the enclosures. BVOC emissions observed in the enclosures will be quantitatively compared to BVOC distributions in ambient air. We explore the presence of possibly unidentified BVOCs in the forest canopy by examining PTR-MS mass spectra of enclosure and ambient air samples based on mass scans between 40 - 210 amu.

Grass crop species, rice and sorghum, that are widely grown in the southeastern Texas region were analyzed for release of biogenic volatile organic compounds (VOCs) in simulated leaf-drying/senescence experiments. VOC release was measured by both online proton transfer reaction mass spectrometry (PTR-MS) and proton transfer ion trap mass spectrometry (PIT-MS) methods, and it was demonstrated that these two grass crops release a large variety of oxygenated VOCs upon drying under laboratory conditions primarily from leaves and not from stems. VOC release from paddy rice varieties was much greater than from sorghum, and major VOCs identified by gas chromatography PTR-MS included methanol, acetaldehyde, acetone, n-pentanal, methyl propanal, hexenol, hexanal, cis-3-hexenal, and trans-2-hexenal. The latter four VOCs, all C6 compounds known to be formed in wounded leaves, were the major volatiles released from drying rice leaves; smaller but substantial amounts of acetaldehyde were observed in all drying experiments. Online detection of VOCs using PIT-MS gave results comparable to those obtained with PTR-MS, and use of PIT-MS with collision-induced dissociation of trapped ions allowed unambiguous determination of the ratios of cis- and trans-hexenals during different phases of drying. As rice is one of the largest harvested crops on a global scale, it is conceivable that during rice senescence releases of biogenic VOCs, especially the reactive C6 wound VOCs, may contribute to an imbalance in regional atmospheric oxidant formation during peak summer/fall ozone formation periods. A county-by-county estimate of the integrated emissions of reactive biogenic VOCs from sorghum and rice production in Texas suggests that these releases are orders of magnitude lower than anthropogenic VOCs in urban areas but also that VOC emissions from rice in southeastern coastal Texas may need to be included in regional air quality assessments during periods of extensive harvesting.

Fluxes of biogenic volatile organic compounds (VOCs) were measured at a hardwood forest in northern Michigan (UMBS, Prophet research site) over the course of the growing and senescing season. Methanol, acetaldehyde, acetone and isoprene were found to be the most abundant biogenic VOCs with maximum fluxes (mixing ratios in ppbv) of 2.0 mg m−2 h−1 (21.0), 1.0 mg m−2 h−1 (2.7), 1.6 mg m−2 h−1 (5.6) and 7.6 mg m−2 h−1 (6), respectively. The emission patterns show distinct seasonal changes and indicate a spring peak for methanol due to rapid leaf expansion and a fall peak for acetone and acetaldehyde most likely attributed to senescing and decaying biomass; isoprene emissions peaked as expected in the summer. We estimate potential source strengths of 8.9 Tg (C) y−1 methanol, 2.7 Tg (C) y−1 acetaldehyde and 7.0 Tg (C) y−1 acetone for deciduous temperate forests, which is a substantial contribution to the global atmospheric VOC budget.

The development of proton-transfer-reaction mass spectrometry (PTR-MS) as a tool for the analysis of volatile organic compounds (VOCs) is described. PTR-MS is based on the rapid, non-dissociative transfer of protons from H3Oþ to most common VOCs, but not to the principal gases in the air sample. Recent developments in the design of PTR-MS instruments allow detection of some VOCs in the parts per trillion by volume range. This sensitivity and the capability of PTR-MS instruments to be operated for extended periods in both laboratory and field settings has allowed exploration of many aspects of VOC analysis in environmental, food and medical applications.