Helmig D., Ortega J., Guenther A., Herrick J.D., and Geron C. (2006) Sesquiterpene emissions from Loblolly Pine and their contribution to biogenic aerosol formation in the southeastern US. Atmos. Environ., Vol.40, pp. 4150–4157.
Sesquiterpene (SQT) and montoterpene (MT) emissions from loblolly pine (Pinus taeda L.) were studied by branch enclosure experiments at Duke Forest in Chapel Hill, NC. Four SQT (b-caryophyllene, a-bergamotene a-humulene, b-farnesene) and six MT (a-pinene, b-pinene, b-myrcene, b-phellandrene, limonene, linalool) were identified. Emission rates of both compound classes increased exponentially with temperature, albeit SQT temperature coefficients (0.12 – 0.18 K-1) were higher than for MT (0.068 – 0.15 K-1), resulting in an increased contribution of SQT to the overall biogenic volatile organic compound (BVOC) flux during warm temperature conditions. The highly correlated variables of light and temperature conditions precludes a rigorous characterization of their individual roles in driving these emissions. However, the observations indicate that there may be both temperature-only and temperature/light dependent components contributing to SQT emission variations. When normalized to 30oC using the best-fit temperature algorithm, total SQT basal emission rate was 450 ng g-1h-1. The contribution of SQT from all pine trees (based on the loblolly pine emission factors) to secondary, biogenic organic aerosol in 12 southeastern U.S. states was estimated to be approximately 7 x 106 kg for the month of September which constitutes an appreciable portion of the overall PM 2.5 emission budget.
Tanner D., Helmig* D., Hueber J. and Goldan P. (2006) A gas chromatography system for the automated, unattended, and cryogen-free monitoring of C2 to C6 non-methane hydrocarbons in the remote troposphere. J. Chrom., 1111, 76-88.
An unattended, automated, on-line, cryogen-free, remotely controlled gas chromatography (GC) system was developed and has been deployed for more than one year for the continuous determination of C2 to C6 hydrocarbons at an observatory located at 2225 m elevation, on the summit caldera of an inactive volcano on the island of Pico, Azores. The GC instrument is tailored to the measurement challenges at this remote and high altitude site. All consumable gases are prepared in situ. Total power use remains below 700 Watts at all times. Sample collection and analysis is performed without use of cryogen. Hydrocarbons are concentrated on a one stage trapping/injection system consisting of a peltier-cooled multi-bed solid adsorbent trap. Analytes are detected after thermal desorption and separation on an Alumina-PLOT (porous-layer open tubular) column by a flame ionization detector (FID). Sample focusing, desorption, separation and detection parameters were thoroughly investigated to ensure quantitative collection and subsequent injection onto the GC system. GC operation is controlled remotely and data is downloaded daily. 600 ml and 3000 ml sample volumes are alternated for analysis of C2 to C3 and C3 to C6 hydrocarbons, respectively. Detection limits are in the low pptv (parts per trillion by volume) range, sufficient for quantification of the compounds of interest at their central North Atlantic lower free troposphere background concentrations.
Pollmann J., Ortega J., and Helmig D. (2005) Sampling of Atmospheric Sesquiterpenes: Sampling Losses and Mitigation of Ozone Interference. Environ. Sci. and Technol. 39, 9620-9629.
Atmospheric standards containing parts-per-billion levels of fourteen semi-volatile hydrocarbon compounds, including eight sesquiterpenes (SQT; longipinene, a-copaene, isolongifolene, a-cedrene, trans-caryophyllene, aromadendrene, a-humulene, d-cadinene), two oxidized sesquiterpenoids (cis-nerolidol, trans-nerolidol), one biogenic ketone (geranylacetone) and three aromatic compounds (1,3,5-tri-isopropylbenzene, diphenylmethane, nonylbenzene) were collected onto four solid adsorbent materials at increasing ozone mixing ratios (0 to 100 ppbV O3) for analysis by thermodesorption-gas chromatography. Substantial sampling losses of up to > 90 % were found for the most reactive SQT, even at the lowest ozone level investigated of 20 ppbV. Loss rates from the ozone-SQT reaction were used to derive estimates of gas-phase ozone reaction rate constants for longipinene, a-copaene, isolongifolene, geranylacetone, aromadendrene, d-cadinene, cis-nerolidol and trans-nerolidol. Three different ozone mitigation techniques were investigated to prevent these sampling losses. These strategies included a) placing glass fiber filters impregnated with sodium thiosulfate (Na2S2O3) into the sampling line, b) titration of ozone in the sampling stream with nitric oxide (NO) and c) catalytically removing ozone with a commercially available manganese dioxide (MnO2) catalyst. All three techniques reduced ozone mixing ratios from 100 ppbV to < 0.6 ppbV at sampling flow rates of 1 l min-1. When the Na2S2O3 filters and the NO-titration techniques were applied, SQT loss rates decreased to 0 % to 5 % for most SQT compounds and from > 90 % to ~ 10 - 50 % for the two most reactive compounds at ozone mixing ratios of up to 100 ppbV. The commercial manganese dioxide scrubber, however, caused complete analyte losses (> 98 %) even at 0 ppbV ozone. These results underline the need and present applicable techniques for removal of ozone in air samples for SQT analysis by solid adsorption.
Helmig, D., Revermann, T. and Hall, B. (2004) Characterization of a Pressurized C5-C16 Hydrocarbon Gas Calibration Standard for Air Analysis. Anal. Chem. 76, 6528-6534.
A compressed gas standard containing parts-per-billion (ppb) amounts of the volatile hydrocarbons methylpentadiene (isoprene, 540 ppb) and iso-octane (259 ppb) and a series of less volatile C12-C16 n-alkanes (n-dodecane, 349 ppb; n-tridecane, 340 ppb; n-tetradecane, 202 ppb; n-pentadecane, 271 ppb and n-hexadecane, 308 ppb) was prepared by a one step (no further gas dilution) microgravimetric method. The gravimetric mixing ratios were confirmed by referencing to a capillary diffusion method. The cylinder was heated to 75oC to minimize condensation losses of analytes to the cylinder walls. Mixing ratios were monitored over a 2.5-year period. Some initial analyte losses (~ 3 20%) were observed for the heavier C14-C16 n-alkanes. Subsequently, analyte loss rates were found to be in the range of < 1.0 % per year for n-dodecane to n-hexadecane, respectively. The developed guidelines for preparation, storage and retrieval of these semi-volatile analytes enable the use of compressed gas standards for calibration and method development purposes in the environmental gas-phase analysis of these compounds.
Detlev Helmig*, Florence Bocquet, Jan Pollmann and Tobias Revermann
Sesquiterpene (SQT) compounds (C15H24) and their oxygenated alcohol and
ketone derivatives are biogenic volatile organic compounds (BVOC) that have been
identified in emissions from vegetation. SQT emission rates and landscape
flux estimates are highly uncertain. Reliable ambient flux measurements
have not been possible because of low ambient concentrations, rapid atmospheric
reactions (prohibiting ambient tower flux measurements), and analytical
challenges and uncertainties that stem from the low volatility of SQT.
Standards from an in-situ capillary diffusion system with 18 SQT compounds and
four other organic compounds (geranylacetone, 1,3,5-tri-isopropylbenzene,
diphenylmethane, nonylbenzene) were used to thoroughly investigate experimental
procedures for SQT emission rate studies by vegetation enclosure techniques.
Recovery rates in tubing materials, sampling bags, leaf cuvettes, on six solid
adsorbent materials (Tenax TA, Tenax GR, Carbotrap, Carbotrap C, Unibeads, Glass
Beads) for gas chromatography analysis, and gas chromatography retention indices
and mass spectral fragmentation patterns were determined. SQT compounds
were found to exhibit a high degree of stickiness to all materials tested.
However, the non-oxygenated SQT can be recovered in enclosure experiments for
quantitative emission rate determination after careful consideration of the
analytical conditions.
Critical parameters are the adsorbent choice. Furthermore it is utmost
important to allow sufficient purging and equilibration times for all materials
in contact with the sample air. Oxygenated SQT were irreversibly lost in
enclosure experiments which made their quantitative measurement prohibitive.
Results for the other organic compounds were similar and indicate that these
data mostly stem from the volatility of these compounds. Consequently, the
findings of this study provide guidelines for the analysis of a wide range of
volatile organic compounds in the ~ C13 – C17volatility range.
Helmig D., Revermann T., Pollmann J., Kaltschmidt O., Jiménez Hernández A., Bocquet F. and David D.
Sesquiterpenes (C15H24, SQT) are semi-volatile organic compounds emitted from vegetation and are of interest for air quality considerations because of their suspected contribution to the formation of secondary aerosol. This article investigates the application of a capillary diffusion method for the generation of standard atmospheres of 16 SQT and four other related semi-volatile compounds. This instrument subsequently has been used in the testing of analytical materials, protocols and calibration of air sampling methods. SQT DB-1 retention indices, vapor pressures at 25oC and 75oC, and diffusion coefficients were determined. A quantitative, on-line GC method yielded improved results (median relative standard deviation of 5.0% and 6.1%) for the diffusion rate determination in comparison to a gravimetric approach (median relative standard deviation 18%). The GC method also allowed identifying errors in the gravimetric method stemming from residual solvent evaporation, impurities, and chemical analyte losses. Stainless steel, glass, nickel and Teflon tubing that were tested for transfer lines and a sampling loop had to be kept at temperatures in excess of ~ 110oC in order to prevent significant analytical errors from the stickiness of SQT to these materials. Besides for SQT analysis, results from this research provide general guidelines for gas-phase analysis of related compounds in the C14 – C16 volatility range.
Karbiwnyk C.M., Mills C.S., Helmig* D. and Birks J.W.
Solid adsorbents have proven useful for determining the vertical profiles of volatile organic compounds (VOCs) using sampling platforms such as balloons, kites and light aircraft, and those profiles provide valuable information about the sources, sinks, transformations and transport of atmospheric VOCs. One of the largest contributions to error in VOC concentrations is the estimation of the volume of air sampled on the adsorbent cartridge. These errors arise from different sources, such as variations in pumping flow rates from changes in ambient temperature and pressure with altitude, and decrease in the sampling pump battery power. Another significant source for sampling rate variations are differences in the flow resistance of individual sampling cartridges. In order to improve the accuracy and precision of VOC measurements, the use of ambient chlorofluorocarbons (CFCs) as internal standards was investigated. A multi-bed solid adsorbent, AirToxic™ (Supelco), was chosen for its wide sampling range (C3-C12). Analysis was accomplished by thermal desorption and dual detection GC/FID/ECD, resulting in sensitive and selective detection of both VOCs and CFCs in the same sample. Long-lived chlorinated compounds (CFC-11, CFC-12, CFC-113, CCl4, and CH3CCl3) banned by the Montreal Protocol and subsequent amendments were studied for their ability to predict sample volumes using both ground-based and vertical profiling platforms through the boundary layer and free troposphere. Of these compounds, CFC-113 and CCl4 were found to yield the greatest accuracy and precision for sampling volume determination. Use of ambient CFC-113 and CCl4 as internal standards resulted in accuracy and precision of generally better than 10 % for the prediction of sample volumes in ground-, balloon- and aircraft-based measurements. Consequently, use of CFCs as reference compounds can yield a significant improvement of accuracy and precision for ambient VOC measurements in situations where accurate flow control is troublesome.
Water vapor can be a significant interference in the analysis of air for non-methane volatile organic compounds (NMVOCs) using solid adsorbent sampling techniques. The adsorbent materials used in sampling cartridges have different hydrophobic characteristics, and it is therefore necessary to characterize solid adsorbent cartridges over a wide range of humidity. Controlled humidity experiments were performed to assess the extent of water vapor interference when samples are collected onto AirToxicsTM solid adsorbent cartridges. It was found that elevating the temperature of the cartridge to 10?C above the temperature of the air sample greatly reduced water vapor adsorption and interferences and resulted in ? 90% recovery of NMVOCs, biogenic VOCs and chlorofluorocarbons (CFCs). Similar collection efficiencies were obtained at ambient temperature by reducing the relative humidity to ? 60% in the sample by dilution with dry, scrubbed ambient air. A procedure also was developed and optimized for dry purging cartridges prior to analysis. However, under optimized conditions, significant losses of C3-C5 compounds still occurred under highly humid conditions. It was determined that these losses were due to reduced retention during sampling rather than loss during the dry purge procedure. The dry purge method was shown to be adequate at high humidities for sampling NMVOCs with retention indices greater than 500.
The temporal and spatial distribution of boundary-layer ozone was studied during June 2000 at Summit, Greenland by surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data and a total of 133 meteorological and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground.
The lower atmosphere at Summit was characterized by the prevalence of high stability conditions with strong surface temperature inversions. These inversions succumbed to neutral to slightly unstable conditions between ~ 9.00 and 18.00 hrs local time with the formation of shallow mixing heights of typically ~70–250 m above the surface.
The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 hours. The diurnal mean ozone mixing ratio showed distinct diurnal cycles indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37 to 76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A ~ 1 to 4 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. Taken together these suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime.
This review of gas chromatographic air analysis considers approximately 170 research articles published in the literature from 1995 to 1998. The focus is on sample collection, injection, separation and detection techniques, with special emphasis on capillary gas chromatography details. These parameters are summarized in two tables, the first one ordered by the chemical groups of compounds analyzed and the second by stationary phase. The reported techniques are analyzed and evident trends are summarized for the parameters and key words: Capillary Columns, Carrier Gas, Detection, Film Thickness, Injection, Liquid Stationary Phase, Multidimensional GC, Oven Programming, Packed Columns, PLOT Columns, and Sampling Techniques. Furthermore, a brief summary of emerging alternative approaches replacing traditional GC methods is given.
Vegetation composition and biomass were surveyed for three specific sites in Atlanta, GA; near Rhinelander, WI; and near Hayden, CO. At each research site emissions of biogenic volatile organic compounds (BVOCs) from the dominant vegetation species were sampled by enclosing branches in bag enclosure systems and sampling the equilibrium head space onto multi-stage solid adsorbent cartridges. Analysis was performed using a thermal desorption technique with gas chromatography (GC) separation and mass spectrometry (MS) detection. Identification of BVOCs covering the GC retention index range (stationary phase DB-1) from approximately 400 to 1400 was achieved (volatilities C4 - C14).
Overall, 63 vegetation species were sampled, and a total of 114 BVOCs were detected and characterized. Structural chemical identification was achieved on approximately 60 % of all compounds, tentative identification on 26 %, and 14 % remained unidentified. Identified compounds include isoprene and BVOCs of the classes of monoterpenes, sesquiterpenes, carbonyl compounds, alcohols, and esters. The MS data was further used to derive emission rate estimates of the identified BVOCs. Even though these data have substantial margins of error, it allows to group BVOCs into the major and minor emissions and derive conclusion on the relative contribution of individual compounds to the overall BVOC flux. Results obtained by this method show that besides terpenoid compounds (isoprene, monoterpenes and sesquiterpenes), oxygenated compounds may contribute a rather significant fraction of the total BVOC flux. Compounds of particular importance are cis-3-hexene-ol and its derivatives. Limitations of the branch enclosure technique and the analytical approach with their sources of error are critically discussed and evaluated.
Landscape flux potentials for biogenic volatile organic compounds (BVOCs) were derived for three ecosystems in the continental U.S. (Fernbank Forest, Atlanta, GA; Willow Creek, Rhinelander, WI; Temple Ridge, CO). Analytical data from branch enclosure measurements were combined with ecological survey data for plant species composition and biomass. Other quantitative flux measurements at the leaf and landscape level were incorporated to scale the results from the enclosure measurements to the landscape level. Flux estimates were derived by using a one week ambient temperature and light record (30 min time resolution) and adjusting all emission rates to these conditions with temperature and light correction algorithms.
Although uncertainties due to the branch enclosure technique limit the conclusions, tentative data which is valuable to define future research attention and needs is derived. Scaled to the landscape level, this technique allowed identification of those plant species that are responsible for the major fraction of the total BVOC landscape flux and to identify the major compounds emitted. It was found that for each of the sites investigated, a very few selected plant species contribute to the major fraction of the total emissions. Northern red oak, post oak, white oak and American beech accounted for 71% of the total BVOC emissions at the Atlanta site. Quaking aspen and Northern red oak were the dominating species at the Wisconsin site with 68% of the total emissions. At the Colorado site, Gambel oak and service berry made up 86% of the BVOC emissions. Total daily average BVOC landscape fluxes determined by this method were 2.0, 4.5 and 2.4 mgC m-2h-1 for the Atlanta, Wisconsin and Colorado sites, respectively. The contribution of isoprene to this overall BVOC flux was calculated to be 45, 38 and 54% at the three sites, respectively.
The branch enclosure technique combined with ecological surveying and landscape-scale isoprene flux measurements proved to be a valuable tool for screening a high number of plant species and for identifying the major and most important emitters for a more thorough investigation. The obtained data, the suitability of this approach and its limitation to derive BVOC fluxes on the ecosystem level are critically evaluated and factors introducing experimental errors are identified.
Biogenic volatile organic compounds (BVOCs) and their role in atmospheric oxidant formation were investigated at a forest site near Oak Ridge, Tennessee, as part of the Nashville Southern Oxidants Study (SOS) in July 1995. Of 98 VOCs detected, a major fraction were anthropogenic VOCs such as chlorofluorocarbons (CFCs), alkanes, alkenes and aromatic compounds. Isoprene was the dominant BVOC during daytime. Primary products from BVOC oxidation were methylvinylketone, methacrolein and 3-methylfuran. Other compounds studied include the BVOCs a-pinene, camphene, b-pinene, p-cymene, limonene and cis-3-hexenyl acetate and a series of light alkanes, aromatic hydrocarbons and seven of the CFCs. The correlation of meteorological parameters, with the mixing ratios of these different compounds, reveals information on atmospheric oxidation processes and transport. Long-lived VOCs show very steady mixing ratio time series. Regionally and anthropogenically emitted VOCs display distinct diurnal cycles with a strong mixing ratio decrease in the morning from the breakup of the nocturnal boundary layer. Nighttime mixing ratio increases of CFCs and anthropogenic VOCs are suspected to derive from emissions within the Knoxville urban area into the shallow nocturnal boundary layer. In contrast, the time series of BVOCs and their oxidation products are determined by a combination of emission control, atmospheric oxidation and deposition, and boundary layer dynamics. Mixing ratio time series data for monoterpenes and cis-3-hexenyl acetate suggest a temporarily emission rate increase during and after heavy rain events. The isoprene oxidation products demonstrate differences in the oxidation pathways during night and day and in their dry and wet deposition rates.
Vertical profiles of volatile organic compounds (VOCs) within the convective boundary layer (CBL) were measured at a tropical forest site in the Peruvian Amazon during July 1996 from a tethered balloon sampling platform. A profiling technique based on the collection of VOCs onto solid adsorbent cartridges was used to take samples at altitudes up to 1600 m above ground. VOC analysis was performed by thermal desorption with gas chromatographic separation and mass spectrometric and flame ionization detection. A total of 26 VOCs were structurally identified. VOCs were dominated by biogenic compounds. Highest concentrations were observed for isoprene, followed by a-pinene, p-cymene, and b-pinene. Combined, all monoterpenes accounted for approximately 15-20% of the total carbon from biogenic VOCs (BVOCs). The isoprene oxidation products methacrolein (MAC), methylvinylketone (MVK), and 3-methylfuran were observed throughout the CBL. Besides the ubiquitous chlorofluorocarbons, anthropogenic VOC concentrations were at the lower end of concentration ranges observed in rural air. From the vertical profiles, BVOC surface flux estimates were derived. Emission rates were estimated from five vertical profiles using the mixed-layer gradient and CBL budget methods. Emission estimates varied depending on method and choice of statistics, but were within 3000-8200 microgram compound m-2 h-1 for isoprene, 120-370 microgram m-2 h-1 for a-pinene, 40-75 microgram m-2 h-1 for b-pinene, about 16 microgram m-2 h-1 for p-cymene, and 40-50 microgram m-2 h-1 for camphene. The changes in the ratios of MAC and MVK to isoprene with altitude were utilized to estimate the mixing times between the surface layer, mixed layer and lower troposphere.