AbstractSimultaneous in situ observations of biogenic volatile organic compounds (BVOCs) and their oxidation products provide information on the rates and mechanisms of BVOC oxidation and the roles of BVOCs in regional ozone production and the formation and growth of secondary organic aerosol (SOA). In this study, Proton Transfer Reaction Mass Spectrometry (PTR-MS) was used to monitor concentrations of common BVOCs and oxidation products such as isoprene, monoterpenes, methyl vinyl ketone (MVK) and methac-rolein (MACR), and to identify prevalent terpene oxidation products and characterize their abundances at Harvard Forest, a rural site in Western Massachusetts. Laboratory studies of authentic samples of known terpene oxidation products were used to characterize the PTR-MS response to these compounds, including identification of parent ion mass-to-charge ratios and fragmentation patterns. During the summer and fall of 2005, PTR-MS measurements of isoprene, total monoterpenes, MVK + MACR, acetone, methanol, and a number of other VOCs were recorded. Isoprene concentrations exhibited a very strong diurnal cycle, with peak values above 5 ppbv at midday and minima around 100 pptv at night. In contrast, maximum monoter-pene concentrations typically were less than 1 ppbv and occurred in the early evening, with a local minimum at midday. The MVK + MACR diurnal cycle lags that of isoprene, with lower daytime maxima and higher nighttime minima, resulting in an average MVK + MACR / isoprene ratio of ~0.35 during daylight hours and ~1.8 at night. For a total of 26 days between July and September 2005, PTR-MS signals at mass-to-charge ratios corresponding to expected monoterpene oxidation products such as nopinone, pinonaldehyde, alpha-pinene oxide, and others were monitored above the forest canopy. Estimated gas-phase concentrations ranged from near the detection limit (several pptv) to approximately 100 pptv. Observations of the diurnal cycles of these compounds and the relationship between their atmospheric concentrations and factors such as season, temperature, radiation levels, and meteorological conditions are used to provide information on the mechanisms controlling their production and loss in the gas phase.

Figure 2. Schematic of the sam-pling system. Air is drawn from the top of a 30-m tower at a rate of 3.5 l min-1 (residence time of ~8 s in the teflon sample tubing). A filter at the inlet removes particles. Calibra-tion gas is added to the main sample flow periodically to determine in-strument sensitivity. Background signals at the masses of interest are determined by two methods: (1) the addition of humidified zero air to the sample line at the top of the tower to exclude ambient air and (2) removal of the analyte mol-ecules using an activated charcoal scrubber. Background levels ob-tained by the two methods typically are in good agreement, resulting in an uncertainty of 20% or less due to the background correction.

Instrumental Methods Measurements of VOC concentrations were made using Proton Transfer Re-action Mass Spectrometry (PTR-MS) [Lindinger, et al., 1998]. PTR-MS has the advantages of low detection limits (<20 pptv for a 1 s dwell time) and fast time re-sponse (integration times from ~0.1 to several seconds, depending on target mass and measurement mode). Isobaric compounds, such as the monoterpenes (C10H16), cannot be distinguished by PTR-MS. Many monoterpene oxidation products (MOPs) also share the same chemical formulas and molecular masses. In these cases, the total ion signal is attributed to the sum of the individual compounds. In-strument sensitivity was determined by standard addition of known amounts of acetone and isoprene and found to agree well (<10%) with that estimated using published proton-transfer rate constants for these compounds. Based on this agree-ment, monoterpene concentrations were calculated using published rate constants (2.5x10-9 cm3 s-1). Because standard mixtures of the monoterpene oxidation prod-ucts were not available and the proton-transfer rate constants of these compounds are not well known, concentrations were estimated using a proton-transfer rate constant of 2x10-9 cm3 s-1 for all mass-to-charge ratios. We emphasize that the re-sulting mixing ratios should be viewed as approximate and may differ from the values shown by up to a factor of 2. Average countrates and estimated concentra-tions and detection limits are shown in Tables 1 and 3.

Figure 1a. Map of Harvard Forest show-ing the location of the EMS site. The forest is ~70 years old and is composed mainly of oak, maple, pine, hemlock, and spruce. 1b. The 30-m sam-pling tower extends ~5 m above the forest canopy. 1c. Satellite photo of Massachusetts show-ing the location of Harvard Forest.

1a 1b

1c.

Figure 9. Diurnal averages of monoterpene oxidation product concentrations. The patterns fall into three categories.A and B. Nighttime minimum, late afternoon maximum. This pattern is likely due to a build-up of these compounds during the day as they are produced from freshly emitted monoterpenes and a decrease during the night as production slows relative to chemical loss processes, mixing, deposition, and aerosol uptake. C. Nighttime maximum, daytime minimum. The daytime minimum indicates an increase in the rate of loss relative to that of production during daylight hours, due to photolysis, oxidation by OH, dilution, or aerosol uptake. Pinonaldehyde (m/z-H2O = 151), which has a relatively short photolysis lifetime (~3 hrs), follows this pattern. D. Double maxima at midday and in evening. The midday maximum may be the result of production from OH oxidation whereas the evening maximum may result from O3 oxidation of monoterpenes. One of the compounds in this group is nopinone (m/z = 139), which is the product of both OH and O3 oxida-tion of b-pinene. Nopinone has a longer OH lifetime (~10 hrs) than does b-pinene (~5 hrs), allowing it to build up during the day. In the evening, the increase in monoterpene concentration gives rise to an in-crease in production from the O3 channel after OH has decayed.

Conclusions and Future Work These results demonstrate our ability to detect a wide variety of terpenes and their reaction products and form the basis for ongoing studies to understand ter-penoid oxidation at Harvard Forest. Future PTR-MS experiments will include eddy covariance measurements of the fluxes of major biogenic hydrocarbons and OVOCs, modeling of chemistry and mixing within and above the canopy, efforts to better quantify MOP concentrations, expansion of the suite of target molecules. Implementation of a system for simultaneous gas-phase measurement by on-line gas chromatography (GC) to obtain information on the concentrations of individual monoterpenes and some oxidation products is in progress. Method development for the characterization of terpene oxidation products via GC/MS is well underway. Compounds such as pinonaldehyde and pinic acid have been indentified in preliminary aerosol samples. During the coming growing sea-son, denuder/filter and filter/PTR-MS sampling trains will allow for separate analy-sis of gas and aerosol fractions to provide information on the volatility of products and their partitioning between the gas and condensed phases. Denuder sampling followed by GC/MS analysis will yield gas samples with longer integration times (hours to days) but a higher degree of structural specificity than PTR-MS to facili-tate identification of individual compounds in the gas phase. Together, these tech-niques will provide significant new insight into the dominant processes and key species in the atmospheric oxidation of terpenes.

Figure 8. Diurnal averages of monoterpenes and total of oxdiation products. Monoterpene concen-trations increase during the morning as the temper-ature rises due to the dependence of emission rate on T and the proximity of the sampling location to emission sources. During midday, monoterpene concentrations level off or decline slightly due to increased oxidation by OH and faster vertical mix-ing. Concentrations reach a peak in the early eve-ning when continued warm temperatures result in sustained emissions while OH oxidation and verti-cal mixing slow. The sum of the subset of oxida-tion products monitored here is roughly equal to the monoterpene concentration at most times of the day and thus should contribute significantly to the total VOC reactivity under these conditions.

Monitored M/Z

Possible Parent Chemical Formula

Possible Parent Compound

Possible Parent Compound Structure Source

81 C6H8 Monoterpene frag. M137-C4H8 Emission

135 C10H14

α−Pinene oxide fragment

-H2O Oxidation of α−pinene

137 C10H16 Monoterpenes Emission

139 C9H14O NopinoneOxidation of

β-pinene

151 C10H14OPinonaldehyde

fragment-H2O Oxidation of

α-pinene

153 C9H12O2 3-Oxo-pina ketoneOxidation of

β-pinene

155

C9H14O2 NorpinonaldehydeOxidation of

α-pinene

C9H14O2

3-Hydroxy- pina ketone

Oxidation of β-pinene

157 C8H12O3, C9H16O2

2,2-Dimethyl-3-formyl-cyclobutyl-

methanoic acidOxidation of β-

pinene159 C8H14O3 Unknown

167 C10H14O2

Pinonic acid fragment M185-H2O

169

C10H16O2

2-Hydroxy-3-pinanone

Oxidation of α−pinene

C9H12O3 Pinic acid fragment-H2O Oxidation of

α- and β-pinene

171 C9H14O3

Pinalic acidOxidation of

α- and β-pinene

Norpinonic acidOxidation of

α-pinene

173 C8H12O4 Norpinic acidOxidation of

α- and β-pinene181 C10H12O3 fragment M199-H2O

185 C10H16O3

Pinonic acidOxidation of

α-pinene

Hydroxy-pinonaldehydes

Oxidation of α- and β-pinene

199 C10H14O4 7-Keto-pinonic acidO3 oxidation of

α-pinene

Table 2. Mass-to-charge ratios, possible chemi-cal formulas, and pos-tulated structures of selected ions. In many cases, several possible structures correspond to a given product ion. M/Z values were se-lected for monitoring based on the structures of known monoterpene oxidation products and their predicted PTR-MS fragmentation patterns. Mass scans were also performed on ambient air samples to identify additional candidates. Selected compounds were synthesized in the laboratory and char-acterized by PTR-MS. Compounds synthesized to date include pinonal-dehyde, norpinonic acid, hydroxy-pinonaldehyde, and hydroxy-pinonic acid. Nopinone, a-pi-nene oxide, and cis-pi-nonic acid were obtained commercially (Aldrich). PTR-MS mass spectra of these compounds are consistent with m/z val-ues listed in Table 2.

Target Compounds

Observations of biogenic VOCs and their oxidation products at a New England forestKarena A. McKinney, Katharine K. Duncan, and John Angiolillo

Amherst College, Department of Chemistry, Amherst, MA (kamckinney@amherst.edu)

Monoterpene Oxidation Products Concentrations of monoterpenes and their oxidation products were monitored in-termittently for a total of 26 days from July to September 2005. The measurements showed monoterpene mixing ratios averaging several hundred parts per trillion by volume (pptv) during the summer months, depending on temperature and boundary layer conditions.

Figure 7. Concentrations of mono-terpenes and oxidation products at m/z=139 (nopinone), m/z=151 (pinonaldehyde), and m/z=159 (unknown) as a function of time between September 8 (Day 251) and September 15 (Day 258), 2005. Data points represent 10-point averages with precisions of ~10 pptv and ~3 pptv for the monoterpenes and oxidation prod-ucts, respectively. Monoterpenes typically exhibit greater point-to-point variability due to emissions and turbulent transport above the canopy. Note the strong correla-tion between the concentrations and temperature, and the differing diurnal cycles among the oxida-tion products, which are distin-guishable in individual daily cy-cles.

Field Sampling Field studies were carried out at the Environmental Measurement Site (EMS) at Harvard Forest in Petersham, MA (42.54 °N, 72.17 °W). Air was sampled from the top of a 30-m tower equipped with a suite of meteorological instrumentation.

m/zAverage Countrate

(cps)Average Mixing

Ratio (pptv)Average Background

Countrate (cps)Dwell

Time (s)Detection Limit

(pptv)81+137 74.5 180.1 8.2 0.5 29.3

135 6.89 20.8 2.5 5 6.5139 16.2 49.0 3.0 5 7.1151 4.17 12.6 1.2 5 4.3153 12.3 37.2 1.9 5 5.6155 3.54 10.7 1.5 5 5.0157 4.53 13.7 2.3 5 6.3159 7.35 22.2 0.58 5 3.1167 1.56 4.7 0.66 5 3.3169 1.79 5.6 0.76 5 3.7171 1.97 5.9 0.95 5 3.9173 0.69 2.4 0.35 10 2.0181 3.70 11.2 0.32 5 2.3185 0.84 2.5 0.45 10 1.9199 0.38 1.2 0.24 10 1.4

Table 3. Observed Signal, Background, and Detection Limits. Detection limits (3s) are determined based on background countrates as-suming counting statistics, and apply to a single inte-gration period. Data shown in figures are averages with correspondingly lower de-tection limits. Masses are measured sequentially for a total measurement cycle time of ~90 s.

B

C

D

A

AcknowledgementsWe would like to thank S. Wofsy and W. Munger of Harvard University for assistance with our work at Har-vard Forest and the National Science Foundation and Amherst College for their support.

Biogenic Hydrocarbons and OVOCsThe major biogenic hydrocarbons and oxygenated VOCs were measured above the forest canopy throughout the summer of 2005.

Figure 3. Time series of the major biogenic hydrocarbons and OVOCs at Harvard Forest during the summer of 2005. All mixing ratios are in parts per billion (ppbv). Points represent 15-minute averages. Isoprene concentra-tions and diurnal and seasonal patterns are consistent with previous measurements at the site [Goldstein, et al., 1998].

Figure 4a-c. Diurnal average cycles of the com-pounds shown in Figure 3. Also shown are average PAR and temperature for the measurement period. Compounds exhibit a late afternoon/early evening maximum, consistent with measurements above a large forest emission source. All of the compounds with direct emission sources show large point-to-point variability, which suggests turbulent vertical transport out of the canopy. The isoprene oxidation products MVK+MACR concentrations have signifi-cantly lower variance. Eddy covariance flux mea-surements are planned for the near future.

B C

A

Evening Behavior of Isoprene and MonoterpenesIsoprene and the monoterpenes often exhibit a large early-evening peak followed by a rapid decay. The peak occurs later than the predicted peak in emission rate. The pro-cesses contributing to this build-up in concentration affects other emitted compounds and photochemical products as well (e.g., methanol, see Figure 4c). Similar diurnal patterns have been observed in other studies [Martin, et al., 1991; Hurst et al., 2001].

Figure 5. A two-day sample of isoprene and monoterpene concentration data from July 10 and 11, 2005. Data points represent 5-s and 1-s integration times for isoprene and monoterpenes, respec-tively, with a cycle time of 22 s. Emis-sion rates (red lines) were calculated us-ing the BEIS 2.3 model using measured temperature and PAR. The data show that in the evening as the emission rates and vertical flux are decreasing, the isoprene and monoterpene concentra-tions reach a maximum, then decrease. The maxima coincide with a quiescent period in the horizontal wind speed (top panel), suggesting that mixing slows and concentrations build up near the surface as the nocturnal boundary layer is established.

Figure 6. To understand the relationship between wind speed and isoprene concentration during the evening ‘lull’, data was separated into three groups: (A) days when the wind speed showed no evening lull; (B) days when there was a small lull in wind speed (>.75 m/s) and no peak in isoprene; and (C) days when there was a substantial lull in wind speed (<.5 m/s) and a large peak in isoprene. The lag time between the decrease in PAR and the evening isoprene decay is ~1.5 h, ~1.75 h, and ~2.5 h for days in groups A, B, and C, respectively. The average decay life-time (fit with a simple exponential function) for the three groups are 2.2 h, 2.7 h, and 1.9 h.

Diurnal Patterns in MOPs

m/zAverage Countrate

(cps)Average Mixing

Ratio (pptv)Average Background

Countrate (cps)Dwell

Time (s)Detection Limit

(ppbv)33 2752 2.7 2051 0.5 0.8759 1475 2.7 57 0.5 0.0869 316 0.83 50 5 0.0571 302 0.52 10 1 0.0273 213 0.50 19 1 0.04

81+137 77 0.15 11 1 0.02

Table 1. Observed Signal, Background, and Detec-tion Limits. Detection limits (3s) are determined based on background countrates assuming counting statis-tics, and apply to a single integration period. Masses are measured sequentially for a total measurement cycle time of ~22 s.

(Lindinger, Hansel et al. 1998), (Goldstein, Goulden et al. 1998), (Hurst, Jr. et al. 2001), (Martin, Westberg et al. 1991)

ReferencesGoldstein, A. H., M. L. Goulden, et al. (1998). “Seasonal course of isoprene emissions from a midlatitude forest.” J. Geophys. Res. 103(D23):

31045-31056.Hurst, J. M., D. J. B. Jr., et al. (2001). “Investigation of the nighttime decay of isoprene.” J. Geophys. Res. 106(D20): 24335-24346.Lindinger, W., A. Hansel, et al. (1998). “On-line monitoring of volatile organic compounds at pptv levels by means of proton-transfer-reaction

mass spectrometry (PTR-MS) - Medical applications, food control and environmental research.” Int. J. of Mass Spectrom. Ion Proc. 173(3): 191-241.

Martin, R. S., H. Westberg, et al. (1991). “Measurement of Isoprene and Its Atmospheric Oxidation-Products in a Central Pennsylvania Deciduous Forest.” Journal of Atmospheric Chemistry 13(1): 1-32.