Measurements of CH4 mixing ratio, vertical gradients and turbulent fluxes were carried out in a tropical forest (Reserva Biológica Cuieiras), about 60 km north of Manaus, Brazil. The methane mixing ...ratio and flux measurements were performed at a height of 53 m (canopy height 35 m). In addition, vertical CH4 gradients were measured within the canopy using custom made air samplers at levels of 2, 16 and 36 m above ground. The methane gradients within the canopy reveal that there is a continuous methane source at the surface. No clear evidence for aerobic methane emission from the canopy was found. The methane fluxes above the canopy are small but consistently upwards with a maximum early in the morning. The measured fluxes are in agreement with the observed CH4 gradient in the canopy. In the morning hours, a strong canopy venting peak is observed for both CH4 and CO2, but for CO2 this peak is then superimposed by photosynthetic uptake, whereas the peak lasts longer for CH4. Monthly averaged diurnal cycles of the CH4 mixing ratio show a decrease during daytime and increase during nighttime. The magnitude of the difference in CH4 mixing ratio between day and night gradually increases throughout the wet season. The fluxes required to explain the nighttime increase are in agreement with the nighttime fluxes measured above the canopy, which implies that the CH4 increase in the nighttime boundary layer originates from local sources.
A system of two plant chambers and a downstream reaction chamber has been set up to investigate the emission of biogenic volatile organic compounds (BVOCs) and possible effects of pollutants such as ...ozone. The system can be used to compare BVOC emissions from two sets of differently treated plants, or to study the photochemistry of real plant emissions under polluted conditions without exposing the plants to pollutants. The main analytical tool is a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS) which allows online monitoring of biogenic emissions and chemical degradation products. The identification of BVOCs and their oxidation products is aided by cryogenic trapping and subsequent
in situ
gas chromatographic analysis.
A system of two plant chambers and a downstream reaction chamber has been set up to investigate the emission of biogenic volatile organic compounds (BVOCs) and possible effects of pollutants such as ozone.
Our understanding of formation processes, physical properties, and climate/health effects of organic aerosols is still limited in part due to limited knowledge of organic aerosol composition. We ...present speciated measurements of organic aerosol composition by two methods: in situ thermal-desorption proton-transfer-reaction mass spectrometry (TD-PTR-MS) and offline two-dimensional gas chromatography with a time-of-flight mass spectrometer (GC × GC/TOF-MS). Using the GC × GC/TOF-MS 153 compounds were identified, 123 of which were matched with 64 ions observed by the TD-PTR-MS. A reasonable overall correlation of 0.67 (r2) was found between the total matched TD-PTR-MS signal (sum of 64 ions) and the total matched GC × GC/TOF-MS signal (sum of 123 compounds) for the Los Angeles area. A reasonable quantitative agreement between the two methods was observed for most individual compounds with concentrations which were detected at levels above 2 ng m−3 using the GC × GC/TOF-MS. The analysis of monocarboxylic acids standards with TD-PTR-MS showed that alkanoic acids with molecular masses below 290 amu are detected well (recovery fractions above 60 %). However, the concentrations of these acids were consistently higher on quartz filters (quantified offline by GC × GC/TOF-MS) than those suggested by in situ TD-PTR-MS measurements, which is consistent with the semivolatile nature of the acids and corresponding positive filter sampling artifacts.
A series of 12 high volume air samples collected from the S2 firn core during the North Greenland Eemian Ice Drilling (NEEM) 2009 campaign have been measured for mixing ratio and stable carbon ...isotope composition of the chlorofluorocarbon CFC-12 (CCl2 F2 ). While the mixing ratio measurements compare favorably to other firn air studies, the isotope results show extreme 13 C depletion at the deepest measurable depth (65 m), to values lower than δ13 C = -80per thousand vs. VPDB (the international stable carbon isotope scale), compared to present day surface tropospheric measurements near -40per thousand. Firn air modeling was used to interpret these measurements. Reconstructed atmospheric time series indicate even larger depletions (to -120per thousand) near 1950 AD, with subsequent rapid enrichment of the atmospheric reservoir of the compound to the present day value. Mass-balance calculations show that this change is likely to have been caused by a large change in the isotopic composition of anthropogenic CFC-12 emissions, probably due to technological advances in the CFC production process over the last 80 yr, though direct evidence is lacking.
We present a novel approach to study the organic composition of aerosol filter samples using thermal-desorption proton-transfer-reaction mass spectrometry (TD-PTR-MS) in the laboratory. The method is ...tested and validated based on the comparison with in situ TD-PTR-MS measurements. In general, we observe correspondence within the levels of uncertainty between in situ and offline TD-PTR-MS measurements for compounds desorbing at temperatures above 100°C and for quartz fiber filters that were sampled for more than one day. Positive sampling artifacts (50–80%, with respect to the in situ measurements) from adsorption of semivolatile organic gas phase compounds are apparent on filters sampled for one day. Detailed chemical analysis shows that these positive sampling artifacts are likely caused by primary emissions that have not been strongly oxidized. Negative sampling artifacts (7–35%, with respect to the in situ measurements) are observed for most filters sampled for two and three days, and potentially caused by incomplete desorption of aerosols (in particular, nitrogen-containing organics) from the filters during the offline measurements and chemical reactions on the filters.
•The offline method to study organic aerosol composition is developed and validated.•The method allows for the extensive use of TD-PTR-MS technique.•Positive sampling artifacts are observed for filters sampled for 1 day.•Negative sampling artifacts are observed for most filters sampled for 2 and 3 days.
The chlorofluorocarbons CFC-11 (CFCl3) and CFC-12 (CF2Cl2) are stable atmospheric compounds that are produced at the earth's surface, but removed only at high altitudes in the stratosphere by ...photolytic reactions. Their removal liberates atomic chlorine that then catalytically destroys stratospheric ozone. For such long-lived compounds, isotope effects in the stratospheric removal reactions have a large effect on their global isotope budgets. We have demonstrated a photolytic isotope fractionation for stable carbon isotopes of CFC-11 and CFC-12 in laboratory experiments using broadband UV-C (190–230 nm) light. 13C/12C isotope fractionations (ε) range from (−23.8±0.9) to (−17.7±0.4) ‰ for CFC-11 and (−66.2±3.1) to (−51.0±2.9) ‰ for CFC-12 between 203 and 288 K, a temperature range relevant to conditions in the troposphere and stratosphere. These results suggest that CFCs should become strongly enriched in 13C with decreasing mixing ratio in the stratosphere, similar to what has been recently observed for CFC chlorine isotopes. In conjunction with the strong variations in CFC emissions before and after the Montréal Protocol, the stratospheric enrichments should also lead to a significant temporal increase in the 13C content of the CFCs at the surface over the past decades, which should be recorded in atmospheric air archives such as firn air.
Many monoterpenes have been identified in forest emissions using gas chromatography (GC). Until now, it has been impossible to determine whether all monoterpenes are appropriately measured using GC ...techniques. We used a proton transfer reaction mass spectrometer (PTR-MS) coupled with the eddy covariance (EC) technique to measure mixing ratios and fluxes of total monoterpenes above a ponderosa pine plantation. We compared PTR-MS-EC results with simultaneous measurements of eight speciated monoterpenes, β-pinene, α-pinene, 3-carene, d-limonene, β-phellandrene, α-terpinene, camphene, and terpinolene, made with an automated, in situ gas chromatograph with flame ionization detectors (GC-FID), coupled to a relaxed eddy accumulation system (REA). Monoterpene mixing ratios and fluxes measured by PTR-MS averaged 30±2.3% and 31±9.2% larger than by GC-FID, with larger mixing ratio discrepancies between the two techniques at night than during the day. Two unidentified peaks that correlated with β-pinene were resolved in the chromatograms and completely accounted for the daytime difference and reduced the nighttime mixing ratio difference to 20±2.9%. Measurements of total monoterpenes by PTR-MS-EC indicated that GC-FID-REA measured the common, longer-lived monoterpenes well, but that additional terpenes were emitted from the ecosystem that represented an important contribution to the total mixing ratio above the forest at night.
We report measurements of ambient atmospheric mixing ratios for methyl chavicol and determine its biogenic emission rate. Methyl chavicol, a biogenic oxygenated aromatic compound, is abundant within ...and above Blodgett Forest, a ponderosa pine forest in the Sierra Nevada Mountains of California. Methyl chavicol was detected simultaneously by three in-situ instruments – a gas chromatograph with mass spectrometer detector (GC-MS), a proton transfer reaction mass spectrometer (PTR-MS), and a thermal desorption aerosol GC-MS (TAG) – and found to be abundant within and above Blodgett Forest. Methyl chavicol atmospheric mixing ratios are strongly correlated with 2-methyl-3-buten-2-ol (MBO), a light- and temperature-dependent biogenic emission from the ponderosa pine trees at Blodgett Forest. Scaling from this correlation, methyl chavicol emissions account for 4–68% of the carbon mass emitted as MBO in the daytime, depending on the season. From this relationship, we estimate a daytime basal emission rate of 0.72–10.2 μgCg−1 h−1, depending on needle age and seasonality. We also present the first observations of its oxidation products (4-methoxybenzaldehyde and 4-methyoxy benzene acetaldehyde) in the ambient atmosphere. Methyl chavicol is a major essential oil component of many plant species. This work suggests that methyl chavicol plays a significant role in the atmospheric chemistry of Blodgett Forest, and potentially other sites, and should be included explicitly in both biogenic volatile organic carbon emission and atmospheric chemistry models.
We developed a system that links a thermal desorption oven for aerosol filter samples to an isotope ratio mass spectrometer (IRMS) via a combustion interface. Organic compounds are desorbed from the ...filter sample in He at 7 temperature steps between 100 and 400°C and subsequently oxidized to CO2, which is analyzed for δ13C. We tested the system for isotopic fractionation, reproducibility and linearity using organic test compounds and aerosol filter samples. An oxalic acid standard with known δ13C value shows no isotopic fractionation when it is desorbed at temperatures well above its melting point. When organic acids are heated at a temperature close to their melting point only a fraction of the compound (10–80%) evaporates which shows depleted δ13C values at this temperature step. At the next higher temperature step, when the rest of the compound is desorbed, δ13C values are enriched. This effect is stronger for compounds with a lower molecular weight than for compounds with a higher molecular weight. In ambient samples this should only have a moderate effect on the overall δ13C value, since hundreds of different compounds are desorbed at each temperature step. Choosing different values for temperature steps does not strongly change the δ13C values for ambient aerosol filter samples. For typical ambient samples the reproducibility lies below ±0.3‰ for oven temperatures below 200°C and below ±0.5‰ for oven temperatures above 200°C. Tests with oxalic acid and ambient filter samples show that the IRMS is linear for peak areas in the operational range of 1–20Vs.
•A step-wise thermal desorption system for measuring δ13C values of organic aerosol.•The results are independent of the chosen temperature steps.•The system is linear, i.e. δ13C values are independent of the analyzed peak area.•No isotopic fractionation for fully desorbed compounds.•The reproducibility of δ13C values is below 0.5‰.
We report the detection of a class of related oxygenated compounds by proton‐transfer‐reaction mass‐spectrometry (PTR‐MS) that have rarely or never been observed as a group using in situ ...instrumentation. Measurements were made as part of the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) 2004 in Chebogue Point, Nova Scotia. The detected class of compounds discussed here includes acetic acid, formaldehyde, acetaldehyde, tentatively identified formic acid and hydroxyacetone, and unidentified compounds detected at mass to charge ratios 85, 87, 99, 101, 113, 115, and 129. Typical concentrations were 800, 2500, 450, 700, 85, 25, 50, 50, 60, 35, 20, and 25 ppt, respectively. The uniqueness of this class of compounds is illustrated by showing they were poorly related to trace gases found in the US outflow, local pollution, primary biogenic emissions and other oxygenated compounds such as acetone, methanol, and MEK measured by other in situ instrumentation. On the other hand these oxidized volatile organic compounds were related to chemical species in aerosols and their abundance was high during nucleation events. Thus they likely are gas phase species that are formed in parallel to biogenic secondary organic aerosol production. We clearly show these compounds do not originate from local sources. We also show these compounds match the oxidation products of isoprene observed in smog chamber studies, and we therefore suggest they must be mainly produced by oxidation of biogenic precursor compounds.
PDF
