Proton transfer reaction mass spectrometry (PTR-MS) based on reactions of H3O+ ions has been used to measure the concentrations of propanol in 46 healthy persons, yielding an average concentration of about 150 ppb. That the measurements were not obscured by other components of the same mass as propanol was proven by comparison of PTR-MS data with separate selected-ion flow-drift tube (SIFDT) investigations of the energy dependences of reactions of H3O+ and H3O+·H2O with isopropanol, n-propanol, acetic acid and methyl formate.

Tropical forests with emissions greater than 1015 g C of reactive hydrocarbons per year strongly affect atmospheric chemistry. Here we report aircraft-borne measurements of organics during March 1998 in Surinam, a largely unpolluted region which is optimally located to study chemical processes induced by tropical forest emissions. Isoprene and its degradation products methylvinyl ketone (MVK) and methacrolein (MACR) and possibly isoprene hydroperoxides (ISOHP), were measured in the nmol mol−1 volume mixing ratio (VMR) range, consistent with estimated emissions and model calculations. In addition, high VMRs of some non-isoprene-derived organics were measured, such as acetone (≈2–4 nmol mol1 up to 12 km altitude), an important source of HO and HO2 in the upper troposphere. Moreover, several masses were measured at significant mixing ratios which could not be identified by reference to previous field measurements or gas-phase isoprene chemistry. High VMRs, almost 0.4 nmol mol−1, were also recorded for a compound which is most likely dimethyl sulphide (DMS). If so, boundary layer loss of HO by reactions with hydrocarbons and their oxidation products strongly prolongs the lifetime of DMS, allowing its transport deep into the Amazon forest south of the intertropical convergence zone (ITCZ). We postulate greater sulphate production and deposition north than south of the (ITCZ) with possible consequences for cloud and ecosystem properties.

[1] There is interest in and significant uncertainty about the emissions of oxygenated volatile organic compounds (oxVOCs) from vegetation to the atmosphere. Here, we measured the fluxes of selected oxVOCs from an alfalfa field, before, during, and after cutting, using a combination of disjunct eddy covariance and proton-transfer-reaction mass spectrometry. Over the course of 1 day a significant methanol flux of 4 mg m−2 h−1 was observed from undisturbed alfalfa with a maximum at 0800 LT, possibly caused by the evaporation of dew. A smaller release of hexenals during this day (0.04 mg m−2 h−1) demonstrated the sensitivity of the method. Other results suggested that acetaldehyde and acetone were released in the afternoon but were lost by dry deposition in the evening and morning; deposition velocities were estimated to be 0.2 cm s−1 (acetaldehyde) and 0.09 cm s−1 (acetone). After the alfalfa was cut the emissions of methanol, acetaldehyde, acetone, and hexenals were significantly enhanced and remained high for three days during which the alfalfa was drying. After a rainstorm the oxVOC emissions from the cut, wet alfalfa increased even more. Nighttime measurements yielded low oxVOC fluxes in general, but the high variability of the concentrations during the night and the high degree of correlation between different oxVOCs suggest that the nighttime releases of oxVOCs from alfalfa were nonzero. This work suggests that the global source of oxVOCs due to the production of hay is of minor importance. The emission flux of methanol from vegetation during the growing season may be very large on a global basis.

Airborne measurements of acetonitrile (CH3CN) were made off the U.S. west coast, over California, and during two transfer flights over the U.S. in April and May of 2002. Acetonitrile was strongly enhanced in the plumes from two forest fires, confirming the usefulness of the measurement as an indicator for biomass burning emissions. The emission ratios relative to CO of acetonitrile in the two plumes were slightly higher than previously reported values for fires burning in other fuel types. No significant acetonitrile release was observed in the Los Angeles basin or from other point sources (ships and a power plant). Acetonitrile concentrations were significantly reduced in the marine boundary layer indicating the presence of an ocean uptake sink. Increased loss of acetonitrile was observed close to the coast, suggesting that acetonitrile was efficiently lost by dissolving in the upwelling ocean water, or by biological processes in the surface water.

Organic compounds were measured by proton transfer reaction-mass spectrometry (PTR-MS) on board the National Oceanic and Atmospheric Administration's research ship Ronald H. Brown during the New England Air Quality Study (NEAQS) in July and August of 2002. PTR-MS has the potential to measure many important organic species with a fast time response, but its validity has not been proven sufficiently. The results obtained by PTR-MS during NEAQS were compared with those from (oxygenated) hydrocarbon measurements by gas chromatography/mass spectrometry (GC-MS), peroxyacyl nitrate measurements by gas chromatography/electron capture detection, and carboxylic acid measurements by mist chamber/ion chromatography. The PTR-MS and GC-MS data for methanol, acetonitrile, acetone, isoprene, benzene, and toluene agreed within the measurement uncertainties. The comparison for C8 aromatics and acetaldehyde was less quantitative due to calibration inaccuracies. In addition, PTR-MS measured the sum of methyl vinyl ketone and methacrolein at 71 amu, the sum of C9 aromatics at 121 amu, and the sum of monoterpenes at 81 and 137 amu. The PTR-MS signal at 61 amu was found to correlate well with data for acetic acid. The signal at 73 amu correlated reasonably well with methyl ethyl ketone data, but the quantitative disagreement suggested interference from other species, possibly methyl glyoxal. The signal at 77 amu correlated well with data for peroxyacetyl nitrate, and the sensitivity inferred from the field data agreed within 30% with the results from laboratory calibrations. Finally, the signal at 105 amu was attributed to styrene and peroxy isobutyryl nitrate. These results prove that many important organic species can be measured accurately and with a fast response time by PTR-MS.

Unexpectedly high values for acetaldehyde have been observed in airborne measurements using a proton-transfer-reaction mass spectrometry instrument. The acetaldehyde values increase with increasing ambient ozone levels with a ratio up to 5 pptv acetaldehyde per ppbv of ozone in the free troposphere. The elevated values of acetaldehyde cannot easily be explained from known tropospheric chemistry. Here, we investigate the possibility that the elevated acetaldehyde signals are due to a sampling artifact. Laboratory experiments show that the elevated signals are not due to changes of the ion chemistry in the instrument, or from the instrument materials reacting with ozone. The heterogeneous oxidation of a number of unsaturated organic compounds is investigated as a possible source for a chemical artifact produced in the instrument inlet. The products of the heterogeneous reactions are consistent with gas phase chemistry, and the ozonolysis of some alkenes does produce acetaldehyde when they have the appropriate hydrocarbon structure. The amount of reactive material in the free troposphere expected to accumulate in the aircraft inlet is unknown, and the exact origin of reactive compounds that contribute to the artifact production remains unresolved.

We present a newly developed instrument that uses proton-transfer ion trap-mass spectrometry (PIT-MS) for on-line trace gas analysis of volatile organic compounds (VOCs). The instrument is based on the principle of proton-transfer reaction-mass spectrometry (PTR-MS): VOCs are ionized using PTRs and detected with a mass spectrometer. As opposed to a quadrupole mass filter in a PTR-MS, the PIT-MS instrument uses an IT-MS, which has the following advantages: (1) the ability to acquire a full mass spectrum in the same time as one mass with a quadrupole and (2) extended analytical capabilities of identifying VOCs by performing collision-induced dissociation (CID) and ion molecule reactions in the IT. The instrument described has, at its current status, limits of detection between 0.05 and 0.5 pbbv for 1-min measurements for all tested VOCs. The PIT-MS was tested in an ambient air measurement in the urban area of Boulder, Colorado, and intercompared with PTR-MS. For all measured compounds the degree of correlation between the two measurements was high (r2 > 0.85), except for acetonitrile (CH3CN), which was close to the limit of detection of the PIT-MS instrument. The two measurements agreed within less than 25%, which was within the combined measurement uncertainties. Automated CID measurements on m/z 59 during the intercomparison were used to determine the contributions of acetone and propanal to the measured signal; both are detected at m/z 59 and thus are indistinguishable in PTR-MS. It was determined that m/z 59 was mainly composed of acetone. An influence of propanal was detected only during a high pollution event. The advantages and future developments of PIT-MS are discussed.

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.

During the summer of 2004 large wildfires were burning in Alaska and Canada, and part of the emissions were transported toward the northeast United States, where they were measured during the NEAQS-ITCT 2k4 (New England Air Quality Study–Intercontinental Transport and Chemical Transformation) study on board the NOAA WP-3 aircraft and the NOAA research vessel Ronald H. Brown. Using acetonitrile and chloroform as tracers the biomass burning and the anthropogenic fraction of the carbon monoxide (CO) enhancement are determined. As much as 30% of the measured enhancement is attributed to the forest fires in Alaska and Canada transported into the region, and 70% is attributed to the urban emissions of mainly New York and Boston. On some days the forest fire emissions were mixed down to the surface and dominated the CO enhancement. The results compare well with the FLEXPART transport model, indicating that the total emissions during the measurement campaign for biomass burning might be about 22 Tg. The total U.S. anthropogenic CO sources used in FLEXPART are 25 Tg. FLEXPART model, using the U.S. EPA NEI-99 data, overpredicts the CO mixing ratio around Boston and New York in 2004 by about 50%.

The NOAA WP-3 aircraft intercepted aged forest fire plumes from Alaska and western Canada during several flights of the NEAQS-ITCT 2k4 mission in 2004. Measurements of acetonitrile (CH3CN) indicated that the air masses had been influenced by biomass burning. The locations of the plume intercepts were well described using emissions estimates and calculations with the transport model FLEXPART. The best description of the data was generally obtained when FLEXPART injected the forest fire emissions to high altitudes in the model. The observed plumes were generally drier than the surrounding air masses at the same altitude, suggesting that the fire plumes had been processed by clouds and that moisture had been removed by precipitation. Different degrees of photochemical processing of the plumes were determined from the measurements of aromatic VOCs. The removal of aromatic VOCs was slow considering the transport times estimated from the FLEXPART model. This suggests that the average OH levels were low during the transport, which may be explained by the low humidity and high concentrations of carbon monoxide and other pollutants. In contrast with previous work, no strong secondary production of acetone, methanol and acetic acid is inferred from the measurements. A clear case of removal of submicron particle volume and acetic acid due to precipitation scavenging was observed.

During the NEAQS-ITCT2k4 campaign in New England, anthropogenic VOCs and CO were measured downwind from New York City and Boston. The emission ratios of VOCs relative to CO and acetylene were calculated using a method in which the ratio of a VOC with acetylene is plotted versus the photochemical age. The intercept at the photochemical age of zero gives the emission ratio. The so determined emission ratios were compared to other measurement sets, including data from the same location in 2002, canister samples collected inside New York City and Boston, aircraft measurements from Los Angeles in 2002, and the average urban composition of 39 U.S. cities. All the measurements generally agree within a factor of two. The measured emission ratios also agree for most compounds within a factor of two with vehicle exhaust data indicating that a major source of VOCs in urban areas is automobiles. A comparison with an anthropogenic emission database shows less agreement. Especially large discrepancies were found for the C2-C4 alkanes and most oxygenated species. As an example, the database overestimated toluene by almost a factor of three, which caused an air quality forecast model (WRF-CHEM) using this database to overpredict the toluene mixing ratio by a factor of 2.5 as well. On the other hand, the overall reactivity of the measured species and the reactivity of the same compounds in the emission database were found to agree within 30%.

We present quantitative, fast time response measurements of formaldehyde (HCHO) onboard an aircraft using a Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS) instrument. The HCHO measurement by PTR-MS is strongly humidity dependent and therefore airborne measurements are difficult and have not been reported. The PTR-MS instrument was run in the normal operating mode, where about 15 volatile organic compounds (VOCs) are measured together with HCHO onboard the NOAA WP-3 aircraft during the CalNex 2010 campaign in California. We compare the humidity dependence determined in the laboratory with in-flight calibrations of HCHO and calculate the HCHO mixing ratio during all flights using the results from both. The detection limit for HCHO was between 100 pptv in the dry free troposphere and 300 pptv in the humid marine boundary layer for a one second acquisition time every 17 s. The PTR-MS measurements are compared with HCHO measurements using a DOAS instrument and a Hantzsch monitor at a ground site in Pasadena. The PTR-MS agreed with both instruments within the stated uncertainties. We also compare HCHO enhancement ratios in the Los Angeles basin and in the free troposphere with literature values and find good agreement. The usefulness of the PTR-MS HCHO measurements in atmospheric observations is demonstrated by following an isolated anthropogenic plume. The photochemical production of HCHO can be observed simultaneously with production of acetaldehyde and the photochemical degradation of aromatic compounds using the PTR-MS.

Volatile organic compound (VOC) mixing ratios were measured with two different instruments at the T1 ground site in Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign in March of 2006. A gas chromatograph with flame ionization detector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while a proton-transfer-reaction ion-trap mass spectrometer (PIT-MS) quantified 12 VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separation system was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluate PIT-MS measurements and to aid in the identification of unknown VOCs. The VOC measurements are also compared to simultaneous canister samples and to two independent proton-transfer-reaction mass spectrometers (PTR-MS) deployed on a mobile and an airborne platform during MILAGRO. VOC diurnal cycles demonstrate the large influence of vehicle traffic and liquid propane gas (LPG) emissions during the night and photochemical processing during the afternoon. Emission ratios for VOCs and OVOCs relative to CO are derived from early-morning measurements. Average emission ratios for non-oxygenated species relative to CO are on average a factor of  2 higher than measured for US cities. Emission ratios for OVOCs are estimated and compared to literature values the northeastern US and to tunnel studies in California. Positive matrix factorization analysis (PMF) is used to provide insight into VOC sources and processing. Three PMF factors were distinguished by the analysis including the emissions from vehicles, the use of liquid propane gas and the production of secondary VOCs + long-lived species. Emission ratios to CO calculated from the results of PMF analysis are compared to emission ratios calculated directly from measurements. The total PIT-MS signal is summed to estimate the fraction of identified versus unidentified VOC species.

A large fraction of atmospheric aerosols are derived from organic compounds with various volatilities. A National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft made airborne measurements of the gaseous and aerosol composition of air over the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico that occurred from April to August 2010. A narrow plume of hydrocarbons was observed downwind of DWH that is attributed to the evaporation of fresh oil on the sea surface. A much wider plume with high concentrations of organic aerosol (>25 micrograms per cubic meter) was attributed to the formation of secondary organic aerosol (SOA) from unmeasured, less volatile hydrocarbons that were emitted from a wider area around DWH. These observations provide direct and compelling evidence for the importance of formation of SOA from less volatile hydrocarbons.