Gas-phase volatile organic compounds (VOCs) were measured at three vertical levels between 0.6 m and 5.4 m in the Arctic boundary layer in Barrow, Alaska, for the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS)-2009 field campaign during March-April 2009. C 4 -C 8 nonmethane hydrocarbons (NMHCs) and oxygenated VOCs (OVOCs), including alcohols, aldehydes, and ketones, were quantified multiple times per hour, day and night, during the campaign using in situ fast gas chromatography-mass spectrometry. Three canister samples were also collected daily and subsequently analyzed for C 2 -C 5 NMHCs. The NMHCs and aldehydes demonstrated an overall decrease in mixing ratios during the experiment, whereas acetone and 2-butanone showed increases. Calculations of time-integrated concentrations of Br atoms, ∫[Br]dt, yielded values as high as (1.34 ± 0.27) × 10 14 cm À3 s during the longest observed ozone depletion event (ODE) of the campaign and were correlated with the steady state Br calculated at the site during this time. Both chlorine and bromine chemistry contributed to the large perturbations on the production and losses of VOCs. Notably, acetaldehyde, propanal, and butanal mixing ratios dropped below the detection limit of the instrument (3 parts per trillion by volume (pptv) for acetaldehyde and propanal, 2 pptv for butanal) during several ODEs due to Br chemistry. Chemical flux calculations of OVOC production and loss are consistent with localized high Cl-atom concentrations either regionally or within a very shallow surface layer, while the deeper Arctic boundary layer provides a continuous source of precursor alkanes to maintain the OVOC mixing ratios.
Mass spectrometry imaging is becoming an increasingly common analytical technique due to its ability to provide spatially resolved chemical information. Here, we report a novel imaging approach combining laser ablation with two mass spectrometric techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, separately and in parallel. Both mass spectrometric methods provide the fast response, rapid data acquisition, low detection limits, and high-resolution peak separation desirable for imaging complex samples. Additionally, the two techniques provide complementary information with aerosol mass spectrometry providing near universal detection of all aerosol molecules and chemical ionization mass spectrometry with a heated inlet providing molecular-level detail of both gases and aerosols. The two techniques operate with atmospheric pressure interfaces and require no matrix addition for ionization, allowing for samples to be investigated in their native state under ambient pressure conditions. We demonstrate the ability of laser ablation-aerosol mass spectrometry-chemical ionization mass spectrometry (LA-AMS-CIMS) to create 2D images of both standard compounds and complex mixtures. The results suggest that LA-AMS-CIMS, particularly when combined with advanced data analysis methods, could have broad applications in mass spectrometry imaging applications.
Abstract. In the lower troposphere, rapid collisions between ions and trace gases result in the transfer of positive charge to the highest proton affinity species and negative charge to the lowest proton affinity species. Measurements of the chemical composition of ambient ions thus provide direct insight into the most acidic and basic trace gases and their ion–molecule clusters – compounds thought to be important for new particle formation and growth. We deployed an atmospheric pressure interface time-of-flight mass spectrometer (APi-ToF) to measure ambient ion chemical composition during the 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) campaign at the United States Department of Energy Atmospheric Radiation Measurement facility in the Southern Great Plains (SGP), an agricultural region. Cations and anions were measured for alternating periods of ∼ 24 h over 1 month. We use binned positive matrix factorization (binPMF) and generalized Kendrick analysis (GKA) to obtain information about the chemical formulas and temporal variation in ionic composition without the need for averaging over a long timescale or a priori high-resolution peak fitting. Negative ions consist of strong acids including sulfuric and nitric acid, organosulfates, and clusters of NO3- with highly oxygenated organic molecules (HOMs) derived from monoterpene (MT) and sesquiterpene (SQT) oxidation. Organonitrates derived from SQTs account for most of the HOM signal. Combined with the diel profiles and back trajectory analysis, these results suggest that NO3 radical chemistry is active at this site. SQT oxidation products likely contribute to particle growth at the SGP site. The positive ions consist of bases including alkyl pyridines and amines and a series of high-mass species. Nearly all the positive ions contained only one nitrogen atom and in general support ammonia and amines as being the dominant bases that could participate in new particle formation. Overall, this work demonstrates how APi-ToF measurements combined with binPMF analysis can provide insight into the temporal evolution of compounds important for new particle formation and growth.
In order to evaluate the error introduced by the binPMF and peak-fitting process, synthetic peaks were generated and analyzed.First, Gaussian peaks at selected positions between m/z 200 and 550 were generated in time-of-flight (ToF) space. Peak widths were equal to real peaks observed at the selected m/z. The peaks were sampled in ToF space at the same interval as the APi-ToF data acquisition. The ToF space to m/z space transformation was calculated as:where p1 and p2 are the fit parameters selected for the simulation and ToF is the time of flight (ns). This function was selected for the simulated data because it was found to the best fit function for real data and was used to fit both positive and negative mode data throughout the campaign. "True" values for p1 and p2 were selected for the simulation. To simulate an upper limit estimate on error in the mass calibration and its impact on the binPMF results, pairs of p1 and p2 values were randomly selected from the set of p1 and p2 values fit from the ambient data using Tofware. Because p1 and p2 do not vary independently, each pair of values consisted of parameters calculated for the same time point in the calibration. This is an upper estimate of our error because it assumes that all shifts in mass calibration contribute error, but there are real shifts in p1 and p2 that result from temperature changes, drift within the instrument, and other factors. Following the ToF to m/z space transformation, the synthetic peaks in m/z space were binned using the same bins and bin widths used for binPMF and fit with a Gaussian to determine the peak center. Error introduced by the m/z calibration was determined using a Monte Carlo method to randomly select many sets of p1 and p2. Root mean squared errors introduced by this method were approximately 50 ppm for both positive and negative mode data. The simulation was also repeated using only the "true" fit parameters to determine whether error in the peak positions originated from simulated error in the mass calibration or from the binning and fitting procedure.Error was negligible (<<1 ppm) when using the "true" fit parameters, suggesting that most error is from the mass calibration and not the fitting procedure. Peak broadening was also evaluated. Peaks may be broadened both by the procedure of binning and fitting peaks to bins and by shifts in the mass calibration throughout the campaign. Figure S1 shows the comparison between the peak in a 15-minute average mass spectrum at m/z 487 and the Gaussian peak fit to the bins at that mass. Minimal broadening is observed. It should also be noted that peak widths have no direct implications for the conclusions of this work.Peak shape was also investigated. Figure S2 compares the high-resolution peak shape calculated in Tofware and a Gaussian
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