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Dimensionality-reduction techniques for complex mass spectrometric datasets: application to laboratory atmospheric organic oxidation experimentsAbstract. Oxidation of organic compounds in the atmosphere produces an immenselycomplex mixture of product species, posing a challenge for both theirmeasurement in laboratory studies and their inclusion in air quality andclimate models. Mass spectrometry techniques can measure thousands of thesespecies, giving insight into these chemical processes, but the datasetsthemselves are highly complex. Data reduction techniques that groupcompounds in a chemically and kinetically meaningful way provide a route tosimplify the chemistry of these systems but have not been systematicallyinvestigated. Here we evaluate three approaches to reducing thedimensionality of oxidation systems measured in an environmental chamber:positive matrix factorization (PMF), hierarchical clustering analysis (HCA),and a parameterization to describe kinetics in terms of multigenerationalchemistry (gamma kinetics parameterization, GKP). The evaluation isimplemented by means of two datasets: synthetic data consisting of athree-generation oxidation system with known rate constants, generationnumbers, and chemical pathways; and the measured products of OH-initiatedoxidation of a substituted aromatic compound in a chamber experiment. Wefind that PMF accounts for changes in the average composition of allproducts during specific periods of time but does not sort compounds intogenerations or by another reproducible chemical process. HCA, on the otherhand, can identify major groups of ions and patterns of behavior andmaintains bulk chemical properties likemore »
Using collision-induced dissociation to constrain sensitivity of ammonia chemical ionization mass spectrometry (NH<sub>4</sub><sup>+</sup> CIMS) to oxygenated volatile organic compounds
Abstract. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2 min.The CIMS mode of the new instrument has sensitivities of upto 67 000 dcps ppbv−1 (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60 pptv at2σ for a 1 s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.
Mechanistic study of the formation of ring-retaining and ring-opening products from the oxidation of aromatic compounds under urban atmospheric conditionsAbstract. Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevantpolluted conditions (NOx∼10 ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH4+ CIMS and I− CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O:C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursorsmore »