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Title: Dimensionality-reduction techniques for complex mass spectrometric datasets: application to laboratory atmospheric organic oxidation experiments
Abstract. 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 like carbon oxidation state that can beuseful for modeling. The continuum of kinetic behavior observed in a typicalchamber experiment can be parameterized by fitting species' time traces tothe GKP, which approximates the chemistry as a linear, first-order kineticsystem. The fitted parameters for each species are the number of reaction stepswith OH needed to produce the species (the generation) and an effectivekinetic rate constant that describes the formation and loss rates of thespecies. The thousands of species detected in a typical laboratory chamberexperiment can be organized into a much smaller number (10–30) of groups,each of which has a characteristic chemical composition and kinetic behavior.This quantitative relationship between chemical and kinetic characteristics,and the significant reduction in the complexity of the system, provides anapproach to understanding broad patterns of behavior in oxidation systemsand could be exploited for mechanism development and atmospheric chemistrymodeling.  more » « less
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Journal Name:
Atmospheric Chemistry and Physics
Page Range / eLocation ID:
1021 to 1041
Medium: X
Sponsoring Org:
National Science Foundation
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