BayCEER Workshop 2016

Chemical composition studies of laboratory-made α-pinene SOA by NMR

P 6.3 in Paving the way for research: Databases, instruments, networks
& Open Session

Introduction

Understanding atmospheric processes such as chemical transformations, nucleation, cloud formation, etc. require studies at a molecular level. Our work was focused on α-pinene secondary organic aerosol (SOA) formation and analysis by nuclear magnetic resonance (NMR), in order to complement information about the aerosol chemical composition and to study reactive intermediates which are often lost during long residence times and sample preparation or analysis by common analytical tools like liquid chromatography (LC).

Material and Methods

α-Pinene SOA was generated in a 30.5 L flow reactor by ozonolysis under low NOx and low RH conditions. SOA was collected on QMF filters and analysed by 1H, 13C-NMR (Bruker DRX 500 MHz) and HPLC-MS (Agilent 1100/6130). Variability in sampling time and residence time allowed to study labile compounds and compounds involved in particle aging.

Results

¹H-NMR spectra of α-pinene SOA at different residence times revealed the same major signals corresponding to identical compounds. The major signals were identified as pinic, pinonic and terpenylic acid. The differences between aged SOA occurred in the composition of minor compounds, and in reciprocal ratio of signals. In expanded ¹H-NMR spectra plenty of protons attributed to aldehydes, hydro peroxides, double bonds and protons to electron-poor sp³ carbon or heteroatoms were recognized. Several of these compounds are reactive enough to be transformed during the sampling process. They were recorded only in short time samples, whereas others appear only in samples with longer residence times, thus on more aged particles.

Conclusions

These experiments allowed us to recognize major compounds on particles and the most labile compounds, which are involved in the mechanistic pathway of monoterpene oxidation and particle formation. Still, additional experiments are required in order to make full structural identification of these intermediates.

Keywords: α-pinene SOA, flow reactor, NMR, terpenoic acids, reactive intermediates