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- Title
The link between organic aerosol mass loading and degree of oxygenation: an α-pinene photooxidation study.
- Authors
Pfaffenberger, L.; Barmet, P.; Slowik, J. G.; Praplan, A. P.; Dommen, J.; Prévôt, A. S. H.; Baltensperger, U.
- Abstract
A series of smog chamber (SC) experiments was conducted to identify driving factors responsible for the discrepancy between ambient and SC aerosol degree of oxygenation. An Aerodyne high resolution time-of-flight aerosol mass spectrometer is used to compare mass spectra from α-pinene photooxidation with ambient aerosol. Composition is compared in terms of the fraction of organic mass measured at m/z 44 (f44), a surrogate for carboxylic/organic acids as well as the atomic oxygen-to-carbon ratio (O : C), vs. f43, a surrogate for aldehydes, alcohols and ketones. Low (near-ambient) organic mass concentrations were found to be necessary to obtain oxygenation levels similar to those of low-volatility oxygenated organic aerosol (LV-OOA) commonly identified in ambient measurements. The effects of organic mass loading and OH (hydroxyl radical) exposure were decoupled by inter-experiment comparisons at the same integrated OH concentration. On average, an OH exposure of 2.9±1.3x107 cm-3 h is needed to increase f44 by 1% during aerosol aging. For the first time, LV-OOA-like aerosol from the abundant biogenic precursor α-pinene was produced in a smog chamber by oxidation at typical atmospheric OH concentrations. Significant correlation between measured secondary organic aerosol (SOA) and reference LV-OOA mass spectra is shown by Pearson's R2 values larger than 0.90 for experiments with low organic mass concentrations between 1.5 and 15 µgm-3 at an OH exposure of 4x107cm-3 h, corresponding to about two days oxidation time in the atmosphere, based on a global mean OH concentration of ~ 1x106cm-3. Not only is the α-pinene SOA more oxygenated at low organic mass loadings, but the functional dependence of oxygenation on mass loading is enhanced at atmospherically-relevant precursor concentrations. Since the degree of oxygenation influences the chemical, volatility and hygroscopic properties of ambient aerosol, smog chamber studies must be performed at near-ambient concentrations to accurately simulate ambient aerosol properties.
- Subjects
ATMOSPHERIC aerosols; OXYGENATION (Chemistry); SMOG; PINENE; PHOTOOXIDATION; HYDROXYL group; ALDEHYDES; TIME-of-flight mass spectrometry
- Publication
Atmospheric Chemistry & Physics Discussions, 2012, Vol 12, Issue 9, p24735
- ISSN
1680-7367
- Publication type
Article
- DOI
10.5194/acpd-12-24735-2012