Abstract
Abstract. A series of smog chamber (SC) experiments was conducted to identify 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 particulate CO2+, a surrogate for carboxylic acids, vs. the fraction of C2H3O+, a surrogate for aldehydes, alcohols and ketones, as well as in the Van Krevelen space, where the evolution of the atomic hydrogen-to-carbon ratio (H : C) vs. the atomic oxygen-to-carbon ratio (O : C) is investigated. 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. An OH exposure between 3 and 25 × 107 cm−3 h is needed to increase O : C by 0.05 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 1.2 and 18 μg m−3 at an OH exposure of 4 × 107 cm−3 h, corresponding to about two days of oxidation time in the atmosphere, based on a global mean OH concentration of ~ 1 × 106 cm−3. α-Pinene SOA is more oxygenated at low organic mass loadings. Because 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.
Highlights
Instrumentation carboxylic acids, vs. the fraction of C2H3O+, a surrogate for aldehydes, alcohols and ketones, as well as in the Van Krevelen space, where the evolution of the atomic hydrogen
secondary organic aerosol (SOA) produced in smog chamber (SC) studies typically falls within the range of ambient semi-volatile oxygenated OA (SV-OOA), almost always showing lower degrees of oxygenation than ambient low-volatility oxygenated organic aerosol (LV-OOA) (Ng et al, 2010), with only a few exceptions (Bahreini et al, 2005; Chhabra et al, 2011)
The aim of this study is to find the main driving factors responsible for the inability of smog chamber studies to yield LV-OOA-like aerosol from the biogenic precursor α-pinene, even after the equivalent of tens of hours of atmospheric aging
Summary
Instrumentation carboxylic acids, vs. the fraction of C2H3O+, a surrogate for aldehydes, alcohols and ketones, as well as in the Van Krevelen space, where the evolution of the atomic hydrogen-. SOA produced in smog chamber (SC) studies typically falls within the range of ambient SV-OOA, almost always showing lower degrees of oxygenation than ambient LV-OOA (Ng et al, 2010), with only a few exceptions (Bahreini et al, 2005; Chhabra et al, 2011). LV-OOA-like aerosol was obtained in a smog chamber, but mostly by starting with oxygenated gas-phase precursors (Chhabra et al, 2011), whereas most primary volatile organic compound emissions are thought to be more hydrocarbon-like These results indicate that the location of SOA in the fpCO+2 -f C2H3O+ as well as in the Van Krevelen space is affected by precursor identity.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
