Abstract
Abstract. Secondary organic aerosol (SOA) particle formation ranks among the least understood chemical processes in the atmosphere, rooted in part in the lack of knowledge about chemical composition and structure at the particle surface, and little availability of reference compounds needed for benchmarking and chemical identification in pure and homogenous form. Here, we synthesize and characterize SOA particle constituents consisting of the isoprene oxidation products α-, δ-, and cis- and trans-β-IEPOX (isoprene epoxide), as well as syn- and anti-2-methyltetraol. Paying particular attention to their phase state (condensed vs. vapor), we carry out a surface-specific and orientationally selective chemical analysis by vibrational sum frequency generation (SFG) spectroscopy of these compounds in contact with a fused silica window. Comparison to the vibrational SFG spectra of synthetic isoprene-derived SOA particle material prepared at the Harvard Environmental Chamber yields a plausible match with trans-β-IEPOX, suggesting it is an abundant species on their surfaces, while the other species studied here, if present, appear to be SFG inactive and thus likely to be localized in a centrosymmetric environment, e.g., the particle bulk. No match is found for authentic SOA particle material collected at the site of the Amazonian Aerosol Characterization Experiment (AMAZE-08) with the surface SFG spectra of the compounds surveyed here, yet we cannot rule out this mismatch being attributable to differences in molecular orientation. The implications of our findings for SOA formation are discussed in the context of condensational particle growth and reactivity.
Highlights
Secondary organic aerosol (SOA) particles are important in the climate system as they can lead to significant negative radiative forcing, especially over the world’s large forest ecosystems (Carlton et al, 2009; Kanakidou et al, 2005; Williams et al, 2011)
We compare the sum frequency generation (SFG) spectra obtained from the epoxides and tetraols prepared here to those obtained from aerosol particles synthesized as a model system from the photochemical reaction of isoprene and OH radicals under low-NO conditions at the Harvard Environmental Chamber (HEC) (Chen et al, 2011; Ebben et al, 2011a), as well as from submicron-sized SOA particles collected in the central Amazon Basin during the Amazonian Aerosol Characterization Experiment (AMAZE-08) campaign (Martin et al, 2010a), chosen as an example of a tropical forest whose air is typically rich in isoprene
The approach for our analysis of SOA particle material prepared at the HEC and collected at the AMAZE-08 field site by SFG has been described in detail in our prior work (Ebben et al, 2012)
Summary
Secondary organic aerosol (SOA) particles are important in the climate system as they can lead to significant negative radiative forcing, especially over the world’s large forest ecosystems (Carlton et al, 2009; Kanakidou et al, 2005; Williams et al, 2011). Molecular studies (Claeys et al, 2009; Constantinescu et al, 2007; Docherty et al, 2005; Gao et al, 2010; Heaton et al, 2009; Kalberer et al, 2004; Kroll and Seinfeld, 2008; Lin et al, 2012; Müller et al, 2008; Tolocka et al, 2004; Yasmeen et al, 2010) aimed at bridging this knowledge gap support the hypothesis that the products formed from the gas-phase oxidization of biogenic volatile organic compounds react with one another or undergo further oxidation to form species with lower and lower vapor pressures, leading to SOA particle formation Heterogeneous processes such as physisorption and surface reactions are important for conditions of condensational particle growth.
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