The oxidation of isoprene is a globally significant source of secondary organic material (SOM) of atmospheric particles. The relative importance of different parallel pathways, however, remains inadequately understood and quantified. SOM production from isoprene photooxidation was studied under hydroperoxyl-dominant conditions for <5% relative humidity and at 20 °C in the presence of highly acidic to completely neutralized sulfate particles. Isoprene photooxidation was separated from SOM production by using two continuously mixed flow reactors connected in series and operated at steady state. Two online mass spectrometers separately sampled the gas and particle phases in the reactor outflow. The loss of specific gas-phase species as contributors to the production of SOM was thereby quantified. The produced SOM mass concentration was directly proportional to the loss of isoprene epoxydiol (IEPOX) isomers from the gas phase. IEPOX isomers lost from the gas phase accounted for (46 ± 11)% of the produced SOM mass concentration. The IEPOX isomers comprised (59 ± 21)% (molecular count) of the loss of monitored gas-phase species. The implication is that for the investigated reaction conditions the SOM production pathways tied to IEPOX isomers accounted for half of the SOM mass concentration.

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.

The development of an efficient oxidative [2,3]-sigmatropic rearrangement of allylic hydrazides, via singlet N-nitrene intermediates, is reported. The requisite allylic hydrazide precursors are readily prepared and undergo smooth sigmatropic rearrangement upon exposure to iodosobenzene. The products of this novel transformation are shown to be useful precursors to a variety of compounds.

This study aims to reliably assign the vibrational sum frequency generation (SFG) spectrum of α-pinene at the vapor/solid interface using a method involving deuteration of various methyl groups. The synthesis of five deuterated isotopologues of α-pinene is presented to determine the impact that removing contributions from methyl group C-H oscillators has on its SFG response. 0.6 cm(-1) resolution SFG spectra of these isotopologues show varying degrees of differences in the C-H stretching region when compared to the SFG response of unlabeled α-pinene. The largest spectral changes were observed for the isotopologue containing a fully deuterated vinyl methyl group. Noticeable losses in signal intensities allow us to reliably assign the 2860 cm(-1) peak to the vinyl methyl symmetric stretch. Furthermore, upon removing the vinyl methyl group entirely by synthesizing apopinene, the steric influence of the unlabeled C9H14 fragment on the SFG response of α-pinene SFG can be readily observed. The work presented here brings us one step closer to understanding the vibrational spectroscopy of α-pinene.

Abstract. Secondary organic aerosol (SOA) particles, formed from gas-phase biogenic volatile organic compounds (BVOCs), contribute large uncertainties to the radiative forcing that is associated with aerosols in the climate system. Reactive uptake of surface-active organic oxidation products of BVOCs at the gas–aerosol interface can potentially decrease the overall aerosol surface tension and therefore influence their propensity to act as cloud condensation nuclei (CCN). Here, we synthesize and measure some climate-relevant physical properties of 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. Following viscosity measurements, we use octanol–water partition coefficients to quantify the relative hydrophobicity of the oxidation products while dynamic surface tension measurements indicate that aqueous solutions of α- and trans-β-IEPOX exhibit significant surface tension depression. We hypothesize that the surface activity of these compounds may enhance aerosol CCN activity, and that trans-β-IEPOX may be highly relevant for surface chemistry of aerosol particles relative to other IEPOX isomers.