The House Observations of Microbial and Environmental Chemistry (HOMEChem) study was a large-scale collaborative experimental investigation probing indoor air composition and chemistry.

The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of toxic trace species, such as PAHs and persistent organic pollutants.

Motivated by the importance of the heterogeneous chemistry of squalene contained within skin oil to indoor air chemistry, the surface reaction of squalene with gas-phase ozone has been investigated. Using direct analysis in real time mass spectrometry (DART-MS) to monitor squalene, the reactive uptake coefficients were determined to be (4.3 ± 2.2) × 10 and (4.0 ± 2.2) × 10 for ozone mixing ratios (MR) of 50 and 25 ppb, respectively, on squalene films deposited on glass surfaces. At an MR of 25 ppb, the lifetime for oxidation was the same as that in an indoor office with an MR between 22 and 32 ppb, suggesting that O was the dominant oxidant in this indoor setting. While the heterogeneous kinetics of squalene and O were independent of relative humidity (RH), the RH significantly affected the reaction products. Under dry conditions (<5% RH), in addition to several products between m/z 300 and 350, the major condensed-phase end products were levulinic acid (LLA) and succinic acid (SCA). Under humid conditions (50% RH), the major end products were 4-oxopentanal, 4-oxobutanoic acid, and LLA. The molar yields of LLA and SCA were quantified as 230 ± 43% and 110 ± 31%, respectively, under dry conditions and 91 ± 15% and <5%, respectively, at 50% RH. Moreover, high-molecular weight (molecular weight of >450 Da) products were observed under dry conditions with indications that LLA was involved in their formation. The mechanism of squalene oxidation is discussed in light of these observations, with indications of an important role played by Criegee intermediates.

This study is focused on the relative humidity (RH) dependence of water-soluble secondary organic aerosol (SOA) aging by photolysis. Particles containing α-pinene SOA and ammonium sulfate, generated by atomization, were exposed to UV radiation in an environmental chamber at three RH conditions (5, 45, and 85%), and changes in chemical composition and mass were monitored using an aerosol mass spectrometer (AMS). Under all RH conditions, photolysis leads to substantial loss of SOA mass, where the rate of mass loss decreased with decreasing RH. For all RH conditions, the less oxidized components of SOA (e.g., carbonyls) exhibited the fastest photodegradation rates, which resulted in a more oxidized SOA after photolytic aging. The photolytic reactivity of SOA material exhibited a dependence on RH likely due to moisture-induced changes in SOA morphology or phase. The results suggest that the atmospheric lifetime of SOA with respect to photolysis is dependent on its RH cycle, and that photolysis may be an important sink for some SOA components occurring on an initial time scale of a few hours under ambient conditions.