Integrating phase and composition of secondary organic aerosol from the ozonolysis of α-pinene

Abstract

Airborne particles are important for public health, visibility, and climate. Predicting their concentrations, effects, and responses to control strategies requires accurate models of their formation and growth in air. This is challenging, as a large fraction is formed by complex reactions of volatile organic compounds, generating secondary organic aerosol (SOA), which grows to sizes important for visibility, climate, and deposition in the lung. Growth of SOA is particularly sensitive to the phase/viscosity of the particles and remains poorly understood. We report studies using a customdesigned impactor with a germanium crystal as the impaction surface to study SOA formed from the ozonolysis of α-pinene at relative humidities (RHs) up to 87% at 297 ± 2 K (which corresponds to a maximum RH of 70-86% inside the impactor). The impaction patterns provide insight into changes in phase/viscosity as a function of RH. Attenuated total reflectance-Fourier transform infrared spectroscopy and aerosol mass spectrometry provide simultaneous information on composition changes with RH. The results show that as the RH at which the SOA is formed increases, there is a decrease in viscosity, accompanied by an increasing contribution from carboxylic acids and a decreasing contribution from higher molecular mass products. In contrast, SOA that is formed dry and subsequently humidified remains solid to high RH. The results of these studies have significant implications for modeling the growth, aging, and ultimately, lifetime of SOA in the atmosphere.aerosol phase | SOA viscosity | oligomers | SOA composition | particle bounce A irborne particles negatively affect health (1) and visibility (2-4), and their effects on climate are substantial (2,3,(5)(6)(7). A large portion of atmospheric particulate matter comprises lowvolatility organics formed during gas phase oxidation of hydrocarbons, including biogenics. This organic contribution, which is typically more than 50% of the total particle mass (8-10), is often underpredicted by models. Although recent measurementmodel comparisons are in closer agreement (11-15), challenges remain because of the complexity of SOA. For example, despite improvements in modeled mass loading, there remain issues when trying to predict specific characteristics such as oxygen-tocarbon ratio and volatility (16) simultaneously.It is vital that models and measurements are in agreement for the right reasons to have robust predictive capability. A potential contributor to the remaining discrepancies could be the assumptions incorporated into the models. Specifically, SOA has been generally considered to be a low-viscosity liquid that equilibrates with the surrounding gases, with no condensed phase diffusion limitations on the scale of the time steps used in the models (17,18). However, there is recent evidence for solid or semisolid phases of SOA, such as much slower evaporation than expected (19,20), SOA composition that is consistent with irreversible condensational growth (21), viscosity measurements (22), time...

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…9 shows a photograph of a typical impaction pattern of trans-3-hexene SOA on a Ge ATR crystal. For comparison, the impaction pattern of SOA formed from a-pinene ozonolysis obtained in a previous study 54 in our lab using the same impactor is also shown. The trans-3-hexene SOA flows away from the centerline that is the initial area of impact on the ATR crystal.…”

“…48,51 In the present study, particle impaction/ATR-FTIR measurements were performed to offer some insight into the phase/viscosity of trans-3-hexene SOA since the impaction patterns on the Ge crystal have been shown to be an indicator of phase. 54 For example, viscous liquid particles hit and stick to the crystal to form a row of spots, whereas solid particles above a certain size bounce and can be subsequently captured on the crystal, forming a disperse pattern dubbed ''clouds''. 54 Fig.…”

“…54 For example, viscous liquid particles hit and stick to the crystal to form a row of spots, whereas solid particles above a certain size bounce and can be subsequently captured on the crystal, forming a disperse pattern dubbed ''clouds''. 54 Fig. 9 shows a photograph of a typical impaction pattern of trans-3-hexene SOA on a Ge ATR crystal.…”

“…We report here an expanded investigation of the ozonolysis of trans-3-hexene in which particle composition is probed as a function of size. In addition, the influence of water vapor, OH and SCI scavengers, as well as the phase state of SOA particles (which plays critical roles in various physical and chemical processes of aerosols) [47][48][49][50][51][52][53][54][55][56] are elucidated and possible mechanisms are examined in light of predictions of a box model for this system. Initial studies of the ozonolysis of the much larger and complex biogenic alkene a-cedrene are also reported, and it is shown that the mechanism of particle formation for this sesquiterpene is different from that for trans-3-hexene.…”

Dynamic viscosity mapping of the oxidation of squalene aerosol particles

“…An aerosol's viscous state also has direct implications on the particle morphology, phase behaviour, optical properties, aging processes and lifetime. At the same time, only few direct measurement methods are available for monitoring such properties 1, [6][7][8][9][10][11][12][13][14][15][16] . Recently, we have demonstrated that viscosity of model organic aerosols, such as oleic acid droplets, and secondary organic aerosols (SOA) produced from the oxidation of myrcene or -pinene can be measured and imaged in a spatially and time-resolved manner using Fluorescence Lifetime Imaging Microscopy (FLIM) of molecular rotors 17 .…”