Understanding mechanisms of formation, growth and physical properties of secondary organic aerosol (SOA) is central to predicting impacts on visibility, health and climate. It has been known for many decades that the oxidation of monoterpenes by ozone in the gas phase readily forms particles. However, the species responsible for the initial nucleation and the subsequent growth are not well established. Recent studies point to high molecular weight highly oxygenated products with extremely low vapor pressures (ELVOC, extremely low volatility organic compounds) as being responsible for the initial nucleation, with more volatile species contributing to particle growth. We report here the results of studies of SOA formed in the ozonolysis of α-pinene in air at 297 ± 2 K using atmospheric solids analysis probe (ASAP) mass spectrometry, attenuated total reflectance (ATR) Fourier transform infrared spectrometry and proton transfer reaction (PTR) mass spectrometry. Smaller particles are shown to be less volatile and have on average higher molecular mass components compared to larger particles, consistent with recent proposals regarding species responsible for the formation and growth of particles in this system. Thus the signatures of species responsible for particle development at various stages are observable even in particles of several hundred nm diameter. Pinonaldehyde and acetic acid were observed to evaporate from a film of impacted SOA at room temperature, from which the ratio of their diffusion coefficients to the square of the average film thickness, D/l(2), could be obtained. For acetic acid and pinonaldehyde, D/l(2) = 6.8 × 10(-6) s(-1) and 5.0 × 10(-6) s(-1) respectively, the relative magnitudes being consistent with the size difference between acetic acid and pinonaldehyde molecules. Limitations to quantifying the film thickness and hence absolute values of the diffusion coefficient are discussed and highlight a need for novel experimental methods for quantifying diffusion coefficients of organic species in SOA.

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 custom-designed 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.

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 custom-designed 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.