Phase State and Saturation Vapor Pressure of Submicron Particles of <i>meso</i>-Erythritol at Ambient Conditions

<i>meso</i>-Erythritol is a sugar alcohol identified in atmospheric aerosol particles. In this work, evaporation of submicron-sized particles of <i>meso</i>-erythritol was studied in a TDMA system including a laminar flow tube under dry conditions at five temperatures (278–308 K) and ambient pressure. A complex behavior was observed and attributed to the formation of particles of three different phase states: (1) crystalline, (2) subcooled liquid or amorphous, and (3) mixed. With respect to saturation vapor pressure, the subcooled liquid and amorphous states are treated to be the same. The particle phase state was linked to initial particle size and flow tube temperature. Saturation vapor pressures of two phase states attributed to the crystalline and subcooled liquid state respectively are reported. Our results suggest a mass accommodation coefficient close to one for both states

Influence of Ozone and Radical Chemistry on Limonene Organic Aerosol Production and Thermal Characteristics

Limonene has a strong tendency to form secondary organic aerosol (SOA) in the atmosphere and in indoor environments. Initial oxidation occurs mainly via ozone or OH radical chemistry. We studied the effect of O₃ concentrations with or without a OH radical scavenger (2-butanol) on the SOA mass and thermal characteristics using the Gothenburg Flow Reactor for Oxidation Studies at Low Temperatures and a volatility tandem differential mobility analyzer. The SOA mass using 15 ppb limonene was strongly dependent on O₃ concentrations and the presence of a scavenger. The SOA volatility in the presence of a scavenger decreased with increasing levels of O₃, whereas without a scavenger, there was no significant change. A chemical kinetic model was developed to simulate the observations using vapor pressure estimates for compounds that potentially contributed to SOA. The model showed that the product distribution was affected by changes in both OH and ozone concentrations, which partly explained the observed changes in volatility, but was strongly dependent on accurate vapor pressure estimation methods. The model–experiment comparison indicated a need to consider organic peroxides as important SOA constituents. The experimental findings could be explained by secondary condensed-phase ozone chemistry, which competes with OH radicals for the oxidation of primary unsaturated products

Parameterization of Thermal Properties of Aging Secondary Organic Aerosol Produced by Photo-Oxidation of Selected Terpene Mixtures

Formation and evolution of secondary organic aerosols (SOA) from biogenic VOCs influences the Earth’s radiative balance. We have examined the photo-oxidation and aging of boreal terpene mixtures in the SAPHIR simulation chamber. Changes in thermal properties and chemical composition, deduced from mass spectrometric measurements, were providing information on the aging of biogenic SOA produced under ambient solar conditions. Effects of precursor mixture, concentration, and photochemical oxidation levels (OH exposure) were evaluated. OH exposure was found to be the major driver in the long term photochemical transformations, i.e., reaction times of several hours up to days, of SOA and its thermal properties, whereas the initial concentrations and terpenoid mixtures had only minor influence. The volatility distributions were parametrized using a sigmoidal function to determine <i>T</i>_VFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. <i>T</i>_VFR0.5 increased by 0.3 ± 0.1% (ca. 1 K), while the steepness increased by 0.9 ± 0.3% per hour of 1 × 10⁶ cm^–3 OH exposure. Thus, aging reduces volatility and increases homogeneity of the vapor pressure distribution, presumably because highly volatile fractions become increasingly susceptible to gas phase oxidation, while less volatile fractions are less reactive with gas phase OH