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

Formation, ageing and thermal properties of secondary organic aerosol

In order to properly represent and predict the effects of aerosol in climate systems, an accurate description of their formation and properties is needed. This thesis describes work done to increase the knowledge of processes and properties of atmospherically relevant secondary organic aerosol (SOA) from both biogenic and anthropogenic origin. The common theme for these projects is the use of a Volatility Tandem Differential Mobility Analyser (VTDMA) setup, which in combination with other observations has generated insight into both detailed chemical mechanisms and physical processes that eventually could be suitable for testing in air quality or climate models. During the course of this work, the experimental facility the Gothenburg Flow Reactor for Oxidation Studies at low Temperatures (G-FROST) and the VTDMA setup, as well as a corresponding data evaluation methodology, have been improved and refined. Thermal properties could be linked to both formation and ageing processes of SOA. Using a VTDMA setup, where the thermal characteristics of SOA were measured at a range of evaporation temperatures, a sigmoidal fit to the data enabled parameterisation of their volatility properties. The parameters extracted were e.g. the temperature corresponding to a volume fraction remaining of 0.5 (TVFR0.5) and the slope factor (SVFR), which are measures of the general volatility and the volatility distribution of the condensed phase products, respectively. A higher TVFR0.5 indicates lower volatility, while an increase of SVFR states a broader distribution of vapour pressures. The response of these parameters from changes in experimental conditions could be linked to processes occurring both in the gaseous and the condensed phase. In photo-chemical experiments, the change in TVFR0.5 and SVFR could be described using the OH dose. The gas phase processes were found to be very important for SOA ageing, driven mainly by OH radical exposure in the outdoor chamber SAPHIR. However, processes in the condensed phase, such as plausible non oxidative ageing processes and non-liquid behaviour of SOA particles, were also observed. Detailed studies of ozonolysis of the boreal forest monoterpenes β-pinene and limonene were enabled by precise control of reaction conditions using the G-FROST. The experimental findings in response to e.g. water and radical conditions emphasized the difference in ozonolysis reaction paths between endo- and exocyclic compounds. The results support the recently suggested decomposition of the stabilized Criegee Intermediate via the hydroperoxide channel in ozonolysis of β-pinene

A reference data set for validating vapor pressure measurement techniques : homologous series of polyethylene glycols

To predict atmospheric partitioning of organic compounds between gas and aerosol particle phase based on explicit models for gas phase chemistry, saturation vapor pressures of the compounds need to be estimated. Estimation methods based on functional group contributions require training sets of compounds with well-established saturation vapor pressures. However, vapor pressures of semivolatile and low-volatility organic molecules at atmospheric temperatures reported in the literature often differ by several orders of magnitude between measurement techniques. These discrepancies exceed the stated uncertainty of each technique which is generally reported to be smaller than a factor of 2. At present, there is no general reference technique for measuring saturation vapor pressures of atmospherically relevant compounds with low vapor pressures at atmospheric temperatures. To address this problem, we measured vapor pressures with different techniques over a wide temperature range for intercomparison and to establish a reliable training set. We determined saturation vapor pressures for the homologous series of polyethylene glycols (H-(O-CH2-CH2)(n)-OH) for n = 3 to n = 8 ranging in vapor pressure at 298 K from 10(-7) to 5 x 10(-2) Pa and compare them with quantum chemistry calculations. Such a homologous series provides a reference set that covers several orders of magnitude in saturation vapor pressure, allowing a critical assessment of the lower limits of detection of vapor pressures for the different techniques as well as permitting the identification of potential sources of systematic error. Also, internal consistency within the series allows outlying data to be rejected more easily. Most of the measured vapor pressures agreed within the stated uncertainty range. Deviations mostly occurred for vapor pressure values approaching the lower detection limit of a technique. The good agreement between the measurement techniques (some of which are sensitive to the mass accommodation coefficient and some not) suggests that the mass accommodation coefficients of the studied compounds are close to unity. The quantum chemistry calculations were about 1 order of magnitude higher than the measurements. We find that extrapolation of vapor pressures from elevated to atmospheric temperatures is permissible over a range of about 100 K for these compounds, suggesting that measurements should be performed best at temperatures yielding the highest-accuracy data, allowing subsequent extrapolation to atmospheric temperatures.Peer reviewe

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 TVFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. TVFR0.5 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

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 TVFR0.5 (the temperature yielding a 50% particle volume fraction remaining) and the steepness of the volatility distribution. TVFR0.5 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

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