Introduction 4116 2. Theoretical Background and Framework for Atmospheric Aerosols 4117 2.1. Saturation Vapor Pressures 4117 2.2. Vapor−Liquid or Vapor−Solid Equilibria over Mixed Solutions 4118 2.3. Equilibria over Curved Surfaces 4118 2.4. Dynamic Evaporation and Condensation from and to an Aerosol Particle 4119 2.5. Ambient Partitioning 4120 3. Experimental Methods 4120 3.1. Knudsen-Cell-Based Methods 4121 3.1.1. Knudsen Mass Loss Methods 4121 3.

Abstract. Secondary organic aerosol (SOA) formation from mixed anthropogenic and biogenic precursors has been studied exposing reaction mixtures to natural sunlight in the SAPHIR chamber in Jülich, Germany. In this study aromatic compounds served as examples of anthropogenic volatile organic compound (VOC) and a mixture of α-pinene and limonene as an example for biogenic VOC. Several experiments with exclusively aromatic precursors were performed to establish a relationship between yield and organic aerosol mass loading for the atmospheric relevant range of aerosol loads of 0.01 to 10 μg m−3. The yields (0.5 to 9%) were comparable to previous data and further used for the detailed evaluation of the mixed biogenic and anthropogenic experiments. For the mixed experiments a number of different oxidation schemes were addressed. The reactivity, the sequence of addition, and the amount of the precursors influenced the SOA properties. Monoterpene oxidation products, including carboxylic acids and dimer esters were identified in the aged aerosol at levels comparable to ambient air. OH radicals were measured by Laser Induced Fluorescence, which allowed for establishing relations of aerosol properties and composition to the experimental OH dose. Furthermore, the OH measurements in combination with the derived yields for aromatic SOA enabled application of a simplified model to calculate the chemical turnover of the aromatic precursor and corresponding anthropogenic contribution to the mixed aerosol. The estimated anthropogenic contributions were ranging from small (≈8%) up to significant fraction (>50%) providing a suitable range to study the effect of aerosol composition on the aerosol volatility (volume fraction remaining (VFR) at 343 K: 0.86–0.94). The aromatic aerosol had higher oxygen to carbon ratio O/C and was less volatile than the biogenic fraction. However, in order to produce significant amount of aromatic SOA the reaction mixtures needed a higher OH dose that also increased O/C and provided a less volatile aerosol. The SOA yields, O/C, and f44 (the mass fraction of CO2+ ions in the mass spectra which can be considered as a measure of carboxylic groups) in the mixed photo-chemical experiments could be described as linear combinations of the corresponding properties of the pure systems. For VFR there was in addition an enhancement effect, making the mixed aerosol significantly less volatile than what could be predicted from the pure systems. A strong positive correlation was found between changes in volatility and O/C with the exception during dark hours where the SOA volatility decreased while O/C did not change significantly. Thus, this change in volatility under dark conditions as well as the anthropogenic enhancement is due to chemical or morphological changes not affecting O/C.

Abstract. 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−CH 2 −CH 2 ) n −OH) for n = 3 to n = 8 ranging in vapor pressure at 298 K from 10 −7 to 5 × 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.

The influence of water and radicals on SOAs produced by β-pinene ozonolysis was investigated at 298 and 288 K using a laminar flow reactor. A volatility tandem differential mobility analyzer (VTDMA) was used to measure the evaporation of the SOA, enabling the parametrization of its volatility properties. The parameters extracted included the temperature at which 50% of the aerosol had evaporated (T(VFR0.5)) and the slope factor (S(VFR)). An increase in S(VFR) indicates a broader distribution of vapor pressures for the aerosol constituents. Reducing the reaction temperature increased S(VFR) and decreased T(VFR0.5) under humid conditions but had less effect on T(VFR0.5) under dry conditions. In general, higher water concentrations gave lower T(VFR0.5) values, more negative S(VFR) values, and a reduction in total SOA production. The radical conditions were changed by introducing OH scavengers to generate systems with and without OH radicals and with different [HO2]/[RO2] ratios. The presence of a scavenger and lower [HO2]/[RO2] ratio reduced SOA production. Observed changes in S(VFR) values could be linked to the more complex chemistry that occurs in the absence of a scavenger and indicated that additional HO2 chemistry gives products with a wider range of vapor pressures. Updates to existing ozonolysis mechanisms with routes that describe the observed responses to water and radical conditions for monoterpenes with endocyclic and exocyclic double bonds are discussed.

BILDE, M.; BARSANTI, K.; BOOTH, M.; CAPPA, C. D.; DONAHUE, N. M.; EMANUELSSON, E. U.; MCFIGGANS, G.; KRIEGER, U. K.; MARCOLLI, C.; TOPPING, D.; et al.; Chem. Rev. (Washington, DC, U. S.) 115 (2015) 10, 4115-4156, http://dx.doi.org/10.1021/cr5005502 ; Dep. Chem., Univ. Aarhus, DK-8000 Aarhus, Den.; Eng.) -Koehler 28-326