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.
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. Several experiments with exclusively anthropogenic 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 (0.5–9%) 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 anthropogenic SOA enabled application of a simplified model to calculate the chemical turnover of the anthropogenic 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 at 343 K: 0.86–0.94). The anthropogenic 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 anthropogenic SOA the reaction mixtures needed a higher OH dose that also increased O/C and provided a less volatile aerosol. 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. This change in volatility under dark conditions is likely 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 − CH2 − CH2)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 volatility of secondary organic aerosol (SOA) from the oxidation of mixtures of biogenic Volatile Organic Compounds (VOC) has been investigated in the SAPHIR facility in Forschungszentrum Julich, Germany, by using a Volatility Tandem Differential Mobility Analyser (VTDMA). The standard VTDMA setup comprises three main parts (Jonsson et al. 2007): 1) An initial DMA, where a nearly monodispers size fraction of the aerosol particles is selected (typically 100 or 150 nm), 2) the oven unit, i.e. four ovens in parallel where each oven includes a heating and adsorption section where the evaporation and adsorption of the volatile fraction occurs and 3) a final SMPS (Scanning Mobility Particle Sizer) system where the residual particle number distribution is measured. For this measurement campaign the set-up also contained a Quadrupol Aerosol Mass Spectrometer (Aerodyne QAMS, Jayne et. al, 2000). The temperature of the ovens can be varied between 298 and well above 573 K. In parallel to the final SMPS the AMS was used for chemical composition and density measurements. When the system was dedicated for AMS measurements the initial DMA was bypassed to improve the aerosol concentration. However, the produced SOA has a narrow size distribution still making it possible to follow small changes in the aerosol peak diameter. A general feature of the thermo-denuder system is that a less volatile SOA gives a larger residual particle size distribution compared to more volatile SOA. The experiments conduced were based on photochemical oxidation of a reference boreal mixture (BMT) of terpenes, consisting of α-pinene, β-pinene, limonene, ∆-carene, and ocimene. In further studies, the sesquiterpenes β-caryophyllene and α-farnesene were added (BMT+SQT). Secondary organic aerosol was formed from the precursor compounds by reaction with O3/H2O/OH in SAPHIR on the first day. The particles were kept in the chamber for up to two further days and were exposed to natural sunlight and OH radicals to initiate close to natural chemical aging. The VTDMA results show that SOA becomes less volatile during ageing and this ageing was further enhanced when the mixtures were exposed to sunlight. The volatility, measured as normalised mode particle diameter (NMDp), was also affected by changes in the terpene mixtures (Figure 1). With the AMS we measured mass spectra of the organic aerosol particles at a reference temperature of 298 K and two additional elevated temperatures. Size distributions of the particles were obtained from the particle-time-of-flight mode of the AMS at sixteen representative m/z values. The residual total mass measured at the elevated oven temperatures was related to the total mass at the reference temperature to obtain the mass fraction remaining (MFR), which is higher for less volatile SOA. In agreement with the decreasing volatility during aging measured with the VTDMA, the MFR increases with time. An effective density of the particles was calculated comparing the mode position of the size distributions measured with the AMS and the SMPS. The effective density increases with ongoing photochemical ageing. In addition, the density of the low volatile residual particles that passed a high-temperature oven is higher than the density of particles at reference temperature. In order to investigate if the observed changes in density and volatility can be attributed to changes of the chemical composition of the particles, the mass spectra obtained at different oven temperatures and different chemical age were compared. We found that the ratio of heavy fragments (m/z > 90) increases with higher temperatures. Furthermore the fraction of the CO2 fragment at m/z 44 to the total mass increases during the ageing process.
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 semi- and low volatile 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 two. 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 Pa 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 to reject outlying data more easily. Most of the measured vapor pressures agreed within the stated uncertainty range. Deviations mostly occurred for vapor pressures 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) suggest that the mass accommodation coefficients of the studied compounds are close to unity. The quantum chemistry calculations were about one 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.
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.
The influence of water and radicals on SOAs produced by {\beta}-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 parameterization of its volatility properties. The parameters extracted included the temperature at which 50% of the aerosol had evaporated (TVFR0.5) and the slope factor (SVFR). An increase in SVFR indicates a broader distribution of vapor pressures for the aerosol constituents. Reducing the reaction temperature increased SVFR and decreased TVFR0.5 under humid conditions but had less effect on TVFR0.5 under dry conditions. In general, higher water concentrations gave lower TVFR0.5 values, more negative SVFR 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. Presence of a scavenger and lower [HO2]/[RO2] ratio reduced SOA production. Observed changes in SVFR 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.
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