Influence of seed aerosol surface area and oxidation rate on vapor wall deposition and SOA mass yields: a case study with α-pinene ozonolysis

Laboratory chambers, invaluable in atmospheric chemistry and aerosol formation studies, are subject to particle and vapor wall deposition, processes that need to be accounted for in order to accurately determine secondary organic aerosol (SOA) mass yields. Although particle wall deposition is reasonably well understood and usually accounted for, vapor wall deposition is less so. The effects of vapor wall deposition on SOA mass yields in chamber experiments can be constrained experimentally by increasing the seed aerosol surface area to promote the preferential condensation of SOA-forming vapors onto seed aerosol. Here, we study the influence of seed aerosol surface area and oxidation rate on SOA formation in α-pinene ozonolysis. The observations are analyzed using a coupled vapor–particle dynamics model to interpret the roles of gas–particle partitioning (quasi-equilibrium vs. kinetically limited SOA growth) and α-pinene oxidation rate in influencing vapor wall deposition. We find that the SOA growth rate and mass yields are independent of seed surface area within the range of seed surface area concentrations used in this study. This behavior arises when the condensation of SOA-forming vapors is dominated by quasi-equilibrium growth. Faster α-pinene oxidation rates and higher SOA mass yields are observed at increasing O3 concentrations for the same initial α-pinene concentration. When the α-pinene oxidation rate increases relative to vapor wall deposition, rapidly produced SOA-forming oxidation products condense more readily onto seed aerosol particles, resulting in higher SOA mass yields. Our results indicate that the extent to which vapor wall deposition affects SOA mass yields depends on the particular volatility organic compound system and can be mitigated through the use of excess oxidant concentrations

Constraining uncertainties in particle-wall deposition correction during SOA formation in chamber experiments

The effect of vapor-wall deposition on secondary organic aerosol (SOA) formation has gained significant attention; however, uncertainties in experimentally derived SOA mass yields due to uncertainties in particle-wall deposition remain. Different approaches have been used to correct for particle-wall deposition in SOA formation studies, each having its own set of assumptions in determining the particle-wall loss rate. In volatile and intermediate-volatility organic compound (VOC and IVOC) systems in which SOA formation is governed by kinetically limited growth, the effect of vapor-wall deposition on SOA mass yields can be constrained by using high surface area concentrations of seed aerosol to promote the condensation of SOA-forming vapors onto seed aerosol instead of the chamber walls. However, under such high seed aerosol levels, the presence of significant coagulation may complicate the particle-wall deposition correction. Here, we present a model framework that accounts for coagulation in chamber studies in which high seed aerosol surface area concentrations are used. For the α-pinene ozonolysis system, we find that after accounting for coagulation, SOA mass yields remain approximately constant when high seed aerosol surface area concentrations ( ≥  8000 µm^2 cm^(−3)) are used, consistent with our prior study (Nah et al., 2016) showing that α-pinene ozonolysis SOA formation is governed by quasi-equilibrium growth. In addition, we systematically assess the uncertainties in the calculated SOA mass concentrations and yields between four different particle-wall loss correction methods over the series of α-pinene ozonolysis experiments. At low seed aerosol surface area concentrations (< 3000 µm^2 cm^(−3)), the SOA mass yields at peak SOA growth obtained from the particle-wall loss correction methods agree within 14 %. However, at high seed aerosol surface area concentrations ( ≥  8000 µm^2 cm^(−3)), the SOA mass yields at peak SOA growth obtained from different particle-wall loss correction methods can differ by as much as 58 %. These differences arise from assumptions made in the particle-wall loss correction regarding the first-order particle-wall loss rate. This study highlights the importance of accounting for particle-wall deposition accurately during SOA formation chamber experiments and assessing the uncertainties associated with the application of the particle-wall deposition correction method when comparing and using SOA mass yields measured in different studies

Influence of seed aerosol surface area and oxidation rate on vapor wall deposition and SOA mass yields: a case study with α-pinene ozonolysis

Laboratory chambers, invaluable in atmospheric chemistry and aerosol formation studies, are subject to particle and vapor wall deposition, processes that need to be accounted for in order to accurately determine secondary organic aerosol (SOA) mass yields. Although particle wall deposition is reasonably well understood and usually accounted for, vapor wall deposition is less so. The effects of vapor wall deposition on SOA mass yields in chamber experiments can be constrained experimentally by increasing the seed aerosol surface area to promote the preferential condensation of SOA-forming vapors onto seed aerosol. Here, we study the influence of seed aerosol surface area and oxidation rate on SOA formation in α-pinene ozonolysis. The observations are analyzed using a coupled vapor–particle dynamics model to interpret the roles of gas–particle partitioning (quasi-equilibrium vs. kinetically limited SOA growth) and α-pinene oxidation rate in influencing vapor wall deposition. We find that the SOA growth rate and mass yields are independent of seed surface area within the range of seed surface area concentrations used in this study. This behavior arises when the condensation of SOA-forming vapors is dominated by quasi-equilibrium growth. Faster α-pinene oxidation rates and higher SOA mass yields are observed at increasing O3 concentrations for the same initial α-pinene concentration. When the α-pinene oxidation rate increases relative to vapor wall deposition, rapidly produced SOA-forming oxidation products condense more readily onto seed aerosol particles, resulting in higher SOA mass yields. Our results indicate that the extent to which vapor wall deposition affects SOA mass yields depends on the particular volatility organic compound system and can be mitigated through the use of excess oxidant concentrations

Chemical characterization of secondary organic aerosol at a rural site in the southeastern US: insights from simultaneous high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO chemical ionization mass spectrometer (CIMS) measurements

The formation and evolution of secondary organic aerosol (SOA) were investigated at Yorkville, GA, in late summer (mid-August to mid-October 2016). The organic aerosol (OA) composition was measured using two online mass spectrometry instruments, the high-resolution time-of-flight aerosol mass spectrometer (AMS) and the Filter Inlet for Gases and AEROsols coupled to a high-resolution time-of-flight iodide-adduct chemical ionization mass spectrometer (FIGAERO-CIMS). Through analysis of speciated organics data from FIGAERO-CIMS and factorization analysis of data obtained from both instruments, we observed notable SOA formation from isoprene and monoterpenes during both day and night. Specifically, in addition to isoprene epoxydiol (IEPOX) uptake, we identified isoprene SOA formation from non-IEPOX pathways and isoprene organic nitrate formation via photooxidation in the presence of NO_x and nitrate radical oxidation. Monoterpenes were found to be the most important SOA precursors at night. We observed significant contributions from highly oxidized acid-like compounds to the aged OA factor from FIGAERO-CIMS. Taken together, our results showed that FIGAERO-CIMS measurements are highly complementary to the extensively used AMS factorization analysis, and together they provide more comprehensive insights into OA sources and composition

Chemical characterization of secondary organic aerosol at a rural site in the southeastern US: insights from simultaneous high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and FIGAERO chemical ionization mass spectrometer (CIMS) measurements

The formation and evolution of secondary organic aerosol (SOA) were investigated at Yorkville, GA, in late summer (mid-August to mid-October 2016). The organic aerosol (OA) composition was measured using two online mass spectrometry instruments, the high-resolution time-of-flight aerosol mass spectrometer (AMS) and the Filter Inlet for Gases and AEROsols coupled to a high-resolution time-of-flight iodide-adduct chemical ionization mass spectrometer (FIGAERO-CIMS). Through analysis of speciated organics data from FIGAERO-CIMS and factorization analysis of data obtained from both instruments, we observed notable SOA formation from isoprene and monoterpenes during both day and night. Specifically, in addition to isoprene epoxydiol (IEPOX) uptake, we identified isoprene SOA formation from non-IEPOX pathways and isoprene organic nitrate formation via photooxidation in the presence of NO_x and nitrate radical oxidation. Monoterpenes were found to be the most important SOA precursors at night. We observed significant contributions from highly oxidized acid-like compounds to the aged OA factor from FIGAERO-CIMS. Taken together, our results showed that FIGAERO-CIMS measurements are highly complementary to the extensively used AMS factorization analysis, and together they provide more comprehensive insights into OA sources and composition

Chemical Kinetics and Mechanisms of Unsaturated Organic Aerosol Oxidation

Understanding the heterogeneous oxidation of organic particulate matter ("aerosol") is an active area of current research in atmospheric and combustion chemistry. The chemical evolution of organic aerosol is complex and dynamic since it can undergo multiple oxidation reactions with gas phase oxidants to form a mixture of different generations of oxidation products that control the average aerosol mass and volatility. In many of these systems, hydrocarbon free radicals, formed by reaction with gas phase oxidants, play key roles as initiators, propagators and terminators of surface reactions. This dissertation presents a detailed study of the reaction kinetics and mechanisms of the heterogeneous oxidation of unsaturated organic aerosol, and aims to provide new molecular and mechanistic insights into the reaction pathways in heterogeneous organic aerosol oxidation.The heterogeneous oxidation of unsaturated fatty acid (oleic acid C18H34O2, linoleic acid C18H32O2 and linolenic acid C18H30O2) aerosol by hydroxyl (OH) radicals is first studied in Chapter 2 to explore how surface OH addition reactions initiate chain reactions that rapidly transform the chemical composition of unsaturated organic aerosol. Oleic acid, linoleic acid and linolenic acid have the same linear C18 carbon backbone structure with one, two and three C=C double bonds, respectively. By studying carboxylic acids with different numbers of C=C double bonds, the role that multiple reactive sites plays in controlling reaction rates can be observed. The kinetic parameter of interest in these studies is the effective uptake coefficient, defined as the number of particle phase unsaturated fatty acid molecules reacted per OH-particle collision. The effective uptake coefficients for the unsaturated fatty acids are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficients for the unsaturated fatty acids decrease with increasing O2 concentration, indicating that O2 promotes chain termination in the unsaturated fatty acid reactions. The kinetics and products of squalene (a C30 branched alkene with 6 C=C double bonds) oxidation are compared to that of the unsaturated fatty acids in Chapters 3 and 4 to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. The squalene effective uptake coefficient, which is also larger than one, is smaller than that of linoleic acid and linolenic acid despite the larger number of C=C double bonds in squalene. In contrast to the unsaturated fatty acids, the squalene effective uptake coefficient increases with O2 concentration, indicating that O2 promotes chain propagation in the squalene reaction. Elemental and product analysis of squalene aerosol shows that O2 promotes particle volatilization in the squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, elemental and product analysis of linoleic acid aerosol shows that O2 does not influence the rate of particle volatilization in the linoleic acid reaction, suggesting that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol. Lastly, depending on the aerosol phase (e.g. solid and semi-solid) and the timescale for homogeneous mixing within the aerosol particle, the chemical composition may vary spatially within an aerosol particle. This necessitates the need for new techniques to characterize the interfacial chemical composition of aerosol particles. In the last portion of the dissertation, direct analysis in real time mass spectrometry (DART-MS) is used to analyze the surface chemical composition of nanometer-sized organic aerosol particles in real time at atmospheric pressure. By introducing a stream of aerosol particles in between the DART ionization source and the atmospheric pressure inlet of the mass spectrometer, the aerosol particles are exposed to a thermal flow of helium or nitrogen gas containing some fraction of metastable helium atoms or nitrogen molecules. In this configuration, the molecular constituents of organic aerosol particles are desorbed, ionized and detected with reduced molecular ion fragmentation, allowing for compositional identification. The reaction of ozone with sub-micron oleic acid particles is also measured to demonstrate the ability of DART-MS to identify products and quantify reaction rates in a heterogeneous reaction