Measurements of Isoprene-Derived Organosulfates in Ambient Aerosols by Aerosol Time-of-Flight Mass Spectrometry—Part 2: Temporal Variability and Formation Mechanisms

Organosulfate species have recently gained attention for their potentially significant contribution to secondary organic aerosol (SOA); however, their temporal behavior in the ambient atmosphere has not been probed in detail. In this work, organosulfates derived from isoprene were observed in single particle mass spectra in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Real-time measurements revealed that the highest organosulfate concentrations occurred at night under a stable boundary layer, suggesting gas-to-particle partitioning and subsequent aqueous-phase processing of the organic precursors played key roles in their formation. Further analysis of the diurnal profile suggests possible contributions from multiple production mechanisms, including acid-catalysis and radical-initiation. This work highlights the potential for additional SOA formation pathways in biogenically influenced urban regions to enhance the organic aerosol burden

Measurements of Isoprene-Derived Organosulfates in Ambient Aerosols by Aerosol Time-of-Flight Mass Spectrometry - Part 1: Single Particle Atmospheric Observations in Atlanta

Organosulfate species have recently been identified as a potentially significant class of secondary organic aerosol (SOA) species, yet little is known about their behavior in the atmosphere. In this work, organosulfates were observed in individual ambient aerosols using single particle mass spectrometry in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Organosulfates derived from biogenically produced isoprene were detected as deprotonated molecular ions in negative-ion spectra measured by aerosol time-of-flight mass spectrometry; comparison to high-resolution mass spectrometry data obtained from filter samples corroborated the peak assignments. The size-resolved chemical composition measurements revealed that organosulfate species were mostly detected in submicrometer aerosols and across a range of aerosols from different sources, consistent with secondary reaction products. Detection of organosulfates in a large fraction of negative-ion ambient spectra − ca. 90−95% during ANARChE and ~65% of submicrometer particles in AMIGAS − highlights the ubiquity of organosulfate species in the ambient aerosols of biogenically influenced urban environments

Characterization and Quantification of Isoprene-Derived Epoxydiols in Ambient Aerosol in the Southeastern United States

Isoprene-derived epoxydiols (IEPOX) are identified in ambient aerosol samples for the first time, together with other previously identified isoprene tracers (i.e., 2-methyltetrols, 2-methylglyceric acid, C5-alkenetriols, and organosulfate derivatives of 2-methyltetrols). Fine ambient aerosol collected in downtown Atlanta, GA and rural Yorkville, GA during the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS) was analyzed using both gas chromatography/quadrupole mass spectrometry (GC/MS) and gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) with prior trimethylsilylation. Mass concentrations of IEPOX ranged from ~1 to 24 ng m^(−3) in the aerosol collected from the two sites. Detection of particle-phase IEPOX in the AMIGAS samples supports recent laboratory results that gas-phase IEPOX produced from the photooxidation of isoprene under low-NO_x conditions is a key precursor of ambient isoprene secondary organic aerosol (SOA) formation. On average, the sum of the mass concentrations of IEPOX and the measured isoprene SOA tracers accounted for about 3% of the organic carbon, demonstrating the significance of isoprene oxidation to the formation of ambient aerosol in this region

Mass Spectrometric Analysis of Organic Aerosol Composition: Laboratory and Ambient

Organic compounds contribute a significant mass fraction of ambient aerosol and play a role in determining the physiochemical properties of ambient aerosol. A significant fraction of organic aerosol is secondary organic aerosol (SOA), which is produced when the volatile organic compounds (VOCs) originated from various anthropogenic and biogenic sources react with atmospheric oxidants such as ozone, hydroxyl radicals, and nitrate radicals to form lower volatility organic compounds, which subsequently partition into the particle phase. Understanding the composition of ambient aerosol is crucial for identifying their sources and formation mechanisms and predicting their properties and effects on various ambient processes. This thesis focuses on investigating the composition of laboratory–generated SOA formed from the oxidation of biogenic VOCs of atmospheric importance (isoprene and β–caryophyllene) and ambient aerosol collected in the field campaigns using advanced mass spectrometric techniques. By comparing the mass spectrometric data collected for the both laboratory–generated SOA and ambient aerosol, we propose reaction pathways and new chemical tracers for these biogenic VOCs, which enhance our knowledge of the composition, sources, and formation pathways of SOA in the atmosphere. With a better knowledge of the SOA composition, a product–specific model is proposed to predict the composition and aerosol mass yields (mass of SOA formed per mass of hydrocarbon reacted) of laboratory–generated α–pinene SOA

Experimental measurements of phase transition and hygroscopic growth of water-soluble organic compounds in atmospheric aerosols

The hygroscopicity, the water uptake property, of atmospheric aerosols describes the interaction between water vapor and aerosols. It affects the phase, size, and chemical composition of aerosols, and thus influences many atmospheric processes such as cloud activation activity, light scattering, and chemical reactions. Recent studies have revealed that organic species derived from biomass burning, amino acids, and macromolecular polyacids contribute to a substantial portion of water-soluble organic compounds (WSOC) and the aerosol mass. This thesis focuses on studying the hygroscopicity of particles of these three major groups of WSOC using the single particle levitation technique in an electrodynamic balance (EDB). The hygroscopic measurements were performed by equilibrating singly levitated particles at different relative humidity (RH) in the EDB for in-situ mass determination. Levoglucosan, marnosan, and galactosan are commonly detected in the biomass burning aerosols, derived from the combustion of cellulose. The particles of these species exist as highly concentrated solutions at RH as low as 5%. Our results suggest that biomass burning aerosols, which contain these non-crystallizing organic species, may not be completely dry and absorb water at low RH. Our results support the reports in the literature that the light scattering coefficient of biomass burning aerosols starts to increase with increasing RH, even at low RH. Crystallization was observed in some amino acid particles (glycine, alanine, serine, glutamine, and threonine) upon evaporation of water while no phase transition was observed in some others (arginine and asparagine), even at 5%RH. Best fittings of the literature and experimental data with the model estimates yield a new set of UNIFAC interaction parameters that give predictions to within 15% of the measurements up to the supersaturation regime for atmospheric application. Water-soluble macromolecular polyacids have molecular structures similar to natural fulvic acids (FA) and are referred to as humic-like substances (HULIS). The role of HULIS on the hygroscopicity of atmospheric aerosols was studied using their model compounds: two natural FA: Nordic Aquatic Fulvic Acid (NAFA) and Suwannee River Fulvic Acid (SRFA). The NAFA and SRFA particles absorbed and desorbed water reversibly without crystallization and retained water at RH < 10%. Our measurements of the crystallization characteristics of these natural FA particles are different from the literature data of another natural FA. The differences are attributed to the difference in the composition of the natural FA, which depends the isolation methods. A standardization of the isolation methods of natural FA and HULIS in atmospheric aerosols is needed for a more systematic understanding their hygroscopicity and their role on the properties of atmospheric aerosols

Mass transfer effects on the hygroscopic growth of ammonium sulfate particles with a water-insoluble coating

Organic coatings have been commonly found in atmospheric particles and can affect the hygroscopicity of atmospheric particles. In this paper. we examine the effect of a water-insoluble coating on the hygroscopic growth and phase transformation properties of ammonium sulfate ((NH4)(2)SO4) particles using an electrodynamic balance. Octanoic acid was utilized as a model compound for water-insoluble coating. (NH4)2SO4 particles coated with octanoic acid (9 and 34wt\% octanoic acid) were subjected to relative humidity (RH) cycling to effect two cycles of deliquescence and crystallization. The coated particles exhibited very similar hygroscopicity in two cycles. Octanoic acid was likely present oil the surface of solid (NH4)(2)SO4 cores and (NH4)(2)SO4 solution droplets throughout the RH cycling. Mass transfer effects were found in the deliquescence and evaporation of the heavily coated (34wt\%) particles. The mass transfer of water molecules to and through octanoic acid coating led to a significant delay in the deliquescence and crystallization of the coated particles, all increase in the observed deliquescence RH and a decrease in observed crystallization RH as compared to that of uncoated (NH4)(2)SO4 particles. A water accommodation coefficient in the order of 10(-3) can explain these observations. When sufficient time was given to ensure equilibrium, the coating had no effect in altering the equilibrium phase transitions and hygroscopic growth of the (NH4)(2)SO4 particles. It is important to allow sufficient time for equilibration in hygroscopic measurements of aerosol particles coated with organic coatings. (c) 2007 Elsevier Ltd. All rights reserved