This study addresses photochemical aging of secondary organic aerosol (SOA) produced from α-pinene ozonolysis. The SOA is aged via hydroxyl radical (OH) reactions with first-generation vapors and UV ...photolysis. OH radicals are created through tetramethylethylene ozonolysis, HOOH photolysis, or HONO photolysis, sources that vary in OH concentration and the presence or absence of UV illumination. Aging strongly influences observed SOA mass concentrations, but the behavior is complex. In the dark or with high concentrations of OH, vapors are functionalized, lowering their volatility, resulting in an increase in OA by a factor of 2–3. However, with lower concentrations of OH under UV illumination SOA mass concentrations decrease over time. We attribute this decrease to evaporation driven by photolysis of the highly functionalized second-generation products. The photolysis rates are rapid, a few percent of the NO2 photolysis frequency, and can thus be highly competitive with other aging mechanisms in the atmosphere.
Secondary organic aerosol formation from volatile precursors can be thought of as a succession of generations of reaction products. Here, we constrain first-generation SOA formation from the α-pinene ...+ OH reaction and also study SOA formation from α-pinene ozonolysis carried out without an OH scavenger. SOA yields from OH oxidation of α-pinene are significantly higher than SOA yields from ozonolysis including an OH scavenger, and the SOA mass yields for unscavenged ozonolysis generally fall within the range of mass yields for α-pinene ozonolysis under various conditions. Taken together, first-generation product yields parametrized with a volatility basis set fit provide a starting point for atmospheric models designed to simulate both the production and subsequent aging of SOA from this important terpene.
According to the pseudo-ideal mixing assumption employed in practically all chemical transport models, organic aerosol components from different sources interact with each other in a single solution, ...independent of their composition. This critical assumption greatly affects modeled organic aerosol concentrations, but there is little direct experimental evidence to support it. A main experimental challenge is that organic aerosol components from different sources often look similar when analyzed with an aerosol mass spectrometer. We developed a new experimental method to overcome this challenge, using isotopically labeled compounds (13C or D) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). We generated mixtures of secondary organic aerosol (SOA) from isotopically labeled toluene and from unlabeled α-pinene and used the HR-ToF-AMS data to separate these different SOA types. We evaluated their interaction by comparing the aerosol mass yields of toluene and α-pinene when the SOA was formed in these mixtures to their yields when the SOA was formed in isolation. At equilibrium, our results are consistent with pseudo-ideal mixing of anthropogenic and biogenic SOA components from these chemically dissimilar precursors.
Hydrogen peroxide (HOOH) is a potentially valuable hydroxyl radical (OH) scavenger in secondary organic aerosol experiments focused on ozonolysis yields. Here, we present results for α-pinene ...ozonolysis. The OH scavenging produces solely HO
2
radicals and the resulting high HO
2
/RO
2
ratio causes an increase in aerosol formation from α-pinene ozonolysis, compared to experiments performed with butanol OH scavengers. The majority of the increase comes in the 100 μg m
−3
volatility range, suggesting that instead of more volatile products formed under higher RO
2
conditions, less volatile, multifunctional hydroperoxides form under the high-HO
2
conditions here. This dependence on the HO
2
/RO
2
ratio can be parameterized in a similar fashion to the way high- and low-NO
x
yields are currently treated in models.
The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary ...organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.
Secondary organic aerosol formation from volatile precursors can be thought of as a succession of generations of reaction products. Here, we constrain first-generation SOA formation from the α-pinene ...+ OH reaction and also study SOA formation from α-pinene ozonolysis carried out without an OH scavenger. SOA yields from OH oxidation of α-pinene are significantly higher than SOA yields from ozonolysis including an OH scavenger, and the SOA mass yields for unscavenged ozonolysis generally fall within the range of mass yields for α-pinene ozonolysis under various conditions. Taken together, first-generation product yields parametrized with a volatility basis set fit provide a starting point for atmospheric models designed to simulate both the production and subsequent aging of SOA from this important terpene. PUBLICATION ABSTRACT
Hydrogen peroxide (HOOH) is a potentially valuable hydroxyl radical (OH) scavenger in secondary organic aerosol experiments focused on ozonolysis yields. Here, we present results for α-pinene ...ozonolysis. The OH scavenging produces solely HO... radicals and the resulting high HO.../RO... ratio causes an increase in aerosol formation from α-pinene ozonolysis, compared to experiments performed with butanol OH scavengers. The majority of the increase comes in the 100 ¨g m... volatility range, suggesting that instead of more volatile products formed under higher RO... conditions, less volatile, multifunctional hydroperoxides form under the high-HO... conditions here. This dependence on the HO.../RO... ratio can be parameterized in a similar fashion to the way high- and low-NO... yields are currently treated in models. (ProQuest: ... denotes formulae/symbols omitted.)
Atmospheric particle matter (PM) negatively affects visibility, climate, and human health. Organic aerosol (OA) is a significant fraction of the submicron PM; this fraction is complex and still ...poorly understood. OA can be formed from chemistry in the atmosphere where volatile organic compounds (VOCs) are oxidized, forming products of lower volatility that can condense. The volatile compounds that then remain in the gas phase can continue to be oxidized, and we call this aging. We look to parameterize this first generation chemistry and illuminate the processes occurring during aging in order to inform an atmospheric chemical transport model (CTM). This thesis focuses on a biogenic system, OA from α-pinene oxidation, isolated in the Carnegie Mellon University smog chamber. α-Pinene is the most abundant monoterpene (a class of biogenic VOCs) and is characteristic of biogenic species with an endocyclic double bond. First we look at aerosol formation from the oxidation of α-pinene via ozonolysis (reaction with ozone), OH reactions, and a system with both oxidants. The aerosol yields from these systems are very dependent on the radical species concentrations in the chamber, and therefore we have parameterized α-pinene ozonolysis yields based on the HO2/RO2 ratio. OH oxidation yields are shown here to be higher than ozonolysis yields, and therefore must also be separately parameterized for use in CTMs. Aging of biogenic species has not been represented in CTMs because a scheme where solely functionalization chemistry occurs overpredicts the ambient OA concentrations. Functionalization chemistry (occurring in the gas phase) adds oxygen to compounds, lowering their volatility and allowing them to condense to form more OA. We have shown in the α-pinene system that fragmentation chemistry, specifically photolysis, can also occur during aging. This photochemistry raises the volatility of the products, and subsequently OA must evaporate to maintain equilibrium with the gas phase. We have examined light and dark aging cases as well as varying the oxidant (OH) concentration to see when fragmentation chemistry dominates over functionalization. With the added sinks of fragmentation and specifically photolysis we can begin to model biogenic OA aging with more realistic results.
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