I am grateful for the professors that served on my candidacy and thesis commit- tees. This includes John Seinfeld, my advisor, Paul Wennberg, Rick Flagan, and Michael Homann (candidacy) and Mitchio Okumura (thesis).
I thank my advisor, John Seinfeld, for inspiring me to pursue graduate studies in atmospheric chemistry. I saw for the rst time the seminal gure on the uncertain role of aerosols on radiative forcing from the IPCC 2007 report in John's seminar at UC Riverside during my senior year of college. I remember leaving that seminar charged with the motivation to better understand aerosolchemistry, and it served as motivation in my successful research proposal for a National Science Foundation Graduate Research Fellowship that supported me while at Caltech. Since being at Caltech, I have appreciated John's advising, as I recall key moments when he took the initiative to address many of my concerns before I had to state them. He has shaped the way I look at and approach scientic problems, and I have always valued his professional mentorship.
– Observed SOA formation in photochemical laboratory experiments scales linearly with aqueous phase OH chemistry. However, a cloud chemistry model falls short by a factor 290–870 in explaining SOA amounts formed in aqueous ammonium sulfate and ammonium sulfate/fulvic acid seed assuming hygroscopic SOA (by a larger factor if SOA is assumed to be non- hygroscopic). It is speculated that this difference is due to additional radical recycling and/or photochemi- cal processes that are not captured by the model. A sim- plified box model that explicitly represents the forma- tion and fate of organic peroxy radicals, RO 2 , in aque- ous solution shows enhanced recombination rates of RO 2 in the highly concentrated aqueous particle phase compared to dilute clouds. The additional photochemi- cal process has been parameterized as a first-order pro- cess with rate constants of 0.8 s −1 < k photochem < 7 s −1 depending on the chemical composition of the aqueous seed particle.
The simulation with SIMPOL and no heterogeneous re- actions gives best agreement with the measured final SOA mass formation (70 and 65 µg m − 3 , respectively). However, this simulation substantially underestimates the SOA forma- tion during the start of the experiment. The best agreement between the model and measurements at the beginning of the experiment is instead reached when we include relatively rapid oligomerization in the particle phase. The results from this simulation also show surprisingly good agreement with the model simulation using the semi-empirical parameteriza- tions from Ng et al. (2007). This again indicates that hetero- geneous reactions are likely to be important for the SOA for- mation. The larger SOA formation from these model simula- tions compared to the measurements can likely be attributed to substantial gas-phase losses directly onto the Teflon walls in the chamber. This effect will be especially pronounced at the end of the experiment, when the surface-area-to-volume ratio is large (see Sect. 3.4.1). Hence for this experiment, the model simulations indicate that the wall corrections (which assume continued uptake of condensable organic compounds onto the wall-deposited particles) do not give an upper esti- mate of the actual (atmospheric relevant) SOA formation (see Sect. 2.2.3).
“modelers” of the group, Philip Stier, Daven Henze, Anne Chen, Candy Tong, Amir Hakami, and Julia Lu for their support.
I am especially grateful to Tomtor and Roya for their patience in teaching me so many things in the lab, from little things like “the difference between a back ferrule and a front ferrule”, to how to operate the many instruments, and for always being there to help me even when they were overwhelmed with their own work. The two and a half years with Jesse in the lab have been incredible. We built our friendship through running hundreds and hundreds of chamber experiments (final checks!), and together we survived the ups and downs of the experiments. Jesse taught me so much about atmospheric chemistry and I am grateful for his patience and guidance. I considered myself extraordinarily lucky to have the opportunity to work so closely with such a great scientist and I treasure our friendship dearly.
I would also like to thank my two biggest mentors in lab, Jesse Kroll and Sally Ng, from whom I learned everything I needed to know about anything, ranging from RO 2 chemistry and aerosol science to Swagelok fittings and chemical syntheses. I had the great opportunity to work with Jesse Kroll on my first project, and overlapped with him at Caltech for a few months. He has introduced to me a dynamic framework to think about organic aerosols. From working with him closely on one of his papers and one of my own, I have learned how to write an interesting paper and to present it coherently and logically. His interest in online videos almost parallels his interest in science, both of which made the roof lab a source of great science and great fun. Among all the people whom I have worked with, Sally is the one who has taught me the most. She has shown me how to run the roof lab chambers, and that is the least important lesson I have learned from her. Her relentless scientific pursuit (always insisting every piece of instrument be run to get the most data) and borderline superstitious attitude have been the most impressive to me. She has also been the most passionate about science, and would always come to lab in the morning with new ideas and questions she thought of overnight. Always cheerful, never down, she has the most positive attitude about everything and everyone, which is infectious upon people around her. She has taught me the importance of returning borrowed items just as they were before they were borrowed, singing during chemical syntheses, and asking for help when it is needed. Together, Jesse and Sally have done some great work during the years they were here, but more importantly, they have made the roof lab the best community one can work in.
344 A. R. METCALF ET AL.
followed by one in which growth is quenched while the cham- ber undergoes dilution. The dilution phase is used as a means to assess SOA volatility by measuring the possible evaporation of coatings on the rBC seed. In the experiments presented here, a 3-λ photoacoustic soot spectrometer is used to measure the optical properties of the uncoated rBC seed, initially, and the coated rBC seed during the course of SOA formation. These measurements, coupled with the application of a core-and-shell Mie scattering model, allow one to infer the optical properties of the SOA. Application of a prototype single-particle angularly- resolved light scattering instrument confirms that the uncoated rBC particles are nonspherical. Important to understanding the effect rBC has on SOA formation is whether or not SOA con- densed onto rBC seed is chemically and optically similar to nu- cleated SOA under dry conditions. High-resolution Aerodyne aerosol mass spectrometer measurements for the three systems considered here, naphthalene photooxidation and photooxida- tion of α-pinene under both high- and low-NO x conditions, confirm that the composition of SOA coating rBC seed particles differs from homogeneously nucleated SOA by no more than condensing SOA on the more conventional ammonium sulfate seed used in many chamber experiments, so that the use of rBC as a seed is not expected to alter the basic chemistry of SOA formation under dry conditions. Both SP2 and PASS-3 measure- ments reveal a change in the SOA coating and particle optical properties during SOA growth in the high-NO x α-pinene sys- tem, which is mirrored by a corresponding change in the AMS mass spectra. The combination of SP2 and AMS measurements in this system suggest that semivolatile species are evaporating from the aerosol during chemical aging. A change in optical properties during SOA growth in the low-NO x α -pinene system is mirrored by a change in organic growth rate and AMS mass spectra, but not in single-particle coating thicknesses. Explo- ration of a fundamental explanation of the chemistry leading to these changes lies beyond the scope of the present work. We have provided a framework by which future studies of SOA optical properties and single-particle growth dynamics may be explored in environmental chambers.
oxidation of SO 2 , SOA mass yields from isoprene under high- and low- NO x conditions, respectively, increase substantially. Because isoprene is estimated to be the largest single contributor to global SOA, these results may help to resolve two existing dilemmas in atmospheric chemistry: (i) Radiocarbon ( 14 C) data consistently indicate that well over half of the ambient SOA is of modern (biogenic) origin (7, 33), whereas correlations between water-soluble organic carbon and anthropogenic tracers, such as CO, suggest that much of the SOA is actually of anthropogenic origin (34, 35); and (ii) comparisons between measured and predicted SOA based on known precursors suggest that there is a substantial amount of “missing urban SOA” not included in current models (35–37). Revising the chemistry of isoprene in regional and global SOA models could lead to a decrease in this discrepancy; however, the measurement and parameterization of aerosol acidity requires additional work.
While we have demonstrated good agreement between simpleGAMMA and GAMMA, the limitations of GAMMA also apply to simpleGAMMA; for example, neither model includes a treatment of oxidative aging of aaSOA at this time due to a lack of kinetic and mechanistic data. As a result, overprediction of total aaSOA mass is likely (Budisulistior- ini et al., 2015). The only sources of aqueous-phase OH in GAMMA are HOOH photolysis or Henry’s law transfer of OH from the gas phase. Therefore, we, like others (Waxman et al., 2013; Ervens et al., 2014), have observed OH-limited chemistry in the aqueous aerosol phase using GAMMA, and this informed the simpleGAMMA formulation. While tran- sition metal ion chemistry, a possible source of OH (Her- rmann et al., 2015), was not included in the first version of GAMMA (McNeill et al., 2012) due to the focus on ammo- nium sulfate aerosols in that study, these mechanisms may be active in ambient aerosols. Preliminary calculations in GAMMA show that including transition metal ion (Fe +3 , Cu +2 , Mn +3 ) chemistry following CAPRAM 3.0 (Chemical Aqueous Phase Radical Mechanism; Herrmann et al., 2005) does not perturb the predicted aaSOA yield or product dis- tribution. Aqueous-phase diffusion is not accounted for in GAMMA or simpleGAMMA, that is, Henry’s law equili- bration is assumed to occur instantaneously and no spatial concentration gradients within the particle are considered. This likely leads to an overestimate of OH chemistry when this highly reactive species is taken up from the gas phase. However, since we have found that aqueous-phase photo- chemistry does not dominate aaSOA formation, inclusion of aqueous-phase diffusion limitations in this calculation would not change our results or the formulation of simpleGAMMA. Aqueous-phase diffusion may also be important for relatively large droplets such as those encountered in marine aerosols.
frequently used as a model monoterpene in chamber studies, this exceptional behavior relative to all other monoterpenes tested is particularly important. We seek an explanation in the initial oxidation steps of this reaction, and note that α-pinene could be expected to produce an internal nitrate rather than a terminal nitrate, such as would be expected in β-pinene, Δ-3- carene, or sabinene (see SI Figure S6). It has been shown that terminal functional groups result in lower vapor pressure than internal, but this observation would seem to be a very extreme manifestation of that trend. We note that the NO 3 + α-pinene reaction forms little organonitrate (Table 4), and this organonitrate was not observed to partition to the aerosol phase. Most of the NO 3 returns to the gas phase as NO 2 , consistent with previous observations that α-pinene chemistry instead produces the diketone pinonaldehyde (SI Figure S6, red pathway), which is expected to be more volatile than the organonitrate alternatives. 37
Prof. Paul Wennberg introduced the CalTecher’s belief and passion in science by teaching me the first course I had at Caltech. Having Paul’s class is always a great challenge but fun. He continuously and convincingly conveys the spirit of exploration and an excitement in regard to teaching and tutoring. Last year, I took atmospheric chemistry II, and I realized that for the first time, I finally got a clear picture of isoprene photochemistry, regardless years of reading in the past. I also find myself always benefiting a lot from discussions or even little conversations with him. His genial nature makes him a good listener, as well as a great advise giver. One of my studies regarding vapor wall losses at humid chamber conditions is originally inspired by our conversations in the coffee room. I am thankful to him for being ever so kind to show interest in my research and for giving his precious advice on my research topic.
In addition, the reaction rates of OH and O 3 with organics have to be quantified and comparable when one investigates the relative role of OH oxidation and ozonolysis in particle formation. To obtain the reaction rates of VOCs with OH, the OH concentration is a required parameter. However, none of these previous studies directly measured the OH concentra- tion, which was either not stated or just modeled. Since the detailed chemistry, including HO x generation pathways, of BVOC photooxidation is still not well understood, modeled OH concentrations may have significant uncertainties (Fuchs et al., 2013; Kaminiski, 2014; Kim et al., 2013; Whalley et al., 2011). Consequently, the relative importance of OH oxi- dation and ozonolysis in particle formation and growth may have large uncertainties when the comparison of both cases is based on modeled OH concentrations and corresponding reaction rates with OH.
(< 10 ppb) conditions using H 2 O 2 as the OH source. Sec-
ondary organicaerosol (SOA) yields (ratio of mass of SOA formed to mass of primary organic reacted) greater than 25 % are observed. Aerosol growth is rapid and linear with the pri- mary organic conversion, consistent with the formation of essentially non-volatile products. Gas- and aerosol-phase ox- idation products from the guaiacol system provide insight into the chemical mechanisms responsible for SOA forma- tion. Syringol SOA yields are lower than those of phenol and guaiacol, likely due to novel methoxy group chemistry that leads to early fragmentation in the gas-phase photoox- idation. Atomic oxygen to carbon (O : C) ratios calculated from high-resolution-time-of-flight Aerodyne Aerosol Mass Spectrometer (HR-ToF-AMS) measurements of the SOA in all three systems are ∼ 0.9, which represent among the high- est such ratios achieved in laboratory chamber experiments and are similar to that of aged atmospheric organicaerosol. The global contribution of SOA from intermediate volatil- ity and semivolatile organic compounds has been shown to be substantial (Pye and Seinfeld, 2010). An approach to rep-
The gas-phase oxidation of isoprene and its major oxidation products are de- scribed in detail. The mechanism is developed with the aim of both providing accurate simulations of the impact of isoprene emissions on HO x and NO x free radical concentrations and to produce realistic representation of the yields of prod- ucts known to be involved in condensed phase processes. The schemes presented represent a synthesis of recent laboratory studies at the California Institute of Tech- nology and elsewhere that have provided a new wealth of detail on the mechanisms at play. Insights from new theoretical approaches are also incorporated. Finally, we present a reduced mechanism appropriate for implementation in chemical trans- port models that retains the essential chemistry required to accurately simulate this chemistry under the typical conditions where isoprene is emitted and oxidized in the atmosphere.
ABOUT THE AUTHOR
Kelvin Hamilton Bates was born on January 9, 1990, in Seattle, Washington, to Tim Bates, an oceanographer, and Susan Hamilton, a biochemist. His parents’ scientific backgrounds did not lead them to name their son after a unit of temperature – Kelvin was, instead, named after his grandfather – but did inspire them to invest heavily in his education. Kelvin’s early schooling emphasized immersive cultural experiences, including travels to Japan, Vietnam, and the Dominican Republic, and instilled in him a profound love of learning. That passion helped him to thrive at Lakeside High School, where he first discovered his interests in organicchemistry and theater. In his eighteen years of childhood in the Pacific Northwest, Kelvin also gained an abiding appreciation for the splendor of the natural world.
The highest SOA formation potential of the gas-phase oxidation products was estimated for the healthy ozonolysis experiment (H-O3) with an F p of 0.032. In contrast, the mass fractions of oxidation products in the aphid-stressed experi- ment (S-O3) were weighted toward the highest volatility bin with a corresponding lower SOA formation potential (F p = 0.005). These ﬁndings are consistent with the explanation that the healthy VOC pro ﬁle was dominated by cyclic mono- terpenes, which produce highly functionalized oxidation products that are more likely to undergo gas-particle partitioning. In contrast, the VOC proﬁle in the aphid-stressed experiment had a higher contribution from acyclic sesquiter- penes, which fragment upon reaction with ozone and generate higher volatility oxidation products that are less likely to undergo gas-particle partitioning. Another explanation for reduced fraction of low volatility oxidation products in the stressed experiments could be attributed to oxidation product scavenging by highly reactive peroxy radicals formed from the acyclic structures. This mechanism of SOA suppression was recently reported from monoterpene/isoprene mixtures, 95 and we did not use an OH scavenger to suppress HOx chemistry. 3.3. Photooxidation Chemistry. The chemical mecha- nisms controlling SOA mass yield in the photooxidation experiments are more challenging to pinpoint than in the dark ozonolysis experiments because the OH radical is more universally reactive with other compounds. In the ozonolysis experiments, we were able to ignore most of the non-terpene volatiles in the chamber, but recall from Figure 2 that there were aromatic compounds in the plant experiments as well. To demonstrate the increased complexity of plant volatile photooxidation chemistry in comparison with α-pinene chemistry, the PTR mass spectra of a healthy and aphid- stressed experiment and an α-pinene experiment are shown ( Figure 8 ). Spectra at the beginning and end of each
Assuming that this observed SOA mass decay is due to wall re-partitioning, this process will not occur in the atmo- sphere, and aqSOA production can be determined using the maximum mass concentration measured at the end of each cloud event. In that case, aqSOA mass yield from isoprene photooxidation in the presence of clouds would be between 0.002 and 0.004 considering our results from the diphasic ex- periments, or between 2 and 4 times higher than mass yields observed for isoprene photooxidation experiments carried out under dry conditions with preliminary manual cleaning (Brégonzio-Rozier et al., 2015). For triphasic experiments, the observed increase of total SOA mass concentration at the end of each cloud event was at least a factor of 2 compared to the gasSOA mass concentrations reached under dry condi- tions prior cloud formation. Hence, it can be assumed that a substantial aqSOA production was observed in both types of experiments. Furthermore, the fact that additional SOA mass was formed in the triphasic system (i.e. in the second mode) seems to demonstrate that the role of cloud chemistry is not just to increase the rate of gas-phase oxidation reactions but is adding new chemistry.
a role in the SOA yield variability: the only studies who used light sources with spectra representing the solar one are those by Zhang et al. (2011) (outdoor chamber), and by Dommen et al. (2006) who used xenon arc lamps like in our study. Although fluorescent lamps used as irradiation source in the other studies (Chan et al., 2010; Chhabra et al., 2010; Ed- ney et al., 2005; Kleindienst et al., 2006; Kroll et al., 2006, 2005) deliver a light intensity equivalent to NO 2 photolysis rates which are close to natural light intensity, they exhibit emission spectra significantly different from the solar spec- trum (with no emission in the longer wavelength regions, i.e above 400 nm). It is thus suggested that some oxidation products contributing to the aerosolformation and growth in studies using fluorescent lamps (under similar NO x con- ditions), could be photolyzed in our experiments, leading to lower SOA yields. It can be noted that the photolysis of α- dicarbonyls, for example methylglyoxal and glyoxal, may occur outside the fluorescent lamp spectrum. Average pho- tolysis wavelengths of methylglyoxal and glyoxal are at 417 and 383 nm, respectively (Carter et al., 1995). This hypoth- esis is thus opposite to the one from Warren et al. (2008) who observed higher SOA yields using an argon arc lamp (which presents a realistic irradiation spectrum) instead of black lights. However, atmospheric chemistry of aromatics is strongly different from that of alkenes, it is thus not sur- prising to observe a different behavior concerning relation between light source and SOA yields for isoprene / NO x sys- tem.
Joshi et al., 1982) and wall interactions (McMurry and Rader, 1985; McMurry and Grosjean, 1985; Pierce et al., 2008).
On the other hand, smog chambers with lower oxidant con- centrations and longer residence times may more closely simulate atmospheric oxidation. All laboratory reactors are imperfect simulations of the atmosphere because they have walls that cause particle loss and can influence the chemistry of semivolatile organics and, thus, particle growth and com- position (Matsunaga and Ziemann, 2010). Therefore, utiliz- ing flow tubes and smog chamber reactors with different de- signs can complement each other, making it possible to ex- tend studies over a range of parameters unattainable by either method individually, and ultimately lead towards a better un- derstanding of atmospheric aerosol processes. The results of laboratory aerosol experiments are used as inputs to climate models. Therefore, the evaluation of experimental uncertain- ties associated with measurements is needed for reliable ap- plication. The characterization of different reactor designs is important to establish the reliability of the experimental techniques.
* now at: NILU, Norwegian Institute for Air Research, Kjeller, Norway
Received: 1 December 2008 – Published in Atmos. Chem. Phys. Discuss.: 29 January 2009 Revised: 11 August 2009 – Accepted: 27 August 2009 – Published: 22 September 2009
Abstract. The role of isoprene as a precursor to secondaryorganicaerosol (SOA) over Europe is studied with the two- way nested global chemistry transport model TM5. The in- clusion of the formation of SOA from isoprene oxidation in our model almost doubles the atmospheric burden of SOA over Europe compared to SOA formation from terpenes and aromatics. The reference simulation, which considers SOA formation from isoprene, terpenes and aromatics, predicts a yearly European production rate of 1.0 Tg SOA yr − 1 and an annual averaged atmospheric burden of about 50 Gg SOA over Europe. A fraction of 35% of the SOA produced in the boundary layer over Europe is transported to higher altitudes or to other world regions. Summertime measurements of or- ganic matter (OM) during the extensive EMEP OC/EC cam- paign 2002/2003 are better reproduced when SOA formation from isoprene is taken into account, reflecting also the strong seasonality of isoprene and other biogenic volatile organic compounds (BVOC) emissions from vegetation. However, during winter, our model strongly underestimates OM, likely caused by missing wood burning in the emission inventories. Uncertainties in the parameterisation of isoprene SOA for- mation have been investigated. Maximum SOA production is found for irreversible sticking (non-equilibrium partitioning) of condensable vapours on particles, with tropospheric SOA production over Europe increased by a factor of 4 in sum- mer compared to the reference case. Completely neglecting SOA formation from isoprene results in the lowest estimate (0.51 Tg SOA yr − 1 ). The amount and the nature of the ab-
Therefore, this work experimentally evaluates the influence of changing AERs on those outcomes for transient SOA indoor formation that occurs for the ozonolysis of limonene and α- pinene when those terpenes are pulse emitted by dynamic consumer product usage indoors over short durations rather than continuously. Though the pulse indoor applications of sources of ter- penes such as air fresheners, cleaners and vegetable oils are dominant indoor terpene emission pathways (Nazaroff and Weschler, 2004), this study of transient formation is the first of its kind in the literature and adds further depth to the current batch or steady state studies available. Further- more, the studies herein are conducted in both ozone-limiting and -excess conditions, to account for the fact that terpenes are often at higher concentrations indoors than ozone, opposite to the situation occurring outdoors and in most previously conducted batch AMF experimental work. This distinction is especially important for limonene, which has two unsaturated carbon-carbon double bonds and rich secondarychemistry potential.