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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2012)

Biogenic volatile organic compounds (BVOC) are one of the most important chemical classes in tropospheric chemistry associated with ozone and secondary organic aerosol formation. Nevertheless, their emission and deposition rates remain unknown for many potentially important plant species due to lack of measurements. Moreover, the number of BVOC species reported for flux measurement by current techniques has been limited to a small fraction of the number of compounds actually emitted or deposited. Thus, large uncertainties still exist in estimating the ecosystem-scale fluxes of BVOCs and their oxidation products (OVOC).

Two intensive field observation campaigns in different vegetation ecosystems took place in summer 2009 and 2010. The Biosphere Effects on AeRosols and Photochemistry EXperiment field campaign during summer 2009 (BEARPEX 2009, June 15 - July 31) was made in a Ponderosa pine plantation on the western slope of the Sierra Nevada Mountain range in California. During summer 2010 (June 25 - July 26), another intensive BVOC concentration and flux measurement campaign was conducted as part of a one-year continuous field campaign (CITRUS 2010, October 2009 - November 2010) in an orange orchard in the Central Valley of California near Visalia.

Through these two field campaigns, this dissertation presents the ecosystem-atmosphere exchange of a wide range of BVOC and OVOC by coupling state of the art VOC measurement techniques (proton transfer reaction mass spectrometry; PTR-MS, and proton transfer reaction - time of flight - mass spectrometry; PTR-TOF-MS) with a well-established micrometeorological approach (eddy-covariance method; EC). Ambient concentrations of BVOC and OVOC were simultaneously measured for investigating fluxes and vertical gradients in both field sites along with the meteorological parameters.

From BEARPEX 2009, I developed an approach of using the flux and gradient relationship from species which could be measured quickly enough to use the EC flux method (e.g. 5 Hz) to determine the eddy diffusivity (K), the apply that K to estimate the flux of species for which sufficiently fast measurements were not available to use the EC method. In the past, K had been inferred from heat, H2O and CO2 which don't have similar exchange characteristics as BVOC, thus I suggest using a universal K derived from multiple BVOC can more accurately be applied where PTR-MS measurements are conducted.

In the CITRUS field campaign we simultaneously used PTR-MS for flux measurements of 3 species and vertical concentration gradient measurements of 27 species, and PTR-TOF-MS for flux and concentration measurements of a much fuller suite of VOCs above the plant canopy. Using the PTR-TOF-MS we demonstrate for the first time that there were significant emission for 27 VOC species for which concentration is commonly measured by PTR-MS, demonstrating not only the strong potential in use of PTR-TOF-MS in EC flux application but also possible underestimation in current knowledge about BVOC emission from land vegetation. For example, acetic acid was missed in flux measurements by the PTR-MS technique; however, acetic acid was here observed to be the third largest emission of any VOC species in the Orange orchard.

I demonstrate for the first time how the application of PTR-TOF-MS can expand our observational capabilities and changes the scope of understanding of VOC exchanges at atmosphere-biosphere interface. Previous work has mostly focused on quantifying emission strength from vegetation for a limited number of BVOC species. However, the results from flux measurements performed during CITRUS 2010 show the existence of significant deposition flux of BVOC and their oxidation products in addition to their emissions. Additionally, active ecosystem-atmosphere exchanges of the vast majority of the VOCs observed (at least 494 species) were discovered by applying a new approach (time shifted absolute value covariance). Through this measurement and analysis, I proved the existence of ecosystem-scale exchanges of species showing vast majority of VOC have bi-directional flux - both upward and downward. This observation is unprecedented, and revealed that most VOC species have at least some active exchange between the biosphere and atmosphere.

The observational evidence of active VOC exchanges in two independent ecosystems in this dissertation provides significant insights of interaction between the biosphere and atmosphere, coupling the VOC exchange and photochemistry within and above the plant canopy.

    Cover page: Quantifying the ecosystem-scale emission and deposition fluxes of biogenic volatile organic compounds (BVOC) and their oxidation products above plant canopies
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2018)
    Oxidation flow reactors (OFRs) allow the concentration of a given atmospheric oxidant to be increased beyond ambient levels in order to study secondary organic aerosol (SOA) formation and aging over varying periods of equivalent aging by that oxidant. Previous studies have used these reactors to determine the bulk OA mass and chemical evolution. To our knowledge, no OFR study has focused on the interpretation of the evolving aerosol size distributions. In this study, we use size-distribution measurements of the OFR and an aerosol microphysics model to learn about size-dependent processes in the OFR. Specifically, we use OFR exposures between 0.09 and 0.9 equivalent days of OH aging from the 2011 BEACHON-RoMBAS and GoAmazon2014/5 field campaigns. We use simulations in the TOMAS (TwO-Moment Aerosol Sectional) microphysics box model to constrain the following parameters in the OFR: (1) the rate constant of gas-phase functionalization reactions of organic compounds with OH, (2) the rate constant of gas-phase fragmentation reactions of organic compounds with OH, (3) the reactive uptake coefficient for heterogeneous fragmentation reactions with OH, (4) the nucleation rate constants for three different nucleation schemes, and (5) an effective accommodation coefficient that accounts for possible particle diffusion limitations of particles larger than 60nm in diameter. We find the best model-to-measurement agreement when the accommodation coefficient of the larger particles (Dp>60nm) was 0.1 or lower (with an accommodation coefficient of 1 for smaller particles), which suggests a diffusion limitation in the larger particles. When using these low accommodation-coefficient values, the model agrees with measurements when using a published H2SO4-organics nucleation mechanism and previously published values of rate constants for gas-phase oxidation reactions. Further, gas-phase fragmentation was found to have a significant impact upon the size distribution, and including fragmentation was necessary for accurately simulating the distributions in the OFR. The model was insensitive to the value of the reactive uptake coefficient on these aging timescales. Monoterpenes and isoprene could explain 24%-95% of the observed change in total volume of aerosol in the OFR, with ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) appearing to explain the remainder of the change in total volume. These results provide support to the mass-based findings of previous OFR studies, give insight to important size-distribution dynamics in the OFR, and enable the design of future OFR studies focused on new particle formation and/or microphysical processes.
      Cover page: Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modeling
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2016)
      The emission, dispersion, and photochemistry of isoprene (C5H8) and related chemical species in the convective boundary layer (CBL) during sunlit daytime were studied over a mixed forest in the southeastern United States by combining ground-based and aircraft observations. Fluxes of isoprene and monoterpenes were quantified at the top of the forest canopy using a high-resolution proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). Snapshot (2 min sampling duration) vertical profiles of isoprene, methyl vinyl ketone (MVK)Cmethacrolein (MACR), and monoterpenes were collected from aircraft every hour in the CBL (100-1000 m). Both ground-based and airborne collected volatile organic compound (VOC) data are used to constrain the initial conditions of a mixed-layer chemistry model (MXLCH), which is applied to examine the chemical evolution of the O3-NOx-HOx-VOC system and how it is affected by boundary layer dynamics in the CBL. The chemical loss rate of isoprene (1 h) is similar to the turbulent mixing timescale (0.1-0.5 h), which indicates that isoprene concentrations are equally dependent on both photooxidation and boundary layer dynamics. Analysis of a modelderived concentration budget suggests that diurnal evolution of isoprene inside the CBL is mainly controlled by surface emissions and chemical loss; the diurnal evolution of O3 is dominated by entrainment. The NO to HO2 ratio (NO :HO2) is used as an indicator of anthropogenic impact on the CBL chemical composition and spans a wide range (1-163). The fate of hydroxyl-substituted isoprene peroxyl radical (HOC5H8OO q; ISOPOO) is strongly affected by NO:HO2, shifting from NO-dominant to NO-HO2-balanced conditions from early morning to noontime. This chemical regime change is reflected in the diurnal evolution of isoprene hydroxynitrates (ISOPN) and isoprene hydroxy hydroperoxides (ISOPOOH).
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      Cover page: Understanding isoprene photooxidation using observations and modeling over a subtropical forest in the southeastern USCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2016)
      South Korea has recently achieved developed country status with the second largest megacity in the world, the Seoul Metropolitan Area (SMA). This study provides insights into future changes in air quality for rapidly emerging megacities in the East Asian region. We present total OH reactivity observations in the SMA conducted at an urban Seoul site (May-June, 2015) and a suburban forest site (Sep, 2015). The total OH reactivity in an urban site during the daytime was observed at similar levels (∼15 s(-1)) to those previously reported from other East Asian megacity studies. Trace gas observations indicate that OH reactivity is largely accounted for by NOX (∼50%) followed by volatile organic compounds (VOCs) (∼35%). Isoprene accounts for a substantial fraction of OH reactivity among the comprehensive VOC observational dataset (25-47%). In general, observed total OH reactivity can be accounted for by the observed trace gas dataset. However, observed total OH reactivity in the suburban forest area cannot be largely accounted for (∼70%) by the trace gas measurements. The importance of biogenic VOC (BVOCs) emissions and oxidations used to evaluate the impacts of East Asian megacity outflows for the regional air quality and climate contexts are highlighted in this study.
        Cover page: OH reactivity in urban and suburban regions in Seoul, South Korea – an East Asian megacity in a rapid transitionCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2016)
        The Amazon Basin plays key role in atmospheric chemistry, biodiversity and climate change. In this study we applied nanoelectrospray (nanoESI) ultra-high-resolution mass spectrometry (UHRMS) for the analysis of the organic fraction of PM2.5 aerosol samples collected during dry and wet seasons at a site in central Amazonia receiving background air masses, biomass burning and urban pollution. Comprehensive mass spectral data evaluation methods (e.g. Kendrick mass defect, Van Krevelen diagrams, carbon oxidation state and aromaticity equivalent) were used to identify compound classes and mass distributions of the detected species. Nitrogen-and/or sulfur-containing organic species contributed up to 60% of the total identified number of formulae. A large number of molecular formulae in organic aerosol (OA) were attributed to later-generation nitrogen-and sulfur-containing oxidation products, suggesting that OA composition is affected by biomass burning and other, potentially anthropogenic, sources. Isoprene-derived organosulfate (IEPOX-OS) was found to be the most dominant ion in most of the analysed samples and strongly followed the concentration trends of the gas-phase anthropogenic tracers confirming its mixed anthropogenic-biogenic origin. The presence of oxidised aromatic and nitro-aromatic compounds in the samples suggested a strong influence from biomass burning especially during the dry period. Aerosol samples from the dry period and under enhanced biomass burning conditions contained a large number of molecules with high carbon oxidation state and an increased number of aromatic compounds compared to that from the wet period. The results of this work demonstrate that the studied site is influenced not only by biogenic emissions from the forest but also by biomass burning and potentially other anthropogenic emissions from the neighbouring urban environments.
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2012)
          2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C(5)H(12)O(6)S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM(2.5)) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM(2.5) collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.
            Cover page: Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2‑Methyl-3-Buten-2-ol (MBO) in the AtmosphereCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2016)
            Isoprene-epoxydiols-derived secondary organic aerosol (IEPOX-SOA) can contribute substantially to organic aerosol (OA) concentrations in forested areas under low NO conditions, hence significantly influencing the regional and global OA budgets, accounting, for example, for 16-36% of the submicron OA in the southeastern United States (SE US) summer. Particle evaporation measurements from a thermodenuder show that the volatility of ambient IEPOX-SOA is lower than that of bulk OA and also much lower than that of known monomer IEPOX-SOA tracer species, indicating that IEPOX-SOA likely exists mostly as oligomers in the aerosol phase. The OH aging process of ambient IEPOX-SOA was investigated with an oxidation flow reactor (OFR). New IEPOX-SOA formation in the reactor was negligible, as the OFR does not accelerate processes such as aerosol uptake and reactions that do not scale with OH. Simulation results indicate that adding ∼ 100μgm-3 of pure H2SO4 to the ambient air allows IEPOX-SOA to be efficiently formed in the reactor. The heterogeneous reaction rate coefficient of ambient IEPOX-SOA with OH radical (kOH) was estimated as 4.0±2.0 × 10-13cm3molec-1s-1, which is equivalent to more than a 2-week lifetime. A similar kOH was found for measurements of OH oxidation of ambient Amazon forest air in an OFR. At higher OH exposures in the reactor (> 1 × 1012molec cm-3s), the mass loss of IEPOX-SOA due to heterogeneous reaction was mainly due to revolatilization of fragmented reaction products. We report, for the first time, OH reactive uptake coefficients (γOH = 0.59±0.33 in SE US and γOH = 0.68±0.38 in Amazon) for SOA under ambient conditions. A relative humidity dependence of kOH and γOH was observed, consistent with surface-area-limited OH uptake. No decrease of kOH was observed as OH concentrations increased. These observations of physicochemical properties of IEPOX-SOA can help to constrain OA impact on air quality and climate.
              Cover page: Volatility and lifetime against OH heterogeneous reaction of ambient isoprene-epoxydiols-derived secondary organic aerosol (IEPOX-SOA)Creative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2021)
              The Korea-United States Air Quality (KORUS-AQ) field study was conducted during May-June 2016. The effort was jointly sponsored by the National Institute of Environmental Research of South Korea and the National Aeronautics and Space Administration of the United States. KORUS-AQ offered an unprecedented, multi-perspective view of air quality conditions in South Korea by employing observations from three aircraft, an extensive ground-based network, and three ships along with an array of air quality forecast models. Information gathered during the study is contributing to an improved understanding of the factors controlling air quality in South Korea. The study also provided a valuable test bed for future air quality-observing strategies involving geostationary satellite instruments being launched by both countries to examine air quality throughout the day over Asia and North America. This article presents details on the KORUS-AQ observational assets, study execution, data products, and air quality conditions observed during the study. High-level findings from companion papers in this special issue are also summarized and discussed in relation to the factors controlling fine particle and ozone pollution, current emissions and source apportionment, and expectations for the role of satellite observations in the future. Resulting policy recommendations and advice regarding plans going forward are summarized. These results provide an important update to early feedback previously provided in a Rapid Science Synthesis Report produced for South Korean policy makers in 2017 and form the basis for the Final Science Synthesis Report delivered in 2020.
                Cover page: The Korea–United States Air Quality (KORUS-AQ) field studyCreative Commons 'BY' version 4.0 license
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