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MAX-DOAS measurements of aerosol, HCHO, and NO2 over Los Angeles from an elevated mountaintop site.
Full text linkMAX-DOAS measurements of aerosol, HCHO, and NO2 over Los Angeles from an elevated mountaintop site. ByRoss CheungDoctor of Philosophy in Atmospheric and Oceanic SciencesUniversity of California, Los Angeles, 2016Professor Jochen Stutz, ChairDifferential Optical Absorption Spectroscopy (DOAS) has become a popular technique for measuring atmospheric trace gases using UV/Vis narrow-band absorption features along a light path through the atmosphere. The UCLA Multi-Axis DOAS instrument (MAX-DOAS) is a ground-based spectrometer currently located at Mt. Wilson, California (1700 meters above sea level) that measures solar scattered light at various viewing elevation angles. Since May of 2010, it has been taking regular measurements of atmospheric pollutants in the boundary layer of the atmosphere in and above the Los Angeles Basin. This thesis presents the experimental setup and spectral retrievals, as well as results of our observations of measurements of NO2 and HCHO from Mt. Wilson. Radiative transfer modeling efforts of the deployment at Mt. Wilson will be presented, as well as our efforts to model and account for the effects of clouds and aerosols on MAX-DOAS measurements. Because of the unique challenges presented by aerosols in the ultraviolet and visible light region in a polluted urban boundary layer, new techniques were developed to account for and quantify these effects. Observations of path-integrated NO2 and HCHO, some of the primary precursors to ozone formation in the lower troposphere, as well as aerosol extinctions using the UCLA MAX-DOAS will be presented, and the advantages of a mountaintop measurement strategy will be discussed in light of the amount of vertical information that can be retrieved from this approach. The techniques developed to improve the optimal estimation of vertical aerosol extinction profiles and trace gas concentration profiles will be discussed. Finally, an application of these observations uses the ratio of HCHO/NO2 to study the dependency of ozone formation on nitrogen oxides and VOCs will be presented
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Nitrous Acid Chemistry in Two Polluted Environments: Urban Boundary Layers and Biomass Burning Plumes
Full text linkNitrous acid (HONO) is an important radical precursor that can influence secondary pollutant levels, especially in areas impacted by urban emissions and wildfire smoke. Due to uncertainties in its emissions and heterogeneous formation mechanisms, models often under predict HONO concentrations. A number of heterogeneous sources at the ground and on aerosols have been proposed but there is no consensus about which play a significant role in these polluted environments. In this thesis, a new one-dimensional chemistry and transport model (PACT-1D) is used to interpret field measurements and analyze the HONO budget. PACT-1D performs surface chemistry based on molecular collisions and chemical conversion, allowing the addition of detailed HONO formation chemistry at the ground and on particles.Model runs were conducted for the 2010 CalNex campaign, finding good agreement with observations of key species such as O3, NOx, and HOx. With the ground sources implemented, the model captures the diurnal and vertical profile of the HONO observations. Primary HOx production from HONO photolysis is 2-3 times more important than O3 or HCHO photolysis at mid-day, below 10 m. The HONO concentration, and its contribution to HOx decreases quickly with altitude. Heterogeneous chemistry at the ground provided a HONO source of 2.5 x 10^11 molecules cm^-2 s^-1 during the day and 5 x 10^10 molecules cm^-2 s^-1 at night. The night time source was dominated by NO2 hydrolysis. During the day, photolysis of surface HNO3/nitrate contributed 45-60% and photo-enhanced conversion of NO2 contributed 20-45%. Sensitivity studies addressing the uncertainties in both photolytic mechanisms show that, while the relative contribution of either source can vary, HNO3/nitrate is required to produce a surface HONO source that is strong enough to explain observations.Two wildfire smoke plumes from the 2019 FIREX-AQ experiment were also successfully simulated. Both plumes show high levels of HONO, with concentrations exceeding 20 ppb in young smoke. PACT-1D captures the rapid decay of HONO with smoke age, due to a mix of chemical loss and dilution. Vertical profiles show highest levels in the plume center that quickly decrease towards the plume top and bottom. The decrease occurs more rapidly than less-reactive species, creating a narrower vertical distribution of HONO. The large quantities of HONO emitted from fires provide the dominant source and result in HONO providing 80% of the total HOx formation in young smoke. Sensitivity studies show that emissions also drive O3 production, with O3 levels reduced up to 10 ppb when HONO emissions are omitted. Low photolysis rates and minimal vertical mixing in the plumes allow these emissions to be transported hours downwind, continuing to provide a source of OH. HONO's contribution to the HOx budget decreases as emissions are lost and by 6 hours downwind, HCHO photolysis becomes the dominant HOx source. Secondary sources of HONO are minor compared to emissions but pNO3 photolysis becomes significant in older smoke and helps HONO continue to contribute at least 5% to HOx production
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Nitrous Acid Formation and Chemistry over Soil and Snow in a Polluted Rural Area
Full text linkDespite of decades of research, the formation mechanism of daytime HONO is still not fully understood and little is known about the daytime behavior of HONO over soil and snow surfaces in polluted rural areas. HONO measurements in two regions heavily affected by oil and gas drilling activities (in Wyoming and Utah) show high levels of daytime HONO (above 500 pptv) during the wintertime. However, questions were raised about the reliability of wintertime HONO observations as recent studies show that many in-situ HONO measurement methods suffer from pernitric acid (PNA) interferences that overestimate HONO concentrations.The UCLA Long-Path (LP) Differential Optical Absorption Spectroscopy (DOAS) instrument measured HONO and HONO gradients in the Uintah Basin (UB), a region highly impacted by oil and gas activities. The UCLA LP-DOAS system measured HONO on three light paths covering height intervals from 2 m to 31, 45, and 68 m above ground level (agl). Due to disparate meteorological conditions in 2012 and 2014, HONO was measured over soil in 2012 and over snow in 2014. HONO mixing ratios averaged 74 ppt in 2012. These levels are lower than HONO measured in polluted urban areas but comparable to levels measured in rural environments. Distinct daytime negative vertical gradients of HONO were observed in 2012, with higher levels of HONO measured on the lower than upper light paths, an indication that HONO had a surface source in 2012. HONO vertical profiles were retrieved using a novel least square minimization approach, in which the average mixing ratios of HONO along the light paths were simulated using a height dependent exponential function. The HONO vertical profiles, together with NOAA micrometeorological data were used to calculate HONO fluxes at 19 m agl. Sunny days hourly average HONO fluxes show that they follow the same trend as solar irradiance, with a maximum of (1.7�0.3) x1010 molec. cm-2 s-1 at noontime, supporting the idea of a photolytic surface HONO source in 2012. Analysis of HONO flux and photolytic surface HONO sources show that under conditions of high NO2 levels, conversion of NO2 on ground surfaces is most likely the source of daytime HONO. In contrast, on days with moderate NO2 levels, photolysis of surface-adsorbed HNO3 and nocturnal uptake of HONO followed by daytime acid displacement might play an important role; however, additional measurements are needed to confirm this theory. Comparison of the missing source of HONO (Punknwon) and HONO surface flux rate shows that the HONO surface flux rate accounts for (63�32%) of the Punknown throughout the day indicating that photolytic surface sources of HONO were the dominant sources of HONO in 2012. HONO mixing ratios measured in 2014 were on average 96 ppt. Comparison of DOAS HONO measurements with four in-situ instruments shows that the DOAS system consistently measured the lowest HONO levels. Preliminary analysis of PNA interferences shows that some in-situ measurements might overestimate HONO concentrations in the presence of PNA (average maximum concentration of 120 ppt measured around noontime), therefore, HONO DOAS were considered the reference measurements in the 2014 study. 2014 HONO vertical profiles also show higher HONO mixing ratios near the surface and a rapid decay of HONO mixing ratios with height. The sunny days hourly average HONO fluxes were also positive with a maximum HONO flux near noontime of (1.1�0.7)x1010 molec. cm-2 s-1. The sunny days hourly average HONO fluxes also follow a similar temporal trend as solar irradiance, further supporting the hypothesis of a photolytic surface HONO source in 2014. Due to insufficient data, we were not able to determine which photolytic surface HONO formation pathway dominated in 2014. However, we were able to determine that the measured 2014 HONO was most probably formed at the top of the snowpack and not in the firn. This result is supported by the surprisingly similar HONO fluxes measured in 2012 and 2014, which indicate that the HONO formation mechanisms during these two years were probably very similar. Comparison of the missing source of HONO (Punknwon) and HONO surface flux rate shows that HONO surface flux rate accounts for (54�% 15) of the Punknown in the early morning and noontime, suggesting that HONO sources at noon and early morning are light dependent and are produced on the snow surface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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Observation-based modeling of ozone chemistry in the Seoul metropolitan area during the Korea-United States Air Quality Study (KORUS-AQ)
Full text linkThe Seoul Metropolitan Area (SMA) has a population of 24 million and frequently experiences unhealthy levels of ozone (O3). In this work, measurements taken during the Korea-United States Air Quality Study (KORUS-AQ, 2016) are used to explore regional gradients in O3 and its chemical precursors, and an observationally-constrained 0-D photochemical box model is used to quantify key aspects of O3 production including its sensitivity to precursor gases. Box model performance was evaluated by comparing modeled concentrations of select secondary species to airborne measurements. These comparisons indicate that the steady state assumption used in 0-D box models cannot describe select intermediate species, highlighting the importance of having a broad suite of trace gases as model constraints. When fully constrained, aggregated statistics of modeled O3 production rates agreed with observed changes in O3, indicating that the box model was able to represent the majority of O3 chemistry.
Comparison of airborne observations between urban Seoul and a downwind receptor site reveal a positive gradient in O3 coinciding with a negative gradient in NOx, no gradient in CH2O, and a slight positive gradient in modeled rates of O3 production. Together, these observations indicate a radical-limited (VOC-limited) O3 production environment in the SMA. Zero-out simulations identified C7+ aromatics as the dominant VOC contributors to O3 production, with isoprene and anthropogenic alkenes making smaller but appreciable contributions. Simulations of model sensitivity to decreases in NOx produced results that were not spatially uniform, with large increases in O3 production predicted for urban Seoul and decreases in O3 production predicted for far-outlying areas. The policy implications of this work are clear: Effective O3 mitigation strategies in the SMA must focus on reducing local emissions of C7+ aromatics, while reductions in NOx emissions may increase O3 in some areas but generally decrease the regional extent of O3 exposure
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Disentangling physical and biological drivers of optical signals for improved monitoring of evergreen needleleaf photosynthesis
Full text linkThe largest source of uncertainty in global climate models is terrestrial carbon cycle feed- backs. One of the most important but most poorly understood vegetation types in the global carbon cycle is evergreen needleleaf forests (ENFs). To address this challenge, a growing appreciation for the stress physiology of photosynthesis has inspired emerging techniques to detect ENF photosynthetic activity with optical signals. This includes the use of solar- induced chlorophyll fluorescence (SIF), a small light signal emitted by plants during the photosynthetic process. SIF has shown a marked improvement over traditional reflectance- based vegetation indices in tracking ENF photosynthesis. However, SIF, as well as other optical signals, in ENF are complicated by photon-plant interactions over complex canopy structures (physical) and unique adaptations to deal with the seasonal stress of winter while retaining their needles (biological).In this dissertation, we identify the physical and biological drivers of optical signals in ENF and connect remote sensing observations with physiological processes to improve monitoring of evergreen needleleaf photosynthesis. In Chapter 2, we provide a broad overview for non-specialists of the biological basis for using optical signals to track evergreen needleleaf photosynthesis. We then explore these topics in more detail by using tower-based remote sensing data across four ENF sites which span the climatic gradient experienced by ENF (details in Chapter 3). In Chapters 4 and 5 we zoom in to a single site in Canada and explore the temporal dynamics of different optical metrics and their biological underpinnings. In Chapter 6 we then show how to combine multiple metrics across multiple sites to improve predictions of forest carbon uptake.Ultimately this work advances our understanding of ENF photosynthesis and our ability to predict the fate of ENFs in a changing climate. Future work will help scale and integrate the understandings gleaned in this dissertation to satellite and modeling frameworks
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Nitrous acid (HONO) Chemistry over Snow in the Uintah Basin, Utah
Full text linkOH radicals are crucial for the formation of secondary organic aerosol (SOA) and ozone in the atmosphere. HONO photolysis is one of the main sources of OH radicals, especially in winter. HONO chemistry in polluted areas has been a widely discussed in the last five dec-ades. However, HONO chemistry is often absent or simplified in air quality models, making its impact on OH radical chemistry uncertain. In this thesis, I investigated the formation of HONO over snow in a polluted rural area.The platform for atmospheric chemistry and vertical transport in one dimension (PACT-1D) model was used to calculate a 4-day cycle (Jan. 18th -Jan. 21st,2014) of atmospheric composi-tion and HONO figures. The model output was compared with observational data from the Uintah Basin Winter Ozone Study 2014(UBWOS 2014). Modeled HONO mixing ratios compared well with observation. HONO fluxes from surface chemistry in the model shows good agreement of shape and magnitude.
In conclusion, PACT-1D model calculations show that chemical HONO formation on the snow explain the observed HONO mixing ratios
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