1,721,172 research outputs found
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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
Comparison of halocarbon measurements in an atmospheric dry whole air sample
Full text linkAbstract The growing awareness of climate change/global warming, and continuing concerns regarding stratospheric ozone depletion, will require continued measurements and standards for many compounds, in particular halocarbons that are linked to these issues. In order to track atmospheric mole fractions and assess the impact of policy on emission rates, it is necessary to demonstrate measurement equivalence at the highest levels of accuracy for assigned values of standards. Precise measurements of these species aid in determining small changes in their atmospheric abundance. A common source of standards/scales and/or well-documented agreement of different scales used to calibrate the measurement instrumentation are key to understanding many sets of data reported by researchers. This report describes the results of a comparison study among National Metrology Institutes and atmospheric research laboratories for the chlorofluorocarbons (CFCs) dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11), and 1,1,2-trichlorotrifluoroethane (CFC-113); the hydrochlorofluorocarbons (HCFCs) chlorodifluoromethane (HCFC-22) and 1-chloro-1,1-difluoroethane (HCFC-142b); and the hydrofluorocarbon (HFC) 1,1,1,2-tetrafluoroethane (HFC-134a), all in a dried whole air sample. The objective of this study is to compare calibration standards/scales and the measurement capabilities of the participants for these halocarbons at trace atmospheric levels. The results of this study show agreement among four independent calibration scales to better than 2.5% in almost all cases, with many of the reported agreements being better than 1.0%
Litter microbial respiration and enzymatic resistance to drought stress
Full text linkMany ecosystems are experiencing an increase in drought conditions as a consequence of climate warming and changing precipitation patterns. The stress imposed by these environmental changes can affect ecosystem processes such as the extracellular enzymatic degradation of carbon-containing leaf litter by soil microbial communities. However, the magnitude of these impacts may depend on the composition and metabolism of the microbial community. Based on the hypothesis of local adaptation, microbial communities native to warm-dry ecosystems should display a greater capacity to degrade leaf litter polymers with extracellular enzymes following exposure to warm-dry conditions. To test this hypothesis, we performed a microcosm study in which we monitored extracellular enzyme activity and respiration of microbial communities from five ecosystems along a southern California climate gradient, ranging from warmer, drier desert to wetter, cooler subalpine forest. To simulate drought and rewetting, we subjected microcosms to periods of high temperature and low moisture followed by a water pulse. We found that enzyme activity of wet-cool communities generally exceeded that of warm-dry communities across enzyme types for the five sites we considered. Additionally, we observed a significant decrease in respiration for all communities after longer durations of drought exposure. Although these findings did not align with our expectations of local adaptation, they suggest litter-inhabiting microbial communities are able to retain metabolic functioning in environmental conditions different from those of their native ecosystems. These results may imply that factors such as litter chemistry impose greater constraints than climate on community metabolic function. Overall, despite differences in local climates, microbial communities from semiarid regions may be metabolically adapted to maintain functioning in the face of drought
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Evaluation of simulated O3 production efficiency during the KORUS-AQ campaign: Implications for anthropogenic NOx emissions in Korea
Full text linkWe examine O3 production and its sensitivity to precursor gases and boundary layer mixing in Korea by using a 3-D global chemistry transport model and extensive observations during the KORea-US cooperative Air Quality field study in Korea, which occurred in May–June 2016. During the campaign, observed aromatic species onboard the NASA DC-8 aircraft, especially toluene, showed high mixing ratios of up to 10 ppbv, emphasizing the importance of aromatic chemistry in O3 production. To examine the role of VOCs and NOx in O3 chemistry, we first implement a detailed aromatic chemistry scheme in the model, which reduces the normalized mean bias of simulated O3 mixing ratios from –26% to –13%. Aromatic chemistry also increases the average net O3 production in Korea by 37%. Corrections of daytime PBL heights, which are overestimated in the model compared to lidar observations, increase the net O3 production rate by ~10%. In addition, increasing NOx emissions by 50% in the model shows best performance in reproducing O3 production characteristics, which implies that NOx emissions are underestimated in the current emissions inventory. Sensitivity tests show that a 30% decrease in anthropogenic NOx emissions in Korea increases the O3 production efficiency throughout the country, making rural regions ~2 times more efficient in producing O3 per NOx consumed. Simulated O3 levels overall decrease in the peninsula except for urban and other industrial areas, with the largest increase (~6 ppbv) in the Seoul Metropolitan Area (SMA). However, with simultaneous reductions in both NOx and VOCs emissions by 30%, O3 decreases in most of the country, including the SMA. This implies the importance of concurrent emission reductions for both NOx and VOCs in order to effectively reduce O3 levels in Korea
Surface ozone in the Colorado northern Front Range and the influence of oil and gas development during FRAPPE/DISCOVER-AQ in summer 2014
Full text linkHigh mixing ratios of ozone (O3) in the northern Front Range (NFR) of Colorado are not limited to the urban Denver area but were also observed in rural areas where oil and gas activity is the primary source of O3 precursors. On individual days, oil and gas O3 precursors can contribute in excess of 30 ppb to O3 growth and can lead to exceedances of the EPA O3 National Ambient Air Quality Standard. Data used in this study were gathered from continuous surface O3 monitors for June–August 2013–2015 as well as additional flask measurements and mobile laboratories that were part of the FRAPPE/DISCOVER-AQ field campaign of July–August 2014. Overall observed O3 levels during the summer of 2014 were lower than in 2013, likely due to cooler and damper weather than an average summer. This study determined the median hourly surface O3 mixing ratio in the NFR on summer days with limited photochemical production to be approximately 45–55 ppb. Mobile laboratory and flask data collected on three days provide representative case studies of different O3 formation environments in and around Greeley, Colorado. Observations of several gases (including methane, ethane, CO, nitrous oxide) along with O3 are used to identify sources of O3 precursor emissions. A July 23 survey demonstrated low O3 (45–60 ppb) while August 3 and August 13 surveys recorded O3 levels of 75–80 ppb or more. August 3 exemplifies influence of moderate urban and high oil and gas O3 precursor emissions. August 13 demonstrates high oil and gas emissions, low agricultural emissions, and CO measurements that were well correlated with ethane from oil and gas, suggesting an oil and gas related activity as a NOx and O3 precursor source. Low isoprene levels indicated that they were not a significant contributor to O3 precursors measured during the case studies
Atmospheric Ethane-Methane Relationship and Implications for the Arctic
Full text linkThe purpose of this project is to evaluate hydrocarbon concentrations, their chemistry in the atmosphere, and the corresponding implications for arctic ecosystems. Recently, methane’s increasing threat as a greenhouse gas has warranted much research in the scientific field as global warming trends continue. A subject of significantly less research but perhaps of equal importance are ethane emissions, which are crucial to the understanding of methane’s growth. Though methane and ethane share anthropogenic sources, methane concentrations in the atmosphere vary due to biogenic sources specific to the chemical compound. Due to this constraint, co-measurement of methane and ethane is important because of the strong correlation between the two chemicals due to their shared anthropogenic sources. A significant upturn in methane growth without a corresponding increase in ethane may indicate releases of methane from biogenic sources such as melting permafrost. Recent studies have shown that ethane concentrations are decreasing worldwide, likely due to sequestration of anthropogenic fossil fuel emissions. However, methane concentrations are becoming a focus of concern for environmentalists
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
Atmospheric Ethane-Methane Relationship and Implications for the Arctic
Full text linkThe purpose of this project is to evaluate hydrocarbon concentrations, their chemistry in the atmosphere, and the corresponding implications for arctic ecosystems. Recently, methane’s increasing threat as a greenhouse gas has warranted much research in the scientific field as global warming trends continue. A subject of significantly less research but perhaps of equal importance are ethane emissions, which are crucial to the understanding of methane’s growth. Though methane and ethane share anthropogenic sources, methane concentrations in the atmosphere vary due to biogenic sources specific to the chemical compound. Due to this constraint, co-measurement of methane and ethane is important because of the strong correlation between the two chemicals due to their shared anthropogenic sources. A significant upturn in methane growth without a corresponding increase in ethane may indicate releases of methane from biogenic sources such as melting permafrost. Recent studies have shown that ethane concentrations are decreasing worldwide, likely due to sequestration of anthropogenic fossil fuel emissions. However, methane concentrations are becoming a focus of concern for environmentalists
An Investigation into Seasonal Trends of Terpene Emissions from Biogenic Sources: A Focus on Monoterpene and Sesquiterpene Emissions of Native Colorado Tree Species
Full text linkPlants naturally emit a variety of highly reactive hydrocarbon compounds, classified as biogenic volatile organic compounds (BVOC), which are known to have large implications for atmospheric chemistry. Once released into the troposphere, BVOC participate in reactive nitrogen, hydroxyl and ozone chemistry as well as secondary organic aerosol formation. The resulting air quality impacts warrant a thorough understanding of BVOC emission behavior, critical for effective regional modeling and legislative decision-making. This work aims to improve the database and algorithms used in biogenic emission models by examining the seasonal trends of two classes of BVOC, monoterpene (MT) and sesquiterpene (SQT) hydrocarbons. A field site established at a local tree nursery provided an opportunity to easily monitor the emissions of naturally growing vegetation throughout a growing season. The emission rates of five tree species native to Colorado forests were measured monthly between February 2009 and February 2010. Biogenic emissions were found to exhibit seasonal variation with higher emission rates observed between spring and late summer, falling to a low through winter months. The seasonal behavior of BVOC emissions proves to be very complex. These findings are discussed and call into question the methods used for estimating annual emission rates
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