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  • 1
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    The role of foehn in the formation of heavy air pollution events in Urumqi, China (2015)
    Wiley
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    Publication Date: 2015-05-17
    Description: The impact of sandwich foehn on air pollution in Urumqi, a gap town located on the northern leeside of the Tianshan Mountains of China, is analyzed. The results show that during days with high pollution, the boundary layer over the city and the down-valley area can be divided into a three-layer structure, with the southeasterly foehn sandwiched between the northwesterly winds on top and the cold-air surface pool beneath. The southeasterly foehn at heights between 480 and 2,100 m results in a very stable boundary layer structure. In combination with the decoupling between the foehn flow and cold air pool, such boundary layer structure prevents vertical mixing of atmospheric pollutants. In the up-valley area from the northern leeside flank to the southern urban area, the ground-based foehn confronts the thermally driven valley breeze and forms a “mini-front,” which moves northwards in the morning and retreats southwards in the afternoon. Although the mini-front disappears in the early evening, the wind shear of the mountain breeze between the southern suburb and downtown areas is still remarkable, which is favorable for a convergence line to persist around the city all day long. In this case, air pollutants emitted from the up-valley and down-valley areas are transported toward the urban area. Therefore, the air pollutants accumulate daily, leading to the frequent occurrence of heavy pollution events in Urumqi. This indicates that the sandwich foehn plays a critical role in the formation of heavy air pollution events in Urumqi.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Seismic Spectral Ratios Between North Korean Nuclear Tests: Implications for Their Seismic Sources (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Seismic spectral ratios between the 3 September 2017 North Korean nuclear test (NKT2017) and four earlier Korean tests (25 May 2009, NKT2009; 12 February 2013, NKT2013; 6 January 20016, NKT2016J; and 9 September 2016, NKT2016S) are investigated. All the observed teleseismic P wave spectral ratios exhibit a unique notch at approximately 2.5 Hz, but it is not observed for spectral ratios of regional P and Lg waves. Meanwhile, for a given event pair, the network‐averaged Lg wave spectral ratio is similar to that of regional P wave, but with the explosions' source corner frequencies significantly reduced, supporting the conjecture by Fisk (2006, https://doi.org/10.1785/0120060023). We demonstrated that the observed notch of teleseismic P wave spectral ratios may be well modeled with interference between pP and P waves with an apparent pP‐P time delay ΔtpP of about 0.4 s for NKT2017, while regional P wave spectral ratios may be well fitted with source spectral ratios predicted by classical explosion source models including Mueller and Murphy (1971, https://doi.org/10.2172/4107998) (MM71), Denny and Johnson (1991, https://doi.org/10.1029/gm065p0001) (DJ91), and their two hybrids. Via the above modeling, yield sizes of the explosions and the depth of burials of NKT2009–2016S are determined as functions of NKT2017's buried depth h0. Our result indicate that, for h0 in the range of 600–1,100 m, the MM71‐related models give yield estimations about 100–300 kt for NKT2017, 3–7 kt for NKT2009, 6–15 kt for NKT2013 and NKT2016J, and 10–25 kt for NKT2016S, while yield sizes obtained by model DJ91 are much smaller. Nevertheless, for all the above source models, the obtained depths of burial estimations of NKT2009–2016S are always significantly smaller than NKT2017's depth of burial h0.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Surface meteorological conditions and boundary‐layer height variations during an air pollution episode in Nanjing, China (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract The evolution of planetary boundary‐layer (PBL) was investigated using observations from laser ceilometer, eddy covariance system and automatic meteorological station in the north of Nanjing city during an air pollution episode in 2016‐2017 winter. Based on 7‐day observation under clean to polluted day, we recorded the temporal variations of backscatter signals observed by the ceilometer, and then inter‐compare planetary boundary‐layer height (PBLH) retrieved from individual methods. The results show that backscatter signal gradient, standard deviation, and wavelet transform analysis methods generated similar PBLH values and PBL diurnal variation patterns. Moreover, the PBL structure varied diurnally, with distinct patterns corresponding to clean and polluted days. Based on these measurements, the relationships between PBLH, weather conditions, and contaminants were analyzed. Results show that: on clean days, strong surface turbulence exchange makes the PBL fully developed and makes the PBLH increased sharply after sunrise, with a maximum of 1483 m; on polluted days, stable synoptic conditions and the weaker wind speeds facilitated the accumulation of air pollutants, leading to smaller net surface radiation and weaker turbulence. Consequently, these conditions during polluted days led to lower PBLH values, which were typically less than 900 m.
    Print ISSN: 2169-897X
    Electronic ISSN: 2169-8996
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Regional air pollution brightening reverses the greenhouse gases induced warming-elevation relationship (2015)
    Wiley
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    Publication Date: 2015-05-27
    Description: Mountain waters, glaciers, hazards and biodiversity are vulnerable to the impacts of global warming. Warming is projected to amplify over mountains by global climate models, yet meteorological records do not show a uniform acceleration of warming with elevation. Here, we explore warming-elevation relationships using records from 2660 meteorological stations, and determine that the vertical gradient of warming rate varies with location. The warming is faster at higher altitudes in Asia and Western North America, but the opposite is observed over Central Europe and Eastern North America which have received more short-wave radiation (brightening) associated with a decrease of aerosols and clouds since the 1980s. We found that altitudinal differences in air pollution (brightening), with observations showing more short-wave radiation received at low altitudes than at mountains, modulate the warming-elevation relationships. The advance in understanding of the drivers of regional climate change will contribute to the formulation of strategies for climate change mitigation at high elevations.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Theoretical bounce resonance diffusion coefficient for waves generated near the equatorial plane (2016)
    Wiley
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    Publication Date: 2016-07-17
    Description: Theoretical bounce resonance diffusion coefficient for waves generated near the equatorial plane with arbitrary wave normal angle distributions is derived. Previous studies either assumed waves to cover the whole bounce trajectory or to have only one wave normal angle for a given frequency. In this work, we theoretically derive a new bounce resonance diffusion coefficient without these limitations. We demonstrate that the pitch angle diffusion from bounce resonance is significant for near-equatorially mirroring particles using a published magnetosonic wave model. Our results suggest that the bounce resonance diffusion by magnetosonic waves could be an important mechanism for pitch angle scattering of near equatorially mirroring particles; therefore, it might be important to include bounce resonance with magnetosonic waves in global radiation belt modeling.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Storm Time Current Distribution in the Inner Equatorial Magnetosphere: THEMIS Observations (2016)
    Wiley
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    Publication Date: 2016-05-12
    Description: For the first time, the current density distribution in the inner equatorial magnetosphere ranging from 4-12 R E (R E is the Earth radius, 6371 km) has been obtained by using THEMIS (P3, P4, P5) three point magnetic measurements. This study mainly focuses on the storm events when the constellation of the three THEMIS spacecraft has relatively small separation distance. Two cases with different storm activities are first displayed to illustrate the effectiveness of the method. The inner magnetospheric equatorial current distribution ranging from 4-12 R E is shown through statistical analysis. The features of current density are separately analyzed for the storm main phase and the recovery phase. The statistical study reveals that with increasing radial distance the predominant ring current density reverses from Eastward (below r = 4.8 R E , where r is the geocentric radial distance) to Westward, but that the distribution behaves differently for the two phases of activity. During the main phase, both the westward and eastward current are enhanced by added signal and are more dynamic so that both radial profile and magnetic local time (MLT) structure is obscured. During the recovery phase, the radial profile of the westward current is smooth and peaks, then falls, between r = 5-7.5R E showing some MLT dependence in this region. Beyond r = 7.5 R E , the current is lower and nearly constant and shows little MLT variation. The results also suggest that the change from eastward to westward current depends on the storm phase and hence storm activity.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Highly relativistic radiation belt electron acceleration, transport, and loss: large solar storm events of March and June 2015 (2016)
    Wiley
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    Publication Date: 2016-07-10
    Description: Two of the largest geomagnetic storms of the last decade were witnessed in 2015. On 17 March 2015, a coronal mass ejection-driven event occurred with a Dst (storm time ring current index) value reaching -223 nT. On 22 June 2015 another strong storm (Dst reaching -204 nT) was recorded. These two storms each produced almost total loss of radiation belt high-energy (E ≳ 1 MeV) electron fluxes. Following the dropouts of radiation belt fluxes there were complex and rather remarkable recoveries of the electrons extending up to nearly 10 MeV in kinetic energy. The energized outer zone electrons showed a rich variety of pitch angle features including strong “butterfly” distributions with deep minima in flux at α = 90°. However, despite strong driving of outer zone Earthward radial diffusion in these storms, the previously reported “impenetrable barrier” at L ≈2.8 was pushed inward, but not significantly breached, and no E ≳ 2.0 MeV electrons were seen to pass through the radiation belt slot region to reach the inner Van Allen zone. Overall these intense storms show a wealth of novel features of acceleration, transport, and loss that are demonstrated in the present detailed analysis.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    A major solar eruptive event in July 2012: Defining extreme space weather scenarios (2013)
    Wiley
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    Publication Date: 2013-09-20
    Description: [1]  A key goal for space weather studies is to define severe and extreme conditions that might plausibly afflict human technology. On 23 July 2012 solar active region 1520 (~141°W heliographic longitude) gave rise to a powerful coronal mass ejection (CME) with an initial speed that was determined to be 2500±500 km/s. The eruption was directed away from Earth toward 125°W longitude. STEREO-A sensors detected the CME arrival only about 19 hours later and made in situ measurements of the solar wind and interplanetary magnetic field. In this paper we address the question of what would have happened if this powerful interplanetary event had been Earthward directed. Using a well-proven geomagnetic storm forecast model, we find that the 23-24 July event would certainly have produced a geomagnetic storm that was comparable to the largest events of the 20th Century (Dst ~ -500nT). Using plausible assumptions about seasonal and time-of-day orientation of the Earth's magnetic dipole, the most extreme modeled value of storm-time disturbance would have been Dst = -1182nT. This is considerably larger than estimates for the famous Carrington storm of 1859. This finding has far reaching implications because it demonstrates that extreme space weather conditions such as those during March of 1989 or September of 1859 can happen even during a modest solar activity cycle such as the one presently underway. We argue that this extreme event should immediately be employed by the space weather community to model severe space weather effects on technological systems such as the electric power grid.
    Print ISSN: 1539-4964
    Electronic ISSN: 1542-7390
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    An Experimental Study on Binderless Briquetting of Low-Rank Coals (2013)
    Wiley
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    Publication Date: 2013-04-06
    Description: The potential of binderless briquetting as a means of transforming low-rank coals into low moisture high grade solid fuel products has been studied. Using two dried low-rank coals, binderless briquettes with high mechanical strength have been successfully produced through mechanical compression. An increase in heating value was achieved as a result of moisture reduction in the briquettes compared to as-received coals. The residue moisture content in the briquettes had a predominant effect on briquetting characteristics and there existed an optimum moisture content for the maximum briquettes strength. The chemical structure and wettability of binderless briquettes were analyzed using FTIR and contact angle measurement. The results showed that hydrophobicity and chemical structure significantly affected the briquette properties. High mechanical strength binderless briquettes using two dried low-rank coals were successfully produced. Drying of low-rank coals prior to briquetting resulted in 30–50 % increase in calorific value. The optimum moisture content for high compressive strength was 12–15 %. The aromaticity of the briquettes was higher than raw coals as a result of decomposition of oxygen functionalities and aliphatic hydrogen groups.
    Print ISSN: 0930-7516
    Electronic ISSN: 1521-4125
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Published by Wiley
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  • 10
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    An efficient and positivity-preserving layer method for modeling radiation belt diffusion processes (2016)
    Wiley
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    Publication Date: 2016-01-08
    Description: An efficient and positivity-preserving layer method is introduced to solve the radiation belt diffusion equation and is applied to study the bounce resonance interaction between relativistic electrons and magnetosonic waves. The layer method with linear interpolation, denoted by LM-L (layer method-linear), requires the use of a large number of grid points to ensure accurate solutions. We introduce a monotonicity- and positivity-preserving cubic interpolation method to be used with the Milstein-Tretyakov layer method. The resulting method, called LM-MC (layer method-monotone cubic), can be used to solve the radiation belt diffusion equation with a much smaller number of grid points than LM-L, while still being able to preserve the positivity of the solution. We suggest that LM-MC can be used to study long-term dynamics of radiation belts. We then develop a 2D LM-MC code and use it to investigate the bounce resonance diffusion of radiation belt electrons by magnetosonic waves. Using a previously published magnetosonic wave model, we demonstrate that bounce resonance with magnetosonic waves is as important as gyro-resonance; both can cause several orders of magnitude increase of MeV electron fluxes within one day. We conclude that bounce resonance with magnetosonic waves should be taken into consideration together with gyro-resonance.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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