1,720,993 research outputs found
Time Resolved EPR of Excited Triplet C60 Aligned in Nematic Liquid Crystals
No full textThe photoexcited triplet states of fullerenes C60 and C70 dissolved in a nematic liquid-crystal solvent have been investigated using time-resolved EPR (TR-EPR) spectroscopy. Triplet spectra composed of two lines with opposite polarization (enhanced absorption and emission) and small line widths have been recorded for C60, but much broader lines are recorded for C70. The Jahn-Teller dynamic distortion of C60 is responsible for the motional narrowing of the triplet lines, and the partial orientation induced by the nematic solvent removes the degeneracy of the two triplet transitions giving rise to a doublet of lines. Broad features appearing at low temperatures in the nematic phase are attributed to the local disorder induced by the dissipation of excess energy in the photoexcitation process. The time decay of the TR-EPR signals is explained by a kinetic model for the spin-level populations including spin relaxation and selective decay of the spin sublevels
Drought and Human Mobility in Africa
Full text linkHuman mobility from droughts is multifaceted and depends on environmental, political, social, demographic and economic factors. Although droughts cannot be considered as the single trigger, they significantly influence people's decision to move. Yet, the ways in which droughts influence patterns of human settlements have remained poorly understood. Here we explore the relationships between drought occurrences and changes in the spatial distribution of human settlements across 50 African countries for the period 1992–2013. For each country, we extract annual drought occurrences from two indicators, the international disaster database EM-DAT and the standardized precipitation evapotranspiration index (SPEI-12) records, and we evaluate human settlement patterns by considering urban population data and human distance to rivers, as derived from nighttime lights. We then compute human displacements as variations in human distribution between adjacent years, which are then associated with drought (or non-drought) years. Our results show that drought occurrences across Africa are often associated with (other things being equal) human mobility toward rivers or cities. In particular, we found that human settlements tend to get closer to water bodies or urban areas during drought conditions, as compared to non-drought periods, in 70%–81% of African countries. We interpret this tendency as a physical manifestation of drought adaptation, and discuss how this may result into increasing flood risk or overcrowding urban areas. As such, our results shed light on the interplay between human mobility and climate change, bolstering the analysis on the spatiotemporal dynamics of drought risks in a warming world
The orientation of the principal axes of the electron dipolar interaction tensor in triplet state C60 monoadducts
No full textThe time resolved EPR spectra of a fullerene monoadduct triplet state has been studied in nematic phase of E7. The EPR lineshape has been explained with a motionally averaged, preferentially oriented, triplet spectrum. From the knowledge of the order parameters of the C-60 adduct, obtained from the analysis of EPR spectra of a very similar C-60-nitroxide derivative dissolved in the same nematic solvent, it was possible to determine that the X principal axis of the triplet state dipolar tensor is parallel to the C-2 symmetry axis of the C-60 monoadduc
Contribution of anthropogenic and hydroclimatic factors on the variation of surface water extent across the contiguous United States
Full text linkHuman pressure and climate variability are significantly threatening freshwater resources, with cascading effects on societies and ecosystems. In this context, it is crucial to understand the anthropogenic and climatic impacts on surface water dynamics. Here, we examine the interaction between the variation of surface water extent and the change in five potential concurrent drivers across river basins of the contiguous United States (CONUS) during the period 1984-2020. In particular, built-up area, population, and irrigated land are regarded as the anthropogenic drivers, while hydroclimatic drivers are represented by precipitation and potential evapotranspiration (PET). We perform statistical analyses in order to quantify the change in the considered variables and then identify significantly different spatial patterns and possible interrelations. Results show that almost 79% (169 out of 204 river basins) of the CONUS experienced an expansion of surface water extent mainly in the continental and temperate climatic regions (mean expansion 158.33 km2). Increasing precipitation is found to be the most widespread driver of the gain in surface water extent, affecting nearly 70% of river basins. The remaining 35 river basins of the CONUS, mostly located in the arid southwestern region of the country, faced a reduction in surface water extent (mean reduction −146.73 km2). The expansion of built-up areas and increasing PET resulted to contribute to the loss of surface water in all the river basins, followed by population growth (in ∼75% of the river basins), decreasing precipitation (in ∼60% of the river basins, all situated in southwestern US), and irrigated land expansion (in ∼55% of the river basins). Our findings shed light on the potential impacts of the variability of anthropogenic and hydroclimatic factors on hydrology and surface water resources, which could support predictive adaptation strategies that ensure water conservation
Identification of the Sites of Chlorophyll Triplet Quenching in Relation to the Structure of LHC-II from Higher Plants. Evidence from EPR Spectroscopy
No full textThe Chlorophyll a (Chl a) molecules involved in the triplet-triplet energy transfer to the central luteins in trimeric LHC-II are identified by time-resolved and pulse EPR techniques. The concept of spin angular momentum conservation during triplet-triplet energy transfer is exploited for the calculation of the spin polarization of the carotenoid triplet states. The sites with the highest probability of forming triplet states, which are quenched by the central luteins, result to be Chl603 and Chl612. "Unquenched" Chl triplet states are produced by photoexcitation in the LHC-II complex. Putative sites of these triplet states are Chl614, Chl611, Chl604, and Chl613 since they do not contribute to the formation of the observed carotenoid triplet state
Optical spectrum of C-60 mono-adducts: assignment of transition bands using time-resolved EPR magneto-photo-selection
No full textThe magneto-photo-selection technique implemented in the time-resolved EPR (TR-EPR) experiment is used for studying the characteristics of the optical spectrum of C60 mono-adducts, in the 410–690 nm wavelength range. The analysis of the shape of the triplet state TR-EPR spectra of the mono-adducts, recorded after laser light pulses having polarization parallel or perpendicular to the magnetic field direction allows to determine the orientation distribution of the excited molecules; whence the direction of transition moments in the molecular frame is inferred. This information provides the assignment of the vibronic states symmetries
Pulsed EPR and ENDOR on the Peridinin Triplet State Involved in the Photoprotective Mechanism in Peridinin-Chlorophyll a-Proteins
No full textThe photoexcited triplet state of the carotenoid peridinin in the peridinin-chlorophyll a-protein (PCP) of the dinoflagellate Heterocapsa pygmaea has been investigated by pulsed electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectroscopies. The alpha- and beta-protons hyperfine couplings of the peridinin-conjugated chain have been derived from Davies and Mims ENDOR experiments. The spectroscopic results have been compared to those obtained for the main form of the PCP complex and for the high-salt PCP form from Amphidinium carterae. The EPR features of the peridinin triplet state are very similar in the antenna complexes belonging to the two different dinoflagellate species, proving that the triplet formation pathway and the triplet localization on one specific peridinin per subcluster are common features of different PCP antennas. No significant variation of the hyperfine couplings of the peridinin triplet state has been detected between the main form of the PCP complex from A. carterae and H. pygmaea. The spectroscopic results confirm the close relationship between the Amphidinium PCP and the corresponding Heterocapsa complex at least in terms of mutual arrangement of the chlorophyll a-peridinin pair involved in photoprotection and in terms of conformation of the peridinin-conjugated chain
Influence of Urban Areas on Surface Water Loss in the Contiguous United States
Full text linkUrbanization is one of the main drivers of surface water loss, which implies a transition from water to land. However, it is still unclear how urban areas affect the spatial pattern of surface water loss. Here, we use remotely sensed data to analyze and model the decrease of surface water extent and, in particular, the frequency of surface water loss as a function of distance from urban areas across the contiguous United States (CONUS). We employ an exponential distance-decay model that confirms the presence of a higher frequency of surface water loss in the proximity of human settlements and provides innovative insights on surface water loss patterns at different spatial scales (i.e., river basins, water resource regions, and the CONUS). These spatial patterns are found to be influenced by climatic conditions, with more widely distributed losses in arid regions with respect to temperate and continental climates. Our results provide a new and deeper understanding of the spatial influence of urban areas on surface water loss, which could be effectively integrated in the definition of sustainable strategies for urbanization, water management, and surface water restoration, focused on both human and environmental water needs
Why the 2022 Po River drought is the worst in the past two centuries
Full text linkThe causes of recent hydrological droughts and their future evolution under a changing climate are still poorly
understood. Banking on a 216-year river flow time series at the Po River outlet, we show that the 2022 hydrological
drought is the worst event (30% lower than the second worst, with a six-century return period), part of an
increasing trend in severe drought occurrence. The decline in summer river flows (−4.14 cubic meters per
second per year), which is more relevant than the precipitation decline, is attributed to a combination of
changes in the precipitation regime, resulting in a decline of snow fraction (−0.6% per year) and snowmelt
(−0.18 millimeters per day per year), and to increasing evaporation rate (+0.013 cubic kilometers per year)
and irrigated areas (100% increment from 1900). Our study presents a compelling case where the hydrological
impact of climate change is exacerbated by local changes in hydrologic seasonality and water use
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