Managing while invisible: how the gig economy shapes us and our cities

By making themselves invisible, platforms cast themselves as digital infrastructures without responsibilities or agency, write Daniel Curto-Millet and Roser Pujada

Influence de la pluviométrie sur la contamination de l'atmosphère et des eaux de pluie par les pesticides

Cette étude a pour objectif d'identifier les facteurs qui influencent la contamination des eaux de pluie par les produits phytosanitaires. Cinq sites contrastés ont été choisis de manière à être représentatifs des zones de productions légumières ou de plein champs et à couvrir les différents modes de contamination des précipitations. Il s'agit des sites de l'Ile de Ouessant, Landivisiau, Plouay, Ploufragan et Rennes. Les évènements pluvieux collectés sont choisis en fonction des caractéristiques de formation de la perturbation et du calendrier des épandages de pesticides.Par ailleurs, les concentrations rencontrées pour le site de Rennes en 2000 (année très humide) ont pu être comparées à celles obtenues lors d'une étude conduite en 1996 sur un site proche mais pour des conditions climatiques plus habituelles (année humide à sèche).Les analyses sont réalisées par extraction en phase solide suivie d'une analyse en chromatographie en phase gazeuse couplée à la spectrométrie de masse, ou par détection azote-phosphore spécifique (NPD) ou détection par capture d'électrons pour les composés halogénés (ECD). Dans ces deux derniers cas, la confirmation de l'identité des produits est réalisée par un système de double colonnes.Sept évènements pluvieux distincts ont été collectés et analysés entre les 15 mars et 15 juillet de l'année 2000. Parmis les produits recherchés six molécules sont régulièrement retrouvées : l'atrazine et son métabolite la déethylatrazine (DEA), l'alachlore, le lindane (gamma HCH) et son isomère le béta HCH ainsi que la desméthryne. Nous avions déjà des observations analogues à l'issu de nos premières investigations de 1996.Les analyses réalisées ont montré l'existence d'une contamination chronique du compartiment atmosphérique par l'atrazine et la DEA pour les zones sous influence agricole. Les niveaux de concentrations rencontrés sont faibles, de l'ordre de 10 ng/L. A ce bruit de fond s'ajoutent en période de traitement, des transferts depuis les parcelles traitées qui conduisent à des valeurs beaucoup plus élevées (de 0,1 à 0,7 µg.L-1). La détection de l'alachlore et de la desmethryne est limitée aux périodes d'application de ces produits.Du lindane, et dans un cas son isomère le béta-HCH, ont pu être mis en évidence sur quelques prélèvements, traduisant une contamination chronique du compartiment aérien due à la rémanence de ce type de composés.Les concentrations dans les pluies, pour un site donné, sont très dépendantes de la pluviométrie. Alors qu'en année de pluviosité normale (1996), les concentrations en période d'application sont élevées, elles restent faibles pour une année humide (2000). Pour s'affranchir de l'effet de dilution, nous avons calculé des retombées massiques (mg.ha-1) pour les deux périodes de collecte de 1996 et 2000. Les résultats de 2000 restent malgré cela très inférieurs à ceux de 1996 (d'un facteur quatre environ). Les surfaces emblavées et les différences d'usage entre les deux années sont trop faibles pour expliquer les écarts obtenus. Ceci nous permet de conclure que c'est l'intensité des transferts sol-air qui détermine les niveaux de contamination de l'atmosphère. L'humidité élevée des sols, pour une année humide, favorise la migration verticale des produits phytosanitaires dans les couches inférieures, réduisant ainsi les concentrations de pesticides susceptibles d'être transférés vers le compartiment aérien.Contamination of rainwater by pesticides was investigated by analyzing samples from 5 locations in Brittany (France). These sites were chosen on the basis of various characteristics including agricultural practices around the site, the direction of prevailing winds (west to east) and the proximity to heavily populated areas. The sites chosen were: Ile de Ouessant (a natural reserve without pesticide application), Landivisiau (a semi-natural zone), Plouay (a rural site with intensive agriculture, mainly cereals and vegetables), Ploufragan (a suburban site) and Rennes (an urban area surrounded by intensive agriculture and orchards). The pesticides to be monitored included the compounds most commonly used in Brittany. New molecules were added to this list to take into account new homologues or uses. Some organochlorines and pesticides with specific application modes were thus included in the list. Analytical techniques involved solid-liquid extraction procedures followed by GC-NPD, GC-ECD or GC-MS after prior derivatisation. Derivatisation was performed with heptafluorobutyric acid to detect urea and sulfonylurea herbicides. Quantification limits were about 5 ng/L for GC-ECD or GC-NPD, and 50 ng/L for molecules detected by GC-MS after derivatisation.Rain episodes were selected according to the prevailing meteorological conditions and the air mass trajectory. Only precipitation events greater than 6 mm were considered, in order to collect sufficient volume for analyses. Thirty-five samples (7 per location) were collected between 15 March and 15 July 2000, corresponding to various weather fronts.Six pesticides were detected: atrazine and its metabolite de-ethylatrazine (DEA), alachlor, desmethryne, lindane and its isomer (beta HCH). Among those detected most concentrations were below 100 ng/L but higher concentrations were occasionally detected during and after spraying in agricultural areas. Lindane was detected at low concentration (<10 ng/L) in several samples collected during different periods and locations. These results demonstrate chronic atmospheric contamination due to the presence of this substance even though its use has been banned in the Economic European Community (EEC) (e.g. in 1999 in France).The most frequently detected compound was atrazine, detected in 60 % of the samples. The presence of atrazine and its metabolite DEA in rainfall indicate chronic contamination of the atmosphere outside of spraying time (mid-March to mid-April). Detection of these compounds was generally limited to agricultural sites, and could also indicate transfer from agricultural areas during soil preparation for maize sowing. These hypotheses will be tested by monitoring atrazine in rainwater during a complete agricultural cycle. Important contamination by atrazine was detected during spraying time at agricultural locations, where concentrations ranged from 10 to 60 ng/L. Detection of alachlor was limited to application periods and rural sites, at levels ranging from 20 to 240 ng/L. However, amounts of alachlor were also detected in Ouessant, even though its use is banned on this isle. This presence was presumably due to the specific conditions of formation of the weather fronts that resulted in rainfall.Concentrations of pesticides in rainwater obtained in 2000 were generally lower than those described in literature or in a previous study done in Brittany. Alachlor and atrazine concentrations were compared to those of a previous study conducted in Brittany in 1996. The two sites chosen were Rennes in 2000 and Le Rheu in 1996. These two locations, separated by 5 km, were not significantly different since Rennes is directly downwind of Le Rheu. Pluviometry was very important in the year 2000, showing a 30 % increase in rainfall compared to the average from 1961-1990. The concentrations of pesticides in both years were also very different. During a year of normal rainfall (e.g., 1996) concentrations of pesticides in application periods were high, but they remained low during wet years (e.g., 2000). These differences could not be explained by a dilution effect - when fall-out was calculated (mg/ha) to correct for the dilution effect, the differences remained important. Maize surfaces (132 000 ha in 1996 compared to 128 000 ha in 2000) and the quantities of pesticides applied (580 tonnes in 1994 in Brittany compared to 350 tonnes for atrazine, and 263 tonnes in 1998 compared to 119 tonnes in 1994 for alachlor) were too similar to explain the differences observed in 1998. Photodegradation processes, which are more important during normal or dry years, should also favour higher concentrations in 2000 than in 1996. Rain concentrations and fallout observed in 2000 remained lower than those obtained in 1996. Thus, it can be concluded that atmospheric contamination is lower for wet years (e.g., 2000) than for normal or dry years (e.g., 1996). Soil humidity, high for wet years, favours the vertical migration of these substances to lower soil layers, reducing pesticide concentrations available to be transferred to the atmosphere

Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.Comment: 10.5 pages, 15 figures. Submitted to Phys. Rev.

Logarithmic asymptotics of the densities of SPDEs driven by spatially correlated noise

We consider the family of stochastic partial differential equations indexed by a parameter \eps\in(0,1], \begin{equation*} Lu^{\eps}(t,x) = \eps\sigma(u^\eps(t,x))\dot{F}(t,x)+b(u^\eps(t,x)), \end{equation*} (t,x)\in(0,T]\times\Rd with suitable initial conditions. In this equation, $L$ is a second-order partial differential operator with constant coefficients, $\sigma$ and $b$ are smooth functions and $\dot{F}$ is a Gaussian noise, white in time and with a stationary correlation in space. Let p^\eps_{t,x} denote the density of the law of u^\eps(t,x) at a fixed point (t,x)\in(0,T]\times\Rd. We study the existence of \lim_{\eps\downarrow 0} \eps^2\log p^\eps_{t,x}(y) for a fixed $y\in\R$. The results apply to a class of stochastic wave equations with $d\in\{1,2,3\}$ and to a class of stochastic heat equations with $d\ge1$.Comment: 39 pages. Will be published in the book " Stochastic Analysis and Applications 2014. A volume in honour of Terry Lyons". Springer Verla

Evidence for an unconventional magnetic instability in the spin-tetrahedra system Cu_2Te_2O_5Br_2

Thermodynamic experiments as well as Raman scattering have been used to study the magnetic instabilities in the spin-tetrahedra systems Cu_2Te_2O_5X_2, X=Cl and Br. While the phase transition observed in the Cl system at T_o=18.2 K is consistent with 3D AF ordering, the phase transition at T_o=11.3 K in the Br system has several unusual features. We propose an explanation in terms of weakly coupled tetrahedra with a singlet-triplet gap and low lying singlets.Comment: 4 pages, 4 figure

Vibration-induced friction control for walkway locomotion interface

Falls represent a major challenge to mobility for the elderly community, a point that has motivated various studies of balance failures. To support this work, we are interested in mechanisms for the synthesis of ground environments that can be controlled to exhibit dynamic friction characteristics. As a first step, we investigate the design and development of such a variable-friction device, a hybrid locomotion interface using a cable-driven vibrotactile mechanism. Measurements on our prototype, consisting of an aluminum tile covered with low-friction polytetrafluoroethylene (PTFE), demonstrate that it can effectively simulate a low coefficient of static friction. As part of the design, we also investigated the role that induced vibration plays in modifying the coefficient of friction. Measurements of sliding on a PTFE-covered tile in a tilted configuration showed a significant influence of normal low-frequency vibration, particularly for frequencies around 20 Hz, regardless of the user's weight

Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2

Low energy Raman scattering of the s=1/2 spin tetrahedra system Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated temperatures this mode shows a soft mode-like decrease in energy pointing to an instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2 with a presumably larger inter-tetrahedra coupling does not show such a low energy mode. Instead its excitation spectrum and thermodynamic properties are compatible with long range Neel-ordering. We discuss the observed effects in the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated Electron

Observations of total peroxy nitrates and aldehydes: measurement interpretation and inference of OH radical concentrations

We describe measurements of total peroxy nitrates (&Sigma;PNs), NO₂, O₃ and several aldehydes at Granite Bay, California, during the Chemistry and Transport of the Sacramento Urban Plume-2001 (CATSUP 2001) campaign, from 19 July&ndash;16 September 2001. We observed a strong photochemically driven variation of &Sigma;PNs during the day with the median of 1.2 ppb at noon. Acetaldehyde, pentanal, hexanal and methacrolein had median abundances in the daytime of 1.2 ppb, 0.093 ppb, 0.14 ppb, and 0.27 ppb, respectively. We compare steady state and time dependent calculations of the dependence of &Sigma;PNs on aldehydes, OH, NO and NO₂ showing that the steady state calculations are accurate to &plusmn;30% between 10:00 and 18:00 h. We use the steady state calculation to investigate the composition of &Sigma;PNs and the concentration of OH at Granite Bay. We find that PN molecules that have never been observed before make up an unreasonably large fraction of the &Sigma;PNs unless we assume that there exists a PAN source that is much larger than the acetaldehyde source. We calculate that OH at the site varied between 2 and 7&times;10⁶ molecule cm^&minus;3 at noon during the 8 weeks of the experiment