Measurements at appropriate spatial and temporal scales are essential for understanding and monitoring spatially heterogeneous environments with complex and highly variable emission sources, such as in urban areas. However, the costs and complexity of conventional air quality measurement methods means that measurement networks are generally extremely sparse. In this paper we show that miniature, low-cost electrochemical gas sensors, traditionally used for sensing at parts-per-million (ppm) mixing ratios can, when suitably configured and operated, be used for parts-per-billion (ppb) level studies for gases relevant to urban air quality. Sensor nodes, in this case consisting of multiple individual electrochemical sensors, can be low-cost and highly portable, thus allowing the deployment of scalable high-density air quality sensor networks at fine spatial and temporal scales, and in both static and mobile configurations. In this paper we provide evidence for the performance of electrochemical sensors at the parts-per-billion level, and then outline results obtained from deployments of networks of sensor nodes in both an autonomous, high-density, static network in the wider Cambridge (UK) area, and as mobile networks for quantification of personal exposure. Examples are presented of measurements obtained with both highly portable devices held by pedestrians and cyclists, and static devices attached to street furniture. The widely varying mixing ratios reported by this study confirm that the urban environment cannot be fully characterised using sparse, static networks, and that measurement networks with higher resolution (both spatially and temporally) are required to quantify air quality at the scales which are present in the urban environment. We conclude that the instruments described here, and the low-cost/high-density measurement philosophy which underpins it, have the potential to provide a far more complete assessment of the high-granularity air quality structure generally observed in the urban environment, and could ultimately be used for quantification of human exposure as well as for monitoring and legislative purposes.

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Uranium-uranium collisions at the energy of the Brookhaven Relativistic Heavy-Ion Collider (100 + 100 GeV per nucleon) are predicted to produce an average of nearly 100 jets with pr > 3 GeV. These jets will on average carry off 70 GeV of transverse energy ET per unit rapidity. Central collisions produce more transverse energy than this; the ET distribution extends up to about (5A 4/3 GeV 4 )/prmin per unit rapidity, which is 4 times the average. It is estimated that the minijets are likely to undergo further collisions and become thermalized.

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The known analytic properties of the Compton amplitude at small Q 2 place significant constraints on its behaviour at large Q 2 . This calls for a re-evaluation of the role of perturbative evolution in past fits to data.