The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. ?? 2013 Elsevier B.V. All rights reserved

Abstract.A global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63 • N to 52 • S and 72 • W to 124 • E has been achieved within the Raman and polarization lidar network Polly NET . This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. Polly NET is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design with different capabilities ranging from single wavelength to multiwavelength systems, and now apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at polly.tropos.de. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Ångström exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the Polly NET locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm as this operating wavelength is available for all Polly lidar systems. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of Polly NET to support the establishment of a global aerosol climatology that covers the entire troposphere.

Lead halide perovskites of the type APbX3 (where A = methylammonium MA, formamidinium FA, or cesium and X = iodide and bromide), in a single‐crystal form or more often as polycrystalline films, have already shown unique optoelectronic properties, comparable with those of the best single‐crystal semiconductors. To form a properly crystalline iodide or iodide/bromide, perovskite and achieve high performance in solar cells, sources containing only iodide and bromide salts (PbI2, PbBr2, MAI, FAI, CsI, MABr) are typically used as precursor materials. However, recently, most of the record perovskites contain MACl as additive to control their crystallization, revisiting the importance of methylammonium cation excess and chloride incorporation in perovskites, previously highlighted by Snaith's group back in 2012. Here, we review the background and recent progress in MACl‐mediated crystallization of perovskites, as well as the impact of the additive in solar cells. In particular, we describe the current understanding of the mechanism of perovskite crystallization process and defect passivation at grain boundaries in the presence of MACl. We then discuss the spectacular results (in terms of record efficiencies, stability, and up‐scaling) that have been delivered by solar cells employing MACl‐incorporated perovskites, and give an outlook of future research avenues that might bring perovskite solar cells closer to commercialization.

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