Showing papers by "Silvana Di Sabatino published in 2015"

The rise of low-cost sensing for managing air pollution in cities

Abstract: Ever growing populations in cities are associated with a major increase in road vehicles and air pollution. The overall high levels of urban air pollution have been shown to be of a significant risk to city dwellers. However, the impacts of very high but temporally and spatially restricted pollution, and thus exposure, are still poorly understood. Conventional approaches to air quality monitoring are based on networks of static and sparse measurement stations. However, these are prohibitively expensive to capture tempo-spatial heterogeneity and identify pollution hotspots, which is required for the development of robust real-time strategies for exposure control. Current progress in developing low-cost micro-scale sensing technology is radically changing the conventional approach to allow real-time information in a capillary form. But the question remains whether there is value in the less accurate data they generate. This article illustrates the drivers behind current rises in the use of low-cost sensors for air pollution management in cities, whilst addressing the major challenges for their effective implementation.

The breathability of compact cities

Abstract: Breathability in dense building arrays with packing densities similar to those of typical European cities is investigated using laboratory measurements and numerical simulations. We focus on arrays made up by regularly spaced square buildings forming a network of streets with right-angle intersections. It is shown that breathability can be evaluated using building ventilation concepts (mean flow rate and age of air) and from vertical mean and turbulent fluxes quantifiable through a bulk exchange velocity. Mean age of air reveals that varying wind angles result in different ventilation, which we explain through mean flow streamlines and exchange velocity. For low wind angles (wind direction almost parallel to the axes of half of the streets of the network), vertical transfer and mean transversal transfers are at minimum and removal of pollutants is associated with mean longitudinal fluxes. Larger wind angles result in better ventilation due to an increase of transversal fluxes and vertical exchange. The latter, for which a formulation is derived, shows a non-negligible contribution of the mean flow which increases with increasing wind angle. Ventilation conditions can be further altered by small differences in the array geometry. These observations are useful for the development of simple urban dispersion models.

A Case Study of the Nocturnal Boundary Layer Evolution on a Slope at the Foot of a Desert Mountain

Abstract: Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in Utah. A case study of nocturnal boundary layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary layer development over the slope could be divided into three distinct phases during this night: 1) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. 2) The flow transition was followed by a 3–4-h period of almost steady-state boundary la...

The effects of trees on micrometeorology in a real street canyon: Consequences for local air quality

Abstract: This study analyses the effects of trees on local meteorology of a Mediterranean City (Lecce, IT) using field measurements and computational fluid dynamics simulations. Measurements were collected for 51 days in a street canyon with trees to cover different meteorological and foliage conditions. Building facades and ground temperatures were estimated from infrared images, flow and turbulence measured by ultrasonic anemometers. In the case of approaching wind parallel to the street axis, trees induce large wind direction fluctuations below tree crowns and velocities up to about 80% lower than those at roof top. This, combined with the obstruction by tree crown, lead to lower ventilation in the bottom part of the street, especially during nocturnal hours, and to in-canyon volume-averaged pollutant concentration about 20% larger than in the tree-free case. Ignoring trapping effects of trees, as typically done in many air quality models, may lead to underestimation of ground level concentrations.

Structure of Turbulence in Katabatic Flows below and above the Wind-Speed Maximum

Abstract: Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer during the MATERHORN Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured at multiple levels at four towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The multi-level observations made during a 30-day long MATERHORN-Fall field campaign in September-October 2012 allowed studying of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence and their variations in katabatic winds. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward) whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.

Field-scale remote sensing of soil moisture based on polarimetric characterization of microwave reflections

Abstract: A field-scale remote soil moisture sensing technique that exploits polarization mode dispersion (PMD) associated with radio frequency (RF) signal propagation is considered in this paper. Microwave polarization responses from rough surface scattering are quantified using a dual-polarized receiver system to estimate PMD responses. Changes in PMD response are elicited by changes in the dielectric properties due to soil moisture changes. The ability of PMD characterizations to remotely detect changes in soil moisture, with responses that exhibit good correlation to ground probe measurements, is demonstrated using a prototype with widely separated transmitter/receiver system deployed at the MATERHORN field experiments conducted at the US Army Dugway Proving Ground, Utah.