1,721,001 research outputs found
Bird-Borne Samplers for Monitoring CO2 and Atmospheric Physical Parameters
Get PDFAir quality monitoring in cities is significant for both human health and environment. Here, an innovative miniaturized active air sampler wearable by free-flying birds is presented. The device integrates a GPS logger and atmospheric calibrated sensors allowing for high spatiotemporal resolution measurements of carbon dioxide (CO2) concentration, barometric pressure, air temperature, and relative humidity. A field campaign, carried out from January to June 2021, involved the repeated release of homing pigeons (Columba livia) from downtown Rome (Italy), to sample the air on their way back to the loft, located in a rural area out of the city. The measurements suggest the importance of green urban areas in decreasing CO2 levels. Moreover, a positive relation between CO2 levels, relative humidity, and air temperature was revealed. In contrast, a negative relation with distance from the point of release, month, and time of day was found. Flight speed and the altitude of flight were related to rising CO2 levels. The easy use of such devices paves the way for the application of miniaturized air samplers to other synanthropic species (i.e., gulls), making birds convenient biomonitors for the urban environment. © 2022 by the authors
Turbulent Schmidt Number Measurements Over Three-Dimensional Cubic Arrays
No full textWe present turbulent Schmidt number (Sct) estimations above three-dimensional urban canopies, where Sct is a property of the flow defined as the ratio of the eddy diffusivity of momentum (KM) to the eddy diffusivity of mass (Dt). Despite the fact that Sct modelling is of great interest, inter alia, for pollutant dispersion simulations conducted via computational fluid dynamics, no universal value is known. Simultaneous measurements of fluid velocity and mass concentration are carried out in a water channel for three staggered arrays of cubical obstacles corresponding to isolated flow, wake-interference, and skimming-flow regimes. A passive tracer is released from a continuous point source located at a height z= 1.67 H where H is the obstacle height. The results show an increase of Sct with height above the canopy for all three arrays, with values at z= 2 H (Sct≈ 0.6) about double compared to that at z= H. The observed Sct agrees well with that modelled by using a simple formulation for Sct based on expressions for KM and Dt published in previous studies. Comparisons with other Sct models found in the literature are also presented and discussed
Turbulence properties of the urban boundary layer
No full textThe work investigates turbulence parameters characterizing urban boundary layers reproduced in the waterchannel; arrays of cubes and parallelepipeds are used to simulate real urban arrangements. The friction velocity above the obstacles depends on the planar area fraction in case of obstacles of equal heights, while this does not happen in the case of arrangements of obstacles with uneven heights. The Prandtl mixing length is constant in the roughness sublayer above the obstacles; then it fits the theoretical behavior in the overlying constant flux layer only for arrangements with obstacles of equal heights
Water-channel estimation of Eulerian and Lagrangian time scales of the turbulence in idealized two-dimensional urban canopies
No full textLagrangian and Eulerian statistics are obtained from a water-channel experiment of an idealized two-dimensional urban canopy flow in neutral conditions. The objective is to quantify the Eulerian (TE) and Lagrangian (TL) time scales of the turbulence above the canopy layer as well as to investigate their dependence on the aspect ratio of the canopy, AR, as the latter is the ratio of the width (W) to the height (H) of the canyon. Experiments are also conducted for the case of flat terrain, which can be thought of as equivalent to a classical one-directional shear flow. The values found for the Eulerian time scales on flat terrain are in agreement with previous numerical results found in the literature. It is found that both the streamwise and vertical components of the Lagrangian time scale, TuL and TwL, follow Raupach’s linear law within the constant-flux layer. The same holds true for TwL in both the canopies analyzed (AR= 1 and AR= 2) and also for TuL when AR= 1. In contrast, for AR= 2 , TuL follows Raupach’s law only above z= 2 H. Below that level, TuL is nearly constant with height, showing at z= H a value approximately one order of magnitude greater than that found for AR= 1. It is shown that the assumption usually adopted for flat terrain, that β= TL/ TE is proportional to the inverse of the turbulence intensity, also holds true even for the canopy flow in the constant-flux layer. In particular, γ/ iu fits well βu=TuL/TuE in both the configurations by choosing γ to be 0.35 (here, iu= σu/ u ̄ , where u ̄ and σu are the mean and the root-mean-square of the streamwise velocity component, respectively). On the other hand, βw=TwL/TwE follows approximately γ/ iw= 0.65 / (σw/ u ̄) for z> 2 H, irrespective of the AR value. The second main objective is to estimate other parameters of interest in dispersion studies, such as the eddy diffusivity of momentum (KT) and the Kolmogorov constant (C0). It is found that C0 depends appreciably on the velocity component both for the flat terrain and canopy flow, even though for the latter case it is insensitive to AR values. In all the three experimental configurations analyzed here, KT shows an overall linear growth with height in agreement with the linear trend predicted by Prandtl’s theory
Pollutant fluxes in two-dimensional street canyons
Get PDFThe turbulent dispersion of a passive tracer emitted by a line source simulating vehicular traffic in an idealized urban canyon in neutral conditions is studied in the water channel by means of simultaneous measurements of velocity and concentration fields. The experiments are conducted for two geometrical arrangements of two-dimensional obstacles reproducing the skimming flow (AR=1) and the wake-interference regime (AR=2), where AR is the canyon aspect ratio. The results show a strict connection between the dynamics of the shear layer developing at the top of the canyon and the vorticity field. For AR=1, it is found that the shear layer flaps upwards and downwards according to two different frequencies. The greatest of them matches the vortex shedding frequency as measured at the canyon top, while the lower is comparable to H/u_* (H is the building height and u_* a reference friction velocity). The shear layer flapping modulates in time the pollutant exchange rate between the canyon and the outer layer. The different characteristics of the shear layer found for the two flow regimes also explain the larger pollutant re-entrainment observed for AR=1, which turns out to be greater than the emission rate at the source. Sweep and ejection modes are identified via quadrant analysis and used to quantify the weights of the factors involved in the turbulent exchanges of tracer and momentum between the canyon and the outer layer. It is found that the venting of polluted fluid at the canyon top increases substantially passing from AR=1 to 2, while an opposite trend is observed for the entrainment of polluted fluid. The vertical flux of pollutant at the canyon top for AR=2 is largely of turbulent nature, with the contribution of the mean flux being practically negligible. On the other hand, the latter becomes comparable or even exceeds the magnitude of the turbulent flux when AR=1. The maps of the first three statistical moments of the pollutant concentration for the two geometrical arrangements are also reported and discussed
Pollutant removal mechanism in two-dimensional street canyons: A laboratory study
No full textVelocity and concentration fields have been measured simultaneously in the water channel to quantify the turbulent dispersion of a passive tracer emitted at street level by a line source within an idealized urban canyon. The experiment has been conducted for an arrangement of two-dimensional obstacles with aspect ratio unity. Statistical moments of velocity and concentration have been calculated with high spatial resolution. Furthermore, the simultaneous measure of velocity and concentration at the same point permitted the determination of the tracer flux in the whole domain as well as to quantify removal and re-entrainment of the pollutant through the canyon top
Evaluation of ansys-fluent model against field data in the framework of the VIEPI project
Get PDFThis paper presents a CFD analysis of the wind velocity simulated in a portion of the campus of the University of Rome “La Sapienza” (Italy). One of the final goals of this study is the evaluation of the pressure field on the surfaces of the buildings in order to quantify outdoor-indoor exchanges of air mass within a street canyon adjacent to the building of interest. Numerical results concerning the wind field have been compared with in situ data collected in the framework of the VIEPI (Integrated Evaluation of Indoor Particulate Exposure) project
Winter warm spells over Italy. Spatial–temporal variation and large‐scale atmospheric circulation
No full textThis article analyses the winter warm spells (WWS) that occurred in central Mediterranean over the period 1993-2022, examining the daily maximum temperatures collected at eight airport sites located in the Italian Peninsula, belonging to different climate zones. According to the definition proposed in 1999 by the Expert Team on Climate Change Detection and Indices (ETCCDI), a WWS is a sequence of at least six consecutive days when daily maximum air temperature exceeds the calendar day 90th percentile centred on a 5-day window for a base period. WWS occurring over the entire Italian territory or only over northern/central/southern Italy have been identified and related to the peculiar synoptic conditions. It was found that December is the month most prone to WWS and, on average, WWS last 9.4 days in northern Italy, 6.6 days in central Italy, and 8.5 days in southern Italy. Over the period under investigation, the Italian Peninsula experienced only one common event characterized by persistent high-pressure systems associated with air subsidence over western Mediterranean and, therefore, with exceptional warming. Finally, it has been proven that the definition of WWS proposed by ETCCDI allows to capture synoptic scale events but, in orographically complex areas such as Italy, underestimates moderate spells, which generally might have a duration of at least 3 days. Consequently, it is important to consider the possibility of reducing the period length threshold used for the detection of WWS when orographically heterogeneous regions are studied.In central Mediterranean and in orographically heterogeneous areas such as Italy, the definition of winter warm spells (WWS) provided by the Expert Team on Climate Change Detection and Indices allows to capture mainly synoptic scale events, losing information on moderate warm spells. When orographically heterogeneous regions are studied, it is important to consider the possibility of reducing the period length threshold used for the detection of WWS. imag
Spatial-temporal assessment of air quality in Rome (Italy) based on anemological clustering
Get PDFThe relationship between atmospheric circulation and air pollution is investigated by analysing in-situ measurements collected at four monitoring stations located in the coastal area of central Italy over the period 2014–2020. The study is based on the prior identification of three typical circulation patterns, obtained via the k-mean clustering of surface anemological data. The present analysis explores the relation between atmospheric dynamics and concentrations of nitrogen oxides (NO and NO2), NO2/NOx (NOx = NO + NO2), ozone (O3), and particulate matters (PM2.5 and PM10). When local circulation systems prevail, the best air quality conditions are observed, as the onset of the sea breeze permits clean, marine air masses to be advected to the urban area of Rome. On the other hand, when synoptic winds persistently blow from the northeast, the highest concentrations of atmospheric pollutants are recorded. Finally, when both synoptic and local winds blow from the southeast, the complex anemological regime results in low ventilation and quite poor air quality conditions. The largest differences among clusters are observed during winter, when the north-easterly winds can persist for more than ten consecutive days, with the enhanced atmospheric stability limiting the development of the mixed layer, causing the increase of ground-level pollutants concentration
Turbulent Schmidt number estimate over urban canopies
Get PDFThis work presents laboratory estimates of the turbulent Schmidt number (Sct) above an idealized three-dimensional urban canopy. To do this, a neutral atmospheric boundary layer has been reproduced in the water channel and a staggered array of cubic obstacles of equal heights placed on the channel bottom has been used to simulate the canopy. A point source emitted a passive tracer at a constant rate above the cube array. Simultaneous measurements of pollutant concentration and velocity permitted the estimation of the vertical fluxes of mass and momentum and, therefore, of Sct. The results show a clear dependence of Sct on the height above the canopy
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