Low-cost sensors are useful tools for the collection of air quality data, augmenting the existing regulatory monitoring networks and providing an unprecedented opportunity to increase their spatial ...coverage. This study presents a calibration process of a low-cost PM sensor (PurpleAir PA-II, PAir) in ambient conditions in the city of Patras, Greece, during 18 months of 2017–2018.
The hourly PM1 and PM2.5 measurements using the original sensor values were reasonably well correlated (R2 = 0.82 for PM1 and R2 = 0.56 for PM2.5) with the reference instrument, but with a high mean bias and root mean square error. There was a small improvement of around 10% for the daily averages. For PM1–2.5 (particles with diameters between 1 and 2.5 μm), PM2.5–10 (diameters between 2.5 and 10 μm) and PM10, the performance of the low-cost sensors was poor in this area with R2 < 0.37 in all cases.
The response of the PAir sensor for PM1 and PM2.5 changed significantly compared to the reference instrument during periods with high dust (or other coarse particle) concentrations. These periods were excluded and a simple linear calibration was then developed for the rest of the fine PM measurements. A method for the identification of these high dust periods based on regional model predictions is proposed. This calibration reduces the relative mean error for hourly PM1 to 19% (1.1 μg m−3) and for PM2.5 to 18% (1.1 μg m−3). The corresponding root mean square errors are 25% (1.4 μg m−3) for hourly PM1 and 25% (1.6 μg m−3) for PM2.5. The biases of the corrected values are, as expected, practically zero. Surprisingly, the relative humidity had a negligible effect on fine PM measurements of the PAir in this location and for the conditions of the study.
Display omitted
•Low-cost PM sensors were calibrated in ambient conditions.•The original sensor values were reasonably well correlated.•The sensor response was different during periods with high dust.•The relative humidity had a negligible effect on fine PMs.
An integral model predicting the mean flow and mixing properties of inclined plane and round turbulent buoyant jets in a motionless environment of uniform density is proposed. The escaping masses ...from the main buoyant jet flow are simulated, and the model can be successfully applied to initial discharge inclinations ${\theta }_{0} $ from 90 to $\ensuremath{-} 7{5}^{\ensuremath{\circ} } $ with respect to the horizontal plane. This complementary approach introduces a concentration coefficient, which is calibrated using experimental evidence. The present model has incorporated the second-order approach and, regarding the jet-core region, a jet-core model based on the advanced integral model for the production of more correct transverse profiles of the mean axial velocities and mean concentrations than the common Gaussian or top-hat profiles. The partial differential equations for momentum and tracer conservation are written in orthogonal and cylindrical curvilinear coordinates for inclined plane and round buoyant jets, respectively, and they are integrated under the closure assumptions of (a) quasi-linear spreading of the mean flow and mixing fields, and (b) known transverse profile distributions. The integral forms are solved by employing the Runge–Kutta algorithm. Since the most important contribution in the present model is the simulation of the escaping masses, the model has been called the escaping mass approach (EMA). Herein EMA is applied to predict the mean flow properties (trajectory characteristics, mean axial velocities and mean concentrations) for inclined plane and round buoyant jets. The results predicted are compared with experimental data available in the literature, and the accuracy obtained is more than satisfactory. The performance of the EMA is up to 56 % better than using classical integral procedures. EMA can be used for design purposes and for environmental impact assessment studies.
Gaseous pollutants negatively affect air quality and consequently human health and wellbeing. Thus, the University of Patras Campus (UPC) air quality is evaluated in comparison to Patras downtown air ...quality, regarding nitrogen oxides (NOx) and carbon monoxide (CO) concentration levels. The air pollution station installed at the suburban area of UPC was monitoring the air quality during the period 2014–2018. The daily average concentrations ± standard deviations recorded by continuous measurements of NO, NO2, NOx, and CO were 0.4 ± 0.5, 4.6 ± 2.7, and 5.0 ± 3.0, and 39.3 ± 64.1 μg m−3, correspondingly. These values are lower than the corresponding ones measured by downtown monitoring stations during 2001–2018. After analyzing the data recorded, the diurnal, weakly and yearly variations of NO, NO2, NOx, CO concentrations and NO/NOx, NO2/NOx, NO/NO2 ratios were obtained. The yearly variations show that the pollutant concentrations at UPC are below the legislation limits. On the contrary, in Patras Downtown only NOx concentrations overcome the limit, while NO2 concentrations found marginally below the limit during the last decade. Statistical analysis showed that concentration levels are affected by seasonality and meteorological parameters. The present findings may be useful in environmental impact assessments necessary for sustainable development projects.
The air temperature variation of a closed room, well insulated, during the initial time of operation of air-conditioning systems up to temperature stabilization, is simulated by a two-dimensional ...integral model as a quasi-steady-state phenomenon. The model equipped with a conservation equation for tracer concentration or relative temperature, including the stratification parameter, is well qualified. The flow leaving the air conditioning device forms an inclined buoyant jet which bends over and meets the room floor, where it spreads sideways forming a layer with jet temperature. A sequence of layers, which affect the jet temperature through entrainment, are produced by a novel bottom-up technique. The layer air temperatures are calculated through the bulk dilution of a near bottom jet cross-section, which feeds each new layer. The model simulated a real case and predicted the transient variation of room air and buoyant jet temperatures up to stabilisation. It also predicted the time needed for stabilisation, the cooling rates of the room and jet air temperatures, the Brunt-Väisälä frequency occurring during the temperature transitions, and more. The results are promising as they agree with observations. Thus, the model could be used to evaluate the effectiveness of relevant HVAC systems operating in such rooms.
The aim of the present work was to evaluate the seasonal and long-term concentration levels of several electrochemical and nutrient parameters in surface water and groundwater of the Alfeios River ...Basin, Greece. Seven seasonal sampling campaigns were conducted, each including 12 sampling sites located along the Alfeios River and its major tributaries, Enipeus, Kladheos, Selinous, Erymanthos, Ladhon, and Lousios. At each station, flow discharge was measured using four quick techniques, on-site measurements were performed, and water samples were taken for laboratory analysis. In locations near the river, on-site measurements were conducted, and water samples were collected from wells and boreholes to assess the groundwater quality. In addition, relative data from previous or later campaigns by other researchers is used to assess the long-term and possible seasonal variation of water quality along the Alfeios River and its tributaries. The results show a decrease in concentration for most of the parameters regarding surface and groundwater quality during the last few years. Statistical analysis showed that the dilution process is significant along the Alfeios River, upstream industrial wastewater discharges or agricultural runoff is responsible for high concentration values, while seasonality is evident only in the surface waters. Finally, comparing the water quality values to legislative limits of drinking water shows that exceedances occur, especially at nutrient compounds. This data could be useful for the evaluation of water quality, which is required by the implementation and update of the Alfeios River Basin Management Plan.
Abstract
Gaseous pollutants negatively affect air quality and consequently human health and wellbeing. Thus, the University of Patras Campus (UPC) air quality is evaluated in comparison to Patras ...downtown air quality, regarding nitrogen oxides (NO
x
) and carbon monoxide (CO) concentration levels. The air pollution station installed at the suburban area of UPC was monitoring the air quality during the period 2014–2018. The daily average concentrations ± standard deviations recorded by continuous measurements of NO, NO
2
, NO
x
, and CO were 0.4 ± 0.5, 4.6 ± 2.7, and 5.0 ± 3.0, and 39.3 ± 64.1 μg m
−3
, correspondingly. These values are lower than the corresponding ones measured by downtown monitoring stations during 2001–2018. After analyzing the data recorded, the diurnal, weakly and yearly variations of NO, NO
2
, NO
x
, CO concentrations and NO/NO
x
, NO
2
/NO
x
, NO/NO
2
ratios were obtained. The yearly variations show that the pollutant concentrations at UPC are below the legislation limits. On the contrary, in Patras Downtown only NO
x
concentrations overcome the limit, while NO
2
concentrations found marginally below the limit during the last decade. Statistical analysis showed that concentration levels are affected by seasonality and meteorological parameters. The present findings may be useful in environmental impact assessments necessary for sustainable development projects.
Thermal plumes issued with mean exit velocities of 0.527 and 0.702 m/s from internally closed annular nozzles are investigated experimentally, theoretically, and numerically. The experimental setup ...and the measurement and analysis methodologies for flow and mixing are described in detail. The experimental setup consists of an air heating box with an annular slot of 0.12-m mean diameter and 0.01-m width, accompanied by equipment for setup operation and measurement of mean velocities and temperatures. The trajectory of maximum velocities and the isovelocity contours indicate reattachment due to plume self-merging at
z
P0
=
0.161
m and around 0.20 m in correspondence to exit velocities. It was found that
z
P0
depends mainly upon initial Froude number and annulus equivalent diameter. Experiments detected a vortex ring right after the exit followed by a stagnation point at
z
P1
≅
0.055
m on the plume centreline. The annular plume core obtains almost uniform temperature and on average 15 times higher dilutions than bulk dilutions of the equivalnt round plume. To predict the self-merging trajectory, a novel integral model was developed named self-merging approach (SMA). SMA is based on the solution of the ordinary differential equations of momentum and buoyancy applied to an infinitesimal sector of the annular plume, which is equivalent to two identical slot plumes. These plumes are reattached due to outer entrainment, inner entrainment starvation, and a series of sinks along the annular plume centreline with strengths calculated by an original relationship. SMA predictions agree very well with the observations. The present findings contribute to a better understanding of the phenomenon and offer new ideas for designing buoyancy-operating disposal systems with low momentum, using annular- or rosette-type diffusers mainly in shallow water bodies.
Key points
Novel prediction of annular plume reattachment assuming a pair of plane plumes with inner entrainment starvation
The reattachment distance increases proportionally with initial Froude number and equivalent diameter of the annulus
Excellent integral model predictions for centreline velocities and concentrations beyond the core of annular plume
The flow formed by the discharge of inclined turbulent negatively round buoyant jets is common in environmental flow phenomena, especially in the case of brine disposal. The prediction of the mean ...flow and mixing properties of such flows is based on integral models, experimental results and, recently, on numerical modeling. This paper presents the results of mean flow and mixing characteristics using the escaping mass approach (EMA), a Gaussian model that simulates the escaping masses from the main buoyant jet flow. The EMA model was applied for dense discharge at a quiescent ambient of uniform density for initial discharge inclinations from 15° to 75°, with respect to the horizontal plane. The variations of the dimensionless terminal centerline and the external edge’s height, the horizontal location of the centerline terminal height, the horizontal location of centerline and the external edge’s return point as a function of initial inclination angle are estimated via the EMA model, and compared to available experimental data and other integral or numerical models. Additionally, the same procedure was followed for axial dilutions at the centerline terminal height and return point. The performance of EMA is acceptable for research purposes, and the simplicity and speed of calculations makes it competitive for design and environmental assessment studies.
This paper deals with the mean flow and mixing properties of a vertical round turbulent buoyant jet in a weak crosscurrent, through an implementation of the integral method. The phenomenon is ...described by the Reynolds averaged partial differential equations of continuity, momentum and conservation of tracer mass formulated in a curvilinear cylindrical coordinate system. Applying second order mathematical approximations, the equations are integrated on a reduced cross-sectional area of the jet under the similarity assumption and the boundary conditions. The reduced area by a cyclical sector provides increased entrainment due to the increase of its perimeter, which reduces the fluxes and affects the dilution, and the model predictions, mainly the dilutions, are considerably improved. A system of ordinary differential equations is produced, which is solved numerically using a 4
th
order Runge-Kutta method. The results obtained for several values of the normalized ratio of ambient over buoyant-jet exit velocity are compared with experimental data of normalized trajectories and dilutions available in the literature. The satisfactory performance of the integral model for weak current velocities makes it suitable for research, for studying effluent discharges in water bodies or in the atmosphere, as well as for design purposes.
Article Highlights
• A curvilinear cylindrical coordinate system is used.
• Reduced cross-sectional area is used in the bent-over phase.
• The integral model predictions for weak currents agree well with experimental data.
The flash flood in the town of Mandra, Attica, in 2017 that had a return period equal to approximately T = 150 years was one of the most disastrous floods in Greece. Four years later, flood ...protection works designed for T = 50 years were built. In this work, we pose the research question 'How would this disastrous flood have behaved if flood protection works were built?' To answer this question, we employed the HEC-RAS 1D/2D model, over a detailed DSM, that we calibrated with field measurements to simulate the flood of 2017. We derived the following conclusions: (1) Flooding of the main streets still occurs with 15-25% lower flow velocities and 19-29% lower water depths. (2) The total extent of the inundation areas is reduced by 17%. (3) Works delay flood arrival by approximately 1.5 hour, thus providing the opportunity for an Early Warning System to respond more effectively.
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