Deposition Velocity Research Articles

Protecting playgrounds: local-scale reduction of airborne particulate matter concentrations through particulate deposition on roadside ‘tredges’ (green infrastructure)

Exposure to traffic-related particulate air pollution has been linked with excess risks for a range of cardiovascular, respiratory and neurological health outcomes; risks likely to be exacerbated in young children attending schools adjacent to highly-trafficked roads. One immediate way of reducing airborne PM concentrations at the local (i.e., near-road community) scale is installation of roadside vegetation as a means of passive pollution abatement. Roadside vegetation can decrease airborne PM concentrations, through PM deposition on leaves, but can also increase them, by impeding airflow and PM dispersion. Critical to optimizing PM removal is selection of species with high particle deposition velocity (Vd) values, currently under-parameterised in most modelling studies. Here, the measured amounts of leaf-deposited magnetic PM after roadside greening (‘tredge’) installation, and measured reductions in playground PM, particle number and black carbon concentrations demonstrate that air quality improvements by deposition can be achieved at the local, near-road, community/playground scale. PM deposition on the western red cedar tredge removed ~ 49% of BC, and ~ 46% and 26% of the traffic-sourced PM2.5 and PM1, respectively. These findings demonstrate that roadside vegetation can be designed, installed and maintained to achieve rapid, significant, cost-effective improvement of air quality by optimising PM deposition on plant leaves.

Indoor ozone removal and deposition using unactivated solid and liquid coffee.

Managing indoor ozone levels is important because ozone is a hazardous pollutant that has adverse effects on human health. Coffee is a popular daily beverage, and thus, coffee beans and spent coffee grounds are common in many places such as offices, homes, aircraft, cafeterias, and such. The most common material used to remove ozone is activated carbon which can be made from coffee beans or spent coffee grounds with proper activation processes. This paper presents a novel idea: to remove ozone at the level of an indoor environment using unactivated coffee products. This paper examines the ozone removal efficiency and the ozone deposition velocity at 130 ppb ozone for two types of coffee: solid coffee (powder) and liquid coffee (beverage). The activated carbon, the deionized water, and the seawater are also included for comparison and validation purposes. The tests show that the fine coffee powder has a removal efficiency of 58.5% and a deposition velocity of 0.62 cm/s. The liquid coffee has a removal efficiency of 34.4% and a deposition velocity of 0.23 cm/s. The chemical inspections indicate that the oxidation reactions with the carbohydrates in solid coffee and the metal/mineral elements in liquid coffee are responsible for ozone removal. These results have confirmed that ozone removal via coffee is effective, controlling indoor air quality by coffee products is thus becoming possible.

Precipitation scavenging of beryllium-7 (7Be): Observation results and parameterization

This study aims to develop a 1D model that makes it possible to calculate the daily total 7Be wet deposition flux. For this purpose, long-term (2015–2021), high-frequency (daily) and time-synchronized series of observations of 7Be wet deposition flux and its atmospheric activity concentration are analyzed in this paper. Daily mean 7Be activity concentration in the atmosphere, daily total 7Be wet deposition flux and mean 7Be activity concentration, washout ratio, deposition velocity and scavenging coefficient with individual precipitation events lie in the range of 0.1–17 mBq m−3, 0.8–117.2 Bq m−2 d−1, 0.4–11.3 Bq L−1, 331–3799, 0.2–24.7 cm s−1 and (0.8–35.6) × 10−5 s−1, respectively. Quantitative estimates of the influence of precipitation parameters (amount, intensity and duration) on the daily total 7Be wet deposition flux, mean 7Be activity concentration in precipitation, washout ratio, deposition velocity and scavenging coefficient with individual precipitation events have been obtained using correlation analysis. It has been found that precipitation amount has the greatest influence on 7Be deposition flux and deposition velocity, precipitation intensity has the greatest influence on washout ratio and scavenging coefficient, and precipitation duration has the greatest influence on 7Be activity concentration in precipitation. The relationships between these parameters have been parameterized. Based on these parameterizations, five 1D models that calculate the daily total 7Be wet deposition flux have been introduced and validated against the observation data. It has been revealed that the model, which is based on deposition velocity parameterization and uses the data on 7Be activity concentration in the atmosphere and the daily amount of precipitation as predictors, reproduces the highest fraction of the observational data (88%) with the lowest average calculation error (32%) compared to the other four models.

Quantifying the capacity of tree branches for retaining airborne submicron particles

Human health risks brought by fine atmospheric particles raise scholarly and policy awareness about the role of urban trees as bio-filters of air pollution. While a large number of empirical studies have focused on the characteristics of vegetation leaves and their effects on atmospheric particle retention, the dry deposition of particles on branches, which plays a significant role in capturing and retaining particles during the defoliation period and contributes substantially to total removal of atmospheric particles, is under-investigated. To fill in this knowledge gap, this case study examined the dry deposition velocities (Vd) of submicron particulate matters (PM1) on the branches of six common deciduous species in Shanghai (China) using laboratory experiments. And the association between Vd and key branch anatomical traits (including surface roughness, perimeter, rind width proportion, lenticel density, peeling, and groove/ridge characteristics) was explored. It was found that surface roughness would increase Vd, as a rougher surface significantly increases turbulence, which is conducive to particle diffusion. By contrast, peeling, branch perimeter, and lenticel density would decrease Vd. Peeling represents the exfoliated remains on the branch surfaces which may flutter considerably with airflow, leading to particle resuspension and low Vd. When branch perimeter increases, the boundary layer of branches thickens and a wake area appears, increasing the difficulty of particles to reach branch surface, and reducing Vd. While lenticels can increase the roughness of branch surface, their pointy shape would uplift airflow and cause a leeward wake area, lowering Vd. This finely wrought study contributes to a better understanding of branch dry deposition during leaf-off seasons and potential of deciduous trees serving as nature-based air filters all year round in urban environments.

A Simple Parameterization to Enhance the Computational Time in the Three Layer Dry Deposition Model for Smooth Surfaces

Optimization of dry deposition velocity calculation has been of great interest. Every time, determining the value of the concentration boundary layer (CBL) thickness led to a waste of numerical calculation time, which appears as a huge time in large-scale climate models. The goal of this study is to optimize the numerical calculation time in the three-layer deposition model for smooth surfaces through the development of a MATLAB code that can parameterize the appropriate concentration boundary layer height (y+cbl) and internal integral calculation intervals for each particle diameter Dp (0.01–100 µm) and friction velocity u* (0.01–100 m/s). The particle concentration, as a solution to the particle flux equation, is obtained and modeled numerically by performing the left Riemann sum using MATLAB software. On the other hand, the number of subdivisions N of the Riemann sum was also parameterized for each Dp and u* in order to lessen the numerical calculation time. From a numerical point of view, the new parameterizations were tested by several computers; about 78% on the average of the computation time was saved when compared with the original algorithm. In other words, on average, about 1.2 s/calculation was gained, which is valuable in climate models simulations when millions of dry deposition calculations are needed.

SITE-SPECIFIC DRY AND WET DEPOSITION VELOCITIES USING 7BE AND MASS INTERCEPTION FACTOR FOR VARIOUS TYPES OF PLANT LEAVES AT NARORA SITE, INDIA.

Comprehensive studies were carried out during the period 2016-19 on the estimation of site-specific wet and dry deposition velocities and the mass interception factor for Narora site based on the concentrations of 7Be in air, air deposits, rain water and vegetation samples. Mean wet and dry deposition velocities for Narora site were found to be 1.64E-2 and 1.58E-3ms-1, respectively. Mass interception factors for vegetation found to be 0.34-2.51m2kg-1 dry weight with a mean of 0.97m2kg-1 dry weight. The obtained parameters shall be used for estimation of impact due to radionuclide related to Narora Atomic Power station.

Numerical Simulation of Cold Spray Bonding for CrFeNi Medium‐Entropy Alloy

As a solid‐state deposition, cold spray maintains the intrinsic characteristics of the feedstock powder. Thus, it provides a unique opportunity for depositing high‐entropy alloys (HEAs) and medium‐entropy alloys (MEAs) that are of great interest for coatings to protect vulnerable materials from aggressive environments. However, considering the high hardness and the complexity of these powder's preparation, the choice of deposition parameters through experiments can be challenging and costly. Herein, a finite element model is presented to simulate the deposition of CrFeNi MEA using cold spray technology. A detailed approach is implemented to model the bonding of the particle to the substrate. The model is able to identify the deposition velocity window and simulate the metallurgical and mechanical interlocking. The particle adheres to the substrate at 500 m s−1 and pushes the substrate down facilitating mechanical interlocking. Furthermore, due to the strain localization induced by the particle, a notable temperature increase is observed at the substrate surface raising to its melting point in agreement with the ejecta eruption phenomenon. The developed numerical method can pave the way for successful experiments in case of costly HEAs and MEAs powders.

A multispecies 1D concentration distribution model for coarse‐particle slurries

AbstractCoarse‐particle (settling) slurry pipelines are key process units in many industries. In Canada's oil sands operations, such pipelines represent hundreds of millions of dollars of infrastructure investment and transport thousands of tonnes of solids every hour. The ability to determine key design/operating parameters of a settling slurry, such as frictional pressure gradient and deposition velocity, is dependent on the accuracy of the concentration distribution model used to determine the local concentration of coarse solids as a function of vertical position within the pipe. Accurate predictions of the concentration profile are also required for physics‐based models of particle‐impact erosion in slurry pipelines. In this study, an improved concentration distribution model was developed. The classic Schmidt‐Rouse turbulent diffusion equation forms the basis for the model. The improvements made here include the use of a more suitable, high‐concentration hindered settling velocity correlation, and a simple semi‐empirical correlation for the particle diffusivity, which is shown to be dependent only upon the terminal particle settling velocity and the Kolmogorov turbulent velocity scale. The model is applicable to slurries with broad size distributions and/or species with different densities. The performance of the model is tested against numerous slurry flow conditions, including particles from 70 μm to 8 mm in diameter, pipe diameters of 76 mm ≤ D ≤ 500 mm, and in situ solids volume concentrations from 0.10 to 0.45. The concentration distribution predictions are shown to be in excellent agreement with the measurements.

Effectiveness of plants for passive removal of particulate matter is low in the indoor environment

Particulate air pollution is a major health concern and is responsible for about one in nine premature deaths worldwide. Significant exposure to particulate matter (PM) may happen indoors because people spend a large fraction of their time inside buildings. Indoor plants have been suggested as a potential solution for removing PM; however, their effectiveness has not been well characterized. We quantified the ability of eleven different plant species to remove airborne PM through experiments conducted in an environmental chamber. By introducing PM into the chamber and measuring its removal rate with and without the plants, we estimated plants’ deposition velocities and clean air delivery rates (CADRs).The average deposition velocities were 93 ± 9 cm/h for the moss plant, between 29 ± 3 cm/h to 37 ± 4 cm/h for the needle-leaved plants, and between 1 ± 2 cm/h to 13 ± 2 cm/h for the broad-leaved plants. Their CADRs were between 0.002 ± 0.004 m3/h to 0.084 ± 0.009 m3/h, which were significantly lower than those of filter-based air purifiers (CADRs = 170–800 m3/h), meaning that passive plant systems cannot compete with conventional air purifiers. We also found that large quantities of plants would be required to achieve even modest reductions in indoor PM concentrations under real-world conditions, thus highlighting their limited role in controlling indoor PM levels.

Generalized non-equilibrium suspended sediment transport model with hindered settling effect for open channel flows

The present paper formulates a generalized model for non-equilibrium suspended sediment transport through an open channel turbulent flow that contains all the existing models in literature as special cases. The two-dimensional (2D) unsteady model for suspended concentration distribution contains the effect of hindered settling and the bottom boundary condition considers deposition velocity together with equilibrium bottom concentration. OpenFOAM, an open-source Finite Volume (FV) method based framework, has been used for the numerical approximation of the model. The proposed model addresses the non-linearity in the governing equation as well as the non-linearity in boundary conditions. Picard iteration approach has been utilized to ensure the convergence of the numerical solution. With different physical parameters and conditions, concentration profiles of suspended sediment particles have been analysed theoretically and graphically. It greatly extends the domain of the numerical solutions for determining sediment distribution in suspension through an open channel turbulent flow in a non-equilibrium state.

Atmospheric particulate-bound mercury (PBM10) in a Southeast Asia megacity: Sources and health risk assessment

Particulate-bound mercury (PBM) is a global environmental concern owing to its large dry deposition velocities and scavenging coefficients, both of which drive Hg into terrestrial and marine ecosystems. PBM observation studies have been widely conducted over East Asia, but comparable studies in Peninsular Southeast Asia (PSEA) remain scarce. This is the first study reporting PBM concentrations for Ho Chi Minh City (HCMC), the biggest metropolitan area in Vietnam. A total of 222 samples were collected in 2018 and contained an average PBM10 (particulate matter – PM with diameter ≤10 μm) concentration and Hg mass fraction (i.e. PBM/PM) of 67.3 ± 45.9 pg m−3 and 1.18 ± 1.12 μg g−1, respectively. Although PBM concentration was lower than those reported in Chinese megacities, the Hg mass fraction was similar to those in China, suggesting strong enrichment from anthropogenic Hg emissions in HCMC. Traffic-induced particulate emission and deposition processes were major factors governing PBM temporal variation at our site. In addition, the prevailing southwest monsoon winds brought air masses that passed through industrial areas and were associated with a higher Hg mass fraction. Statistically significant positive correlations (R2 = 0.11–0.52, p < 0.01) were observed for PBM with PM and the Hg mass fraction, indicating similar PM and Hg sources or oxidized Hg adsorption onto PM via gas-particle partitioning. Moreover, PCA results revealed a higher contribution of primary sources than secondary sources to PBM concentration variability in HCMC. A health risk assessment indicated that the PBM concentrations at HCMC posed minimal non-carcinogenic risks (HI < 1) for children and adults, but dermal contact may act as an important exposure route since lightweight clothing is common among residents. This PBM dataset over PSEA, a region with high atmospheric Hg emissions, provides a valuable resource for the Hg scientific community to improve our understanding of Hg biogeochemical cycle.

Impacts of Changes in Land Use and Land Cover Between 2001 and 2018 on Summertime O3 Formation in North China Plain and Surrounding Areas–A Case Study

AbstractChanges in land use and land cover (LULC) influence meteorological fields and biogenic emissions, further affecting the atmospheric chemistry and air quality. Combining the satellite measurements and WRF‐Chem model simulations, we evaluate the impacts of the LULC change between 2001 and 2018 on the summertime ozone (O3) formation in North China Plain and surrounding areas (NCPs). Satellite measurements have revealed that from Taihang to Yanshan Mountain, the fraction of broadleaf and needle forest coverage has increased by 5%–20% and the urban area has increased by up to 25% in the NCP. Additionally, the vegetation density has increased significantly in the NCPs except for urban areas. The LULC change generally enhances biogenic volatile compounds emissions in the NCPs, particularly over Taihang and Yanshan mountain, but the O3 variation is divergent. The maximum daily 8‐ihr average (MDA8) O3 concentrations are reduced by 1%–7% over Taihang and Yanshan Mountain because the raised vegetation density increases O3 dry deposition velocity to accelerate the O3 loss. The raised vegetation density enhances the evapotranspiration to decrease the near‐surface temperature by 0.1°C–1.5°C, which also generates a divergence in the low‐level atmosphere in the NCPs, causing secondary northerly or easterly winds in the NCP. The O3 enhancement along the coastal areas of the NCP is attributed to the perturbation of wind fields and photolysis induced by the LULC change. The divergent variation of the MDA8 O3 concentrations in the NCP is generally caused by the variations of biogenic emissions and photolysis.

Effect of low-cost recirculating portable air filtration on aerosol particle deposition and concentration in a conference room: Experiment, theory, and simulation comparison

Respiratory particles exhaled when an individual speaks and breathes can facilitate respiratory virus transmission. Portable air purification units can potentially reduce transmission risk by decreasing the overall infectious particle concentration in a room. This is particularly important for rooms with an inability to be ventilated with adequate amounts of filtered or outdoor air. While the efficacy of portable air filtration units on particle concentrations has been well-studied, the impact of portable air purification units on particle deposition and, hence, indirect aerosol transmission, has received lesser study but may be important for smaller spaces with greater surface area-to-volume ratios. We conducted a series of experiments in a simulated, small conference room (34.4 m3, mimicking a common office meeting room) assessing (1) particle removal rates and (2) changes in deposition rate owing to the presence of a low-cost box fan air purifier (BFAP). To mimic an infectious aerosol source, a breathing simulator manikin was utilized in these experiments, which ejected fluorescein tagged particles in the 1–3 μm size range throughout each experimental condition, with a prescribed, periodic, breathing waveform mimicking human exhalation. We measured deposition flux on upward- and downward-facing horizontal surfaces throughout the room and determined aerosol mass concentrations using impingers; combined, these data enabled estimates of surface-specific deposition velocities. Aerosol size distributions within the room were also quantified using an optical particle spectrometer. CFD simulations and theoretical calculations were simultaneously conducted for comparison to the experimental data. Results suggested the BFAP unit, which had an effective clean air delivery rate of 540 m3 h−1, increased the effective air exchange rate by 4.4x over baseline (20.1 vs. 4.5 h−1). Notably, BFAP unit operation introduced greater deposition to surfaces near the breathing simulator manikin vs. baseline, with deposition velocities within 2 m of the breathing simulator being 100 cm s−1 during BFAP operation, compared to 10−2 cm s−1 during baseline. Deposition velocities and air exchange rates generated from the CFD and theoretical calculations largely agreed with those derived from experiments. Taken together, data suggest that in smaller rooms, such as conference rooms, deposition is not negligible as a mechanism of aerosol clearance, even for particles near 1–3 μm in diameter.

Species-specific efficiency in PM2.5 removal by urban trees: From leaf measurements to improved modeling estimates

The growing population in cities is causing a deterioration of air quality due to the emission of pollutants, causing serious health impacts. Trees and urban forests can contribute through the interception and removal of air pollutants such as particulate matter (PM). The dry deposition of PM by vegetation depends on air pollutant concentration, meteorological conditions, and specific leaf traits. Several studies explored the ability of different plant species to accumulate PM on leaf structures leading to the development of models to quantify the PM removal. The i-Tree Eco is the most used model to evaluate ecosystem services provided by urban trees. However, fine particulate matter (PM2.5) removal is still calculated with a poorly evaluated function of deposition velocity (which depends on wind speed and leaf area) without differentiating between tree species. Therefore, we present an improvement of the standard model calculation introducing a leaf trait index to distinguish the species effect on PM net removal. We also compared model results with measurements of deposited leaf PM by vacuum filtration. The index includes the effect of morphological and functional leaf characteristics of tree species using four parameters: leaf water storage, deposition velocity, resuspension rate and leaf washing capacity. Leaves of 11 common urban tree species were sampled in representative areas of the city of Ferrara (Italy) and at different times of the year from 2018 to 2021. This includes four deciduous broadleaf trees (Tilia cordata, Platanus acerifolia, Acer platanoides, Celtis australis), three evergreen broadleaf trees (Quercus ilex, Magnolia grandiflora, Nerium oleander), and four conifers (Thuja orientalis, Cedrus libani, Pinus pinaster, Picea abies). The results provide significant advancement in assessing PM removal using decision support tools such as models to properly select tree species for future urban tree planting programs aimed at improving air quality.

Solubility of aerosol minor and trace elements in Xiamen Island, Southeast China: Size distribution, health risk and dry deposition

Aerosol element solubility is essential to evaluate the damage to the environment and human health. In this work, the size distribution of total and soluble elements in eight particle size ranges with diameter <0.25, 0.25–0.44, 0.44–1.0, 1.0–1.4, 1.4–2.5, 2.5–10, 10–16 and >16 μm was investigated in Xiamen Island, southeast China from March 2018 to June 2020. The results showed that both total and soluble elements exhibited significant size dependence without obvious seasonal variations, and their relative contributions to PM1 mass were much lower than in particles larger than 1 μm. The correlations between some elements in soluble fraction were quite different from those in total fraction and the correlations also varied with particle size due to their different solubility. The solubility of Al, Fe, Ag and Cr was relatively low compared with other elements. Moreover, the solubility of Na, Mg, Ca, Mn and Ag was less dependent on particle size while Al, Fe and other trace elements exhibited the highest solubility in PM1 and the lowest in PM>10. Overall, the solubility of elements is primarily a function of aerosol origin and size. The carcinogenic risks of metal exposure via inhalation for children (3.31 × 10−6) and adults (4.42 × 10−6) were slightly higher than the guideline of cancer risk with >60 % from V. As for non-carcinogenic risk, the hazard index values for children and adults were 1.59 and 0.53, respectively, with Mn, V and Ni together accounting for >85 % of the risk. >85 % of the size-dependent dry deposition fluxes of the selected soluble elements over the Xiamen Bay were contributed by particles larger than 10 μm due to their high deposition velocities. The atmospheric inputs of bioavailable Fe and Cu to the sea exceeded the required amounts relative to inorganic nitrogen to meet the growth of phytoplankton.

Limitations in representation of physical processes prevent successful simulation of PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; during KORUS-AQ

Abstract. High levels of fine particulate matter (PM2.5) pollution in East Asia often exceed local air quality standards. Observations from the Korea–United States Air Quality (KORUS-AQ) field campaign in May and June 2016 showed that development of extreme pollution (haze) occurred through a combination of long-range transport and favorable meteorological conditions that enhanced local production of PM2.5. Atmospheric models often have difficulty simulating PM2.5 chemical composition during haze, which is of concern for the development of successful control measures. We use observations from KORUS-AQ to examine the ability of the GEOS-Chem chemical transport model to simulate PM2.5 composition throughout the campaign and identify the mechanisms driving the pollution event. At the surface, the model underestimates sulfate by −64 % but overestimates nitrate by +36 %. The largest underestimate in sulfate occurs during the pollution event, for which models typically struggle to generate elevated sulfate concentrations due to missing heterogeneous chemistry in aerosol liquid water in the polluted boundary layer. Hourly surface observations show that the model nitrate bias is driven by an overestimation of the nighttime peak. In the model, nitrate formation is limited by the supply of nitric acid, which is biased by +100 % against aircraft observations. We hypothesize that this is due to a large missing sink, which we implement here as a factor of 5 increase in dry deposition. We show that the resulting increased deposition velocity is consistent with observations of total nitrate as a function of photochemical age. The model does not account for factors such as the urban heat island effect or the heterogeneity of the built-up urban landscape, resulting in insufficient model turbulence and surface area over the study area that likely results in insufficient dry deposition. Other species such as NH3 could be similarly affected but were not measured during the campaign. Nighttime production of nitrate is driven by NO2 hydrolysis in the model, while observations show that unexpectedly elevated nighttime ozone (not present in the model) should result in N2O5 hydrolysis as the primary pathway. The model is unable to represent nighttime ozone due to an overly rapid collapse of the afternoon mixed layer and excessive titration by NO. We attribute this to missing nighttime heating driving deeper nocturnal mixing that would be expected to occur in a city like Seoul. This urban heating is not considered in air quality models run at large enough scales to treat both local chemistry and long-range transport. Key model failures in simulating nitrate, mainly overestimated daytime nitric acid, incorrect representation of nighttime chemistry, and an overly shallow and insufficiently turbulent nighttime mixed layer, exacerbate the model's inability to simulate the buildup of PM2.5 during haze pollution. To address the underestimate in sulfate most evident during the haze event, heterogeneous aerosol uptake of SO2 is added to the model, which previously only considered aqueous production of sulfate from SO2 in cloud water. Implementing a simple parameterization of this chemistry improves the model abundance of sulfate but degrades the SO2 simulation, implying that emissions are underestimated. We find that improving model simulations of sulfate has direct relevance to determining local vs. transboundary contributions to PM2.5. During the haze pollution event, the inclusion of heterogeneous aerosol uptake of SO2 decreases the fraction of PM2.5 attributable to long-range transport from 66 % to 54 %. Locally produced sulfate increased from 1 % to 25 % of locally produced PM2.5, implying that local emissions controls could have a larger effect than previously thought. However, this additional uptake of SO2 is coupled to the model nitrate prediction, which affects the aerosol liquid water abundance and chemistry driving sulfate–nitrate–ammonium partitioning. An additional simulation of the haze pollution with heterogeneous uptake of SO2 to aerosol and simple improvements to the model nitrate simulation results in 30 % less sulfate due to 40 % less nitrate and aerosol water, and this results in an underestimate of sulfate during the haze event. Future studies need to better consider the impact of model physical processes such as dry deposition and nighttime boundary layer mixing on the simulation of nitrate and the effect of improved nitrate simulations on the overall simulation of secondary inorganic aerosol (sulfate + nitrate + ammonium) in East Asia. Foreign emissions are rapidly changing, increasing the need to understand the impact of local emissions on PM2.5 in South Korea to ensure continued air quality improvements.

Impact of filtering methods on ultrafine particles turbulent fluxes by eddy covariance

In the present work, a comparison between Linear Detrending (LDT) and a Recursive Digital Filter (RDF) in removing the low-frequency contribution to the vertical turbulent fluxes was performed. The two methods were applied in order to obtain a correct evaluation of ultrafine particles, sensible heat and momentum turbulent fluxes. Exchange velocity was also evaluated separating the positive cases (the so called deposition velocity Vd) from negative cases (named emission velocity Ve). The low-frequency time scales (τc) required by the RDF were obtained by means of an ogive analysis of turbulent fluxes for different atmospheric stability conditions (i.e. unstable, stable and neutral). RDF was applied also with a constant low-frequency time scale (RDF300, τc = 300s). The stationarity test proposed by Mahrt (1998 - MST98) has been applied to momentum, kinematic temperature and particle number fluxes before and after applying LDT and RDF methods, in order to investigate the impact of filtering criteria on stationarity of time series. Results emphasised that there were no significant differences in stationary cases for different filtering procedures. The comparison analysis on the main turbulent variables highlighted that wider discrepancies occurred between LDT and RDF300, showing on average an increase in turbulent number particles flux obtained through RDF methods, especially in unstable atmospheric conditions. On the other hand, a mean decrease for momentum and sensible heat fluxes was observed. Filtering procedures led to a slight increase of exchange velocity, although an underestimation occurred for emission and deposition velocities when considered separately.

Sulfur deposition in the Beijing-Tianjin-Hebei region, China: Spatiotemporal characterization and regional source attributions

Elevated atmospheric sulfur (S) deposition, which can affect ecosystems and human health, is a severe environment problem in the Beijing-Tianjin-Hebei (BTH) region. Here we investigated the spatiotemporal characteristics of S deposition and its regional sources in January (winter) and July (summer), 2015 in BTH using the CMAQ model coupled with the Integrated Source Apportionment Method (CMAQ-ISAM). The results showed that S deposition in high ecological risk areas (regions with S deposition significantly exceeding the critical load) of BTH was higher in summer (1.99 kg S ha−1 mon−1) than in winter (1.18 kg S ha−1 mon−1), with 82% (18% for winter) attributed to wet deposition and the rest to dry deposition. The daily variation of S dry deposition was mainly dependent on sulfur dioxide (SO2) dry deposition velocity in winter while S concentration in summer. SO2 dry deposition velocity was mainly caused by the change of resistance to surface resistance (Rc). In summer, during the periods of significant wet deposition due to precipitation, S concentration decreased significantly by ∼70%. The results of the regional source appointment suggested that local emissions (inside BTH) contributed ∼65% to dry deposition but only 8% and 28% to wet deposition in January and July respectively. For the contribution from non-local emission (outside BTH), Shandong contributed most to S dry deposition (17% on average). The contributions from Shandong (23%) and Shanxi (12%) to S wet deposition were higher in July than in January (10% and 3%, respectively). Analysis of daily contributions suggests that the contribution from non-local emissions to dry deposition could be enhanced by the southwest background wind. Local emissions took an important part in S total deposition (∼70%) during dry days while non-local emissions contributed significantly (∼65%) to total deposition during wet days. For seasonally, S total deposition was contributed more by local emissions (56%) in January but was dominated by non-local emissions (65%) in July. It demonstrates the need for a comprehensive strategy on local and non-local emissions for pollutant control against ecological risk caused by deposition.

A database of modeled gridded dry deposition velocities for 45 gaseous species and three particle size ranges across North America

The dry deposition process refers to the flux loss of an atmospheric pollutant due to uptake of the pollutant by the earth's surfaces. Dry deposition flux of a chemical species is typically calculated as the product of its surface-layer concentration and its dry deposition velocity (Vd). Field measurement based Vd data are very scarce or do not exist for many chemical species considered in chemistry transport models. In the present study, gaseous and particulate dry deposition schemes were applied to generate a database of hourly Vd for 45 gaseous species and three particle size ranges for two years (2016–2017) at a 15 km by 15 km horizontal resolution across North America. Hourly Vd of the 45 gaseous species ranged from < 0.001 to 4.6 cm/sec across the whole domain, with chemical species-dependent median (mean) values being in the range of 0.018–1.37 cm/sec (0.05–1.43 cm/sec). The spatial distributions of the two-year average Vd showed values higher than 1–3 cm/sec for those soluble and reactive species over certain land types. Soluble species have the highest Vd over water surfaces, while insoluble but reactive species have the highest Vd over forests. Hourly Vd of PM2.5 across the whole domain ranged from 0.039 to 0.75 cm/sec with median (mean) value of 0.18 (0.20) cm s−1, while the mean Vd for PM2.5–10 is twice that of PM2.5. Uncertainties in the modeled Vd are typically on the order of a factor of 2.0 or larger, which needs to be considered when applying the dataset in other studies.

Blasius and Sakiadis flow of a Casson hybrid nanofluid over a moving plate

ABSTRACT It is generally known that adding a particular amount of nanoparticles to a traditional liquid improves its thermal conductivity. The cause of this dramatic improvement is yet unclear. As a result, determining the proper thermal impact of particles at the nanoscale requires understanding the kinematics of nanoparticle suspension. Bearing this in mind, the primary goal of this research is to investigate thermophoretic particle deposition on a Blasius-Sakiadis flow of a Casson hybrid nanoliquid flow over a moving plate. In this case, a 50% Ethylene glycol (EG) carrier liquid is used to suspend copper and silver nanoparticles. The governing equations are reduced to ordinary differential equations (ODEs) using appropriate similarity transformations. Then these ODEs are solved using the fourth-fifth order Runge–Kutta Fehlberg's technique (RKF-45) with the shooting methodology. The influence of various non-dimensional factors on involved fields is then discussed in depth using appropriate graphs. The results show that for both Blasius and Sakiadis flows, rise in values of the radiation parameter increase fluid heat transfer. Furthermore, for both Blasius and Sakiadis flows, an increase in the thermophoretic coefficient and thermophoretic parameter degrades the liquid's mass transfer and thermophoretic diffusive deposition velocity.