Downwind Regions Research Articles

Temperature elevation and trajectory in the downwind region of rectangular fire sources in cross-winds

The effect of cross-wind on the temperature elevation in the downwind region of rectangular diffusion burners was investigated in wind tunnel experiments. The experimental parameters were the geometry of the diffusion burner (the side length parallel and orthogonal to the cross-wind L1=0.2–0.8m and L2=0.4–2.0m, respectively, and the aspect ratio r=1–10), the heat release rate (Q˙=38.3–383.3 kW), and the inflow velocity of the cross-wind (U∞=0.59–1.49 m/s). Following the concept of the previously proposed point fire source model, the fractional contribution of the cross-wind velocity on the velocity along the trajectory N was introduced to close the system of governing equations, and a relevant line fire source model was derived. The line fire source model was correlated by the measurement results of the present experiment for the diffusion burner with the largest aspect ratio (r=10). Although the predicted temperature elevation along the trajectory by the present model was similar to that of the existing models, there was a substantial discrepancy in the predicted trajectory. For the unified analysis of temperature elevation and trajectories of the fire sources with various aspect ratios, the point fire source model was firstly tested to correlate the measurement results. As expected, the agreement was less successful for the results of the fire sources with large aspect ratios, especially in the region close to the fire source. Thus, the point fire source model was modified to consider the physical effects of the aspect ratio of the fire source and the attachment of the fire plume to the ground by invoking the correlations for the line fire source. With the modified model, the agreement between the predicted and measured results significantly improved. The proposed model is useful for assessing the hazards of wind-blown fire plumes in large outdoor fires on human activities such as firefighting or the evacuation of residents.

Rare earth elements in the East Sea (Japan Sea): Distributions, behaviors, and applications

The distributions of rare earth elements (REEs) were measured at 14 stations from 0 to 3365 m (n = 176) in the entire East Sea for the first time. The East Sea has its own deep-convection system, which operates on a time scale that is approximately one tenth that of the global ocean; it is also a downwind region of Asian dust. The vertical distributions of REE concentrations increased with depth, similar to those in the North Atlantic Ocean. However, the concentrations of REEs were 1.3–2.7 fold higher than those in the North Atlantic Ocean, associated with relatively larger atmospheric depositions of dust from the Asian continent. In this environmental condition, the removal of Ce relative to the other REEs was about an order of magnitude more efficient than that in the North Pacific Ocean. As such, in the deep layer below 750 m, large increases of heavy-REEs (HREEs) relative to light-REEs (LREEs), opposite to the re-mineralization trend, were observed suggesting that LREEs were preferentially re-scavenged by sinking particles. In addition, the boundary scavenging of REEs was also observed for all REEs in the bottom layer of the East Sea. On the other hand, the warm surface waters of REEs, Tsushima Warm Water (TWW) showed large fractionations, relative to the other water masses. This REE pattern traced the deep physical mixing proportions of TWW up to 750 m (∼10%). Thus, the unique distribution patterns of REEs in this sea illustrate the geochemical scavenging characteristics of REEs, which are concealed in the major oceans, as well as water mass mixing patterns in the upper ocean.

Emissions of non-methane volatile organic compounds from a landfill site in a major city of India: impact on local air quality

Emissions from landfills are a significant source of non-methane volatile organic compounds (NMVOCs) in urban environments. NMVOCs play an important role in atmospheric chemistry, and elevated concentrations of some compounds are responsible for air quality deterioration. This study is based on the measurements of a suite of 20 C2–C8 NMVOCs at 21 upwind and downwind sites of the largest landfill in western India. Ethane, ethylene and aromatics were the dominant compounds; the concentrations of BTEX in the downwind regions were up to three times higher than their concentrations at upwind sites. The emission ratios of BTEX and other NMVOCs were different from those for residential, commercial, and industrial sources characterizing the emissions from burning and decomposition of organic material. The slope of ΔToluene/ΔBenzene of 0.64 is about three times higher than that determined at the main road junctions of the city. Ranking by Prop-Equiv, the top NMVOCs were isoprene, cis-2-Butene, m + p-xylenes, propylene, ethylene and trans-2-Butene account for 72–75% of the total Prop-Equiv concentrations. Alkenes played the dominant role in ozone formation, followed by aromatic and alkane groups. In addition to landfill emissions, contributions from traffic-related emissions to ambient concentrations of aromatic VOCs were also significant at some sites. Although the experiment was not designed to characterize the emissions from a specific source, the analysis suggests the substantial contributions from both decomposition and burning of landfill materials. The main difficulty in characterizing VOC emissions from landfills is the spatial and temporal variability of emissions from a large area.

Synergistic enhancement of urban haze by nitrate uptake into transported hygroscopic particles in the Asian continental outflow

Abstract. Haze pollution is affected by local air pollutants, regional transport of background particles and precursors, atmospheric chemistry related to secondary aerosol formation, and meteorological conditions conducive to physical, dynamical, and chemical processes. In the large, populated and industrialized areas like the Asian continental outflow region, the combination of regional transport and local stagnation often exacerbates urban haze pollution. However, the detailed chemical processes underlying the enhancement of urban haze induced by the combined effect of local emissions and transported remote pollutants are still unclear. Here, we demonstrate an important role of transported hygroscopic particles in increasing local inorganic aerosols, by studying the chemical composition of PM2.5 collected between October 2012 and June 2014 in Seoul, a South Korean megacity in the Asian continental outflow region, using the ISORROPIA II thermodynamic model. PM2.5 measured under the condition of regional transport from the upwind source areas in China was higher in mass concentration and richer in secondary inorganic aerosol (SIA) species (SO42-, NO3-, and NH4+) and aerosol liquid water (ALW) compared to that measured under non-transport conditions. The secondary inorganic species and ALW were both increased, particularly in cases with high PM2.5 levels, and this indicates inorganic species as a major driver of hygroscopicity. We conclude that the urban haze pollution in a continental outflow region like Seoul, particularly during the cold season, can be exacerbated by ALW in the transported particles, which enhances the nitrate partitioning into the particle phase in NOx- and NH3-rich urban areas. This study reveals the synergistic effect of remote and local sources on urban haze pollution in the downwind region and provides insight into the nonlinearity of domestic and foreign contributions to receptor PM2.5 concentrations in numerical air quality models.

A Predictive Analytical Model for Surface Shear Stresses and Velocity Profiles Behind a Surface Roughness Jump

A simple analytical model is developed for the flow downwind of a step change of the aerodynamic roughness length in the atmospheric boundary layer. The region downwind of the roughness transition is assumed to be composed of two equilibrium layers, corresponding to the downwind and upwind conditions, and separated by a third, transition, layer. The key assumption in deriving the model is that the eddy viscosity in the transition region is composed of a linearly varying component and an augmentation, which is parabolic in the vertical coordinate. The model is fully predictive up to two parameters that need to be specified. The first parameter is the ratio between the equilibrium boundary-layer height and the internal boundary-layer height. The second parameter controls the degree of nonlinearity of the eddy viscosity augmentation. An empirical relation is developed for the first parameter, while a range of values (between 0.1 and 0.3) is recommended for the second. The model is tested with results from field observations, wind-tunnel experiments, and numerical simulations of the flow behind a sudden jump in surface roughness. These tests show that the flow field and surface stresses are predicted accurately for smooth-to-rough as well as rough-to-smooth transitions.

Timing of rainfall occurrence altered by urban sprawl

The impact of urban changes from 1989 to 2015 on urban rainstorms is assessed for the rapidly expanding Indian city of Hyderabad. A typical event that occurred during 12–14 June 2015 is simulated, and the possible association between altered urban-sprawl and shifts in the rainfall timing and patterns are analyzed. The simulation results revealed that the consideration of urban footprint changes was necessary to correctly simulate the heavy rainfall in the high-resolution Weather Research and Forecasting (WRF) model. With the urban expansion and associated anthropogenic heating (AH), the rainfall was more distributed, and its occurrence delayed by ~1–2 h. The results suggest that urbanization can affect rainfall both in terms of the timing as well as the location with respect to the urbanized area. The urban downwind region received an increased rainfall, and this spatial shift was more pronounced with increasing urbanization - from being over the urban area to over rural-urban boundary downwind of the city. The analysis of hydrometeors indicated that rain and cloud quantity decreased with an increasing AH in the urban area. Study results highlight the possibility that the change in the rainfall timing is another climatological signature of urban modification of rainfall.

O3 and CO in the South Asian outflow over the Bay of Bengal: Impact of monsoonal dynamics and chemistry

Abstract Monsoonal dynamics strongly impacts the composition over the South Asian region, however, in situ measurements remain limited mostly to the continental region. Whereas, remote oceanic regions of the Bay of Bengal (BoB) remain largely unexplored where shipping emissions can infuse into the South Asian outflow. In this direction, we performed ship-based measurements of ozone (O3) and carbon monoxide (CO) over the BoB during July‒August 2018 and corroborated the analyses with model results and satellite retrievals. Surface level mixing ratios of O3 and CO varied in the range of 14–45 and 50–164 ppbv, respectively during the expedition. Elevated O3 and CO levels were observed in coastal region downwind of a tropical megacity (Chennai), referred to as the Chennai-plume. Analysis of the Copernicus Atmosphere Monitoring Service (CAMS) model simulations suggests volatile organic compound (VOC)-limited O3 production in morning over most of the plume and east coast followed by gradual shift to NOx-limited regime over the BoB in afternoon. While the southern BoB (7–10° N) received generally pristine airmasses from the equatorial Indian Ocean, impacts of shipping emissions near 6° N are observed. Consequently, higher values of CO (as much as 120 ppbv) and net O3 production by 1.1–1.3 ppbv h−1 with a diurnal amplitude of 4–8 ppbv are observed over the southern BoB. Satellite-based observations from the Measurements of Pollution in the Troposphere (MOPITT) and results from CAMS model show decreasing trend (4.1–5.2 ± 1 ppbv decade−1) of surface CO, as also inferred from in situ measurements. Zonal fluxes of CO across the east coast show the outflow over the BoB with higher values (2–3 × 1019 molecules m−2 s−1) over the northern BoB. Positive vertical fluxes (~2 × 1016 molecules m−2 s−1) show the transport of CO from marine boundary layer to the free-troposphere. Due to strong updrafts, CO levels in upper and lower troposphere are seen to be comparable during monsoon. These findings in conjunction with higher methane and nitrogen dioxide in the upper-troposphere manifest the influences of monsoonal convection and trapping of trace gases in the anticyclone.

Paradigm shift in aerosol chemical composition over regions downwind of China

A rapid decrease in PM2.5 concentrations in China has been observed in response to the enactment of strong emission control policies. From 2012 to 2017, total emissions of SO2 and NOx from China decreased by approximately 63% and 24%, respectively. Simultaneously, decreases in the PM2.5 concentration in Japan have been observed since 2014, and the proportion of stations that satisfy the PM2.5 environmental standard (daily, 35 µg/m3; annual average, 15 µg/m3) increased from 37.8% in fiscal year (FY) 2014 (April 2014 to March 2015) to 89.9% in FY 2017. However, the quantitative relationship between the PM2.5 improvement in China and the PM2.5 concentration in downwind regions is not well understood. Here, we (1) quantitatively evaluate the impacts of Chinese environmental improvements on downwind areas using source/receptor analysis with a chemical transport model, and (2) show that these rapid emissions reductions improved PM2.5 concentrations both in China and its downwind regions, but the difference between SO2 and NOx reduction rates led to greater production of nitrates (e.g., NH4NO3) due to a chemical imbalance in the ammonia–nitric acid–sulfuric acid–water system. Observations from a clean remote island in western Japan and numerical modeling confirmed this paradigm shift.

Importance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

Abstract. Theoretical, laboratory, and chamber studies have shown fast regeneration of the hydroxyl radical (OH) in the photochemistry of isoprene, largely due to unimolecular reactions which were previously thought not to be important under atmospheric conditions. Based on early field measurements, nearly complete regeneration was hypothesized for a wide range of tropospheric conditions, including areas such as the rainforest where slow regeneration of OH radicals is expected due to low concentrations of nitric oxide (NO). In this work the OH regeneration in isoprene oxidation is directly quantified for the first time through experiments covering a wide range of atmospherically relevant NO levels (between 0.15 and 2 ppbv – parts per billion by volume) in the atmospheric simulation chamber SAPHIR. These conditions cover remote areas partially influenced by anthropogenic NO emissions, giving a regeneration efficiency of OH close to 1, and areas like the Amazonian rainforest with very low NO, resulting in a surprisingly high regeneration efficiency of 0.5, i.e. a factor of 2 to 3 higher than explainable in the absence of unimolecular reactions. The measured radical concentrations were compared to model calculations, and the best agreement was observed when at least 50 % of the total loss of isoprene peroxy radicals conformers (weighted by their abundance) occurs via isomerization reactions for NO lower than 0.2 ppbv. For these levels of NO, up to 50 % of the OH radicals are regenerated from the products of the 1,6 α-hydroxy-hydrogen shift (1,6-H shift) of Z-δ-RO2 radicals through the photolysis of an unsaturated hydroperoxy aldehyde (HPALD) and/or through the fast aldehydic hydrogen shift (rate constant ∼10 s−1 at 300 K) in di-hydroperoxy carbonyl peroxy radicals (di-HPCARP-RO2), depending on their relative yield. The agreement between all measured and modelled trace gases (hydroxyl, hydroperoxy, and organic peroxy radicals, carbon monoxide, and the sum of methyl vinyl ketone, methacrolein, and hydroxyl hydroperoxides) is nearly independent of the adopted yield of HPALD and di-HPCARP-RO2 as both degrade relatively fast (<1 h), forming the OH radical and CO among other products. Taking into consideration this and earlier isoprene studies, considerable uncertainties remain on the distribution of oxygenated products, which affect radical levels and organic aerosol downwind of unpolluted isoprene-dominated regions.

The impacts of transported wildfire smoke aerosols on surface air quality in New York State: A case study in summer 2018

Abstract Wildfire smoke aerosols, once emitted, can transport over long distances and affect surface air quality in downwind regions. In New York State (NYS), fine particulate matter (PM2.5) concentration continues to decrease due to anthropogenic emission reductions and regulatory initiatives in recent years. Smoke aerosols, however, are projected to increase, making them the dominant source of PM2.5. Thus, the influences of smoke aerosols could become more important in the future. In this study, the long-range transport of smoke aerosols, and their impacts on local air quality over NYS in mid-August 2018 were investigated using satellite measurements, ground-based networks, and model products. Satellite measurements showed extensive fire activities over the northwestern United States (US) during August 8th – 10th. Air quality monitoring sites in NYS reported a threefold increase in average PM2.5 concentration (from 8.4 ± 3.4 μg m−3 to 24.8 ± 4.0 μg m−3) on August 15th – 16th, while the ground-based profiler network detected aerosol layers at 2–5 km across the state. Analysis of backward trajectories revealed that the plumes originated from wildfires, transported through southern Canada and arrived at the east coast during a period of 5–7 days. The increased PM2.5 in NYS can be attributed to boundary layer entrainment and vertical mixing of the aloft transported smoke aerosols down to the surface. The NYS mesoscale weather network (NYSM), which is originally designed for severe weather monitoring, demonstrate the capability to probe the lower atmosphere and provide the vertical extent information of air pollutants.

Gas temperature rise and flame length induced by two buoyancy-controlled propane burners aligned parallel to the cross wind

This paper investigates the gas temperature rise and flame length of buoyant turbulent diffusion flames under cross wind, which does exist in fuel leakage fire accidents. Two identical propane burners with the same heat release rate were employed as fire sources. The burner spacing, heat release rate and cross wind velocity were changed. The results show that with the increase of the cross wind velocity, the gas temperatures in downwind region are strongly increased, resulting in a strong heat transfer to nearby energy loads. On the other hand, the non-monotonic trend of the flame length is caused by the cross wind effect. In addition, a segmented function predicting the flame length under cross wind is developed for merging and non-merging flames, which involves the heat release rate, burner spacing, pool size and cross wind velocity. The findings of this work have potential practical applications for optimal arrangements of energy loads, which allow predictions to be made regarding the possible threat and the establishment of the necessary safety distance to prevent the damage of multiple fires.

Temporal variation of dust aerosol pollution in northern China

By employing the sandstorm dataset from meteorological station observations, the temporal and spatial variations in dust outbreak duration in the source regions and visibility variations over the downwind regions of northern China were analyzed over the past four decades. The statistics and wind field analyses reveal that after the early 2000s, the duration of dust outbreaks over the potential source areas of northern China had decreased by 63% compared with that in the 1970s, which suggests that the dust aerosol emission intensities in the region were significantly reduced. However, although the potential dust aerosol emission intensities were significantly reduced, the visibility during dust aerosol emission periods increased by only 25%. The emerging potential source areas in the eastern regions, variations in the wind field, increases in up-air winds, and decreases in surface winds all contribute to the observed continued high dust aerosol pollution in northern China since the early 2000s.

Heliospheric Structure as Revealed by the 3–88 keV H ENA Spectra

Energetic neutral atoms (ENA) are an important tool for investigating the structure of the heliosphere. Recently, it was observed that fluxes of ENAs (with energy $\le$ 55 keV) coming from the upwind and downwind regions of the heliosphere are similar in strength. This led the authors of these observations to hypothesize that the heliosphere is bubble-like rather than comet-like, meaning that it has no extended tail. We investigate the directional distribution of the ENA flux for a wide energy range (3--88 keV) including the observations from IBEX (Interstellar Boundary Explorer), INCA (Ion and Neutral Camera, on board Cassini), and HSTOF (High energy Suprathermal Time Of Flight sensor, on board SOHO, Solar and Heliospheric Observatory). An essential element is the model of pickup ion acceleration at the termination shock (TS) proposed by Zank. We use state of the art models of the global heliosphere, interstellar neutral gas density, and pickup ion distributions. The results, based on the "comet-like" model of the heliosphere, are close in flux magnitude to ENA observations by IBEX, HSTOF and partly by INCA (except for the 5.2-13.5 keV energy channel). We find that the ENA flux from the tail dominates at high energy (in agreement with HSTOF, but not INCA). At low energy, our comet-like model produces the similar strengths of the ENA fluxes from the upwind and downwind directions, which, therefore, removes this as a compelling argument for a bubble-like heliosphere.

Sensitivity Experiments of the Local Wildland Fire with WRF-Fire Module

In this paper, it is discussed the performance of the Weather Research and Forecasting (WRF) model coupled with a wildland fire-behavior module (WRF-Fire model) by the observational data collected in an experiment with a low-intensity prescribed fire (LIPF) conducted in the New Jersey Pine Barrens (NJPB) on March 6, 2012. The sensitivity experiments of the WRF-Fire model are carried out to investigate the interactions between the wildland fire and the atmospheric planetary boundary layer. The two-way WRF-Fire model conofigured with fire and large eddy simulation (LES) mode is used to explore the fire characteristics of perimeter shape, intensity, spread direction and external factors of wind speed, and to discuss how these external parameters affect the fire, and the interactions between the atmosphere and fire. Results show that the sensitive experiments can provide the meteorological elements close to observations, such as the temperatures, winds and turbulent kinetic energy near the surface in the vicinity of the fire. The simulations also can reproduce the fire spread shape and speed, fire intensity, and heat flux released from fire. From the view of energy, the heat flux feed back to the atmospheric model and heat the air near the surface, which will induce strong thermal and dynamic instability causing strong horizontal convergence and updraft, and form the fire-induced convective boundary layer. The updraft will be tilted downstream of the fire area based on the height of the ambient winds. Due to the effect of the this updrafts, the particles and heat from the fuel combustion can be transported to the downwind and lateral regions of the fire area. Meanwhile, there is a downdraft flow with higher momentum nearby the fire line transporting fresh oxygen to the near surface, which will increase winds behind the fire line, accelerate the rate of spread (ROS) and make the fire spread to a larger area. Ultimately, a fire-induced climate is established.

Irrigation Impact on Water and Energy Cycle During Dry Years Over the United States Using Convection‐Permitting WRF and a Dynamical Recycling Model

AbstractAn irrigation scheme is implemented in the Weather Research and Forecasting (WRF) model to investigate irrigation impacts over the Continental U.S. (CONUS). Four major irrigated regions and two downwind regions were chosen to understand irrigation impacts over different climate regimes with a focus on irrigation‐induced changes on the water and energy cycles. The Dynamic Recycling Model (DRM) is employed to quantify precipitation induced by irrigation and the precipitation recycling ratios over each irrigated region. With the irrigation scheme, WRF improves the simulated precipitation, surface skin temperature, and energy fluxes compared to reference datasets. For the energy cycle, irrigation increases latent heat flux over the irrigated regions along with reduced sensible heat flux. The evaporative cooling effect induced by irrigation leads to a cooler surface and less outgoing longwave radiation at the surface. Irrigation also intensifies the hydrological cycle over the irrigated regions, reflected by the increased precipitation, evapotranspiration, recycling ratio, and moisture export. Downwind regions exhibit increased precipitation and evaporation, decreased moisture flux divergence, and less consistent variations in recycling ratio. The precipitation increases over the irrigated regions can be partly explained by the more unstable low‐level conditions, while reduced net moisture export is coincident with the precipitation increases over the downwind regions.

Improved Dust Emission Reduction Factor in the ADAM2 Model Using Real-Time MODIS NDVI

The Korea Meteorological Administration has employed the Asian Dust Aerosol Model 2 (ADAM2) to forecast Asian dust events since 2010, where the dust emission flux is proportional to the fourth power of the friction velocity. Currently, the dust emission reduction factor (RF) is determined by the normalized difference vegetation index (NDVI). This study aims to improve the forecasting capability of ADAM2 by developing a daily dust RF using both monthly (January 2007 to December 2016) and real-time moderate resolution imaging spectroradiometer (MODIS) NDVI data. We also developed a look-up table to transform the RF using NDVI and a system to update the RF by producing MODIS NDVI data for the last 30 days. Using these data, new RFs can be produced every day. To examine the impact of RF modification, the current (CTL) and new (EXP) RFs are compared during the period from March to May 2017. The simulations are verified by ground-based PM10 observations from China and Korea. Accordingly, root mean square errors (RMSEs) are reduced by 11.58% when RF is updated using real-time NDVI data. The results suggest that recent daily NDVI data contribute positively to the forecasting ability of ADAM2, in the dust source and downwind regions.

Central Mediterranean explosive volcanism and tephrochronology during the last 630 ka based on the sediment record from Lake Ohrid

Lake Ohrid, located in the Central Mediterranean region downwind of the Italian volcanoes, has great potential for developing a detailed record of the explosive activity of these volcanoes. Electron microprobe and Laser Ablation Inductively Coupled Plasma Mass Spectrometry geochemical analyses of juvenile glass fragments of previously not analysed tephra layers of the DEEP site sediment record of Lake Ohrid expand the existing tephrostratigraphy for the interval Marine Isotope Stages 1–16. The geochemical signatures of all tephra layers suggest an origin exclusively from the Italian volcanism.Eight of these tephra layers have potential equivalents in marine and terrestrial archives of the Central Mediterranean region (OH-DP-1966/SC5/A7; OH-DP-1955/CES1/A2; OH-DP-1055/S1; OH-DP-1006/S2; OH-DP-0997/S3, OH-DP-0710/S8/C-51; OH-DP-0766/ODP5/P-13; OH-DP-0505/ODP4). Nine tephra layers, which have not been described so far, help to reconstruct the late volcanic activity of Mount Vulture (OH-DP-2439) and the Paleovulsini (OH-DP-1998), the pre- and post Rio Rava activity of the Roccamonfina volcano (OH-DP-1911/-1812/-1733/-1640/-1527), the early activity of the Aeolian Arc (OH-DP-1513/-1520), and the Bracciano activity of the Monti Sabatini volcanic district (OH-DP-1175). The multi-proxy dataset and chronology of the DEEP site sediments allowed definition of the climatostratigraphic position for all tephra layers and constrain the ages of so far unknown and/or undated tephra layers. The presented data provide a crucial increase of knowledge of the Middle Pleistocene tephrostratigraphy in the Central Mediterranean back to 630 ka and allows new insights of the volcanic activity of Roccamonfina, the Roman Province, the Campanian Volcanic Zone (CVZ), the Aeolian Arc, Pantelleria, and Mount Vulture.

Experimental and numerical investigation of the influence of roughness and turbulence on LUT airfoil performance

Vertical-axis wind turbines (VAWTs) have been widely used in urban environments, which contain dust and experience strong turbulence. However, airfoils for VAWTs in urban environments have received considerably less research attention than those for horizontal-axis wind turbines (HAWTs). In this study, the sensitivity of a new VAWT airfoil developed at the Lanzhou University of Technology (LUT) to roughness was investigated via a wind tunnel experiment. The results show that the LUT airfoil is less sensitive to roughness at a roughness height of < 0.35 mm. Moreover, the drag bucket of the LUT airfoil decreases with increasing roughness height. Furthermore, the loads on the LUT airfoil during dynamic stall were studied at different turbulence intensities using a numerical method at a tip-speed ratio of 2. Before the stall, the turbulence intensity did not considerably affect the normal or tangential force coefficients of the LUT airfoil. However, after the stall, the normal force coefficient varied obviously at low turbulence intensity. Moreover, as the turbulence intensity increased, the normal and tangential force coefficients decreased rapidly, particularly in the downwind region of the VAWT.

Numerical Investigation on the Effects of Airfoil Leading Edge Radius on the Aerodynamic Performance of H-Rotor Darrieus Vertical Axis Wind Turbine

This paper numerically investigates the effects of airfoil leading edge radius on the aerodynamic characteristics of H-rotor Darrieus vertical axis wind turbine (VAWT). 10 modified airfoils are generated by changing the leading edge radius of the base NACA 0015 airfoil from 1%c to 9%c, respectively. A 2D unsteady Computational Fluid Dynamics (CFD) model is established and validated with the previously published experimental data. The power, torque, and flow field characteristics of the rotors are analyzed. The results indicate that the maximum and minimum power coefficient at the optimum tip speed ratio (TSR) are obtained for the LE-5%c and LE-1%c model, respectively. The best aerodynamic characteristics are determined by the LE-5%c model below the optimum TSR and the LE-3%c model beyond the optimum TSR. The torque characteristics and pressure distribution for the single blades with different airfoil leading edge radius show an obvious difference in the upwind region and a very small difference in the downwind region. Moreover, the airfoil leading edge radius influences the strength, region, and diffusion rate of the vortices, being the main reason for the observed differences in instantaneous torque coefficient and power coefficient. The vortices of the LE-1%c model are stronger, larger, and diffuse slower than those of the LE-2%c and LE-5%c model at the optimum TSR.

Eight-year dry deposition of atmospheric mercury to a tropical high mountain background site downwind of the East Asian continent

Atmospheric deposition, either dry or wet, has been identified as an important pathway of mercury (Hg) input to terrestrial and aquatic systems. Although East Asia is the major atmospheric Hg emission source region, very few studies have been conducted to quantify atmospheric Hg deposition in its downwind region. In this study, 8-year (2009–2016) atmospheric Hg dry deposition was reported at the Lulin Atmospheric Background Station (LABS), a high mountain forest site in central Taiwan. Dry deposition of speciated Hg was estimated using a bi-directional air-surface flux exchange model for gaseous elemental mercury (GEM) and dry deposition models for gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM), making use of the monitored speciated atmospheric Hg concentrations. Annual total Hg dry deposition ranged from 51.9 to 84.9 μg m−2 yr−1, with a multi-year average of 66.1 μg m−2 yr−1. Among the three forms of atmospheric Hg, GEM was the main contributor to the total dry deposition, contributing about 77.8% to the total, due to the high density of forest canopy as well as the much higher concentration of GEM than GOM and PBM at LABS. Mercury dry deposition is higher in winter and spring than in summer and fall, partly due to the elevated Hg concentrations associated with air masses from East and Southeast Asia where with high atmospheric Hg emissions. The mean annual dry/wet deposition ratio of 2.8 at LABS indicated that Hg deposition to forest landscape was governed by dry rather than wet deposition.