scholarly journals Constant flux layers with gravitational settling: links to aerosols, fog and deposition velocities

2021 ◽  
Vol 21 (24) ◽  
pp. 18263-18269
Author(s):  
Peter A. Taylor
Keyword(s):  
Friction Velocity ◽  
Roughness Length ◽  
Settling Velocity ◽  
Deposition Velocity ◽  
Surface Boundary ◽  
Surface Deposition ◽  
Gravitational Settling ◽  
Constant Flux ◽  
Deposition Velocities ◽  
Roughness Lengths

Abstract. Turbulent boundary layer concepts of constant flux layers and surface roughness lengths are extended to include aerosols and the effects of gravitational settling. Interactions between aerosols and the Earth's surface are represented via a roughness length for aerosol which will generally be different from the roughness lengths for momentum, heat or water vapour. Gravitational settling will impact vertical profiles and the surface deposition of aerosols, including fog droplets. Simple profile solutions are possible in neutral and stably stratified atmospheric surface boundary layers. These profiles can be used to predict deposition velocities and to illustrate the dependence of deposition velocity on reference height, friction velocity and gravitational settling velocity.

scholarly journals Constant Flux Layers with Gravitational Settling: with links to aerosols, fog and deposition velocities over water

2021 ◽  
Author(s):  
Peter Allan Taylor
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Surface Boundary ◽  
Vertical Profiles ◽  
Surface Deposition ◽  
Gravitational Settling ◽  
Constant Flux ◽  
Deposition Velocities ◽  
Roughness Lengths

Abstract. Turbulent boundary layer concepts of constant flux layers and surface roughness lengths are extended to include the effects of gravitational settling. These impact vertical profiles and surface deposition of aerosols, including fog droplets, especially over water. Simple profile solutions are possible in neutral and stably stratified atmospheric surface boundary layers.

Measured Effects of Reduced Flow Velocity in a Laminar Flow Cleanroom

1994 ◽  
Vol 37 (3) ◽  
pp. 41-46
Author(s):  
Peter Carr ◽  
Angelo Rapa ◽  
William Fosnight ◽  
Robert Baseman ◽  
Douglas Cooper
Keyword(s):  
Laminar Flow ◽  
Particle Deposition ◽  
Deposition Velocity ◽  
Equivalent Diameter ◽  
Surface Deposition ◽  
Deposition Velocities ◽  
Airborne Concentration ◽  
Class 1 ◽  
Surface Particle ◽  
Contamination Levels

Vertical laminar flow (VLF) cleanrooms generally operate with airflows between 60 feet per minute (fpm) and 110 fpm. Tests were performed to evaluate the effects on particle contamination levels when the airflow velocity in a FED-STD-209 Class 1 VLF cleanroom was reduced. The cleanroom normally operates at 90 fpm. Measurements of surface particle concentrations were made on settling monitor wafers and on wafers carried in an open cassette. Optical particle counters measured the airborne particle concentrations at several locations. Results at 100 fpm and 50 fpm, respectively, for particles larger than or equal to 0.3 μm in optical equivalent diameter were −0.0002 and +0.0029 particles/sq cm/hr for the settling wafers and 0.009 and 0.024/sq cm total for the wafers carried for 5 hr in the open cassettes. Summary statistics are provided for the airborne and surface particle counts. Deposition velocity is the ratio of surface deposition rate to airborne concentration. For the monitor wafers at the lower flow, the calculated particle deposition velocities were near 0.003 cm/sec, which is within the range expected from theory and experiments in the literature.

scholarly journals A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest

2009 ◽  
Vol 9 (6) ◽  
pp. 26881-26924
Author(s):  
L. Ahlm ◽  
E. D. Nilsson ◽  
R. Krejci ◽  
E. M. Mårtensson ◽  
M. Vogt ◽  
...  
Keyword(s):  
Dry Deposition ◽  
Rain Forest ◽  
Particle Deposition ◽  
Particle Flux ◽  
Friction Velocity ◽  
Deposition Velocity ◽  
Dry Season ◽  
Wet Season ◽  
Deposition Velocities ◽  
Amazon Rain Forest

Abstract. Vertical number fluxes of aerosol particles and vertical fluxes of CO2 were measured with the eddy covariance method at the top of a 53 m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. The primary goal is to quantify the dry deposition sink and to investigate whether particle deposition velocities change when going from the clean wet season into the more polluted dry season. Furthermore, it is tested whether the rain forest is always a net sink of particles in terms of number concentrations, or if particle emission from the surface under certain circumstances may dominate over the dry deposition sink. The particle deposition velocity vd increased linearly with increasing friction velocity in both seasons and the relations are described by vdd=(2.7 u* −0.2)×10−3 (dry season) and vdw=2.5 u*×10−3 (wet season), where u* is the friction velocity. The fact that the two relations are very similar to each other indicates that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle deposition velocities in this study are low compared to studies over boreal forests. The reason is probably domination of accumulation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. Net particle deposition fluxes prevailed in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO2 fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of natural biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO2, and being released from the canopy when conditions become more turbulent in the morning.

scholarly journals Surface deposition of marine fog and its treatment in the WRF model

2021 ◽  
Author(s):  
Peter Allan Taylor ◽  
Zheqi Chen ◽  
Li Cheng ◽  
Soudeh Afsharian ◽  
Wensong Weng ◽  
...  
Keyword(s):  
Water Surface ◽  
Roughness Length ◽  
Deposition Velocity ◽  
Wrf Model ◽  
Liquid Water Content ◽  
Water Droplets ◽  
Surface Deposition ◽  
Near Surface ◽  
Fog Water ◽  
Model Studies

Abstract. There have been many studies of marine fog, some using WRF and other models. Several model studies report over-predictions of near surface liquid water content (Qc) leading to visibility estimates that are too low. This study has found the same. One possible cause of this overestimation could be the treatment of a surface deposition rate of fog droplets at the underlying water surface. Most models, including the Advanced Research Weather Research and Forecasting (WRF-ARW) Model, available from the National Center for Atmospheric Research (NCAR), take account of gravitational settling of cloud droplets throughout the domain and at the surface. However, there should be an additional deposition as turbulence causes fog droplets to collide and coalesce with the water surface. A water surface, or any wet surface, can then be an effective sink for fog water droplets. This process can be parameterized as an additional deposition velocity with a model that could be based on a roughness length for water droplets, z0c, that may be significantly larger than the roughness length for water vapour, z0q. This can be implemented in WRF either as a variant of the Katata scheme for deposition to vegetation, or via direct modifications in boundary-layer modules.

scholarly journals Surface deposition of marine fog and its treatment in the Weather Research and Forecasting (WRF) model

2021 ◽  
Vol 21 (19) ◽  
pp. 14687-14702
Author(s):  
Peter A. Taylor ◽  
Zheqi Chen ◽  
Li Cheng ◽  
Soudeh Afsharian ◽  
Wensong Weng ◽  
...  
Keyword(s):  
Water Surface ◽  
Roughness Length ◽  
Deposition Velocity ◽  
Wrf Model ◽  
Weather Research And Forecasting ◽  
Water Droplets ◽  
Surface Deposition ◽  
Near Surface ◽  
Fog Water ◽  
Weather Research

Abstract. There have been many studies of marine fog, some using Weather Research and Forecasting (WRF) and other models. Several model studies report overpredictions of near-surface liquid water content (Qc), leading to visibility estimates that are too low. This study has found the same. One possible cause of this overestimation could be the treatment of a surface deposition rate of fog droplets at the underlying water surface. Most models, including the Advanced Research Weather Research and Forecasting (WRF-ARW) Model, available from the National Center for Atmospheric Research (NCAR), take account of gravitational settling of cloud droplets throughout the domain and at the surface. However, there should be an additional deposition as turbulence causes fog droplets to collide and coalesce with the water surface. A water surface, or any wet surface, can then be an effective sink for fog water droplets. This process can be parameterized as an additional deposition velocity with a model that could be based on a roughness length for water droplets, z0c, that may be significantly larger than the roughness length for water vapour, z0q. This can be implemented in WRF either as a variant of the Katata scheme for deposition to vegetation or via direct modifications in boundary-layer modules.

scholarly journals Correlation of Dry Deposition Velocity and Friction Velocity over Different Surfaces for PM2.5 and Particle Number Concentrations

2014 ◽  
Vol 2014 ◽  
pp. 1-12 ◽  
Author(s):  
Antonio Donateo ◽  
Daniele Contini
Keyword(s):  
Air Quality ◽  
Dry Deposition ◽  
Friction Velocity ◽  
Particle Number ◽  
Deposition Velocity ◽  
Atmospheric Stability ◽  
Linear Increase ◽  
Climate Modelling ◽  
Average Value ◽  
Deposition Velocities

Dry deposition of particles is an important way of aerosol removal from the atmosphere and a key process in surface-atmosphere exchanges. The deposition velocities, Vd, are often parameterised in air quality and climate modelling as function of the friction velocity,u*, atmospheric stability, and particle size (if size-segregated information is available). In this work, a study of the correlation between Vd andu*over different surfaces is presented for both PM2.5 and particle number fluxes. Results indicate an almost linear increase of Vd withu*with slopes similar for PM2.5 fluxes and particle number fluxes over the different surfaces analysed. This means that the ratios Vd/u*tend to collapse over similar values even if Vd andu*are significantly different becauseu*take into account most of the surface effects. There is a limited difference between stable cases and unstable/neutral cases with slightly lower deposition velocities in stable cases for fixed values ofu*. The average value of Vd/u*is 0.010 ± 0.0017 (median 0.0062 ± 0.0015) (considering all stabilities) and 0.0097 ± 0.002 (median 0.005 ± 0.001) for stable cases. This could be the base for an empirical parameterisation of deposition velocities in air quality models.

scholarly journals Roughness Lengths for Momentum and Heat Derived from Outdoor Urban Scale Models

2007 ◽  
Vol 46 (7) ◽  
pp. 1067-1079 ◽  
Author(s):  
M. Kanda ◽  
M. Kanega ◽  
T. Kawai ◽  
R. Moriwaki ◽  
H. Sugawara
Keyword(s):  
Wind Direction ◽  
Roughness Length ◽  
Scale Dependence ◽  
Scale Model ◽  
Small Scale ◽  
Physical Scale ◽  
Scale Models ◽  
Roughness Lengths ◽  
Urban Sites ◽  
Obstacle Height

Abstract Urban climate experimental results from the Comprehensive Outdoor Scale Model (COSMO) were used to estimate roughness lengths for momentum and heat. Two different physical scale models were used to investigate the scale dependence of the roughness lengths; the large scale model included an aligned array of 1.5-m concrete cubes, and the small scale model had a geometrically similar array of 0.15-m concrete cubes. Only turbulent data from the unstable boundary layers were considered. The roughness length for momentum relative to the obstacle height was dependent on wind direction, but the scale dependence was not evident. Estimated values agreed well with a conventional morphometric relationship. The logarithm of the roughness length for heat relative to the obstacle height depended on the scale but was insensitive to wind direction. COSMO data were used successfully to regress a theoretical relationship between κB−1, the logarithmic ratio of roughness length for momentum to heat, and Re*, the roughness Reynolds number. Values of κB−1 associated with Re* for three different urban sites from previous field experiments were intercompared. A surprising finding was that, even though surface geometry differed from site to site, the regressed function agreed with data from the three urban sites as well as with the COSMO data. Field data showed that κB−1 values decreased as the areal fraction of vegetation increased. The observed dependency of the bulk transfer coefficient on atmospheric stability in the COSMO data could be reproduced using the regressed function of Re* and κB−1, together with a Monin–Obukhov similarity framework.

scholarly journals Revising global ozone dry deposition estimates based on a new mechanistic parameterisation for air-sea exchange and the multi-year MACC composition reanalysis

2017 ◽  
Author(s):  
Ashok K. Luhar ◽  
Matthew T. Woodhouse ◽  
Ian E. Galbally
Keyword(s):  
Chemical Reaction ◽  
Dry Deposition ◽  
Tropospheric Ozone ◽  
Surface Resistance ◽  
Chemical Reactivity ◽  
Deposition Velocity ◽  
Water Layer ◽  
Turbulent Transfer ◽  
Dominant Term ◽  
Deposition Velocities

Abstract. Dry deposition at the Earth’s surface is an important sink of atmospheric ozone. Currently, dry deposition of ozone to the ocean surface in atmospheric chemistry models has the largest uncertainty compared to deposition to other surface types, with implications for global tropospheric ozone budget and associated radiative forcing. Most models assume that the dominant term of surface resistance in the parameterisation of ozone dry deposition velocity at the oceanic surface is constant. We present a consistent, process-based parameterisation scheme for air-sea exchange in which the surface resistance accounts for the simultaneous waterside processes of ozone solubility, molecular diffusion, turbulent transfer, and a first-order chemical reaction of ozone with dissolved iodide. The new scheme makes the following realistic assumptions: (a) the thickness of the top water layer is of the order of a reaction-diffusion length scale (a few micrometres) within which ozone loss is dominated by chemical reaction and the influence of waterside turbulent transfer is negligible; (b) in the water layer below, both chemical reaction and waterside turbulent transfer act together and are accounted for; and (c) iodide (hence chemical reactivity) is present through the depth of the oceanic mixing layer. The asymptotic behaviour of the new scheme is consistent with the known limits when either chemical reaction or turbulent transfer dominates. It has been incorporated into the ACCESS-UKCA global chemistry-climate model and the results are evaluated against dry deposition velocities from currently best available open-ocean measurements. In order to better quantify the global dry deposition loss and its interannual variability, the modelled 3-h ozone deposition velocities are combined with the 3-h MACC (Monitoring Atmospheric Composition and Climate) reanalysis ozone for the years 2003–2012. The resulting ozone dry deposition is found to be 98.4 ± 4.5 Tg O3 yr−1 for the ocean and 722.8 ± 20.9 O3 yr−1 globally. The new estimate of the ocean component is approximately a third of the current model estimates. This reduction corresponds to an approximately 20 % decrease in the total global ozone dry deposition, which is equivalent to an increase of approximately 5 % in the modelled tropospheric ozone burden and a similar increase in tropospheric ozone lifetime.

scholarly journals Snow saltation threshold measurements in a drifting-snow wind tunnel

2006 ◽  
Vol 52 (179) ◽  
pp. 585-596 ◽  
Author(s):  
Andrew Clifton ◽  
Jean-Daniel Rüedi ◽  
Michael Lehning
Keyword(s):  
Particle Size ◽  
Wind Tunnel ◽  
Friction Velocity ◽  
Field Measurements ◽  
Drifting Snow ◽  
Velocity Relationship ◽  
Roughness Lengths ◽  
Surface Particle ◽  
Macroscopic Objects ◽  
Natural Snow

AbstractWind tunnel measurements of snowdrift in a turbulent, logarithmic velocity boundary layer have been made in Davos, Switzerland, using natural snow. Regression analysis gives the drift threshold friction velocity (u*t), assuming an exponential drift profile and a simple drift to friction velocity relationship. Measurements over 15 snow covers show that u*t is influenced more by snow density and particle size than by ambient temperature and humidity, and varies from 0.27 to 0.69 ms–1. Schmidt’s threshold algorithm and a modified version used in SNOWPACK (a snow-cover model) agree well with observations if small bond sizes are assumed. Using particle hydraulic diameters, obtained from image processing, Bagnold’s threshold parameter is 0.18. Roughness lengths (z0) vary between snow covers but are constant until the start of drift. Threshold roughness lengths are proportional to . The influence of macroscopic objects on the roughness length is shown by the lower values measured over the smooth and flat snow surface of the wind tunnel (0.04 ≤ z0 ≤ 0.13 mm), compared to field measurements. Mean drifting-snow grain sizes for mainly new and partly decomposed snow are 100–175 μm, and independent of surface particle size.

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