Particle Deposition Velocity Research Articles

Study on particle deposition in vertical square ventilation duct flows by different models

A proper representation of the air flow in a ventilation duct is crucial for adequate prediction of the deposition velocity of particles. In this paper, the mean turbulent air flow fields are predicted by two different numerical models (the Reynolds stress transport model (RSM) and the realizable k– ε model). Contours of mean streamwise velocity deduced from the k– ε model are compared with those obtained from the Reynolds stress transport model. Dimensionless deposition velocities of particles in downward and upward ventilation duct flows are also compared based on the flow fields presented by the two different numerical models. Trajectories of the particles are tracked using a one way coupling Lagrangian eddy-particle interaction model. Thousands of individual particles are released in the represented flow, and dimensionless deposition velocities are evaluated for the vertical walls in fully developed smooth vertical downward and upward square duct flows generated by the RSM and realizable k– ε model. The effects of particle diameter, dimensionless relaxation time, flow direction and air speed in vertical upward and downward square duct flows on the particle deposition velocities are discussed. The effects of lift and gravity on the particle deposition velocities are evaluated in vertical flows presented by the RSM. It is shown that the particle deposition velocities based on the RSM and realizable k– ε model have subtle differences. The flow direction and the lift force significantly affect the particle deposition velocities in vertical duct flows. The simulation results are compared with earlier experimental data and the numerical results for fully developed duct flows. It is shown that the deposition velocities predicted are in agreement with the experimental data and the numerical results.

Particle fluxes over forests: Analyses of flux methods and functional dependencies

Particle number fluxes at two forests are computed using data from condensation particle counters and both the eddy covariance (EC) and relaxed eddy accumulation (REA) methods applied offline, post data collection. The results indicate that a 30‐min integration period for flux calculation frequently applied for other scalars is also appropriate for particle number fluxes. Half‐hour average particle number fluxes computed using REA and EC are statistically indistinguishable. Less than 1% of all fluxes derived using EC and REA differ by more than the mutual uncertainty bounds, and the average ratio of fluxes computed using the two approaches is 1.00 ± 0.01. This finding tends to support assertions that, for the size of particles under study (with diameters <100 nm), the assumption of similarity with heat transfer is likely robust. Data from both sites indicate higher particle deposition velocities (vd) for particle ensembles with lower number geometric mean diameter (GMD), for GMD of 10 to 100 nm, although the decrease with increasing GMD is less marked than is predicted by the deposition models such as that of Slinn. The relationships between median and mean vd conditionally sampled by GMD are well described by power law fits. Particle vd exhibits a linear dependence on friction velocity for the range of conditions studied and also enhancement under unstable conditions. Similarities between average particle vd conditionally sampled by number GMD, friction velocity, and stability from the two forest sites point to the generalizability of the empirical fits derived herein.

Atmospheric deposition of nitrogen, sulfur and chloride in Thessaloniki, Greece

In the present study, the wet and dry depositions of particulate NO 3 −, SO 4 2−, Cl − and NH 4 + were measured using a wet/dry sampler as a surrogate surface. Gas phase compounds of nitrogen, sulfur and chloride (HNO 3, NH 3, SO 2 and HCl) were measured by an annular denuder system (ADS) equipped with a back up filter for the collection of particles with diameter ≤ 5 μm. Ambient concentrations of NO, NO 2 and SO 2 were also taken into consideration. Sampling was conducted at an urban site in the center of the city of Thessaloniki, northern Greece. The presence of the aerosol species was examined by cold/warm period and the possible compounds in dry deposits were also considered. Dry deposition fluxes were found to be well correlated with ambient particle concentrations in order to be used for the calculation of particle deposition velocity. Average particulate deposition velocities calculated were 0.36, 0.20, 0.20 and 0.10 cm s − 1 for Cl −, NO 3 −, SO 4 2− and NH 4 +, respectively. Total dry deposition fluxes (gas and particles) were estimated at 3.24 kg ha − 1 year − 1 for chloride (HCl + p-Cl), 9.97 kg ha − 1 year − 1 for nitrogen oxidized (NO + NO 2 + HNO 3 + p-NO 3), 5.32 kg ha − 1 year − 1 for nitrogen reduced (NH 3 + p-NH 4) and 15.77 kg ha − 1 year − 1 for sulfur (SO 2 + p-SO 4). 70–90% total dry deposition was due to gaseous species deposition. The contribution of dry deposition to the total (wet + dry) was at the level of 60–70% for sulfur and nitrogen (oxidized and reduced), whereas dry chloride deposition contributed 35% to the total. The dry-to-wet deposition ratio of all the studied species was found to be significantly associated with the precipitation amount, with nitrogen species being better and higher correlated. Wet, dry and total depositions measured in Thessaloniki, were compared with other countries of Europe, US and Asia.

Watershed Processing of Atmospheric Polychlorinated Biphenyl Inputs

Indirect atmospheric deposition of PCBs was examined in subwatersheds of the Delaware River Estuary. Tributary PCB loads and atmospheric PCB concentrations were used to understand the pass-through efficiencies for nine rivers/ creeks for which PCB inputs appeared to be dominated by atmospheric deposition. The pass-through efficiency, E, was calculated from tributary loads and atmospheric deposition fluxes. Unfortunately, uncertainties in the gaseous and dry particle deposition velocities, vg and vd, respectively, render the calculated atmospheric deposition fluxes highly uncertain. In order to circumvent this problem, export of PCBs from the watershed was related directly to atmospheric PCB concentrations via a new mass transfer coefficient, the watershed delivery rate or vws, which describes the process by which the watershed transfers PCBs from the airto the River's main stem. vws increases with increasing chlorination and is significantly correlated with vapor pressure. This trend suggests that the transfer of PCBs from the atmosphere to the River via the watershed is more efficient for high molecular weight PCBs than for low molecular weight PCBs. This may indicate that the selected watersheds are at or close to equilibrium with respect to gaseous exchange of PCBs, such that lower molecular weight congeners undergo substantial revolatilization after deposition. The magnitude of the pass-through efficiency, E, depends on the deposition velocities used to calculate the atmospheric deposition flux, but when congener-specific deposition velocities are used, E is independent of vapor pressure and is relatively constant at about 3%.

Atmospheric Deposition of Reactive Nitrogen on Turf Grassland in Central Japan: Comparison of the Contribution of Wet and Dry Deposition

The atmospheric deposition of reactive nitrogen on turf grassland in Tsukuba, central Japan, was investigated from July 2003 to December 2004. The target components were ammonium, nitrate, and nitrite ions for wet deposition and gaseous ammonia, nitric and nitrous acids, and particulate ammonium, nitrate, and nitrite for dry deposition. Organic nitrogen was also evaluated by subtracting the amount of inorganic nitrogen from total nitrogen. A wet-only sampler and filter holders were used to collect precipitation and the atmospheric components, respectively. An inferential method was applied to calculate the dry deposition velocity of gases and particles, which involved the effects of surface wetness and ammonia volatilization through stomata on the dry deposition velocity. The mean fraction of the monthly wet to total deposition was different among chemical species; 37, 77, and 1% for ammoniacal, nitrate-, and nitrite-nitrogen, respectively. The annual deposition of inorganic nitrogen in 2004 was 47 and 48 mmol m−2 yr−1 for wet and dry deposition, respectively; 51% of atmospheric deposition was contributed by dry deposition. The annual wet deposition in 2004 was 20, 27, and 0.07 mmol m−2 yr−1, and the annual dry deposition in 2004 was 35, 7.4, and 5.4 mmol m−2 yr−1 for ammoniacal, nitrate-, and nitrite-nitrogen, respectively. Ammoniacal nitrogen was the most important reactive nitrogen because of its remarkable contribution to both wet and dry deposition. The median ratio of the organic nitrogen concentration to total nitrogen was 9.8, 17, and 15% for precipitation, gases, and particles, respectively.

Particle deposition in indoor environments: Analysis of influencing factors

In this paper, several factors influencing particle deposition in indoor environments are analyzed with an analytical model and a three-dimensional drift flux model combined with the particle deposition boundary conditions for wall surfaces. The influences of flow conditions near the wall surfaces, surface roughness and particle concentration distribution on particle deposition indoors are studied. By modeling particle deposition onto surfaces with the analytical model, it is found that larger friction velocity near the wall surfaces and rougher surface may lead to larger particle deposition velocity when the particle size is small, but when particle size is large enough (the range is up to the actual friction velocity and in this study it is about 1–5 μm), the influence of the friction velocity and roughness could be neglected. Furthermore, the three-dimensional numerical simulations indicate that particle concentration distribution may be very different even for the same particle source and air change rate, which cause a different deposited particle flux. As the particle concentration distribution may not be uniform in most cases, especially for the ventilated rooms, it is important to incorporate particle concentration distribution when analyzing particle deposition in indoor environments. Some suggestions or rules for particle deposition controlling are also presented based on the analysis.

Measurements of aerosol particle dry deposition velocity using the relaxed eddy accumulation technique

The continuous measurements of aerosol particle deposition velocity have been performed from January 2004 to January 2005 using a REA technique with dynamic deadband.We measured aerosol particle deposition velocity in the size range of 10–150 nanometer with 5–10 nanometer steps using differential mobility analyser for sizing.We were able to measure two size classes simultaneously. One size class was changed at one month intervals, another we kept constant at 30 nm to investigate the effect of seasonal and meteorological variation on deposition velocity. We found that the 80–100 nanometer size particles had the lowest deposition velocity, about 0.4 cm s−-1. Deposition velocity increased with decreasing or increasing particle diameter from 80–100 nanometer size. We also found that deposition velocity increases as a function of friction velocity.

Air Pollution Processing by Radiation Fogs

Several fog episodes occurred in California’s San Joaquin Valley during winter 2000/2001. Measurements revealed the fogs to generally be less than 50 m deep, but to contain high liquid water contents (frequently exceeding 200 mg/m3) and large droplets. The composition of the fog water was dominated by ammonium (median concentration = 608 μN), nitrate (304 μN), and organic carbon (6.9 ppmC), with significant contributions also from nitrite (18 μN) and sulfate (56 μN). Principal organic species included formate (median concentration = 32 μN), acetate (31 μN), and formaldehyde (21 μM). High concentrations of ammonia resulted in high fog pH values, ranging between 5.8 and 8.0 at the core measurement site. At this high pH aqueous phase oxidation of dissolved sulfur dioxide and reaction of S(IV) with formaldehyde to form hydroxymethanesulfonate are both important processes. The fogs are also effective at scavenging and removal of airborne particulate matter. Deposition velocities for key solutes in the fog are typically of the order of 1–2 cm/s, much higher than deposition velocities of precursor accumulation mode aerosol particles. Variations were observed in deposition velocities for individual constituents in the order NO2− > fogwater > NH4+ > TOC ∼ SO42− > NO3−. Nitrite, observed to be enriched in large fog drops, had a deposition velocity higher than the average fogwater deposition velocity, due to the increase in drop settling velocity with size. Species enriched in small fog drops (NH4+, TOC, SO42−, and NO3−) all had deposition velocities smaller than observed for fogwater. Typical boundary layer removal rates for major fog solute species were estimated to be approximately 0.5–1 μg m−3 h−1, indicating the important role regional fogs can play in reducing airborne pollutant concentrations.

Effect of ventilation duct as a particle filter

This paper discusses the effect of ventilation duct as a particle filter by modeling particle deposition in ventilation ducts, which is the reason that ventilation ducts could “filter” particles. An Eulerian model is employed to predict the particle deposition velocity onto the wall and floor from fully developed turbulent flow in ventilation ducts [Zhao B, Wu J. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmospheric Environment 2006;40:457–66], while an empirical equation is proposed to predict the particle deposition velocity onto the ceiling combined with experimental data and, another empirical equation by McFarland et al. [Aerosol deposition in bends with turbulent flow. Environmental Science and Technology 1997;31:3371–7] is used for predicting the particle penetration through the bends, which are hard to analyze by theoretical method.Losses through ventilation duct are thus evaluated for the particle size range 0.01–100μm. Both straight duct (implies fully developed turbulent flow) and duct bend (implies developing turbulent flow) cases are analyzed. Then particle penetration through the supply duct of a swimming pool, including some straight ducts and bends, is analyzed as an application case. The results show that when particle diameter is larger than 5μm, the filtration of the duct might not be ignored as the obvious loss by deposition.

Modeling particle deposition onto rough walls in ventilation duct

An improved Eulerian model is proposed to predict particle deposition velocity onto rough walls in fully developed turbulent duct flow. The model treats the turbulent flow over rough walls as three different regimes of turbulent boundary layer (hydraulically smooth, transition and completely rough) according to the value of roughness Reynolds number (or called dimensionless roughness), k +. For each regime, the velocity and concentration boundary layer has a different thickness compared to that of the separated free shear layer, as flow separation behind the roughness is different, and this results in different shifted distance of virtual origin of velocity boundary layer. Thus a fitted equation is proposed based on measured data to estimate the shifted distance of velocity boundary layer for different values of k +, when calculating particle deposition velocity onto rough walls. Besides, as a successive study of previous one by the authors, the model also accounts for turbophoresis as well as Brownian diffusion, turbulent diffusion and gravitational settling. Turbophoretic velocity is calculated by employing a function of dimensionless normal distances in boundary layer to the duct walls. It only needs the friction velocity as the input. The predicted results agree well with published measured data for rough walls in most cases, and also agree better with measurement in ventilation ducts than those using the existing model.

Size-resolved particle deposition velocities of sub-100 nm diameter particles over a forest

Despite their importance to accurate prediction of the particle ensemble and associated impacts, few empirical studies have attempted to quantify the dependence of deposition velocities ( v d) on friction velocity ( u *) and particle diameter for sub-100 nm particles. Data from a pilot study conducted over a Beech forest in Denmark are used here in to test an indirect method of assessing these dependencies where particle fluxes derived using eddy covariance are conditionally sampled based on parameters of the particle size distribution. The data indicate strongly bi-directional fluxes, with almost 1 3 of all 1 2 -h periods exhibiting upwards fluxes particularly during particle formation events. Vertical velocities for particle ensembles with number geometric mean diameter (GMD) of 20–30, 30–40, 40–50, 50–60 and 60–70 nm have an approximately equal magnitude during deposition and emission periods which may imply that surface uptake does not strongly limit deposition rates of these particles. The results also indicate a marked size dependence of v d. During deposition periods the median v d for particle ensembles with a number GMD of 20–30 nm is 4.5 mm s −1 and decreases to 1.5 mm s −1 for particle ensembles with a GMD of 60–70 nm. The median v d for periods when the GMD was approximately 55 nm is 2.7 mm s −1 and the mean is 4.2 mm s −1, which agrees well with the mean v d of 4.3 mm s −1 for 50 nm particles over a Scots Pine forest at Hyytiälä in central Finland. Also, as in the data from Hyytiälä, the v d for GMD of 50–60 nm increases markedly with u * from 1 to 1.8 mm s −1 at u *<0.4 m s −1, to 7.5 mm s −1 at u *>1 m s −1.

A passive sampler for airborne coarse particles

The personal aeroallergen sampler (PAAS) with a structure resembling a gimbal to enable a particle collection surface continuously directed upward regardless of inclination of the sampler was developed. To evaluate the particle size-dependent collection characteristics by the PAAS, the particle size distributions obtained by the PAAS were compared with those by an existing reference active sampler for sampling durations of 5 h to 6 days. The results showed good correlations between the two methods, suggesting usability of the PAAS for long-term personal monitoring of airborne coarse particles such as aeroallergens. The effective particle deposition velocities experimentally obtained for the personal samplings by the PAAS were 0.17 , 0.35 , 0.66 and 1.6 cm s - 1 for particles with projected area diameters of 10–20, 20–30, 30–50 and 50– 100 μ m , respectively. Unlike existing active samplers, the PAAS is convenient for personal air sampling because of simplicity of handling. It can be easily used for epidemiological studies to investigate relationships between aeroallergen exposures and allergic airway diseases.

Critical Velocities for High Speed Particle Deposition in Kinetic Spraying

During kinetic spraying (or cold gas dynamic spraying), small solid particle can be deposited onto the substrate by a supersonic velocity impaction. A critical velocity of flying particle was found for the minimum deposition speed. Generally, it was thought that higher impact velocity caused higher deposition efficiency. However, some different phenomena were observed during spraying processing in laboratory.In this study, individual particle impact tests have been carried out to probe the effect of impact velocity. Instead of deposition efficiency (DE) of coatings, ratio of bonds, which was defined as the number fraction of attached particles to total impact particles (Craters + Bonds) in unit area of impact surface, was measured in our test. A maximum velocity for particle deposition was observed, because of which a high fraction of rebounded particles occurred at a high impact velocity. A model considering the adhesion and rebound energy was built up to estimate the particle/substrate interaction. The particle deposition behavior was a result of the competition between adhesion and rebound energies during the impact process. Only when the adhesion energy was higher than the rebound energy, the impacting particles could be attached onto the substrate.

Aerosol Number Concentrations during Kosa Events on Suburban Hills in Japan

Measurements of aerosol size-separated number concentrations were performed in March 2001 on the Tama Hills of Japan, located near regions of heavy industry. The concentrations of particles larger than 1.0 μ m in diameter dramatically increased during the Kosa phenomena on 6–8 and 18–24 March. The number concentrations of coarse particles (> 3.0 μm) showed distinct diurnal variations, especially during the Kosa events. Fine particles smaller than 0.5 μm increased when the relative humidity was high and the wind speed was low. On the other hand, the coarse particles concentrations were well correlated with wind speed and temperature. The dry deposition velocity of the coarse particles at the forest canopy was estimated from the nighttime decrease in the number concentrations.

Modeling particle deposition from fully developed turbulent flow in ventilation duct

This paper proposes an improved Eulerian model to predict particle deposition velocity in fully developed turbulent duct flow. The model is modified based on the three-layer model by Lai and Nazaroff (Journal of Aerosol Science, 31, 463–476, 2000), accounting for turbophoresis as well as Brownian diffusion, turbulent diffusion and gravitational settling. An expression relating the turbophoretic velocity to particle relaxation time, friction velocity and the normal distance to the wall surface is presented to model the turbophoresis. Similar with previous one by Lai and Nazaroff, the model only needs to input friction velocity, which makes it easy to apply. The predicted results agree well with measurement data for floor and vertical walls. And then deposition velocity of airborne particles to smooth walls in straight steel ducts is predicted by the modified model. The results agree with the published measured data, especially for floor and vertical walls of ventilation duct. Thus it is expected to be applied for predicting particle deposition in ventilation duct for indoor air quality control or evaluation. Furthermore, the condition to ignore turbophoresis for particle deposition is discussed by comparing the results of the improved model and that of Lai and Nazaroff.

Indoor Deposition and Forced Re-suspension of Respirable Particles

The rate at which particles deposit and re-suspend from indoor surfaces is an essential parameter in determining human exposure to aerosol particles. In this study, we investigate experimentally particle deposition and forced re-suspension in a cubical enclosure for three particle sizes (0.7, 1.5 and 4 m). For particle deposition, the results are presented in terms of particle deposition velocity. It was observed that the particle deposition velocity is particularly significant for the floor surface. Deposition on vertical and ceiling surfaces becomes important only when these surfaces are electrically charged. Air humidity did not influence the particle deposition apart from when it was close to saturation. Re-suspension of particles from the floor surface due to household activities was also studied. It was observed that small particles are more difficult to detach from the floor than bigger ones. Besides, there is a pressure above which re-suspension is very likely to occur (minimum re-suspension pressure). This minimum pressure is dependent on the particle size.

Numerical analysis of particle deposition in ventilation duct

This paper adopts computational fluid dynamics (CFD) to numerically analyze particle deposition in the ventilation duct. A three-dimensional drift-flux model combined with particle deposition boundary conditions for wall surfaces is presented. The numerical method is used to analyze the particle deposition velocity and deposited particle mass flux in the ventilation duct after validation. Twelve groups of particle size, two average air speeds in ducts are investigated to understand the particle deposition in the straight ventilation duct, which ensures a fully developed turbulent duct flow. And then, the particle accumulation by deposition in the ventilation duct is analyzed according to the cleaning code for air duct system in heating, ventilation and air conditioning (HVAC) systems of China. The cases with or without air filter installed are studied by assuming that the duct inlet particle concentration is that of outdoor air in Beijing city, China. The simulated results of dimensionless deposition velocity onto floor agree well with the measured data from others, while the discrepancies of vertical wall and ceiling are obvious. Both the simulated results in this paper and measured data from literature show that particle deposition onto the floor (upward wall) in the ventilation ducts is the most significant. The deposition velocity onto the floor is about 2 orders of magnitude larger than that onto the other walls of the duct. The filter is effective to defend the ventilation duct against particle pollution by deposition. A higher efficiency filter is helpful to postpone the cleaning time for ventilation duct, so the filter should be replaced often to maintain the filter efficiency.

Airborne particle deposition onto the ocular surface

Eye irritation is one of the most frequently reported symptoms in relation to working in office-like environments. So far, no consistent association has been found between concentration of airborne particles and prevalence of eye irritation. External physical factors not hitherto considered could have a large influence on particle deposition velocity onto the ocular surface. This may have obscured the role airborne particles play. Based on previously published semi-empirical models, the paper describes the influence of turbulence, gravitational settling, electrical fields, and thermophoresis on deposition velocity. A probabilistic approach was used to determine percentile ranges in deposition velocity when the magnitude of these parameters varied within typical ranges. The calculations suggest that differences in external factors other than particle size may cause differences in the deposition velocity of one order of magnitude or more. Studies trying to find associations between airborne particle concentration levels and eye irritation should take into consideration the influence of external physical factors on deposition velocity. External physical factors other than concentration are likely to have a large influence on the deposition rate of airborne particles onto the ocular surface. Thus, future studies on associations between airborne particle levels and eye symptoms should only be conducted if particle concentration measurements are suitably size resolved. Other relevant external physical factors should also be assessed and included in a multivariate analysis of exposure-response associations.

Effect of thinning on surface fluxes in a boreal forest

Thinning is a routine forest management operation that changes tree spacing, number, and size distribution and affects the material flows between vegetation and the atmosphere. Here, using direct micrometeorological ecosystem‐scale measurements, we show that in a boreal pine forest, thinning decreases the deposition velocities of fine particles as expected but does not reduce the carbon sink, water vapor flux, or ozone deposition. The thinning decreased the all‐sided leaf area index from 8 to 6, and we suggest that the redistribution of sources and sinks within the ecosystem compensated for this reduction in foliage area. In the case of water vapor and O3, changes in light penetration and among‐tree competition seem to increase individual transpiration rates and lead to larger stomatal apertures, thus enhancing also O3 deposition. In the case of CO2, increased ground vegetation assimilation and decreased autotrophic respiration seem to cancel out opposite changes in canopy assimilation and heterotrophic respiration. Current soil‐vegetation‐atmosphere transfer models should be able to reproduce these observations.

Atmospheric Dry Deposition to Marble and Red Stone

This paper presents dry deposition flux and deposition velocity of atmospheric particles on white marble and red stone at Dayalbagh, a suburban site of semi arid region, which is 10 km away from the industrial sector of the Agra city where due to agricultural practices vegetation predominates. The wind speed at Agra is mostly in the range of 1–2 m s−1. The atmospheric calm conditions at Agra in summer, monsoon, and winter seasons are 47%, 35%, and 76%, respectively. Industrial areas of the city are away from Dayalbagh and are located in the NE, E, SE, and SW sectors. The main industrial activities, which are in operation in Agra city and its outskirts, are foundry and forging industry. The other industrial activities in Agra are rubber processing, lime oxidation and pulverization, chemicals, engineering and brick refractory kilns. Dry deposition samples were collected on dry days on white marble and red stone (0.224 m × 0.224 m × 0.02 m) using surface washing method. Both slabs were fixed to an iron stand (1.5 m height) at an angle of about 80∘ from the horizontal and exposed for 24 h on the roof of the faculty building. The order of deposition flux on white marble is NH4+ > Mg2+ > Ca2+ > Na+ > Cl− > K+ > NO3− > SO42− > F− and that on red stone is NH4+ > Mg2+ > Ca2+ > SO42− > Na+ > NO3− > K+ > F− > Cl−. Average dry deposition flux of major ions varies from 3.4 to 128.5 μM m−2 d−1. The sum of major cations on white marble and red stone are 516.4 and 450.4 μ eq m−2 d−1, respectively while sum of major anions are 425.3 and 400.4 μ eq m−2 d−1 on white marble and red stone, respectively. Higher deposition of all ions was observed when wind blows from NE as most of the Agra Iron foundries and Ferozabad glass industries lie in this direction. The mean values of dry deposition velocity of ions vary between 0.22 cm s−1 to 1.49 cm s−1. Deposition velocity for all ions is higher on white marble than red stone inspite of rougher surface of red stone as compared to white marble. This could be due to the chemical nature of white marble, which is made of dolomite and hence adds significant amount of ions by dissolution during washing. Seasonally the deposition velocity was highest in winter.