Particle Scavenging Research Articles

Aerosol particle and droplet scavenging by snow crystals

Aerosol particle and droplet scavenging by snow crystals

Trace Metals and Dissolved Organic Carbon in an Estuary with Restricted River Flow and a Brown Tide Bloom

This study was designed to establish the distributions of trace metals (Cd, Co, Cu, Ni, Pb, and Zn), dissolved organic carbon (DOC), and inorganic nutrients (PO4 and H4SiO4) in the water column of the small, relatively pristine Peconic River estuary. We were also able to examine the effects of a harmful microalgal bloom, known as the brown tide, which occurred in the area during our study. Because river inflow to the Peconic estuary is restricted by a small dam at the head of the estuary, direct evaluation of the relative importance of riverine inputs on estuarine metal distributions was possible. The simultaneous analyses of geochemical carrier metals (Al, Fe, and Mn), an indicator of sewage (Ag), and other ancillary parameters (e.g., suspended particulate matter, dissolved O2, chlorophylla) were used to describe the major processes controlling metal concentrations in the dissolved phase. The trace metal distributions indicated two distinct biogeochemical regimes within the estuary: an anthropogenically perturbed region with high metal levels (e.g., Ag, 165 pM; Cu, 51 nM; Zn, 57 nM) at the head (Flanders Bay), and a larger outer region with relatively low metal concentrations. The very similar distributions of some metals (e.g., Mn, Ni) in the Peconic estuary compared to those in estuaries having much higher river flow demonstrated the dominant role of internal processes (e.g., diagenetic remobilization) in controlling these metal patterns. An inverse relationship between dissolved Fe and DOC with cell counts of the brown tide microalgaeAureococcus anophagefferens in our field study suggested a close association with the bloom, although a similar relationship was observed between dissolved Al and brown tide cell counts, implying that removal of Fe could be due to particle scavenging rather than biological uptake.

The influence of diffusive convection on sedimentation from buoyant plumes

Diffusive convection driven by the differential diffusion of density altering fluid properties may enhance the scavenging of particles from natural buoyant plumes. For single-phase (fluid–solute–heat) systems this phenomenon has been extensively studied because salt fingering generated at the oceanic thermocline is a major mechanism of salt transport in the oceans. However, the influence of this process on particle laden plumes, for example, fluvial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown. In this paper, we present direct experimental measurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes. Particle flux is measured using a light attenuation technique employing a chain of photodiodes which average concentration over a large number of fingers. The results are in good general agreement with earlier studies based on finger velocity. Flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude. For very small particles double diffusive (salt finger) theory is applicable to two component particulate systems as suggested by earlier studies [Green, T., 1987. The importance of double diffusion to the settling of suspended material. Sedimentology 34, 319–331]. Two component diffusive convection theory is extended to three components in order to predict particle scavenging from marine fluvial plumes which involve the diffusion of sediment, salt and heat. For larger particles which settle significantly the flux can be approximated by adding the double-diffusive and settling fluxes. A theory to predict particle transport through the lower layer and sedimentation at the bed is developed based on the observation of strong convection below the plume. Application of our theory and experimental results indicates that while double diffusion may significantly influence the longitudinal distribution and vertical sorting of deposits from lacustrine plumes, the diffusive convection process is generally insignificant in marine plumes. Observational evidence for lacustrine double diffusion based on water column measurements is presented.

Rates of particle scavenging and particulate organic carbon export estimated using 234Th as a tracer in the subtropical and equatorial Atlantic Ocean

The export flux of particulate organic carbon and particle scavenging rates were calculated using a 234Th-based approach along a transect through the mid-Atlantic from 35°S to 10°N during May and June of 1996. The median residence time of dissolved 234Th along the transect was 135 days, with intense scavenging from 5°N to 6° N ( τ ̄ d =86±17 days). Vertical profiles and depth-integrated samples (surface-100 m) of dissolved (<0.7-μm), suspended particulate (0.7–53-μm), and large particulate (>53- μm) 234 Th activities were collected to estimate export fluxes of particulate organic carbon (POC). A steady-state model of the 234Th activity balance in the upper 100 m was combined with measurements of the POC/ 234Th ratio on >53-μm particles. POC export ranged from 1.5 mmol C m −2 d −1 in the central South Atlantic to 31 mmol C m −2 d −1 at 2.5°S. Export production along the Atlantic equator (5°S–5°N, 25°E–5°W) is estimated to be 0.22 Gt C yr −1. The 234Th-derived POC export estimates reported here and from other studies are comparable with model simulations of export production from the Princeton General Circulation Model (GCM) for the Atlantic Ocean.

Formula omitted] and [formula omitted] in the South-equatorial Atlantic: : distribution and disequilibrium in the Upper 500 m

The distribution of 210 Po and 210 Pb (grand daughter–parent) in the dissolved and total (dissolved+particulate) phase of sea water samples has been measured in the upper 500 m at four stations along a western transect in the South-equatorial Atlantic. Such a transect allows one to make inference about the particle-associated scavenging processes in the equatorial versus subtropical regions of the South Atlantic. A common feature of the 210 Po profiles is that the dissolved activities in the surface waters are about one-third of the equilibrium concentrations, suggesting weeks to a month removal times of 210 Po by adsorption on settling particles. The mean dissolved 210 Po / 210 Pb activity ratios in the equatorial and South Atlantic are 0.27 and 0.39, respectively, representing a pronounced disequilibrium in surface waters. The box-model calculation yields a residence time of about 73 and 130 days for dissolved 210 Po in the surface water at the equatorial and southern sites, respectively. The gross deficiency of 210 Po extends throughout the upper 500 m; however, the 210 Po residence time in the intermediate depths is longer than that in surface water. The observed 210 Po – 210 Pb disequilibrium has been examined with respect to POC concentration, which yields a significant positive correlation ( r 2=0.61), suggesting that the 210 Po deficiency is mainly associated with biological removal. The 210 Pb distribution shows distinctively higher concentrations in the equatorial surface waters (19–22 dpm/100 l), suggesting an enhanced atmospheric flux of 210 Pb . The residence time of dissolved 210 Pb (400–780 days), with respect to atmospheric input, is longer than dissolved 210 Po and 234 Th residence times, and also shows a latitudinal decrease toward the equator in the surface ocean. The shorter residence time in the surface equatorial ocean (0–20 m) seems associated with higher productivity and more rapid particulate scavenging.

The distribution of dissolved Cu, Zn, Ni, Co and Cr in English coastal surface waters

The distribution of trace metals was determined in coastal waters around England during cruises in December 1993 (CH108), October 1994 (CH115), January 1995 (CH117) and April 1995 (CH118). Cu and Ni distributions were pronouncedly influenced by mixing of river waters with seawater during CH108 coinciding with seasonally enhanced freshwater inputs, but less so during the subsequent cruises. These seasonal variations indicate the importance of processes other than conservative mixing on the distributions of Cu and Ni: benthic and atmospheric inputs, removal by biological uptake and particle scavenging. This paper is one of the first to report on coastal dissolved Cr distributions. Non-conservative behaviour was observed for Cr (CH108), with generally low concentrations which were attributed to particle scavenging. Dissolved Zn exhibited a non-conservative behaviour during the spring and late summer cruises, suggesting that processes other than mixing between water masses was influencing its behaviour. Co showed non-conservative behaviour during the late summer cruise (CH115), which was attributed to the influence of benthic inputs and particle scavenging on its distribution.

Biogeochemistry of Nutrients and Trace Metals in the Estuarine Region of the Patos Lagoon (Brazil)

The major aim of this research was to assess processes occurring in the low (0–5) salinity region and the role of biological processes (i.e. uptake and remineralization in the transport and fate of trace metals in a coastal lagoonal system. The characteristics of Patos Lagoon which provide sufficient resolution to evaluate these processes are the long residence time/slow mixing rate of the system and the high utilization of the riverine supply of nutrients within the system. Samples were collected from approximately 24 stations over an approximately 80 nautical mile transect through the estuarine region of the Patos Lagoon over the two day period, 14–15 December 1995. Based on the results, Patos Lagoon can be divided into three zones within each of which certain processes dominate the fate and transfer of materials. In the first zone (salinity zero toc.5–7) nutrient and particle removal, as fresh water mixes with seawater, are the dominant features and reflect high primary production, flocculation and particle scavenging. The second zone (up to salinityc.25–27) is characterized by rapidly increasing salinity with distance along the transect. Within this region, nutrient regeneration suggests that organic matter remineralization is a dominant process and that metal release, in association with this process, may account for metal distributions in this region. However, other processes, such as mobilization from bottom sediment, may also explain these observations. The third zone (salinities greater thanc.27) is dominated by conservative mixing.

The Importance of Snow Scavenging of Polychlorinated Biphenyl and Polycyclic Aromatic Hydrocarbon Vapors

Recently, experimental data on the scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from the atmosphere by snow were interpreted assuming that the distribution of chemical between particles and dissolved phase measured in the meltwater reflects the state of the chemical during the scavenging process (Franz, T. P.; Eisenreich, S. J. Environ. Sci. Technol. 1998, 32, 1771−1778). A consequence of this assumption is that vapor scavenging is found to be unimportant relative to particle scavenging. An alternative interpretation is presented that during melting repartitioning occurs from the dissolved phase to the particle-sorbed phase. Further, it is argued that a constant particle scavenging ratio may apply to all chemicals of the same class in the same precipitation event, and its value can be estimated from the scavenging characteristics of predominantly particle-sorbed, high molecular mass chemicals. This analysis suggests that for more volatile PCBs and PAHs vapor scavenging is an important, if not the dominating, snow scavenging process. Gas scavenging ratios obtained with this method are, as expected, negatively correlated with the vapor pressure of a substance, indicating that adsorption to the air-ice interface is the process responsible for vapor scavenging.

The effects of snow and ice on the environmental behaviour of hydrophobic organic chemicals

A review is presented of the roles of snow and ice as they influence the environmental fate of hydrophobic organic chemicals (HOCs). Measurements of HOC concentrations in snow are reviewed and present information on the partitioning and depositional and post-depositional behaviour of HOCs in snow is described and implications for environmental monitoring and assessment of fate are discussed. It is concluded that snow is an efficient scavenger of HOCs from the atmosphere both by adsorption of gaseous HOCs to the ice interface, and by particle scavenging. The post-depositional fate of HOCs in ageing snow packs is poorly understood. Suggested structures of quantitative models describing HOC interactions with ice and snow are presented. Key parameters in these models include the interfacial area of snow and the extent of HOC sorption to the ice surface. Recent laboratory determinations of these parameters are reviewed. Finally, research needs and gaps are identified with a view to compiling more accurate estimates of net atmospheric wet deposition of HOCs, establishing their fate in snow packs, developing reliable sampling protocols and assessing the usefulness of the glacial record as an indicator of past atmospheric compositions.

Snow Scavenging of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons in Minnesota

Snow and rain events were collected with concurrent air samples during the winter of 1991−1992 at a suburban site in Minnesota to investigate atmospheric scavenging of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs). Particle scavenging was the dominant contributor to the total concentrations in snow and was less important in rain. Gas scavenging was only important for low molecular weight PCB congeners and PAHs. Enrichment of dissolved-phase concentrations over levels predicted from temperature-corrected Henry's Law constants was observed for low molecular weight compounds in both rain and snow. The particulate fraction (φ) of atmospheric PCBs and PAHs was the best indicator of total scavenging (WT) by snow according to the equations log WT = 0.71 (±0.08) log φ + 5.34 (±0.08) for PCB congeners (r = 0.62) and log WT = 0.89 (±0.12) log φ + 6.07 (±0.13) for PAHs (r = 0.73).

Particle Scavenging in the Upper Water Column off Mindoro Island, Philippine:234Th/238U Disequilibria

Vertical profiles of dissolved and particulate234Th in the upper water column of four stations in the narrow channel adjacent to Mindoro Island, Philippine were determined during November 1991. Calculated from the degree of disequilibrium between particle-reactive234Th and soluble parent,238U, the residence times of dissolved and particulate234Th range from 20 to 42 days and from 9 to 17 days, respectively. Vertical234Th fluxes of 4300–7000dpm m−2day−1from the upper 200m are required to balance the deficiency of234Th.

Interaction of Aerosol Particles and Clouds

With the spectral scavenging and microphysics model DESCAM coupled to a 2D dynamic framework the transport and scavenging of aerosol particles by a medium-sized convective cloud are investigated. For typical marine conditions the author has found a depletion of the marine boundary layer of about 70%. Also, about 70% of the vented up aerosol particle mass entered cloud drops due to nucleation. For these simulations the author found an increase in relative humidity, as well as number concentration of Aitken and large and giant particles near cloud top. The increase in relative humidity and large and giant particle concentration was coupled to cloud outflow regions, while the increase in Aitken particles can be attributed to lifting of free tropospheric air due to cloud-top rising. Sensitivity tests varying the number of free tropospheric sulfate particles changed the number of cloud drops but had little effect on precipitation formation and evolution.

A field study of particle scavenging by raindrops of different sizes using monodisperse trace aerosol

A field experiment was conducted to measure particle scavenging processes in the atmosphere during real precipitation events. Monodisperse aerosol particles (mean radii varying from 0.19 to 1.8 μm) tagged by a fluorescent dye tracer are spread during several precipitation events in the atmosphere. Raindrops which cross the aerosol volume are collected according to size by freezing them in liquid nitrogen (Guttalgor method). A continuous decrease in the chemical concentration of the dye in the raindrops with increasing drop size is observed for the scavenging of particles with a radius of 1.8 μm. For the scavenging of smaller particles a maximum of the chemical concentration of the dye tracer is found for drop radii between 0.175 and 0.35 mm. The concentration maximum shifts towards small drop radii with increasing aerosol particle radius. For the scavenging of aerosol particles smaller than 0.6 μm radius the concentration maximum is found at a drop radius of 0.35 mm. Results of this study establish a relationship between the mean of the scavenged particle size and the most effective raindrop radius. Scavenging coefficients in the range 10 −6−10 −9s −1 are derived as a function of drop size in field experiments. They increase with raindrop radii between 0.1 and 0.6 mm and decrease with increasing particle size up to about 1.2 μm. The scavenging coefficient then increases to larger particle sizes. Results are compared to theory which predicts this minimum at smaller particle sizes.

A Sensitivity Analysis of a Radiological Assessment Model for Arctic Waters

A model based on compartment analysis has been developed to simulate the dispersion of radionuclides in Arctic waters for an assessment of doses to man. The model predicts concentrations of radionuclides in the marine environment and doses to man from a range of exposure pathways. A parameter sensitivity analysis has identified components of the model that are potentially important contributors to the predictive accuracy of doses to individuals of critical groups as well as to the world population. The components investigated include features associated with water transport and mixing, particle scavenging, water-sediment interaction, biological uptake, ice transport and fish migration. Two independent evaluations of the release of radioactivity from dumped nuclear waste in the Kara Sea have been used as source terms for the dose calculations.

Aerosol Particle Scavenging by Fog and Haze Droplets

Droplet imprints in thin gelatin layers have been used for the investigation of coarse particles embedded in fog (haze) droplets in comparison to interstitial particles. Droplet samples were taken in less polluted regions of Missouri and Illinois and also in more polluted marine-urban environment in Italy. Considerable differences in particle size distribution and particle location inside the droplets have been found in samples collected on four sampling sites. The deduced relationships can be applied for the description of the aerosol scavenging process in fogs. The pollution of fog (haze) droplets may be related to the droplet mean volume radius and may depend on temperature and on atmospheric humidity. An orientative study of the deposited particle composition indicated important relationships between coarse particles of different origin (soil minerals, industrial and urban pollutants, sea salt).

Speciation and cycling of trace metals in Esthwaite Water: A productive English lake with seasonal deep-water anoxia

The chemical speciation of Cu, Ni, and Cr and the dissolved and unfiltered concentrations of Mo, Ti, Co, Fe, Mn, and Zn were determined in the water column of a lake with a seasonally anoxic hypolimnion (Esthwaite Water, Cumbria, UK) during three surveys in the summer of 1991. Cathodic stripping voltammetry (CSV) with ligand competition was used for this speciation study. The metal data, in conjunction with physical and nutrient data, were used to assess the biogeochemical processes controlling the distribution and speciation of the trace metals in the lake and their seasonal variation. Thermodynamic calculations were used to assess the dissolved trace metal speciation and the saturation state of metal-sulphide phases.The development of anoxia in the lake had an important influence on the redox speciation of dissolved Cr, with Cr(III) forming the major Cr species under anoxic conditions. Cobalt showed enhanced concentrations in the hypolimnion during anoxia, coinciding with enhanced Fe and Mn levels. Dissolved Cu, Ni, and Zn concentrations in the hypolimnion decreased during anoxia, which could possibly be explained by particle scavenging (iron sulphides) and a decline in hypolimnetic supply as the season develops. Regeneration processes of sunken organic detritus on the sediment-water interface were the most likely causes for enhanced hypolimnetic concentrations of Ni, Cu, and Zn, during periods when the lake was completely oxic.Substantial organic Ni and Cu complexation was observed in Esthwaite Water, which was attributed to the eutrophic character of the lake. Between 25 and 80% of the dissolved Ni in Esthwaite Water was strongly complexed by natural ligands. The highest proportions of Ni complexation were observed in the epilimnion and hypolimnion, with a minimum in the thermocline. Strong Cu complexing ligands were observed in the lake, with dissolved ligand concentrations between 6.8 and 29.4 nM and conditional stability constants between 10 11.1 and 10 14.4. More than 94% of the dissolved Cu in Esthwaite Water was complexed by the natural ligands.

The concentration, speciation and sources of mercury in Chesapeake Bay precipitation

Mercury concentrations in wet deposition to a coastal and an inland site within the Chesapeake Bay watershed ranged from less than 10 pM to around 400 pM. Most concentrations were, however, below 150 pM. Speciation measurements of precipitation at the Chesapeake Biological Laboratory (CBL) (average total Hg 88 pM) show that 15% of the total Hg is reactive with methylmercury less than 0.5%, on average. Atmospheric particulate Hg measurements at the CBL site averaged 18 pgm −3, with somewhat higher winter concentrations. There is no strong indication of gaseous ionic Hg species in the Chesapeake Bay atmosphere, in contrast to what has been found elsewhere. Mercury in rain exhibited scavenging-type characteristics with the highest concentrations being found for the smaller rain events. However, a comparison of the ratio of Hg to other constituents in particulate with that of precipitation indicates that other processes besides particulate scavenging are contributing Hg to wet deposition. Thus, atmospheric oxidation of elemental mercury during rain formation and during atmospheric transport must be occurring.

Scavenging of aerosol particles by large water drops: 3. Washout coefficients, half‐lives, and rainfall depths

Using the average values of collection efficiencies obtained from our experiments [Pranesha and Kamra, 1996, this issue] washout coefficients for the drops in the diameter (D) range 3.6≤D≤5.0 mm collecting micron‐sized aerosol particles have been calculated when the drops are neutral, charged, or falling in an electric field. Compared with the neutral case, the values of washout coefficients are higher in both electrical cases, the increase being more pronounced for smaller particles. Washout coefficients show a maximum for a drop charge of 10−12 to 10−11 C. With an increase in electric field, the washout coefficients increase linearly for ∼1‐μm particles, show a maximum for ∼4‐μm particles, and change insignificantly for ∼7‐μm particles. Combining our experimental values of collection efficiencies with the theoretical collision efficiencies of McGann and Jennings [1991] for smaller drops, washout coefficients, half‐lives, and rainfall depths have been computed for the raindrop size distribution extending from 0.1‐ to 5 mm‐diameter. Results show that raindrops of diameter &gt;1 mm contribute dominantly in removing particles of diameter 1–2 μm and their contributions increase with the rainfall rate. When the effect of the raindrops of diameter &gt;1 mm is included, the values of washout coefficient increase by about 2 orders of magnitude for particles of diameter 1–2 μm and by about 1 order of magnitude for particles of diameter &gt;2 μm. It can be concluded from the estimates of rainfall depth that a heavy rainfall over a short duration is more efficient in removing the particles of diameter &lt;2.2 μm, whereas a lower rainfall spread over a longer duration is more efficient in removing the particles of diameter &gt;2.2 μm.

Scavenging of aerosol particles by large water drops: 2. The effect of electrical forces

The effect of electrical forces on the collection efficiency of millimeter‐sized water drops collecting micron‐sized aerosol particles has been investigated in a laboratory experiment. The observations show higher collection efficiencies for drops of 3.6‐ to 4.8‐mm diameters than reported in some of the earlier studies for smaller drops. The limited and sparse data obtained in our experiments show that the collection efficiency of a drop is higher when it is charged or interacts with the aerosol in the presence of an electric field. The collection efficiency shows a maximum when the drop charge of either polarity is in the range of 10−12 to 10−11 C. The data show that the drop surface charge density required for this maximum decreases with the increase in drop size but is independent of the particle size. However, the peak value of collection efficiency is higher for larger particles. Moreover, the total charge on the drop required for this maximum remains almost constant at about 2–3×10−12C. The collection efficiency increases with the increase in the electric field, and the effect of the electric field is stronger for larger drops. In high fields, the drop collection efficiency shows a maximum for particles of diameter between 3.5 and 5 μm. The change in collection efficiency for the same change in particle size is larger for higher electric fields. Distortion of large drops and the consequent charge accumulation on the rim of the drop has been proposed to explain the results. The decrease in collection efficiency for large values of drop charge and electric field support the drop‐to‐particle charge transfer during their interactions.

Field experimental investigation of below-cloud scavenging processes

For the first time it is possible to investigate field experimental particle scavenging processes as a function of aerosol particle size and raindrop size simultaneously. The results for scavenging efficiencies prove the suitability of the described experiments to characterise below-cloud scavenging processes.