Electrostatic forces in wind-pollination—Part 2: Simulations of pollen capture

During fair-weather conditions, a 100 V m⁻¹ electric field exists between positive charge suspended in the air and negative charge distributed on the surfaces of plants and on the ground. The fields surrounding plants are highly complex reaching magnitudes up to 3×10⁶ V m⁻¹. These fields possibly influence the capture of charged wind-dispersed pollen grains. In this article, we model the electric fields around grounded conductive spherical “plants” and then estimate the forces and resulting trajectories of charged pollen grains approaching the plants. Pollen grain capture depends on many factors: the size, density, and charge of the pollen; the size and location of the plant reproductive structures; as well as wind speed, ambient electric field magnitude, and air viscosity. Electrostatic forces become increasingly important as pollen grain charge increases and pollen grain size (mass) decreases. A positively charged pollen grain is attracted to plants, while a negatively charged pollen grain is repelled. The model suggests that a pollen grain (10 μm radius, carrying a positive charge of 1 fC) is captured if passing within 2 mm of the plant. A similar negatively charged pollen grain is repelled and frequently uncapturable. The importance of electrostatic forces in pollen capture is limited by wind, becoming virtually irrelevant at high wind speeds (e.g. 10 m s⁻¹). However, during light wind conditions (e.g. 1 m s⁻¹), atmospheric electricity may be a significant factor in the capture of wind-dispersed pollen.     

The effect of humidity and state of water surfaces on deposition of aerosol particles onto a water surface

Deposition processes of particles with dry diameter larger than about 10 μm are dominated by gravitational settling, while molecular diffusion and Brownian motion predominate the deposition processes of particles smaller than 0.1 μm in dry diameter. Many air pollution derived elements exhibit characteristics common to sub-micron particles. The objective of the present study is to examine the effects of meteorological conditions within the turbulent transfer layer on the deposition velocity of particles with dry diameter between 0.1 and 1 μm. It is for these sub-micron particles that particle growth by condensation in the deposition layer, the broken water surface effect and the enhanced transfer process due to atmospheric turbulence in the turbulent transfer layer play important roles in controlling the particle deposition velocity. Results of the present study show that the `dry air’ assumption of Williams’ model is unrealistic. Effects of ambient air relative humidity and water surface temperature cannot be ignored in determining the deposition velocity over a water surface. Neglecting effects of ambient air relative humidity and water surface temperature will result in defining atmospheric stability incorrectly. It is found that the largest effect of air relative humidity on deposition velocity occurs at an air–water temperature difference corresponding to the point of `displaced neutral stability'. For a given wind speed of U=5 m s⁻¹ the additive effects of water surface temperature, T_w, changes from 5 to 25°C and ambient air relative humidity variations from 85 to 60%, respectively, lead to a maximum difference in v_d of about 20%. For a higher wind speed of 10 m s⁻¹, however, the corresponding change in v_d reduces to less than 5%. This is further confirmation that wind speed is one of the strongest variables that governs the magnitude of v_d. The present study also found that the broken surface transfer coefficient, k_bs, given as a multiple of the smooth surface transfer coefficient, k_ss, is physically more meaningful than assigning it a constant value independent of particle size. The method used in this study requires only a single level of atmospheric data coupled with the surface temperature measurement. The present method is applicable for determining deposition velocity not only at the conventional measurement height of 10 m but also at any other heights that are different from the measurement height.     

Simulation of the evolution of particle size distributions containing coarse particulate in the atmospheric surface layer with a simple convection-diffusion-sedimentation model

The Fugitive Dust Model (FDM) and Industrial Source Complex (ISC), widely used coarse particulate dispersion models, have been shown inaccurate due to the neglect of vertical variations in atmospheric wind speed and turbulent diffusivity (Vesovic et al., 2001), omission of the gravitational advection velocity, and an underestimation of the ground deposition velocity (Kim and Larson, 2001). A simple, transient two-dimensional convection-diffusion-sedimentation model is proposed to simulate the evolution in particle size distribution of an aerosol ‘puff’ containing coarse particulate in the atmospheric surface layer. Monin-Okhubov similarity theory, accompanied by empirical observations made by Businger et al. (1971), is adopted to characterize the surface layer wind speed and turbulent diffusivity profiles over a wide range of atmospheric conditions. A first order analysis of the crossing trajectories effect suggests simulation data presented here are not significantly affected by particle inertia. The model is validated against Suffield experimental data in which coarse particulate deposition was measured out to a distance of 800 m from the source (Walker, 1965). Good agreement is found for the decay in ground deposits with distance from the source for stable atmospheres. Deposition data was also simulated for unstable atmospheric stratification and the current model was determined to modestly underestimate the peak concentration with increasing accuracy further downwind of the release. The current model's effective deposition velocity was compared to that suggested by Kim et al. (2000) and shows improvement with respect to FDM. Lastly, the model was used to simulate the dispersion of nine lognormal aerosol puffs in the lowest 50 m of the atmospheric surface layer for four classes of atmospheric stability. The simulated mass median aerodynamic diameters (MMAD) at multiple downwind sampling locations were calculated and plotted with distance from the source. The first 50 m from the source was found to have a substantial impact on the evolution of MMAD for stable atmospheric conditions. Away from the source, it was observed that particle size distributions were truncated by removal of all particles larger than about 60 μm. A particle Peclet number was also defined to quantify the relative importance of turbulent dispersion and sedimentation on particle motion in the vertical direction.     

Dry deposition of particles to wave surfaces: II. Wind tunnel experiments

Wind tunnel measurements of particle dry deposition to wavy and flat surfaces were made to estimate the enhancement of deposition rates due to waves on water surfaces. Measurements were made of 4.0 and 6.7 μm uranine particles at wind speeds of 5 and 10 m s⁻¹ to sinusoidal waves with height to length ratios 2a/λ=0.1 and 0.03 and to flat surfaces. Results showed that deposition was greatest to the upslope portion of the wave, accounting for 40–45% of the total mass, followed by the trough (30%), downslope (15%), and crest (10–15%). These results generally agreed within experimental variability with modeling predictions (Zufall et al., 1999). Deposition was enhanced at the upslope due to the effects of particle interception and impaction on the wave. Total deposition to the wave surfaces was greater than deposition to the flat surface for a large majority of the cases. The average increase in deposition to both wave surfaces for both particle sizes and wind speeds over deposition to the flat surface was 80%.     

Measurement of the surface charge of ultrafine particles from laser printers and analysis of their electrostatic force

Ultrafine particles (UFPs) released from laser printers are electrostatically charged during the working processes of the devices, and the electrostatic force can obviously influence the dynamics of the particles. Due to the measurement difficulty and scarcity of relevant research, this issue was not reported. This study tried to address this issue through experimental measurement of the surface charge of UFPs and numerical investigation on the influence of electrostatic force on the dynamics of UFPs. A test chamber was set up to collect the UFPs, and the Scanning Electron Microscope was used to observe the morphologies of the UFPs. Based on the particle diameter and surface zeta potential, the surface charge of UFPs was calculated. The measurement results gave that particle emitted from laser printers are negatively charged and the average surface charge of particle emissions for four laser printers is in a range of about ?4.16 × 10^?17 to ?6.07 × 10^?17 C (～?260 to ?379 e). This paper also discussed the influence of electrostatic force on the dynamics of UFPs. According to the numerical investigation, it was found that, in the absence of electric field, the electrostatic force has to be considered when the surface charge is larger than 1 × 10^?16 C and when the UFP is very close to the wall with a distance of less than 0.01 m. These findings will guide constructively in predicting the dispersion and deposition of particles emitted from laser printers.     

Modeling PAH uptake by vegetation from the air using field measurements

We examined PAH uptake by Norway spruce needles following the emergence of new buds in spring 2004–June 2005. Atmospheric PAH concentrations (gaseous phase and particle-bound) were monitored during this period, and PAH concentrations from these three environmental media were then used to calculate deposition and transfer velocities. Benzo(a)pyrene was found almost exclusively associated to particles and thus was used to determine a particle-bound deposition velocity of 10.8 m h^?1. PAHs present in both compartments had net gaseous transfer velocities ranging from negligible values to 75.6 m h^?1 and correlated significantly with log K_OA. The loss velocities thereafter calculated were found to be higher for more volatile PAHs. Using the calculated average atmospheric PAH concentrations and deposition velocities, it was thus possible to model PAH uptake by vegetation through time. We demonstrate that this approach can be used to determine deposition velocities without the use of a surrogate surface. In considering both particulate-bound and gaseous deposition processes this model can be used not only to study air–foliage exchange of semi-volatile organic compounds, but also to illustrate the relative contribution of gaseous deposition and particulate-bound deposition in the overall atmospheric vegetation uptake of semi-volatile organic compounds.     

Influence of the surface microlayer on atmospheric deposition of aerosols and polycyclic aromatic hydrocarbons

Atmospheric dry deposition is an important process for the introduction of aerosols and pollutants to aquatic environments. The objective of this paper is to assess, for the first time, the influence that the aquatic surface microlayer plays as a modifying factor of the magnitude of dry aerosol deposition fluxes. The occurrence of a low surface tension (ST) or a hydrophobic surface microlayer has been generated by spiking milli-Q water or pre-filtered seawater with a surfactant or octanol, respectively. The results show that fine mode (<2.7 μm) aerosol phase PAHs deposit with fluxes 2–3 fold higher when there is a low ST aquatic surface due to enhanced sequestration of colliding particles at the surface. Conversely, for PAHs bound to coarse mode aerosols (>2.7 μm), even though there is an enhanced deposition due to the surface microlayer for some sampling periods, the effect is not observed consistently. This is due to the importance of gravitational settling for large aerosols, rendering a lower influence of the aquatic surface on dry deposition fluxes. ST (mN m⁻¹) is identified as one of the key factor driving the magnitude of PAH dry deposition fluxes (ng m⁻² d⁻¹) by its influence on PAH concentrations in deposited aerosols and deposition velocities (v_d, cm s⁻¹). Indeed, v_d values are a function of ST as obtained by least square fitting and given by Ln(v_d)=−1.77 Ln(ST)+5.74 (r²=0.95) under low wind speed (average 4 m s⁻¹) conditions.     

Effect of forest edges on deposition of radioactive aerosols

The possible enhancement of aerosol deposition at forest edges was investigated in a wind tunnel and in the field. The wind tunnel study was carried out using 0.82 μm mass median aerodynamic diameter uranium particles and a composite canopy of rye grass and spruce saplings. The field study was undertaken at a coniferous woodland near to BNFL Sellafield, Cumbria, UK. Two transects were set through the woodland to determine the influence of the forest edge on atmospheric deposition of radionuclides released under authorisation from the Sellafield site. Results from the wind tunnel study showed that the deposition flux of uranium particles decreased with distance downwind from the grass–tree edge towards the interior of the canopy. The deposition flux at the edge was maximal at about 4×10⁻⁷ μg of U cm⁻² s⁻¹. This was 3 times higher than that observed over grass where a constant flux of about 1.32×10⁻⁷ μg of U cm⁻² s⁻¹ occurred. Results from the field study showed a clear influence of the forest edge on the atmospheric deposition of ²⁴¹Am and ¹³⁷Cs. Activity depositions of around 4750 and 230 Bqm⁻² for ¹³⁷Cs and ²⁴¹Am, respectively, were measured in front of the woodland. Activity deposition inside the forest edge, however, rose to levels of between 20,200 and 50,900 Bq m⁻² and 1100 and 3200 Bq m⁻² for ¹³⁷Cs and ²⁴¹Am, respectively, depending upon the transect. Similar activity concentrations were measured in the pasture to the front and behind Lady Wood. Results from these studies corroborate those obtained from various studies on air pollutants including radionuclides. This underlines the importance of deposition at the edge of forests and its contribution to the overall canopy deposition. The edge effect is therefore an important factor that should be considered in the assessment of fallout impact, whether this is to be made by either direct sampling or by modelling.     

Concentration, size distribution, and dry deposition rate of particle-associated metals in the Los Angeles region

Daily averaged atmospheric concentrations and dry deposition fluxes of particulate metals were measured seasonally at six urban sites and one non-urban coastal site in the Los Angeles region using a conventional total suspended particulate matter (TSP) filter, surrogate surface deposition plates, and a Noll Rotary Impactor (NRI), which provides information about particle size distribution in four size ranges above 6 μm. With the exception of the non-urban site, particulate metal concentrations and deposition fluxes were remarkably uniform spatially and temporally. At all sites there were significant metal concentrations on particles greater than 10 μm, a commonly used upper limit for many air quality monitoring studies, and these large particles were estimated to be responsible for most of the deposited mass of metals. Annual averaged values of deposition rates measured with a surrogate surface were in good agreement with values estimated using theoretical deposition velocities in conjunction with measured size-segregated particle concentrations. Image analysis of particles deposited on NRI stage A, which collects all particles greater than 6 μm, indicated nighttime metal concentrations and deposition at the non-urban coastal site was higher than in the day time due to offshore advection of urban air associated with the diurnal land breeze. Measured enrichments of crustal elements and metals were correlated, indicating efficient mixing of natural and anthropogenic material from different sources, hypothesized to be the result of cyclical resuspension and deposition of dust by moving vehicles and wind.     

Deposition rates on smooth surfaces and coagulation of aerosol particles inside a test chamber

Because aerosol particle deposition is an important factor in indoor air quality, many empirical and theoretical studies have attempted to understand the process. In this study, we estimated the deposition rate of aerosol particles on smooth aluminum surfaces inside a test chamber. We investigated the influence of turbulent intensity due to ventilation and fan operation. We also investigated two important processes in particle deposition: turbophoresis, which is significant for micron particles, and coagulation, which is relevant to ultrafine particles (UFP diameter <0.1 μm) at high particle concentrations. Our analysis included semi-empirical estimates of the deposition rates that were compared to available deposition models and verified with simulations of an aerosol dynamics model. In agreement with previous studies, this study found that induced turbulent intensity greatly enhanced deposition rates of fine particles (FP diameter <1 μm). The deposition rate of FP was proportional to the ventilation rate, and it increased monotonically with fan speed. With our setup, turbophoresis was very important for coarse particles larger than 5 μm. The coagulation of aerosol particles was insignificant when the particle concentration was less than 10⁴ cm^?3 during fan operation. The model simulation results verified that the aerosol dynamics module incorporated in our Multi-Compartment and Size-Resolved Indoor Aerosol Model (MC-SIAM) was valid. The behavior of aerosol particles inside our chamber was similar to that found in real-life conditions with the same ventilation rates (0.018–0.39 h^?1) and similar air mixing modes. Therefore, our findings provide insight into indoor particle behavior.     

Observations of particle growth at a remote,Arctic site

Observations of particle size distributions suggest that particles grow significantly just above the snow surface at a remote, Arctic site. Measurements were made at Summit, Greenland (71.38°N and 31.98°W) at approximately 3200 m above sea level. No new particle formation was observed locally, but growth of ultrafine particles was identified by continuous evolution of the geometric mean diameter (GMD) during four events. The duration of the growth during events was between 24 and 115 h, and calculated event-average growth rates (GR) were 0.09, 0.30, 0.27, and 0.18 nm h^?1 during each event, respectively. Four-hour GR up to 0.96 nm h^?1 were observed. Events occurred during below- and above-average temperatures and were independent of wind direction. Correlation analysis of hourly-calculated GR suggested that particle growth was limited by the availability of photochemically produced precursor gases. Sulfuric acid played a very minor role in particle growth, which was likely dominated by condensation of organic compounds, the source of which was presumably the snow surface. The role of boundary layer dynamics is not definite, although some mixing at the surface is necessary for the observation of particle growth. Due to the potentially large geographic extent of events, observations described here may provide a link between long-range transport of mid-latitude pollutants and climate regulation in the remote Arctic.     

Salinity of marine aerosols in a Brazilian coastal area—Influence of wind regime

This paper presents salinity data of marine aerosols in a Brazilian coastal area at sites closer to the shore and discusses the influence of wind regime. Results show that measurements of marine salt deposition are strongly influenced by wind speeds above the critical value of 3.0 m s⁻¹. Although there is no agreement in literature yet as to how much this threshold is, the level of 3.0 m s⁻¹ is similar to the one found in a study carried out in Spain. An exponential variation of salt concentration with wind speed is presented in previous published papers. However, in the studied case, another relationship is proposed, which is based on chloride deposition on the wet candle device and wind speeds higher than 3.0 m s⁻¹ weighted by the accumulated time in which these wind speeds are observed, what seems to be particularly useful when wind speed ranges around the critical value.     

Urban scale modeling of particle number concentration in Stockholm

A three-dimensional dispersion model has been implemented over the urban area of Stockholm (35×35 km) to assess the spatial distribution of number concentrations of particles in the diameter range 3–400 nm. Typical number concentrations in the urban background of Stockholm is 10 000 cm⁻³, while they are three times higher close to a major highway outside the city and seven times higher within a densely trafficked street canyon site in the city center. The model, which includes an aerosol module for calculating the particle number losses due to coagulation and dry deposition, has been run for a 10-day period. Model results compare well with measured data, both in levels and in temporal variability. Coagulation was found to be of little importance in terms of time averaged concentrations, contributing to losses of only a few percent as compared to inert particles, while dry deposition yield particle number losses of up to 25% in certain locations. Episodic losses of up to 10% due to coagulation and 50% due to deposition, are found some kilometers downwind of major roads, rising in connection with low wind speed and suppressed turbulent mixing. Removal due to coagulation and deposition will thus be more significant for the simulation of extreme particle number concentrations during peak episodes.The study shows that dispersion models with proper aerosol dynamics included may be used to assess particle number concentrations in Stockholm, where ultrafine particles principally originate from traffic emissions. Emission factors may be determined from roadside measurements, but ambient temperature must be considered, as it has a strong influence on particle number emissions from vehicles.     

Measurement of dry deposition to surfaces in deciduous and pine canopies

The importance of dry deposition was assessed at perimeter and interior locations in two vegetative canopies. Dry deposition was measured directly by washing particles from leaves. Ambient particles and gases were also collected at both locations within the canopies. Ambient concentrations on the canopy interior were decreased relative to perimeter concentrations due to dry deposition scavenging by the canopy. The least scavenging was found for SO(4)(2-) and NH(4)(+) and the highest scavenging was found for HNO(3). Dry deposition of all species was higher to perimeter vegetative and surrogate surfaces than to interior surfaces, due both to the lower concentrations and the lower wind speeds in the sheltered interior. Deposition velocities compared well with other experimental and theoretical values.     

Nitrogen oxide fluxes between corn (Zea mays L.) leaves and the atmosphere

In the United States, fertilized corn fields, which make up approximately 5% of the total land area, account for approximately 45% of total soil NO_x emissions. Leaf chamber measurements were conducted of NO and NO₂ fluxes between individual corn leaves and the atmosphere in (1) field-grown plants near Champaign, IL (USA) in order to assess the potential role of corn canopies in mitigating soil–NO_x emissions to the atmosphere, and (2) greenhouse-grown plants in order to study the influence of various environmental variables and physiological factors on the dynamics of NO₂ flux. In field-grown plants, fluxes of NO were small and inconsistent from plant to plant. At ambient NO concentrations between 0.1 and 0.3 ppbv, average fluxes were zero. At ambient NO concentrations above 1 ppbv, NO uptake occurred, but fluxes were so small (14.3±0.0 pmol m⁻² s⁻¹) as to be insignificant in the NO_x inventory for this site. In field-grown plants, NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.9 ppbv (the NO₂ compensation point), with the highest rate of emission being 50 pmol m⁻² s⁻¹ at 0.2 ppbv. NO₂ was assimilated by corn leaves at ambient NO₂ concentrations above 0.9 ppbv, with the maximum observed uptake rate being 643 pmol m⁻² s⁻¹ at 6 ppbv. When fluxes above 0.9 ppbv are standardized for ambient NO₂ concentration, the resultant deposition velocity was 1.2±0.1 mm s⁻¹. When scaled to the entire corn canopy, NO₂ uptake rates can be estimated to be as much as 27% of the soil-emitted NO_x. In greenhouse-grown and field-grown leaves, NO₂ deposition velocity was dependent on incident photosynthetic photon flux density (PPFD; 400–700 nm), whether measured above or below the NO₂ compensation point. The shape of the PPFD dependence, and its response to ambient humidity in an experiment with greenhouse-grown plants, led to the conclusion that stomatal conductance is a primary determinant of the PPFD response. However, in field-grown leaves, measured NO₂ deposition velocities were always lower than those predicted by a model solely dependent on stomatal conductance. It is concluded that NO₂ uptake rate is highest when N availability is highest, not when the leaf deficit for N is highest. It is also concluded that the primary limitations to leaf-level NO₂ uptake concern both stomatal and mesophyll components.     

Measurements of ammonia concentrations,fluxes and dry deposition velocities to a spruce forest 1991–1995

The dry deposition velocities and fluxes of ammonia have been estimated from measurements of the vertical gradient of ammonia and micrometeorology above a spruce forest in western Jutland, Denmark. Measurements have been made in seven periods, each lasting about one week and covering all seasons and different meteorological situations. Different deposition characteristics were observed, depending on the ammonia concentration and the relative humidity. At conditions with westerly winds, the wind brings air masses from the North Sea with low concentration levels of ammonia to the site, while at conditions with easterly winds, the air have passed central Jutland with large emission areas. Some of the relatively low deposition velocities or emissions were observed during conditions with low ammonia concentration and westerly winds. These observations might relate to a compensation point of the forest, i.e. an ammonia concentration below which the trees and/or the surface emit ammonia due to an equilibrium with the ammonia inside the needles or on the surface. Emission of ammonia was also observed at relatively high ammonia concentration levels (above 2 μg NH₃–N m^-3), mainly during one measuring period characterized by easterly winds with dry conditions and high ammonia concentrations, and the emissions might relate to evaporation from ammonia saturated surfaces or emission from mineralization in the forest soil. In general, relatively high net deposition velocities were observed during conditions with relative humidity above 80% or at ammonia concentrations moderate higher than a given (temperature dependent) compensation point. During stable conditions some observations revealed that the gradient above the canopy not necessarily represents the exchange with the canopy.     

Dry deposition and internal circulation of nitrogen,sulphur and base cations to a coniferous forest

The dry deposition of sulphur, nitrogen and base cations to a spruce stand was estimated during a five year period using a surrogate surface resembling needles, throughfall and bulk deposition measurements. The deposition was calculated from the ratio between the deposition of an ion and sodium on the surrogate surface and the net throughfall of sodium to the forest. The dry deposition represented a large fraction of the total atmospheric input of base cations. For Na⁺, Mg²⁺, Ca²⁺, and K⁺ they were 66, 67, 53 and 42%, respectively. The internal circulation was 95% of non-marine net throughfall fro K⁺ and 76% for Ca²⁺. The dry deposition of SO₂ to the canopies regulates the internal circulation of Ca²⁺. The dry deposition of SO₂ to the canopies regulates the internal circulation of Ca²⁺. The dry depositions of ammonium and nitrate are close to the net throughfall of Kjeldahl-N and nitrate, respectively. The obtained deposition velocities are comparable to other studies. The calculated dry deposition of ammonium was compared to the net throughfall of ammonium at three nearby forest stands receiving different ammonium amounts on the soils. No correlation to nitrogen level was found, but most ammonium was lost and converted to organic nitrogen in the canopies of the wettest forest stand.     

Ozone uptake by citrus trees exposed to a range of ozone concentrations

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO_x). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon (Citrus limon), mandarin (Citrus reticulata), and orange (Citrus sinensis), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s^?1 m^?2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.     

Resuspension of small particles from tree surfaces

A detailed study of resuspension of 1.85 μm MMAD silica particles from five horizontal layers within a small scale spruce canopy was carried out in a wind tunnel in which saplings were exposed to a constant free stream wind speed of 5 m s⁻¹. This provided quantitative estimates of the potential for a tree canopy contaminated with an aerosol deposit to provide (i) an airborne inhalation hazard within the forest environment and (ii) a secondary source of airborne contamination after an initial deposition event. Resuspension occurred with a flux of 1.05×10⁻⁷ g m⁻² s⁻¹ from spruce saplings initially contaminated at a level of 4.1×10⁻² g m⁻². An average resuspension rate (Λ) of 4.88×10⁻⁷ s⁻¹ was obtained for the canopy as a whole. Values of Λ were significantly different (ANOVA, p<0.001) between canopy layers and Λ was markedly greater at the top of the canopy than lower down although there was a slight increase in Λ at the base of the canopy. The resuspended silica particles deposited onto the soil surface at an average rate of about 5.3×10⁻⁸ μg cm⁻² s⁻¹. It is concluded that resuspension under wind velocities similar to that used in the reported experiments is likely to pose a relatively small inhalation hazard to humans and a relatively minor source of secondary contamination of adjacent areas. Furthermore, resuspension rates are likely to diminish rapidly with time. The results are discussed in relation to the growing interest in the tree planting schemes in urban areas to reduce the impacts of air pollution.     

Wet deposition of phosphorus in Florida

Wet deposition of phosphorus was measured at 10 sites across Florida originally established as part of the Florida Atmospheric Mercury Study conducted between 1992 and 1996. Monthly integrated samples were collected and analyzed using a total analytical protocol that incorporated “clean lab” conditions for sample equipment preparation and Aerochem Metrics collectors modified for suitability to use for ultra-trace elements. Samples also were collected aboard 15 m towers to minimize any influence on measured deposition by insects, etc., and locally originating particles that do not contribute to true net deposition. Extensive replication of samples in the field was conducted (ca. 83%). The average absolute difference between replicates was 16.2%, with a median absolute difference of 9.5%. Replicate precision was poorest for concentrations above 0.080 mg P l⁻¹, suggesting that concentrations above this level are contaminated.The wet deposition concentrations and fluxes of phosphorus measured in this study are appreciably lower than those reported by previous investigators for wet deposition in Florida, and lie at the lower end of measurements reported in the recent literature. For example, the volume weighted mean concentration and flux for wet deposition across all our study sites averaged 0.005 mg P l⁻¹ and 7.5 mg P m⁻² yr⁻¹, respectively, which is approximately 50% and 32% lower than that reported by Hendry et al. (1981 in Atmospheric Pollutants in Natural Waters. Ann Arbor Science, Ann Arbor. MI, pp. 199–215). Our lower measurements likely reflect three factors: (1) the ultra-trace element sampling and analytical protocols; (2) improved collector design to eliminate sampling artifacts (e.g., splash-off contamination and transfer of contaminants from the dry bucket); and (3) placement of collectors off the ground surface. Lower VWM concentrations were observed near the Florida coast; otherwise, strong spatial patterns across the state were absent. Seasonal variations in VWM also were not pronounced, although deposition fluxes were highest during the summer wet season in response to the strong seasonal distribution of rainfall.