Aerosol Deposition Velocity Research Articles

Atmospheric deposition of polycyclic aromatic hydrocarbons to water surfaces: A mass balance approach

A mass balance model was developed to explain the movement of polycyclic aromatic hydrocarbons (PAH) into and out of Siskiwit Lake, which is located on a wilderness island in northern Lake Superior. Because of its location, the PAH found in this lake must have originated exclusively from atmospheric sources. Using gas Chromatographie mass spectrometry, 11 PAH were quantified in rain, snow, air, lake water, sediment core and sediment trap samples. From the dry deposition fluxes, an aerosol deposition velocity of 0.99 ± 0.15 cm s −1 was calculated for indeno[1,2,3- cd]pyrene and benzo[ ghi]perylene, two high molecular weight PAH which are not found in the gas phase. The dry aerosol deposition was found to dominate the wet removal mechanism by an average ratio of 9:1. The dry gas flux was negative, indicating that surface volatilization was taking place; it accounted for 10–80 % of the total output flux depending on the volatility of the PAH. The remaining PAH were lost to sedimentation. From the dry gas flux, an overall mass transfer coefficient for PAH was calculated to be 0.18 ± 0.06 m d −1. In this case, the overall mass transfer is dominated by the liquid phase resistance.

A particle dry‐deposition parameterization scheme for use in tracer transport models

A particle dry‐deposition parameterization scheme for use in tracer transport models is presented. The scheme produces particle deposition velocities relative to the bottom transport model level expressed in terms of wind speed, temperature, and air density at the bottom model level; surface momentum drag coefficient; particle size and density; characteristics of the surface roughness elements; and several parameters derived for different surface types. A two‐layer boundary layer model is employed to calculate the surface deposition velocities. Formulas for the particle deposition velocity are derived for four surface types: smooth surfaces, surfaces with bluff roughness elements, ocean surfaces, and vegetative canopies. The following processes are included in the model: turbulent transport through the boundary layer, gravitational settling, deposition by Brownian diffusion, interception and impaction on the surface roughness elements, diffusiophoresis on water surfaces, and, in limited cases, particle bounce‐off. Electrical and thermophoretic effects are ignored as well as particle blow‐off. As an illustrative example, the scheme is applied to a general circulation model to calculate the deposition velocities of ambient tropospheric aerosols. The calculated deposition velocities show a strong dependence upon the wind field in the bottom model level, the particle size distribution, and, to a lesser extent, the surface type. Particle mass deposition velocities of continental aerosols are found to lie in the range 0.003–0.036 cm/s for the accumulation mode and in the range 0.5–2.5 cm/s for the coarse mode.

Studies on the dry deposition of aerosol particles on vegetation and plane surfaces

The deposition velocities of polydisperse and monodisperse test aerosols with diameters between 0.4–17 μm were determined experimentally. Deposition experiments on grass show that the deposition velocity depends on the friction velocity and on the dry weight per unit area of the vegetation, and especially on the particle diameter. At a mean friction velocity of about 25 cm/s and a representative dry weight per unit area of 0.017 g/cm 2, deposition velocities of 0.1, 0.4, 2.0 and 4.6 cm/s are obtained for particles with diameters of 4, 5, 7 and 10 μm, respectively. Values below 0.1 cm/s result for smaller diameters. The deposition rate on clover is higher than that on grass by a factor of 2, and on bare, plane surfaces and on water it is lower by a factor of 2–4. Estimates result in a deposition velocity for forests higher by an order of magnitude than that on grass so that a deposition velocity in the order of magnitude of 1 cm/s is realistic for particles with diameters of a few μm.

An evaluation of Gaussian plume-depletion models with dual-tracer field measurements

To evaluate atmospheric diffusion-deposition models, airborne tracer concentrations were measured during simultaneous releases of depositing (ZnS) and nondepositing (SF 6) tracers. The effective deposition velocity of the ZnS tracer, a polydisperse aerosol, was obtained from the near-surface, crosswind-integrated, dual tracer concentrations, using a method based on the surface depletion model of Horst (1977, Atmospheric Environment 11, 41–46.). These deposition velocities were in good agreement with predictions based on the particle size distribution of the ZnS tracer and on wind tunnel measurements of the deposition velocities of monodisperse aerosols. Four Gaussian plume-depletion models were evaluated with the dual tracer data. Source depletion and constant eddy-diffusivity models were found to overestimate the depositing/nondepositing concentration ratio near the surface, but the estimates of these models were noticeably improved by a profile correction to the source depletion model and a mass-conservation correction to the K model. Of the four models tested, the corrected source depletion model gave the best agreement with the observations.

Aerosol deposition velocities on the Pacific and Atlantic oceans calculated from 7Be measurements

The concentrations of 7Be have been measured in Pacific and Atlantic ocean water for the past several years to determine the deposition velocity of aerosol particles on the ocean surface. 7Be is produced at a relatively constant rate in the atmosphere by spallation reactions of cosmic rays with atmospheric nitrogen and oxygen. Immediately after its formation 7Be becomes attached to aerosol particles, and therefore can serve as tracers of the subsequent behavior of these particles. Isopleths of 7Be surface water concentrations, 7Be inventory in the ocean, and deposition velocity have been prepared for the Pacific Ocean from 30°S to 60°N and for the Atlantic Ocean from 10°N to 55°N. The concentrations, inventories and deposition velocities tended to be higher in regions where precipitation was high, and generally increased with latitude. The average flux of 7Be across the ocean surface was calculated to be 0.027 atoms cm −2 s −1 which is probably not significantly greater than the worldwide average 7Be flux across land and ocean surfaces of 0.022 atoms cm −2 s −1 calculated by Lal and Peters. The average deposition velocity was calculated to be 0.80 cm s −1. This value may be 10–50% too low, since it was calculated using atmospheric 7Be concentrations which were measured at continental stations. Measurements of atmospheric 7Be concentrations at ocean stations suggest that the concentrations at the continental stations averaged 10–50% higher than the concentrations over the ocean.

Nutrient Loading of Southern Lake Michigan by Dry Deposition of Atmospheric Aerosol

Aerosol samples and micrometeorological data were collected at 87°00’W 42°00’N in the mid southern Lake Michigan basin from May through September, 1977, to determine total phosphorus and nitrate-nitrite nitrogen particulate (TP + N) loadings. Hi-volume samplers with cellulose fiber filters and a meterological data collection system were operated on board the USEPA's R/V Roger R. Simons. A diabatic drag coefficient method was used to estimate aerosol deposition velocity (V d), which had a mean value of 0.65 cm/s. TP + N concentration and V d were determined for 42 distinct sampling periods of at least three to four hours each. A climatological data base was used to compute weighted average loading rates for TP + N. Dry deposition loading for the southern basin was found to be 0.15 to 0.18 × 10 6 kg/year for P; N loading was 3.5 to 5.1 × 10 6 kg/year. TP + N inputs via dry deposition account for 15% or more of all atmospheric nutrient inputs, and are significant nutrient sources to midlake waters.

Trace element loading of southern Lake Michigan by dry deposition of atmospheric aerosol

Aerosol samples and meteorological data were collected at a mid-southern Lake Michigan site (87° 00′ W, 42° 00′ N) from May through September 1977. Hi-volume samplers with cellulose fiber filters and a digital meteorological data recording system were operated on board the U.S. EPA's R/VRoger R. Simons during four intensive sampling periods. Aerosol samples were analyzed by atomic absorption spectroscopy for seventeen trace elements. A diabatic drag coefficient method was used to determine aerosol deposition velocity (vd) overlake. A meanvd of 0.5 cm s−1 was found for the 0.1<diameter<2.0 μm aerosol. By relating the observed trace element concentrations andvd to a long-term climatological record, annual dry deposition loadings to the southern basin for nine elements were estimated. For four elements, Fe, Mn, Pb, and Zn, dry deposition loadings to the southern basin alone of at least 500, 30, 250, and 100 (×103 kg yr−1) were found. For Fe and Mn, these loadings represent about 15% of the total of all inputs to Lake Michigan. for Zn and Pb, about one-third to one-half of the annual loading from all sources is from dry deposition of atmospheric aerosol.

The Relative Quantity of Airborne Plutonium Deposited in the Respiratory Tract and on the Skin of Rats

In assessing the hazard of exposure to airborne plutonium, questions arise as to the relative importance of material deposited within the respiratory tract and on skin. To provide relevant deposition data, shaved rats were exposed to a 238Pu(No3)4 aerosol in a low-speed windtunnel. Half were oriented with noses facing upstream, the other half with noses downstream. Samples of the airstream were taken during exposure to determine the aerosol concentration and its aerodynamic equivalent size distribution. Calculations of the quantity inhaled were made from the aerosol concentration, minute volume and exposure time; skin deposition was estimated from the aerosol deposition velocity, area of exposed skin and exposure time. Immediately after exposure, the rats were sacrificed, skinned and dissected and the plutonium activity on the skin compared to that in lungs, head and intestines. Rats with their noses downstream inhaled only 66% as much as the upstream group. In both cases, about 40% of the inhaled material was in the lower respiratory tract, about 40% in the head and 20% in the intestines. Plutonium deposited on the skins of the two groups was almost identical, being 22–33% of the respiratory tract value. Measured values agreed with the calculated values within a factor of two.