Measurement of dry deposition of ammonia on a forest

Ammonia concentration gradients above a Douglas fir canopy were measured from 16 August to 31 December 1989 by two automated high-precision thermodenuders at the location Speulderbos in The Netherlands. Concentration gradients were used to calculate the dry deposition flux of ammonia via flux-gradient theory. Meteorological data were obtained from a nearby tower. Ammonia concentrations were highly variable with highest values during the night. Concentration gradients were very small during daytime and quite large at night. Median values of the calculated deposition flux and deposition velocity were 0.1 microg m(-2) s(-1) and 3.2 cm s(-1) respectively (N=1624).     

Numerical modeling of dry deposition coupled to 22 photochemical reactions

Three Eulerian models for the dry deposition of photochemically reactive species were formulated and evaluated: a K-theory model with independent transport and deposition of each species, a K-theory model coupled with 22 gas-phase reactions, and a second-order flux-budget model coupled with 22 reactions. Operator splitting was used to separately solve the reaction and dispersion terms in the models including photochemistry. In an evaluation of numerical method performance, the Adams–Moulton method with a pseudo-steady-state approximation for the free radicals was found to be consistent with, but more computationally efficient than Gear’s method for the solution of the reaction terms. The sensitivity of profiles of vertical concentration and flux to the Eulerian model formulation varied according to the species’ Damköhler number. Although a K-theory model is adequate for weakly depositing species with Damköhler numbers <10^-3, a second-order flux-budget model is required for species with Damköhler numbers that exceed unity, such as nitrogen oxides, free radicals, and those sensitive to net flux production by chemical reactions. Selection of an appropriate model formulation for the entire system depended on the most reactive species. Simple K-theory models may not accurately predict dry deposition fluxes in the urban surface layer.     

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.     

Measurement of the dry deposition flux of NH3 on to coniferous forest

The dry deposition flux of NH3 to coniferous forest was determined by the micrometeorological gradient method using a 36 m high tower. Aerodynamic characteristics of the site were studied, using a second tower erected in the forest 100 m from the first. Fluxes and gradients of heat and momentum measured on both towers indicated a fairly homogeneous turbulent flow field over the studied area of the forest. Site specific flux profile functions for heat were derived from continuous measurements of turbulent fluxes and gradients. These functions were used to derive fluxes from the observed gradients of NH3. In total, eighty 90-min NH3 flux runs were performed. The results indicate a strong nonstomatal uptake of NH3 by the forest. A representative dry deposition velocity for NH3 of 3.6 cm(-1) s was derived. The annual average flux was roughly estimated to be equivalent to 50 kg N ha(-1) yr, significantly higher than the critical load for coniferous forest.     

The electrical analogy does not apply to modeling dry deposition of particles

The most commonly used expresion for dry deposition of particles is based on the electrical analogy. Because the electrical analogy is not consistent with the mass conservation equation, this expression for dry deposition velocity cannot be justified. This paper presents the correct expression.     

Particulate deposition from dry unpaved roadways

Atmospheric transport and deposition of dust generated by vehicular traffic on dry unpaved roadways is quantified by use of a mass-conserving numerical model. Input to the model includes meteorological conditions, surface roughness, source and receptor heights, and measured particulate-size distribution. Model output includes mass deposited and effective area coverage per vehicle pass at various downwind distances. The model can be used to quantify nuisance levels resulting from fugitive road dust, reduction in efficiency of solar collectors, reduction in photosynthesis, and sedimentation in aquatic bodies caused by airborne dust.     

Wintertime wet and dry deposition in northern Michigan

Wet and dry deposition were monitored at the University of Michigan Biological Station in rural northern Michigan for three winters. Dry deposition was measured by both the conventional bucket method and by measuring increases in concentration in exposed, elevated snow samples. Average results of the two methods were in reasonable agreement. The cumulative wet and dry deposition quantities are in good agreement with snowpack accumulations until the first thaw period. Dry deposition to snow accounts for less than 15% of the total H⁺, SO²⁻₄, NO⁻₃ and NH⁺₄ and approximately 25% of the Ca ²⁺, Mg ²⁺, Na⁺, K⁺ and Cl⁻ during an average precipitation year. Snowpack measurements were also made under deciduous and red pine canopies. Decreases in H⁺ and NO⁻₃ were observed under the red pine canopy.     

Wet and dry deposition monitoring in Southeastern Michigan

Wet and dry deposition as collected by a bucket were measured at two sites in southeastern Michigan for two years. The precipitation had an average pH of 4.27 and a SO²⁻₄ to NO⁻₃ ratio of 2.0. Particulate dry deposition velocities of 0.6 cm s⁻¹ for SO²⁻₄ and NO⁻₃ and > 2 cm s⁻¹ for Cl⁻, Ca²⁺, Mg²⁺,Na⁺ and K⁺ were calculated. The ambient particle composition, dry bucket collection and wet deposition were compared at two sites, one urban and the other rural. Higher ambient particle concentrations and dry deposition rates were measured at the urban site than the rural site, indicating the influence of local emissions. However, local emissions had no effect on the wet deposition concentrations. The influence of more distant source regions was examined by separating the precipitation events by wind direction. The events from the south and east had the highest SO²⁻₄ to NO⁻₃ ratios, which corresponded to the areas with the highest sulfur emissions. NO⁻₃ showed no directional dependence.Wet deposition was examined for the effect of storm type and seasonal trends. Contrary to a recent study on Long Island, we found higher concentrations of H⁺, SO²⁻₄ and NH⁺₄ in winter rain compared to snow. The wet deposition concentrations of H⁺, SO²⁻₄, and NH⁺₄ were highest in the summer, while only Na⁺ and Cl⁻ concentrations were highest in the winter, presumably due to winter road salting. The total deposition of acidic ions was highest in the summer and lowest in the winter, due both to lower concentrations and lower precipitation volumes in the winter. The dry deposition as collected by a bucket accounted for 1 % of total H⁺ deposition, 21 % of SO²⁻₄ deposition, 27% of NO⁻₃ deposition, 50% of Cl⁻ deposition and 61 % of Ca²⁺ deposition.     

Measurement of gas-phase total peroxides at the summit of Mount Tai in China

Measurement of ambient gas-phase total peroxides was performed at the summit of Mount Tai (Mt. Tai, 1534 m above sea level) in central-eastern China during March 22–April 24 and June 16–July 20, 2007. The hourly averaged concentration of peroxides was 0.17 ppbv (± 0.16 ppbv, maximum: 1.28 ppbv) and 0.55 ppbv (± 0.67 ppbv, maximum: 3.55 ppbv) in the spring and summer campaigns, respectively. The average concentration of peroxides at Mt. Tai, which is in a heavily polluted region, was much lower than hydrogen peroxide measurements made at some rural mountain sites, suggesting that significant removal processes took place in this region. An examination of diurnal variation and a correlation analysis suggest that these removal processes could include chemical suppression of peroxide production due to the scavenging of peroxy and hydroxy radicals by high NO_x, wet removal by clouds/fogs rich in dissolved sulfur dioxide which reacts quickly with peroxides, and photolysis. These sinks competed with photochemical sources of peroxides, resulting in different mean concentrations and diurnal pattern of peroxides in the spring and summer. A principal component analysis was conducted to quantify the major processes that influenced the variation of peroxide concentrations. This analysis shows that in the spring photochemical production was an important source of peroxides, and the major sink was scavenging during upslope transport of polluted and humid air from the lower part of the planetary boundary layer (PBL) and wet removal by synoptic scale clouds. During the summer, highly polluted PBL air (with high NO_x) was often associated with very low peroxides due to the chemical suppression of HO₂ by high NO_x and wet-removal by clouds/fogs in this sulfur-rich atmosphere, especially during the daytime. Higher concentrations of peroxides, which often appeared at mid-nighttime, were mainly associated with subsidence of air masses containing relatively lower concentrations of NO_y.     

Wet and dry deposition of trace elements onto the Greenland ice sheet

Trace element concentrations have been measured in the air and in fresh and older surface snow during the summers of 1978 and 1979 at Dye 3, south-central Greenland. These data have been used with other information in the literature to calculate a total (wet plus dry) deposition velocity on the order of 0.5 cms⁻¹ for trace element transport to the ice sheet on an annual basis. The data suggest that dry deposition contributes less than 25 % of the total transport for elements with large enrichment factors. Crustal elements may be more significantly influenced by dry deposition.     

Deliquescence induces eddy covariance and estimable dry deposition errors

The effects of deliquescent processes on eddy covariance estimates of dry deposition are considered, both theoretically and using some representative data. Turbulent and latent heat fluxes near the Earth's surface imply a vertical “saturation ratio flux” (Fairall, C.W., Atmospheric Environment, 18 (1984) 1329). Deliquescence, instantaneous relative to turbulent time-scales, responds to saturation ratio (or relative humidity) fluctuations and induces covariance between vertical winds and size-resolved number concentrations of hygroscopic aerosols. The induced covariance represents an error in the estimation of surface exchange by direct application of the eddy covariance technique. Under deliquescent conditions (high relative humidity), resulting errors in dry deposition estimates are shown to be often as large as typical deposition velocities reported for small particles, depending on the shape of the number distribution and the magnitudes of heat and vapor fluxes in the boundary layer.     

Preliminary data from the USEPA dry deposition network: 1989

The objective of the National Dry Deposition Network is to determine patterns and trends of dry deposition for various sulfur and nitrogen species at roughly 50 locations throughout the continental USA. Each site is equipped for collection of continuous meteorological and ozone data and weekly average concentrations of SO4(2-), NO3-, SO2 and HNO3, using a three-stage filter pack. Results from 40 eastern US sites operational throughout 1989 show species-dependent variability from site to site, season to season, and day to night. Annual average concentrations of atmospheric SO4(2-), NO3-, SO2 and HNO3 ranged from 2.7 to 7.9, 0.2 to 3.9, 2.4 to 23.2 and 0.7 to 3.6 microg/m(-3), respectively. Seasonal variability was considerable for all constituents. Day/night data indicate that SO2 and HNO3, but not SO4(2-) and NO3-, are typically found at moderately to substantially lower concentrations at night, especially during spring and summer. Estimated dry deposition for SO2 and HNO3 appear to be much greater than for SO4(2-) and NO3-, respectively. Comparison of measured wet deposition and estimated dry deposition at numerous sites suggests that the two are similar in magnitude over much of the eastern USA.     

Wet and dry deposition of sulfur and nitrogen compounds in Ontario

Measurements have been made of sulfur and nitrogen compounds in precipitation since 1980 and in air since 1981 in Ontario. This paper presents results of the atmospheric deposition measurement program to the end of 1985. As is to be expected from the distribution of emission sources, annual concentrations of SO₄²⁻ andNO₃⁻ in precipitation, and of SO₂,SO₄²⁻ andNO₃⁻ in air are higher in southern Ontario than in northern Ontario. The corresponding distribution pattern for deposition is similar to that of concentration. A wet SO₄²⁻ deposition rate of 20 kg ha¹⁻ y¹⁻, a value considered critical for the acidification of sensitive water bodies, is exceeded in all of central and southern Ontario. On a province-wide basis, sulfur wet deposition is about four times higher than sulfur dry deposition. For nitrogen, wet and dry deposition are more comparable, though the former is still higher. The S- and N-species display different seasonal trends in concentration and deposition reflecting a dependence on meteorological factors, and on the associated chemical transformation rates. On the other hand, year to year variations are small.     

Input of chlorinated hydrocarbons through dry and wet deposition to the Western Mediterranean

Dissolved and particulate phases of rain samples were analysed over a three month period in a coastal station of the Mediterranean sea. Results show that dry deposition seems to be more important for the input of chlorinated compounds than wet deposition. Fluxes to the western Mediterranean sea are estimated through this experiment.     

珠江口大气氮磷干湿沉降通量及其污染特征

为准确核定入海污染负荷和系统实施入海污染物总量控制,观测了珠江口地区大气氮磷的干湿沉降通量,并分析了其时空变异规律、化学组成特征以及主要的影响因素。结果显示,珠江口地区大气总氮和总磷的平均月沉降通量分别约为299.00、4.12kg/km2,其中湿沉降通量占总沉降通量的60%以上。干沉降和湿沉降通量均表现出明显的时空变异规律,前者的季节性变化主要受农业活动氮磷挥发的影响,而后者与年内降雨量分布以及季风作用下降雨云团来源的关系更为密切;沉降负荷的空间分布规律基本与不同区域受人类活动影响的特点相一致。由于不同污染来源和季风气候的影响,氮沉降化学组成的季节差异特征也较为明显,氨氮在秋冬季沉降中所占的比例明显高于春夏季,而有机氮则与之相反。     

Atmospheric nitrogen deposition on the east coast of Spain: relevance of dry deposition in semi-arid Mediterranean regions

Bulk deposition composition and pine branch washing were measured from April 1999 to March 2000 on the east coast of Spain. The main objective was to characterise N deposition patterns with special emphasis on dry deposition. Bulk deposition in the region is dominated by neutralisation processes by Ca2+ and HCO3-, ClNa of marine origin and a high correlation between NO3- and SO4(2-). SO4(2-) concentrations show a decrease with respect to previous studies in the region in agreement with generalized sulfur emission decreases while the remaining ions, including NO3-, are higher due to their general increase as well as to the inclusion of dry deposition in bulk collectors in the present study. An enrichment in NO3- has been observed in dry deposition composition branch washing) with respect to bulk deposition, while an impoverishment has been observed in the case of NH4+. Annual bulk deposition varies between 7.22-3.1 and 3.5-1.8 Kg ha(-1) year(-1) for S- SO4(2-) and N- NO3-, respectively. N total deposition goes from 9.78 to 6.8 Kg ha(-1) year(-1) at most stations, with the lowest deposition at the control station and Alcoi. The relative dry deposition with respect to the total was over 40% at most stations, going up to 75% at the southern station. N-deposition is expected to be higher considering that N-NH4+ deposition has been underestimated in this study.     

Concentration and dry deposition of atmospheric sulfur and nitrogen compounds in Hungary

The level of regional air pollution is regularly monitored at three stations in Hungary. The comparison of regional concentrations of SO₂ and NO₂ to those measured in Budapest shows that urban level concentration of SO₂ is ten times higher than the value for background conditions. The corresponding figure for NO₂ is five. An increase eastwards across the country can be observed for NO₂ and SO₂, while particulate sulphate, nitrate and ammonium have practically identical concentrations. The concentration of gaseous ammonia has a summer maximum, while the annual variation of particulate ammonium suggests a winter maximum. The ratio of the level of nitric acid to aerosol nitrate is higher than unity in summer, while in winter it is less than 1. The dry deposition of sulfur and oxidized nitrogen compounds is comparable to their wet deposition. However, in the case of NH_x (x = 3 or 4) the wet deposition exceeds the dry deposition by an order of magnitude.