Dry Deposition Model Research Articles

Surface Flux Modeling for Air Quality Applications

For many gasses and aerosols, dry deposition is an important sink of atmospheric mass. Dry deposition fluxes are also important sources of pollutants to terrestrial and aquatic ecosystems. The surface fluxes of some gases, such as ammonia, mercury, and certain volatile organic compounds, can be upward into the air as well as downward to the surface and therefore should be modeled as bi-directional fluxes. Model parameterizations of dry deposition in air quality models have been represented by simple electrical resistance analogs for almost 30 years. Uncertainties in surface flux modeling in global to mesoscale models are being slowly reduced as more field measurements provide constraints on parameterizations. However, at the same time, more chemical species are being added to surface flux models as air quality models are expanded to include more complex chemistry and are being applied to a wider array of environmental issues. Since surface flux measurements of many of these chemicals are still lacking, resistances are usually parameterized using simple scaling by water or lipid solubility and reactivity. Advances in recent years have included bi-directional flux algorithms that require a shift from pre-computation of deposition velocities to fully integrated surface flux calculations within air quality models. Improved modeling of the stomatal component of chemical surface fluxes has resulted from improved evapotranspiration modeling in land surface models and closer integration between meteorology and air quality models. Satellite-derived land use characterization and vegetation products and indices are improving model representation of spatial and temporal variations in surface flux processes. This review describes the current state of chemical dry deposition modeling, recent progress in bi-directional flux modeling, synergistic model development research with field measurements, and coupling with meteorological land surface models.

Measurement and modeling of concentrations of ambient air particles, chromium, copper and lead pollutants concentrations, as well as dry deposition in central Taiwan

The main purpose of this investigation was to monitor ambient air particles and metallic elements (Cr, Cu and Pb) in total suspended particulates (TSP) concentration, dry deposition. Furthermore, the calculated/measured dry deposition flux ratios of ambient air particles and metallic elements (Cr, Cu and Pb) were evaluated using three dry deposition models at five characteristic sampling sites for the 2009–2010. The results show that the average concentrations of metallic elements Cr, Cu and Pb in TSP were highest at Bei-shi (suburban/coastal) and the lowest at Gao-mei (wetland) among the five characteristic sampling sites during the years 2009-2010. This study also shows that the mean rates of dry deposition of metallic elements Cr and Cu were highest at Quan-xing (industrial) and lowest at Bei-shi (suburban/coastal) and Chang-hua (downtown). Finally, the Noll and Fang model was found to yield better predictions of the dry deposition of ambient air particles and metallic elements Cr and Cu than any of the other deposition models.

Measuring and modeling atmospheric particles and particle bound mercury Hg(p) at five characteristic-sampling sites

The main purpose of this study was to monitor ambient air particles and particulate-bound mercury Hg(p) in total suspended particulates (TSP) concentration, dry deposition at five characteristic-sampling sites. In addition, the calculated/measured dry deposition flux ratios of ambient air particles and particulate-bound mercury Hg(p) using three dry deposition models during the years of 2009–2010 were also studied. The results showed that the order of seasonal variations for mean particulate-bound mercury Hg(p) concentrations in TSP at Bei-shi (suburban/coastal), Chang-hua (downtown) and He-mei (residential) were all shown as spring > fall, winter > summer, respectively. However, the order of seasonal variations mean particulate-bound mercury Hg(p) concentrations in TSP that occurred at Gao-mei (wetland) were shown as spring > fall > winter > summer, respectively. Finally, the results of this study also indicated that using the Noll and Fang model exhibited batten prediction results for ambient air particles and particulate-bound mercury Hg(p) dry deposition when compared with the other depositions models at any of the characteristic-sampling sites for this study.

Component-based development and sensitivity analyses of an air pollutant dry deposition model

The Urban Forest Effects-Deposition model (UFORE-D) was developed with a component-based modeling approach. Functions of the model were separated into components that are responsible for user interface, data input/output, and core model functions. Taking advantage of the component-based approach, three UFORE-D applications were developed: a base application to estimate dry deposition at an hourly time step, and two sensitivity analyses based on Monte Carlo simulations with a Latin hypercube sampling (LHS-MC) and a Morris one-at-a-time (MOAT) sensitivity test. With the base application, dry deposition of CO, NO 2, O 3, PM10, and SO 2 in the city of Baltimore was estimated for 2005. The sensitivity applications were performed to examine UFORE-D model parameter sensitivity. In general, dry deposition velocity was sensitive to temperature and leaf area index (LAI). Temperature had a non-linear effect on all pollutants, while LAI was important to NO 2 deposition with a nearly linear effect. PAR and wind speed had limited effects on dry deposition of all pollutants; dry deposition was affected by PAR and wind speed only up to their threshold values. The component-based approach allows for seamless integration of new model elements, and provides model developers with a platform to easily interchange model components.

Dry deposition modelling in a Lagrangian dispersion model

Revisions to the dry deposition scheme in the Lagrangian dispersion model, NAME, are described. Improvements are made for situations where material is not well mixed within the boundary layer and for sedimenting heavy particles. The revised scheme overcomes problems of computational noise, common in Lagrangian dispersion models, with a manageable or no increase to the computational cost. The result is a scheme that is well suited to all situations and that has better interaction between the advection, deposition and sedimentation.

Bi‐directional air‐surface exchange of atmospheric ammonia: A review of measurements and a development of a big‐leaf model for applications in regional‐scale air‐quality models

The air‐surface exchange of atmospheric ammonia (NH3) and measurements of the canopy and stomatal compensation points (χcp and χst, respectively) and the stomatal and soil emission potentials (Γst and Γg, respectively) are reviewed. A database of these values has been developed, to be used for the development of input parameters and for model evaluation. The compensation points are dependent on canopy type, nitrogen (N) status, temperature, growth stage, and meteorological conditions. Canopies that receive high atmospheric nitrogen input generally have high χcp values. χcp values also tend to be higher over intensively managed vegetated surfaces than semi‐natural vegetation, due to the higher nitrogen content in these surfaces. Increased nitrogen concentrations from fertilization and cutting practices have been observed to increase the compensation points and therefore the emission from these canopies. The decomposition of litter leaves has been found to play a dominant role and significantly increase the values of χcp over agricultural vegetation and fertilized grasslands. By modifying an existing big‐leaf dry deposition model to allow NH3 emission from leaf stomata and soil surfaces, a bi‐directional air‐surface exchange model has been developed for applications in regional‐scale air‐quality models. The model predicts χcp values that vary with canopy type, nitrogen content, and meteorological conditions. χcp values predicted by the model are around 1–2 μg m−3 over forest canopies and 4–10 μg m−3 over grasslands and agricultural canopies during a typical summer daytime; χcp values are ∼10 and ∼3 times lower over forests and agricultural lands, respectively, in nighttime and/or winter conditions. These results are similar to the range of measured values. The new bi‐directional air‐surface exchange model will reduce the dry deposition fluxes by 20–100 ng m−2 s−1 compared to the original dry deposition model over low‐N forests and agricultural lands during typical summer daytime conditions, which can be the difference between whether deposition or emission occurs. For example, this new model produces emissions fluxes of 0–90 ng m−2 s−1 over croplands when the atmospheric NH3 concentrations are below 10 μg m−3.

Measure and modeling the ambient air particles and particle bound mercury Hg(p) at a traffic sampling site

Ambient air samples were collected between Dec 9, 2008 and Jan 14, 2009 at a highway traffic site located in Sha-Lu, central Taiwan. Atmospheric particulates and particulate bound mercury Hg(p) dry deposition fluxes, concentrations and dry deposition velocities were also studied. The objective of this study was to: 1) measure the dry deposition concentrations of ambient air particles and particle bound mercury Hg(p) following one and two days of sampling events using the total suspended particulates (TSP) sampler, and dry deposition plate (DDP); 2) modeling the dry deposition fluxes of particles and particle bound mercury Hg(p) with calculated deposition velocities then to obtain a calculated dry deposition fluxes; and 3) compared the results of calculated dry deposition fluxes to measured dry deposition fluxes. All the results were presented with one and two days of sampling events with different deposition collection media (i.e., quartz filter and overhead projection film). The results indicated that the average calculated/measured dry deposition fluxes ratios for bound mercury (Hg(p)) for 24 sampling group were 2.27. The results also indicated that the one day calculated/measured dry deposition flux ratios for Hg(p) for overhead projection film and quartz filter were 2.62 and 1.92, respectively. In addition, the results indicated that the average calculated/measured dry deposition fluxes ratios for 24 sampling group were 4.03. The results also indicated that the two days of calculated/measured dry deposition flux ratios for Hg(p) for overhead projection film and quartz filter were 4.64 and 3.42, respectively. The results of particle bound mercury Hg(p) calculated dry deposition model are slightly higher than that of measured dry deposition flux in either one day or two days of sampling period for both of overhead projection and quartz filter collection medium. .Though there are no significant differences of mean values for both particulates and Hg(p) using overhead projection film or quartz filter as a collection media for the dry deposition-study (Fang et al., 2009), quartz filter exhibits better calculated/measured ratio in the prediction of particle bound mercury (Hg(p)) than that of overhead projection film for either one day of the two days of sampling periods.

Influence of intensive agriculture on dry deposition of aerosol nutrients

A procedure is presented for quantification of sources contributing to atmospheric aerosol chemical nutrient concentrations and dry deposition fluxes. Source apportionment using principal component analysis (PCA) and multiple linear regression analysis (MLRA) was followed by application of a size-segregated particle dry deposition model. In a rural region of southeast Brazil, biomass burning, products of secondary reactions, and soil dust re-suspension explained 43%, 31% and 21% of PM2.5 mass, respectively. Re-suspension and biomass burning contributed 22% and 19%, respectively, to PM10 mass, and re-suspension accounted for approximately half of the mass of coarse particles. At least 40% of NO3--N, 20% of phosphorus and 55% of potassium deposited originated from agriculture-related emissions. Deposition to tropical forest is currently higher than the minimum under natural conditions by factors of 12.2 (N), 6.2 (P) and 2.6 (K).

Dry deposition velocity of atmospheric nitrogen in a typical red soil agro-ecosystem in Southeastern China

Atmospheric dry deposition is an important nitrogen (N) input to farmland ecosystems. The main nitrogen compounds in the atmosphere include gaseous N (NH3, NO2, HNO3) and aerosol N (NH4+/NO3-). With the knowledge of increasing agricultural effects by dry deposition of nitrogen, researchers have paid great attention to this topic. Based on the big-leaf resistance dry deposition model, dry N deposition velocities (Vd) in a typical red soil agro-ecosystem, Yingtan, Jiangxi, Southeastern China, were estimated with the data from an Auto-Meteorological Experiment Station during 2004-2007. The results show that hourly deposition velocities (Vdh) were in the range of 0.17-0.34, 0.05-0.24, 0.57-1.27, and 0.05-0.41 cm/s for NH3, NO2, HNO3, and aerosol N, respectively, and the Vdh were much higher in daytime than in nighttime and had a peak value around noon. Monthly dry deposition velocities (Vdm) were in the range of 0.14-0.36, 0.06-0.18, and 0.07-0.25 cm/s for NH3, NO2, and aerosol N, respectively. Their minimum values appeared from June to August, while their maximum values occurred from February to March each year. The maximum value for HNO3 deposition velocities appeared in July each year, and Vdm(HNO3) ranged from 0.58 to 1.31 cm/s during the 4 years. As for seasonal deposition velocities (Vds), Vds(NH3), Vds(NO2), and Vds(aerosol N) in winter or spring were significantly higher than those in summer or autumn, while Vds(HNO3) in summer were higher than that in winter. In addition, there is no significant difference among all the annual means for deposition velocities (Vda). The average values for NH3, NO2, HNO3, and aerosol N deposition velocities in the 4 years were 0.26, 0.12, 0.81, and 0.16 cm/s, respectively. The model is convenient and feasible to estimate dry deposition velocity of atmospheric nitrogen in the typical red soil agro-ecosystem.

Partitioning of ozone deposition over a developed maize crop between stomatal and non-stomatal uptakes, using eddy-covariance flux measurements and modelling

Ozone (O 3) flux measurements, obtained by eddy-covariance technique, over a developed maize crop were used to partition the overall O 3 deposition between stomatal and non-stomatal uptakes. Data were analysed using a big-leaf model, which was developed from current knowledge on O 3 deposition. The classical parameters used in dry deposition models (i.e. the in-canopy aerodynamic resistance R ac , the intrinsic ground resistance R ig and the cuticular resistance R cut ) were determined for the maize crop from the relationship between the experimental non-stomatal conductance ( g ns ) and the friction velocity ( u *) in dry conditions (relative humidity ( RH) < 60%). g ns was determined as the difference between the O 3 canopy conductance ( g c ) and the O 3 stomatal conductance ( g s ), where g s was estimated by a method which combines the Penman–Monteith approach and the use of the CO 2 assimilation flux. Data analysis revealed that chemical reactions between O 3 and nitrogen monoxide (NO) between the canopy top and the O 3 flux measurement level ( z m ) could induce high values of the observed O 3 conductance, not representative of ozone deposition to the canopy. The actual O 3 canopy conductance was derived from the observed O 3 conductance by including a correction term function of z m and the NO concentration at this height, based on the previous studies on O 3 destruction above canopies. The estimations of R ac , R ig and R cut given by the non-linear regression of g ns vs u * are in agreement with previously published results. Our analysis also confirms previous studies which have shown that the cuticular conductance ( g cut ) increases exponentially with RH, and we propose a new parameterization of g cut as a function of RH, based on experimental evidence. Using our model to partition the total O 3 deposition to the canopy, we showed that the relative contributions of stomatal and non-stomatal uptakes varied strongly with the physiological activity of the maize and the meteorological conditions. This point is of major importance for studies dedicated to the impact of ozone on plant physiology, since it emphasizes the necessity to determine accurately the amount of O 3 actually absorbed by the plants via their stomatal activity.

Effect of the soil wetness state on the stomatal ozone fluxes over Hungary

A coupled Eulerian photochemical reaction-transport model and a detailed ozone dry deposition model have been utilised for the estimation of stomatal ozone fluxes over Hungary. Ozone concentrations were modelled on an unstructured triangular grid using a method of lines approach to the solution of the reaction-diffusion-advection equations describing ozone formation, transport and deposition. The model domain covers Central-Europe including Hungary, which was located at the centre of the domain and covered by a high resolution nested grid. The dry deposition velocity of ozone was calculated based on the aerodynamic, quasi-laminar boundary layer and canopy resistance. The effect of soil water content on the stomatal ozone flux was analysed. The stomatal ozone flux calculations were performed for two cases, with and without taking into account the effect of the soil moisture stress on the ozone deposition. The meteorological data were generated by the ALADIN meso-scale limited area numerical weather prediction model. It was found that soil water deficiency can strongly reduce the stomatal conductance and hence the ozone flux through it.

An extended dry deposition model for aerosols onto broadleaf canopies

This paper presents a model to predict transport and dry deposition of aerosols in and to a canopy composed of plane leaves such as a grassland, a crop field or a broadleaf forest under conditions of neutral stability. This model is based on an approach detailed by Petroff et al. [2008b. Aerosol dry deposition on vegetative canopies. Part II: A new modeling approach and applications. Atmospheric Environment, 42, 3654–3683]. Collection parameterizations specific to plane obstacles are derived here for the processes of Brownian diffusion, interception, inertial and turbulent impactions, and gravitational settling. For the process of interception, an original formulation is derived based on the potential flow theory applied to inclined plane surfaces. The validity of the model predictions is evaluated using existing experiments both on the scale of isolated obstacles such as inclined plates and broadleaf twigs, and on the scale of entire canopies of grass, moorland and forest. Under these different configurations, the model agrees reasonably with measurements. Moreover, this comparison emphasizes the strong influence of canopy and aerosol properties on deposition and explains, at least partly, the variability of existing measurement data sets obtained in similar aerodynamic conditions.

Effects of Strong Wind and Ozone on Localized Tree Decline in the Tanzawa Mountains of Japan

The numerical simulation of wind and ozone (<TEX>$O_3$</TEX>) transport in mountainous regions was performed with a computational fluid dynamics technique. A dry deposition model for <TEX>$O_3$</TEX> was designed to estimate <TEX>$O_3$</TEX> deposition in complex terrain, and the qualitative validity of the predicted <TEX>$O_3$</TEX> concentration field was confirmed by comparison with observed data collected with passive samplers. The simulation revealed that wind velocity increases around ridge lines and peaks of mountains. The areas with strong wind corresponded well with the sites of tree decline at high altitudes, suggesting that it is an important factor in the localization of tree/forest decline. On the other hand, there is no direct relationship between forest decline and <TEX>$O_3$</TEX> concentration. The <TEX>$O_3$</TEX> concentration, however, tends to increase as wind velocity becomes higher, thus the <TEX>$O_3$</TEX> concentration itself may be a potential secondary factor in the localized decline phenomena. While the diffusion flux of <TEX>$O_3$</TEX> is not related to localized tree decline, the pattern of advection flux is related to those of high wind velocity and localized tree decline. These results suggest that strong wind with large advection flux of <TEX>$O_3$</TEX> may play a key role in the promotion of tree/forest decline at high mountain ridges and peaks.

Regional spore dispersal as a factor in disease risk warnings for potato late blight: A proof of concept

This study develops and tests a novel approach for including regional risk factors in operational disease risk warnings against potato late blight. The central premise is that fungicide inputs can be reduced by omitting applications on days when conditions are unsuitable for the atmospheric transport of viable sporangia. The decision support system first decides whether a specific day is ‘high risk’ (suitable for disease development in planta). Simulation studies revealed that on such high risk days, the capacity of the atmosphere to transport sporangia viably over relevant distances varies widely. An additional rule assesses this capacity, which is high when weather conditions allow a large number of spores to be released from the canopy and transported viably over long distances. When this capacity is high the original spray advice is followed, and when it is low a no-spray advice is given. The concept is implemented using the published decision support system SIMCAST complemented with models for spore release from sporangiophores, spore escape from the canopy, a newly developed model for spore dispersal and dry deposition of spores, spore survival during transportation, and weather forecast data from the mesoscale meteorological model MM5. Cultivar resistance was also incorporated into spray advice. The concept was tested in a field experiment in 2007 with three cultivars, representing a range in resistance to potato late blight from susceptible to highly resistant, and compared to a ‘stand-alone’ version of SIMCAST. In a period with normal ‘infection pressure’ (risk of disease) one third of the spray recommendations made by SIMCAST alone were modified and negated by the new system for the highly resistant cultivar. These savings came on top of a reduced, resistance-level dependent dose rate of Shirlan (a.i. Fluazinam). The results demonstrate the feasibility of including dispersal modeling and forecasted meteorology in disease warnings against Phytophthora infestans, even if the whereabouts of sources is unknown. The principles can be used in many decision contexts, but further work is needed to test and refine the method before it can be used in practice.

Sensitivity analysis of an ozone deposition model

In this study, sophisticated sensitivity analyses of a detailed ozone dry deposition model were performed for five soil types (sand, sandy loam, loam, clay loam, clay) and four land use categories (agricultural land, grass, coniferous and deciduous forests). Deposition velocity and ozone flux depend on the weather situation, physiological state of the plants and numerous surface-, vegetation-, and soil-dependent parameters. The input data and the parameters of deposition-related calculations all have higher or lower spatial and temporal variability. We have investigated the effect of the variability of the meteorological data (cloudiness, relative humidity and air temperature), plant-dependent (leaf area index and maximum stomatal conductance) and soil-dependent (soil moisture) parameters on ozone deposition velocity. To evaluate this effect, two global methods, the Morris method and the Monte Carlo analysis with Latin hypercube sampling were applied. Additionally, local sensitivity analyses were performed to estimate the contribution of non-stomatal resistances to deposition velocity. Using the Monte Carlo simulations, the ensemble effect of several nonlinear processes can be recognised and described. Based on the results of the Morris method, the individual effects on deposition velocity are found to be significant in the case of soil moisture and maximum stomatal conductance. Temperature and leaf area index are also important factors; the former is primarily in the case of agricultural land, while the latter is for grass and coniferous forest. The results of local sensitivity analyses reveal the importance of non-stomatal resistances.

A new model of bi-directional ammonia exchange between the atmosphere and biosphere: Ammonia stomatal compensation point

A new multi-layer canopy resistance model of bi-directional NH 3 exchange is presented. This new model, which is based on the Multi-Layer BioChemical deposition (MLBC) model [Wu, Y., Brashers, B., Finkelstein, P.L., Pleim, J.E., 2003a. A multiplayer biochemical dry deposition model. I. Model formulation. J. Geophys. Res. 108, D1; Wu, Y., Brashers, B., Finkelstein, P.L., Pleim, J.E., 2003b. A multiplayer biochemical dry deposition model. II. Model evaluation. J. Geophys. Res. 108, D1], incorporates a parameterization for the ammonia stomatal compensation point that is theoretically derived to consider the effects of leaf temperature and apoplastic concentrations of NH 4 + and H +. The new ammonia stomatal compensation point scheme accounts for the effects of deposition, emission and leaf temperature on the dynamics of apoplast [NH 4 +] and [H +]. The new model is evaluated against bidirectional NH 3 fluxes measured over fertilized soybean. The general patterns of observed deposition and emission are successfully reproduced when the ammonia stomatal compensation point is included. Driven by the effects of deposition, emission and leaf temperature, modeled apoplastic [NH 4 +] and [H +] display significant diurnal variation when the buffer effect of the underlying metabolic processes generating or consuming NH 4 + were ignored. Model predictive capability is improved slightly by incorporating the feedback into a dynamic stomatal compensation point. A simple implementation of the feedback mechanism in the current model provides opportunities for improvement. While the stomatal flux is shown to be an important process in the regulation of canopy-scale fluxes, it appears that exchange with leaf surface water and soil may also be important.

Mercury in the air, water and biota at the Great Salt Lake (Utah, USA)

The Great Salt Lake, Utah (USA), is the fourth largest terminal lake on Earth and a stop-over location for 35 million birds on the Pacific Flyway. Recently, the Utah Department of Health and Utah Division of Wildlife Resources issued tissue mercury (Hg) consumption advisories for several species of birds that consume the lake's brine shrimp. We hypothesized that the chemistry of the atmosphere above the Great Salt Lake would facilitate atmospheric deposition of Hg to the water. Because little information was available on Hg at the Great Salt Lake, and to begin to test this hypothesis, we measured atmospheric elemental (Hg 0) and reactive gaseous mercury (RGM) concentrations as well as Hg concentrations in water and brine shrimp five times over a ~year. Surrogate surfaces and a dry deposition model were applied to estimate the amount of Hg that could be input to the lake surface, and HYSPLIT model back trajectories were developed to investigate potential sources of RGM to the lake. Atmospheric Hg 0 concentrations were similar to global ambient background values and RGM concentrations were similar to those reported for rural areas. Both Hg 0 and RGM exhibited regular diel variability. Model estimated deposition velocities for RGM to the lake ranged from 0.9 to 3.0 cm s − 1 while that determined for surrogate surfaces ranged from 2.8 to 7.8 cm s − 1 . Filtered total and methyl Hg concentrations in Great Salt Lake surface waters were consistent throughout the year (3.6 ± 0.8 ng L − 1 and 0.93 ± 0.59 ng L − 1 , respectively), while brine shrimp concentrations had a statistically significant increase from summer to fall. Data collected and data analyses indicated no direct local or regional source of Hg to the lake and that factors within the Great Salt Lake basin are important in controlling Hg 0 and RGM concentrations.

Quantifying air pollution removal by green roofs in Chicago

The level of air pollution removal by green roofs in Chicago was quantified using a dry deposition model. The result showed that a total of 1675 kg of air pollutants was removed by 19.8 ha of green roofs in one year with O 3 accounting for 52% of the total, NO 2 (27%), PM 10 (14%), and SO 2 (7%). The highest level of air pollution removal occurred in May and the lowest in February. The annual removal per hectare of green roof was 85 kg ha −1 yr −1. The amount of pollutants removed would increase to 2046.89 metric tons if all rooftops in Chicago were covered with intensive green roofs. Although costly, the installation of green roofs could be justified in the long run if the environmental benefits were considered. The green roof can be used to supplement the use of urban trees in air pollution control, especially in situations where land and public funds are not readily available.

A size‐resolved model and a four‐mode parameterization of dry deposition of atmospheric aerosols

To deposit on the ground surface, an airborne aerosol particle needs to pass through an aerodynamic resistance layer and a quasi‐laminar sublayer just adjacent to the surface. Traditionally, it is believed that the main mechanisms through which a particle overcomes the resistance from the quasi‐laminar sublayer are Brownian diffusion, interception, inertial impaction, and gravitational settling. In this study, a size‐resolved dry deposition model is formulated, which includes a new proposed dry deposition mechanism, the burst effect of eddy turbulence. The effects on dry deposition of particles by Brownian diffusion and inertial impaction are parameterized through the Schmidt number and the Stokes number, respectively. On the basis of the similarity between deposition of particles on walls of pipes and deposition of atmospheric particles on the ground surface, the burst effect of eddy turbulence is parameterized with the roughness Reynolds number. Through the roughness Reynolds number the effect on dry deposition by the surface roughness is parameterized into the model. Since the dry deposition velocity depends on particle size strongly, a four‐mode parameterization of dry deposition is proposed for use in atmospheric dispersion models. For each aerosol mode the parameterization of the bulk dry deposition velocity is made up of two terms: the Reynolds term and another term that is a power law function of friction velocity. The dry deposition velocities predicted by the model and the four‐mode parameterization are compared with the measurements reported in the literature, and good agreements are achieved.

Measurements and estimates of leaf wetness over agricultural grassland for dry deposition modeling of trace gases

Leaf wetness is an important and frequent phenomenon for the surface–atmosphere exchange of some atmospheric trace gases that are well soluble in water, such as ammonia (NH 3 and SO 2), as well as for plant disease epidemiology. This study shows a comparison of two different techniques to measure leaf wetness; namely a painted flat-plate grid sensor and a system of four clip sensors. Although both techniques gave comparable results, the flat-plate grid sensor was favored, because of its stable signal and its ease of use. In this technique, the measurement height turned out to be of great importance for the leaf wetness duration (LWD); the flat-plate sensor at 1.0 m systematically underestimated LWD, while the flat-plate sensor at 0.1 m better represented the actual LWD. To obtain a representative signal, leaf wetness should be measured close to the surface. Using the available leaf wetness measurements, a comparison was made between three physical and four empirical leaf wetness models. Without any optimization, the physical model that calculates the potential condensation at the leaf surface gave the best results. However, after optimizing the humidity thresholds in the empirical leaf wetness models, the optimized model based on the difference between the actual and saturated specific humidity at the surface gave best results. For practical applications in atmospheric transport models, like for the calculation of dry deposition of well-soluble gases, the relative humidity (RH) threshold model might be easiest to implement. This study showed that different thresholds should be used for different vegetation types. In this study, an optimized RH threshold of 71% was derived for agricultural grassland.