Estimates Of Dry Deposition Velocities Research Articles

Observation- and Model-Based Estimates of Particulate Dry Nitrogen Deposition to the Oceans.

Anthropogenic nitrogen (N) emissions to the atmosphere have increased significantly the deposition of nitrate (NO3-) and ammonium (NH4+) to the surface waters of the open ocean, with potential impacts on marine productivity and the global carbon cycle. Global-scale understanding of the impacts of N deposition to the oceans is reliant on our ability to produce and validate models of nitrogen emission, atmospheric chemistry, transport and deposition. In this work, ~2900 observations of aerosol NO3- and NH4+ concentrations, acquired from sampling aboard ships in the period 1995 - 2012, are used to assess the performance of modelled N concentration and deposition fields over the remote ocean. Three ocean regions (the eastern tropical North Atlantic, the northern Indian Ocean and northwest Pacific) were selected, in which the density and distribution of observational data were considered sufficient to provide effective comparison to model products. All of these study regions are affected by transport and deposition of mineral dust, which alters the deposition of N, due to uptake of nitrogen oxides (NOx) on mineral surfaces. Assessment of the impacts of atmospheric N deposition on the ocean requires atmospheric chemical transport models to report deposition fluxes, however these fluxes cannot be measured over the ocean. Modelling studies such as the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), which only report deposition flux are therefore very difficult to validate for dry deposition. Here the available observational data were averaged over a 5° × 5° grid and compared to ACCMIP dry deposition fluxes (ModDep) of oxidised N (NOy) and reduced N (NHx) and to the following parameters from the TM4-ECPL (TM4) model: ModDep for NOy, NHx and particulate NO3- and NH4+, and surface-level particulate NO3- and NH4+ concentrations. As a model ensemble, ACCMIP can be expected to be more robust than TM4, while TM4 gives access to speciated parameters (NO3- and NH4+) that are more relevant to the observed parameters and which are not available in ACCMIP. Dry deposition fluxes (CalDep) were calculated from the observed concentrations using estimates of dry deposition velocities. Model - observation ratios, weighted by grid-cell area and numbers of observations, (RA,n) were used to assess the performance of the models. Comparison in the three study regions suggests that TM4 over-estimates NO3- concentrations (RA,n = 1.4 - 2.9) and under-estimates NH4+ concentrations (RA,n = 0.5 - 0.7), with spatial distributions in the tropical Atlantic and northern Indian Ocean not being reproduced by the model. In the case of NH4+ in the Indian Ocean, this discrepancy was probably due to seasonal biases in the sampling. Similar patterns were observed in the various comparisons of CalDep to ModDep (RA,n = 0.6 - 2.6 for NO3-, 0.6 - 3.1 for NH4+). Values of RA,n for NHx CalDep - ModDep comparisons were approximately double the corresponding values for NH4+ CalDep - ModDep comparisons due to the significant fraction of gas-phase NH3 deposition incorporated in the TM4 and ACCMIP NHx model products. All of the comparisons suffered due to the scarcity of observational data and the large uncertainty in dry deposition velocities used to derive deposition fluxes from concentrations. These uncertainties have been a major limitation on estimates of the flux of material to the oceans for several decades. Recommendations are made for improvements in N deposition estimation through changes in observations, modelling and model - observation comparison procedures. Validation of modelled dry deposition requires effective comparisons to observable aerosol-phase species concentrations and this cannot be achieved if model products only report dry deposition flux over the ocean.

Applications of satellite remote sensing data for estimating dry deposition in eastern Texas

The sensitivity of dry deposition velocities and air quality model predictions to land use/land cover (LULC) is examined. Four LULC data sets are employed; three of the data sets are derived from recent satellite imagery and the fourth data set is the LULC database currently used in air quality models by the State of Texas. Effects of the land cover data on estimates of dry deposition velocities and modeled ozone concentrations are examined using the Comprehensive Air Quality Model with extensions (CAMx) during an 22 August 2000–6 September 2000 episode developed for the Houston/Galveston area. All land cover data sets showed good general agreement with a few significant differences in classification and spatial extent of certain land cover categories. Dry deposition decreased in urban areas with the new land cover data during peak hours on some days, leading to ozone concentrations that were approximately 2 ppb higher in the Houston area. Ozone concentrations were lower west of Houston due to increased stomatal uptake by increased agricultural land coverage in the new data sets. These results indicate that the satellite-based LULC data, specifically the data derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, can be used to provide periodic updates to land cover information used in CAMx and similar photochemical models.

Ammonia and hydrogen sulphide flux and dry deposition velocity estimates using vertical gradient method at a commercial beef cattle feedlot

Ammonia and hydrogen sulphide flux and dry deposition velocity were estimated using micrometeorological vertical gradient flux method at a commercial cattle feedyard of approximately 50,000 head of beef cattle and average 14.4 m²/head (150 ft²/head) stocking density. Ammonia-N and H2S-S loss had general diurnal patterns with the highest fluxes in daytime and lowest fluxes in nighttime that correlated to temperature changes and active evaporation process during daytime. The highest average deposition velocities also occurred during daytime with unstable atmospheric conditions and the lowest during nighttime with very stable conditions. There are exponential relationship between NH3-N flux and ambient temperature with R² = 0.57 for NH3 (NH3-N flux = – 1.46 + 7.96e0.077*Temperature) and R² = 0.22 for H2S-S (H2S-S flux = – 0.75 + 0.8e-0.013*Temperature).

A revised parameterization for gaseous dry deposition in air-quality models

Abstract. A parameterization scheme for calculating gaseous dry deposition velocities in air-quality models is revised based on recent study results on non-stomatal uptake of O3 and SO2 over 5 different vegetation types. Non-stomatal resistance, which includes in-canopy aerodynamic, soil and cuticle resistances, for SO2 and O3 is parameterized as a function of friction velocity, relative humidity, leaf area index, and canopy wetness. Non-stomatal resistance for other chemical species is scaled to those of SO2 and O3 based on their chemical and physical characteristics. Stomatal resistance is calculated using a two-big-leaf stomatal resistance sub-model for all gaseous species of interest. The improvements in the present model compared to its earlier version include a newly developed non-stomatal resistance formulation, a realistic treatment of cuticle and ground resistance in winter, and the handling of seasonally-dependent input parameters. Model evaluation shows that the revised parameterization can provide more realistic deposition velocities for both O3 and SO2, especially for wet canopies. Example model output shows that the parameterization provides reasonable estimates of dry deposition velocities for different gaseous species, land types and diurnal and seasonal variations. Maximum deposition velocities from model output are close to reported measurement values for different land types. The current parameterization can be easily adopted into different air-quality models that require inclusion of dry deposition processes.

Analysis of measurement techniques to determine dry deposition velocities of aerosol particles with diameters less than 100 nm

Modeled aerosol dry deposition velocities to natural surfaces have not been verified against experimental data because data is not available. However due to recent instrumentation developments measuring vertical number fluxes of aerosols with sizes down to 10 nm became possible. Additional instrument modifications which are discussed here allows measuring size dependent fluxes of 10– 100 nm aerosols and would fulfill the data gap. This study analyzes uncertainties in such measurements. Low particle count is identified as a major contributor to the measurement uncertainty, limiting the accurate estimation of size-resolved dry deposition velocities in remote environment to particles with diameters less than 50 nm using current instrumentation. Even in this size range high concentrations are required. In the case of two particle counters with different detection limits dry deposition velocity estimates can be also systematically biased, depending on the aerosol size distribution. A difference in detection limits of counters about 10 nm is narrow enough to avoid bias in size-dependent dry deposition velocities in most circumstances.

Comparison of dry deposition velocities for SO2, HNO3 and SO4 2? estimated with two inferential models

Dry deposition velocity estimates of SO, HNO3 and SO42− were computed for six locations in eastern North America using two different inferential models; a Big-Leaf model utilized by the U. S. National Dry Deposition Network (NDDN) and, a land-use based model (LUM) that has been used in the past to estimate the relative importance of dry versus wet deposition over selected Canadian regions. There were consistent differences between models that were related to the surface type, chemical species and time of year. Mean monthly dry deposition velocities based upon the 1990–91 time period were compared at two locations. The seasonal cycles in deposition velocity were similar between models, but there were considerable differences in the amplitude of the cycles. The LUM predicted about a 400% increase in S042- deposition velocity from the winter to the summer months, while there was a 50 to 100% increase in the NDDN model estimates, depending upon location. According to the LUM, HN03 deposition to crop land increased by about a factor of 6 from winter to summer, while the big leaf model predicted a 50% increase. Overall, there was better agreement for SO2. Averaged over 12 months, the differences in deposition velocity between models were smaller and generally within the range of uncertainty associated with inferential models. For all six sites, the mean percent difference between models in deposition velocity for SO2, HNO3 and SO42− were 13, 35 and 79, respectively. These differences highlight the effect of using different methods for estimating dry deposition and the importance of applying the same model when examining regional patterns in dry/total deposition rates.

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real‐time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor‐estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42− serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

Dry deposition velocity estimates of SO2 from models and measurements over a deciduous forest in winter

Measurements were made of concentrations and vertical flux densities (using the eddy correlation technique) for SO2 over a deciduous forest during March and April 1990. These were compared with estimates of dry deposition velocities, obtained from resistance analogue models that employ measured meteorological data as input. The models include: (1) the dry deposition module that forms part of a larger Eulerian air quality model, known as ADOM (Acid Deposition and Oxidant Model), (2) a modified version of ADOM and (3) a new parameterization for Rc. The observations yielded Vdvalues of about 0.5 cm s−1 for the daytime and 0.1 cm s−1 at night. ADOM's estimates were much larger, averaging about 3.0 cm s−1 during the daytime and about 2.0 cm s−1 at night. When the ADOM canopy resistance was increased by modifying its formulation, the modified dry deposition estimates were slightly larger during the day than at night, averaging about 0.5 cm s−1. The new parameterization for Rcyielded smaller Vdestimates with no diurnal variation. An attempt is made to relate the observed diurnal cycle of the dry deposition velocity to meteorological parameters.

Using measured variances to compute surface fluxes and dry deposition velocities: A comparison with measurements from three surface types

Abstract Fluxes of temperature, water vapour, O3, SO2 and CO2 were estimated from the measurement of their variances, taken over a wetland region in northern Ontario (Canada) during the summer of 1990 and over a deciduous forest when it was fully leafed during the summer of 1988 and when it was leafless during the winter of 1990. A set of flux‐variance relations was employed, including empirical forms of universal functions that could be adjusted with some constants. Results from the present study show that these constants needed to be adjusted with site‐specific data in order to achieve a closer agreement between estimated and observed fluxes. Best estimates were obtained for the fluxes of temperature and water vapour and it was found that the flux estimates of O3, SO2 and CO2 correlated better with water vapour than with temperature. For these trace gases the flux‐variance method yielded estimates of dry deposition velocities that were either comparable with or larger than those obtained from a resistan...

An investigation of the ADOM dry deposition module using summertime O3 measurements above a deciduous forest

The dry deposition module in the Acid Deposition and Oxidant Model (ADOM) has been isolated from the complete model to accept local meteorological measurements in order to provide local estimates of dry deposition velocities. These are compared with observed O3 data obtained during the summer of 1988 over a deciduous forest. The comparison shows that the model systematically overpredicts by about 70% the dry deposition velocity for two selected days and for the average diurnal variations, obtained from the entire observed time series for July and August of 1988. The canopy resistance, to transfer of O3, is the major contributor to the model's total resistance for O3. In addition to the possible errors due to the “Big Leaf” approximation, sensitivity studies show that key parameters for reducing the systematic bias in the model are the Leaf Area Index, as well as mesophyll, stomatal, ground and cuticle resistances. For dry deposition velocities of SO2, the model also shows sensitivity to these parameters as well as the canopy wetness. A brief discussion on some mathematical approximations in the model is provided.

Dry deposition of reactive nitrogen compounds: A review of leaf, canopy and non-foliar measurements

Abstract This review summarizes dry deposition data for several reactive nitrogen compounds (gases and particles) for both foliar and non-foliar sites of deposition, and differentiates these data with respect to the techniques by which they were obtained. Brief summaries of the atmospheric behavior of reactive nitrogen gases, the processes governing their deposition, and a discussion of the relative importance of dry deposition to total landscape N deposition are also included. Estimates of dry deposition velocities for these species vary significantly depending on the method used and vegetation studied. Leaf-level conductances to the principal reactive nitrogen gases are typically 4–20 times lower than canopy deposition velocity measurements, but this difference can be partially reconciled if canopy leaf area is accounted for when scaling from the leaf to canopy level. Field data inficate that dry deposition of N compounds to plant surfaces is comparable to wet deposition, contributing from 20 to 70% of total atmospheric nitrogen inputs.