Re-thinking the Dry Deposition Multiple Resistance Framework

Abstract. The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total (wet + dry) N deposition has been extensively determined in temperate regions, only very few data sets of N wet deposition exist for tropical ecosystems, and moreover, reliable experimental information about N dry deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002) and were a part of the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke, Aerosols, Clouds, Rainfall, and Climate) 2002 campaign. Ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), nitrogen dioxide (NO2), nitric oxide (NO), ozone (O3), aerosol ammonium (NH4+) and aerosol nitrate (NO3-) were measured in real-time, accompanied by simultaneous meteorological measurements. Dry deposition fluxes of NO2 and HNO3 are inferred using the ''big leaf multiple resistance approach'' and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH3 and HONO are estimated by applying a ''canopy compensation point model''. N dry and wet deposition is dominated by NH3 and NH4+, which is largely the consequence of biomass burning during the dry season. The grass surface appeared to have a strong potential for daytime NH3 emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH3 emissions from cattle excreta. NO2 also significantly accounted for N dry deposition, whereas HNO3, HONO and N-containing aerosol species were only minor contributors. Ignoring NH3 emission from the vegetation surface, the annual net N deposition rate is estimated to be about −11 kgN ha-1 yr-1. If on the other hand, surface-atmosphere exchange of NH3 is considered to be bi-directional, the annual net N budget at the pasture site is estimated to range from −2.15 to −4.25 kgN ha-1 yr-1.

Estimates of dry and wet deposition of nitrogen and sulphur compounds in the Czech Republic for the years 1994 and 1998 are presented. Deposition has been estimated from monitored and modeled concentrations in the atmosphere and in precipitation, where the most important acidifying compounds are sulphur diOxide, nitrogen Oxides, ammonia, and their reaction products. Measured atmospheric concentrations of SO2, NOx, NH3, and aerosol particles (SO4, NO3, and NH4), along with measured concentrations of SO4, NO3, and NH4 in precipitation, weighted by precipitation amounts, were interpolated with Kriging technique on a 10- x 10-km grid covering the whole Czech Republic. Wet deposition was derived from concentration values for SO4, NO3, and NH4 in precipitation and from precipitation amounts. Dry deposition was derived from concentrations of gaseous components and aerosol in the air, and from their deposition velocities. A multiple resistance model was used for calculation of SO2, NOx, and NH3 deposition velocities. Deposition velocities of particles were parameterized. It was estimated that the annual average deposition of SOx in the Czech Republic decreased from 1384 to 1027 mol H ha a between 1994 and 1998. The annual average NOy deposition was estimated to be 972 and 919 mol H a in 1994 and 1998, respectively. The annual average NHx deposition was estimated to be 887 mol H a and 779 mol H a in 1994 and 1998, respectively. It was estimated that the annual average of the total potential acid deposition decreased from 3243 to 2725 mol H a between 1994 and 1998. Sulphur compounds (SOx) contributed about 38%, Oxidized nitrogen species (NOy) 34%, and reduced nitrogen species (NHx) 28% to the total potential acid deposition in 1998. The wet deposition contributed 42% to the total potential acid deposition in 1998.

On the dry deposition of ozone and reactive nitrogen species

The United Kingdom budgets of oxidised and reduced nitrogen (N) species, major components of acidic deposition, are analysed with a long-range transport model. In order to describe adequately the surface sources of trace species, a simple description of vertical diffusion has been incorporated, along with a multiple resistance dry deposition velocity scheme. Also, a fully diurnal chemical scheme is utilised which speciates oxidised N species into nitrate and nitrite aerosols, nitrous and nitric acids, and peroxyacetyl nitrate. The model was validated against measurements of various species and was found to perform well for oxidised N species, although concentrations and subsequent dry deposition of peroxyacetyl nitrate is over-estimated. However, the over-estimation of peroxyacetyl nitrate affects the partitioning of dry deposited oxidised N species only. Dry deposition of reduced N was simulated well, according to other estimates, but wet deposition of reduced N was under-estimated by a factor of approximately 2. It is suggested that the cause of this is that the UK emissions inventory of NH3 is too small. In attribution studies, it was found that modelled results differed significantly from those of EMEP. The modelled contribution of non-UK sources of nitrogen oxides was approximately 50% which may be compared with the EMEP estimation for the same year's emissions of 37%. The model showed a non-linear response to changes in emissions which is explicable and in accord with current understanding of atmospheric processes. Minor sources of nitrogen oxides (lightning, aircraft and soils), not usually considered in long-range transport modelling were investigated: aircraft and lightning contribute trivial amounts of deposited N to the UK, but soil sources resulted in an additional 5% oxidised N deposition.

Because there is no simple device capable of measuring the dry deposition rates of small particles and trace gases directly, much current activity is focused on the use of an inferential technique. In this method, measurements of atmospheric concentration (C) of selected chemical species are coupled with evaluations of appropriate deposition velocity (V d ) to yield estimates of dry deposition rate from their product. Difficulties arise concerning the ability to measure C, and especially regarding the poor knowledge of V d for many species. A multiple resistance routine for deriving deposition velocities is presented here. Current knowledge of biological processes is incorporated into a first-generation lsbig leaf’ model; formulations of resistances appropriate for describing individual leaves are combined to simulate the canopy as a whole. The canopy resistance is combined with estimates of aerodynamic and boundary-layer resistances to approximate the total resistance to transfer, from which deposition velocity is then computed. Special emphasis is given to the influence of the diurnal cycle, to the way in which the various transfer resistances can be inferred from routine data, and to the role of canopy factors (e.g., leaf area index, wetness, temperature response, and sunshade fractions).

Results from numerical investigations regarding the dry deposition of reactive trace gases like NO, NO2, and Og are presented. The investigations were carried out with a numerical model of the atmospheric boundary layer, which simulates the meteorological and photochemical processes as well as the heat and moisture transport processes within the vegetation-soil system as a function of height (depth) and time. The model is briefly described here. The model results show that the dry deposition fluxes of reactive trace gases are not only influenced by meteorological and plant-physiological parameters, but also by chemical reactions. In most cases, the trace gas fluxes vary strongly with height and often even show a change in the direction. The fluxes differ considerably from those obtained with the widely used 'big leaf multiple resistance approach. Hence, the constant flux approximation, on which this resistance approach is based, seems to be inappropriate for determining dry deposition fluxes and deposition velocities of reactive trace gases.

This study presents >5 cumulative years of tropospheric mercury (Hg) speciation measurements, over the period of 2003–2009, for eight sites in the central and eastern United States and one site in coastal Puerto Rico. The purpose of this research was to identify local and regional processes that impact Hg speciation and deposition (wet + dry) across a large swath of North America. Sites sampled were selected to represent both a wide range of mercury exposure and environmental conditions. Seasonal mean concentrations of elemental Hg (1.27 ± 0.31 to 2.94 ± 1.57 ng m−3; ± σ), reactive gaseous mercury (RGM; 1.5 ± 1.6 to 63.3 ± 529 pg m−3), and fine particulate Hg (1.2 ± 1.4 to 37.9 ± 492 pg m−3) were greatest at sites impacted by Hg point sources. Diel bin plots of Hgo and RGM suggest control by a variety of local/regional processes including impacts from Hg point sources and boundary layer/free tropospheric interactions as well as from larger‐scale processes affecting Hg speciation (i.e., input of the global Hg pool, RGM formed from oxidation of Hgo by photochemical compounds at coastal sites, and elemental Hg depletion during periods of dew formation). Comparison of wet Hg deposition (measured), RGM and fine particulate Hg dry deposition (calculated using a multiple resistance model), and anthropogenic point source emissions varied significantly between sites. Significant correlation between emission sources and dry deposition was observed but was highly dependant upon inclusion of data from two sites with exceptionally high deposition. Findings from this study highlight the importance of environmental setting on atmospheric Hg cycling and deposition rates.

A soil/vegetation/atmosphere transfer (SVAT) scheme for determining the dry deposition and/or emission fluxes of NO, NO2, and O3 in the atmospheric surface layer over horizontally uniform terrain covered with fibrous canopy elements is presented and discussed. This transfer scheme is based on the micrometeorological ideas of the transfer of momentum, heat and matter near the Earth's surface, where chemical reactions between these trace gases are included. The fluxes are parameterized by first-order closure principles. The uptake processes by vegetation and soil are described in accord with Deardorff (1978). The SVAT scheme requires only routine data of wind speed, dry- and wet-bulb temperatures, short wave and long wave radiation, and the concentrations of O3 and nitrogen species provided by stations of monitoring networks. First model results indicate that the dry deposition fluxes of NO, NO2, and O3 are not only influenced by meteorological and plant-physiological parameters, but also by chemical reactions between these trace species and by NO emission from the soil. Furthermore, a small displacement in the concentrations of NO, NO2, and O3 within in the range of the detection limits of the chemical sensors can produce large discrepancies in the flux estimates, which are manifested here by the shift from height-invariant fluxes substantiated by the photostationary state to strongly height-dependent fluxes caused by the departure from that state. Especially in the case of these nitrogen species the widely used ‘big leaf’ multiple resistance approach, which is based on the constant flux approximation seems to be inappropriate for computing dry deposition fluxes and deposition velocities.