Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 2 5 2009 10.5194/amtd-2-2423-2009 http://www.atmos-meas-tech-discuss.net/2/2423/2009/ http://www.atmos-meas-tech-discuss.net/2/2423/2009/amtd-2-2423-2009.html http://www.atmos-meas-tech-discuss.net/2/2423/2009/amtd-2-2423-2009.pdf 2423 2482 2009-10-08 Aerodynamic gradient measurements of the NH3-HNO3-NH4NO3 triad using a wet chemical instrument: an analysis of precision requirements and flux errors V. Wolff veronika.wolff@mpic.de I. Trebs C. Ammann F. X. Meixner Max Planck Institute for Chemistry, Biogeochemistry and Air Chemistry Department, P.O. Box 3060, 55020 Mainz, Germany Agroscope ART, Air Pollution and Climate Group, 8046 Zürich, Switzerland Department of Physics, University of Zimbabwe, P.O. Box MP 167, Harare, Zimbabwe The aerodynamic gradient method is widely used for flux measurements of ammonia, nitric acid, particulate ammonium nitrate (the NH3-HNO3-NH4NO3 triad) and other water-soluble reactive trace compounds. The surface exchange flux is derived from a measured concentration difference and micrometeorological quantities (turbulent exchange coefficient). The significance of the measured concentration difference is crucial for the significant determination of surface exchange fluxes. Additionally, measurements of surface exchange fluxes of ammonia, nitric acid and ammonium nitrate are often strongly affected by phase changes between gaseous and particulate compounds of the triad, which make measurements of the four individual species (NH3, HNO3, NH4+, NO3–) necessary for a correct interpretation of the measured concentration differences. We present here a rigorous analysis of results obtained with a multi-component, wet-chemical instrument, able to simultaneously measure gradients of both gaseous and particulate trace substances. Basis for our analysis are two field experiments, conducted above contrasting ecosystems (grassland, forest). Precision requirements of the instrument as well as errors of concentration differences and micrometeorological exchange parameters have been estimated, which, in turn, allows the establishment of thorough error estimates of the derived fluxes of NH3, HNO3, NH4+, and NO3–. Derived median flux errors for the grassland and forest field experiments were: 39 and 50% (NH3), 31 and 38% (HNO3), 62 and 57% (NH4+), and 47 and 68% (NO3–), respectively. Additionally, we provide the basis for using field data to characterize the instrument performance, as well as subsequent quantification of surface exchange fluxes and underlying mechanistic processes under realistic ambient measurement conditions. Alsheimer,~M.: Charakterisierung räumlicher und zeitlicher Heterogenitäten der Transpiration unterschiedlicher montaner Fichtenbestände durch Xylemflussmessungen, Bayreuther Forum Ökologie, 1–143, 1997. Ammann,~C.: On the Applicability of Relaxed Eddy Accumulation and Common Methods for Measuring for Measuring Trace Gas Fluxes, PhD, Geographisches Institut, ETH, Zürich, 229 pp., 1998. Ammann,~C., Flechard,~C R., Leifeld,~J., Neftel,~A., and Fuhrer,~J.: The carbon budget of newly established temperate grassland depends on management intensity, Agr. Ecosyst. Environ., 121, 5–20, 2007. Andersen,~H V., Hovmand,~M F., Hummelshoj,~P., and Jensen,~N O.: Measurements of ammonia flux to a~spruce stand in Denmark, Atmos. Environ. A Gen. Top., 27, 189–202, 1993. Andersen,~H V. and Hovmand,~M F.: Ammonia and nitric acid dry deposition and throughfall, Water Air Soil Pollut., 85, 2211–2216, 1995. Andersen,~H V., Hovmand,~M F., Hummelshoj,~P., and Jensen,~N O.: Measurements of ammonia concentrations, fluxes and dry deposition velocities to a~spruce forest 1991–1995, Atmos. Environ., 33, 1367–1383, 1999. Arya, S. P.: Introduction to Micrometeorology, 2nd ed., International geophysics series, edited by: Dmoswska, R., Holton, J. R., and Rossby, H. T., Academic Press, 2001. Ayers,~G P.: Comment on regression analysis of air quality data, Atmos. Environ., 35, 2423–2425, 2001. Brodeur,~J J., Warland,~J S., Staebler,~R M., and Wagner-Riddle,~C.: Technical note: Laboratory evaluation of a~tunable diode laser system for eddy covariance measurements of ammonia flux, Agr. Forest Meteorol., 149, 385–391, 2009. Businger,~J A.: Evaluation of the accuracy with which dry deposition can be measured with current micrometeorological techniques, J. Climate Appl. Meteorol., 25, 1100–1124, 1986. Businger,~J A. and Delany,~A C.: Chemical sensor resolution required for measuring surface fluxes by 3 common micrometeorological techniques, J. Atmos. Chem., 10, 399–410, 1990. Calvert,~J G., Lazrus,~A., Kok,~G L., Heikes,~B G., Walega,~J G., Lind,~J., and Cantrell,~C A.: Chemical mechanisms of acid generation in the troposphere, Nature, 317, 27–35, 1985. Cantrell,~C A.: Technical Note: Review of methods for linear least-squares fitting of data and application to atmospheric chemistry problems, Atmos. Chem. Phys., 8, 5477–5487, 2008. Cape,~J N., van der Eerden,~L J., Sheppard,~L J., Leith,~I D., and Sutton,~M A.: Evidence for changing the critical level for ammonia, Environ. Pollut., 157, 1033–1037, 2009. Cellier,~P. and Brunet,~Y.: Flux gradient relationships above tall plant canopies, Agr. Forest Meteorol., 58, 93–117, 1992. Duyzer,~J.: Dry deposition of ammonia and ammonium aerosols over heathland, J. Geophys. Res.-Atmos., 99, 18757–18763, 1994. Duyzer,~J H., Verhagen,~H L M., Weststrate,~J H., and Bosveld,~F C.: Measurement of the dry deposition flux of NH3 on to coniferous forest, Environ. Pollut., 75, 3–13, 1992. Erisman,~J W., Vermetten,~A W M., Asman,~W A H., Waijersijpelaan,~A., and Slanina,~J.: Vertical-distribution of gases and aerosols – the behavior of ammonia and related components in the lower atmosphere, Atmos. Environ., 22, 1153–1160, 1988. Erisman,~J W. and Wyers,~G P.: Continuous measurements of surface exchange of SO$_(2)$ and NH$_(3)$ – Implications for their possible interaction in the deposition process, Atmos. Environ. A Gen. Top., 27, 1937–1949, 1993. Erisman,~J W., Draaijers,~G., Duyzer,~J., Hofschreuder,~P., VanLeeuwen,~N., Romer,~F., Ruijgrok,~W., Wyers,~P., and Gallagher,~M.: Particle deposition to forests – Summary of results and application, Atmos. Environ., 31, 321–332, 1997. Erisman,~J W. and Schaap,~M.: The need for ammonia abatement with respect to secondary PM reductions in Europe, Environ. Pollut., 129, 159–163, 2004. Erisman,~J W., Bleeker,~A., Hensen,~A., and Vermeulen,~A.: Agricultural air quality in Europe and the future perspectives, Atmos. Environ., 42, 3209–3217, 2008. Falge,~E., Reth,~S., Bruggemann,~N., Butterbach-Bahl,~K., Goldberg,~V., Oltchev,~A., Schaaf,~S., Spindler,~G., Stiller,~B., Queck,~R., Kostner,~B., and Bernhofer,~C.: Comparison of surface energy exchange models with eddy flux data in forest and grassland ecosystems of Germany, Ecol. Model., 188, 174–216, 2005. Farmer,~D K., Wooldridge,~P J., and Cohen,~R C.: Application of thermal-dissociation laser induced fluorescence (TD-LIF) to measurement of \chemHNO_3, $§igma$ alkyl nitrates, $§igma$ peroxy nitrates, and \chemNO_2 fluxes using eddy covariance, Atmos. Chem. Phys., 6, 3471–3486, 2006. Flechard,~C R. and Fowler,~D.: Atmospheric ammonia at a~moorland site. II: Long-term surface-atmosphere micrometeorological flux measurements, Q. J. Roy. Meteorol. Soc., 124, 759–791, 1998. Flechard,~C R., Neftel,~A., Jocher,~M., Ammann,~C., and Fuhrer,~J.: Bi-directional soil/atmosphere \chemN_2O exchange over two mown grassland systems with contrasting management practices, Global Change Biol., 11, 2114–2127, 2005. Foken,~T.: Lufthygienisch-bioklimatische Kennzeichnung des oberen Egertales (Fichtelgebirge bis Karlovy Vary), Bayreuther Forum Ökologie, 1–70, 2003. Foken,~T.: Angewandte Meteorologie, 2nd edn., Springer, 2006. Galloway,~J N., Dentener,~F J., Capone,~D G., Boyer,~E W., Howarth,~R W., Seitzinger,~S P., Asner,~G P., Cleveland,~C C., Green,~P A., Holland,~E A., Karl,~D M., Michaels,~A F., Porter,~J H., Townsend,~A R., and Vorosmarty,~C J.: Nitrogen cycles: past, present, and future, Biogeochemistry, 70, 153–226, 2004. Garland,~J A.: The dry deposition of sulphur dioxide to land and water surfaces, Proc. R. Soc. London, 245–268, 1977. Garratt,~J R.: Flux profile relations above tall vegetation, Q. J. Roy. Meteorol. Soc., 104, 199–211, 1978. Garratt,~J R.: The Atmospheric Boundary Layer, Cambridge University Press, 1992. Hanson,~P J. and Lindberg,~S E.: Dry deposition of reactive nitrogen compounds: A~review of leaf, canopy and non-foliar measurements, Atmos. Environ. A Gen. Top., 25, 1615–1634, 1991. Held,~A. and Klemm,~O.: Direct measurement of turbulent particle exchange with a~twin CPC eddy covariance system, Atmos. Environ., 40, S92–S102, 2006. Hicks,~B B., Baldocchi,~D D., Meyers,~T P., Hosker,~R P., and Matt,~D R.: A~preliminary multiple resistance routine for deriving dry deposition velocities from measured quantities, Water Air Soil Pollut., 36, 311–330, 1987. Hirsch,~R M. and Gilroy,~E J.: Methods of fitting a~straight-line to data – examples in water-resources, Water Resour. Bull., 20, 705–711, 1984. Hogstrom,~U.: Analysis of turbulence structure in the surface-layer with a~modified similarity formulation for near neutral conditions, J. Atmos. Sci., 47, 1949–1972, 1990. Huebert,~B J. and Robert,~C H.: The dry deposition of nitric-acid to grass, J. Geophys. Res.-Atmos., 90, 2085–2090, 1985. Huey,~L G.: Measurement of trace atmospheric species by chemical ionization mass spectrometry: Speciation of reactive nitrogen and future directions, Mass Spectrom. Rev., 26, 166–184, 2007. Jaeggi,~M., Ammann,~C., Neftel,~A., and Fuhrer,~J.: Environmental control of profiles of ozone concentration in a~grassland canopy, Atmos. Environ., 40, 5496–5507, 2006. Jensen,~N O. and Hummelshoj,~P.: Derivation of canopy resistance for water-vapor fluxes over a~spruce forest, using a~new technique for the viscous sublayer resistance, Agr. Forest Meteorol., 73, 339–352, 1995. Jensen,~N O. and Hummelshoj,~P.: Derivation of canopy resistance for water vapor fluxes over a~spruce forest, using a~new technique for the viscous sublayer resistance (vol 73, 339 p., 1995), Agr. Forest Meteorol., 85, 289–289, 1997. Kaiser,~H. and Specker,~H.: Bewertung und Vergleich von Analyseverfahren, Z. analyt. Chem., 149, 46–66, 1956. Kleijn,~D., Kohler,~F., Baldi,~A., Batary,~P., Concepcion,~E D., Clough,~Y., Diaz,~M., Gabriel,~D., Holzschuh,~A., Knop,~E., Kovacs,~A., Marshall,~E J P., Tscharntke,~T., and Verhulst,~J.: On the relationship between farmland biodiversity and land-use intensity in Europe, Proc R. Soc. B Biol. Sci., 276, 903–909, 2009. Klemm,~O., Held,~A., Forkel,~R., Gasche,~R., Kanter,~H J., Rappengluck,~B., Steinbrecher,~R., Muller,~K., Plewka,~A., Cojocariu,~C., Kreuzwieser,~J., Valverde-Canossa,~J., Schuster,~G., Moortgat,~G K., Graus,~M., and Hansel,~A.: Experiments on forest/atmosphere exchange: climatology and fluxes during two summer campaigns in NE Bavaria, Atmos. Environ., 40, S3–S20, 2006. Kruit,~R J W., van Pul,~W A J., Otjes,~R P., Hofschreuder,~P., Jacobs,~A F G., and Holtslag,~A A M.: Ammonia fluxes and derived canopy compensation points over non-fertilized agricultural grassland in The Netherlands using the new gradient ammonia – high accuracy – monitor (GRAHAM), Atmos. Environ., 41, 1275–1287, 2007. Krupa,~S V.: Effects of atmospheric ammonia (\chemNH_3) on terrestrial vegetation: a~review, Environ. Pollut., 124, 179–221, 2003. Meyers,~T P., Huebert,~B J., and Hicks,~B B.: \chemHNO_3 deposition to a~deciduous forest, Bound.-Lay. Meteorol., 49, 395–410, 1989. Milford,~C., Hargreaves,~K J., Sutton,~M A., Loubet,~B., and Cellier,~P.: Fluxes of \chemNH_3 and \chemCO_2 over upland moorland in the vicinity of agricultural land, J. Geophys. Res.-Atmos., 106, 24169–24181, 2001. Miller, J. C. and Miller, J. N.: Statistics for analytical chemistry, 2nd edition ed., Ellis Horwood series in analytical chemistry, edited by: Chalmers, R. A., and Masson, M., W. Sussex, 1988. Mozurkewich,~M.: The dissociation constant of ammonium nitrate and its dependence on temperature, relative humidity and particle size, Atmos. Environ. A Gen. Top., 27, 261–270, 1993. Mueller,~H., Kramm,~G., Meixner,~F., Dollard,~G J., Fowler,~D., and Possanzini,~M.: Determination of nitric acid dry deposition by modified Bowen ratio and aerodynamic profile techniques, Tellus Ser. B Chem. Phys. Meteorol., 45, 346–367, 1993. Neftel,~A., Flechard,~C., Ammann,~C., Conen,~F., Emmenegger,~L., and Zeyer,~K.: Experimental assessment of \chemN_2O background fluxes in grassland systems, Tellus Ser. B Chem. Phys. Meteorol., 59, 470–482, 2007. Neftel,~A., Spirig,~C., and Ammann,~C.: Application and test of a~simple tool for operational footprint evaluations, Environ. Pollut., 152, 644–652, 2008. Nemitz,~E., Sutton,~M A., Wyers,~G P., Otjes,~R P., Schjoerring,~J K., Gallagher,~M W., Parrington,~J., Fowler,~D., and Choularton,~T W.: Surface/atmosphere exchange and chemical interaction of gases and aerosols over oilseed rape, Agr. Forest Meteorol., 105, 427–445, 2000. Nemitz,~E. and Sutton,~M A.: Gas-particle interactions above a~Dutch heathland: III. Modelling the influence of the \chemNH_3-\chemHNO_3-\chemNH_4NO_3 equilibrium on size-segregated particle fluxes, Atmos. Chem. Phys., 4, 1025–1045, 2004. Nemitz,~E., Sutton,~M A., Wyers,~G P., and Jongejan,~P A C.: Gas-particle interactions above a~Dutch heathland:~I. Surface exchange fluxes of \chemNH_3, \chemSO_2, \chemHNO_3 and HCl, Atmos. Chem. Phys., 4, 989–1005, 2004a. Nemitz,~E., Sutton,~M A., Wyers,~G P., Otjes,~R P., Mennen,~M G., van Putten,~E M., and Gallagher,~M W.: Gas-particle interactions above a~Dutch heathland: II. Concentrations and surface exchange fluxes of atmospheric particles, Atmos. Chem. Phys., 4, 1007–1024, 2004b. Nemitz,~E., Jimenez,~J L., Huffman,~J A., Ulbrich,~I M., Canagaratna,~M R., Worsnop,~D R., and Guenther,~A B.: An eddy-covariance system for the measurement of surface/atmosphere exchange fluxes of submicron aerosol chemical species – First application above an urban area, Aerosol Sci. Technol., 42, 636–657, 2008. Norman,~M., Spirig,~C., Wolff,~V., Trebs,~I., Flechard,~C., Wisthaler,~A., Schnitzhofer,~R., Hansel,~A., and Neftel,~A.: Intercomparison of ammonia measurement techniques at an intensively managed grassland site (Oensingen, Switzerland), Atmos. Chem. Phys., 9, 2635–2645, 2009. Phillips,~S B., Arya,~S P., and Aneja,~V P.: Ammonia flux and dry deposition velocity from near-surface concentration gradient measurements over a~grass surface in North Carolina, Atmos. Environ., 38, 3469–3480, 2004. Plassmann,~K., Edwards-Jones,~G., and Jones,~M L M.: The effects of low levels of nitrogen deposition and grazing on dune grassland, Sci. Total Environ., 407, 1391–1404, 2009. Pryor,~S C., Barthelmie,~R J., Jensen,~B., Jensen,~N O., and Sorensen,~L L.: \chemHNO_3 fluxes to a~deciduous forest derived using gradient and REA methods, Atmos. Environ., 36, 5993–5999, 2002. Rattray,~G. and Sievering,~H.: Dry deposition of ammonia, nitric acid, ammonium, and nitrate to alpine tundra at Niwot Ridge, Colorado, Atmos. Environ., 35, 1105–1109, 2001. Rebmann,~C., Gockede,~M., Foken,~T., Aubinet,~M., Aurela,~M., Berbigier,~P., Bernhofer,~C., Buchmann,~N., Carrara,~A., Cescatti,~A., Ceulemans,~R., Clement,~R., Elbers,~J A., Granier,~A., Grunwald,~T., Guyon,~D., Havrankova,~K., Heinesch,~B., Knohl,~A., Laurila,~T., Longdoz,~B., Marcolla,~B., Markkanen,~T., Miglietta,~F., Moncrieff,~J., Montagnani,~L., Moors,~E., Nardino,~M., Ourcival,~J M., Rambal,~S., Rannik,~U., Rotenberg,~E., Sedlak,~P., Unterhuber,~G., Vesala,~T., and Yakir,~D.: Quality analysis applied on eddy covariance measurements at complex forest sites using footprint modelling, Theor. Appl. Climatol., 80, 121–141, 2005. Remke,~E., Brouwer,~E., Kooijman,~A., Blindow,~I., Esselink,~H., and Roelofs,~J G M.: Even low to medium nitrogen deposition impacts vegetation of dry, coastal dunes around the Baltic Sea, Environ. Pollut., 157, 792–800, 2009. Richardson, A. D., Hollinger, D. Y., Burba, G. G., Davis, K. J., Flanagan, L. B., Katul, G. G., Munger, J. W., Ricciuto, D. M., Stoy, P. C., Suyker, A. E., Verma, S. B., and Wofsy, S. C.: A multi-site analysis of random error in tower-based measurements of carbon and energy fluxes, Agricultural and Forest Meteorology, 136, 1–18, 2006. Schmidt,~A. and Klemm,~O.: Direct determination of highly size-resolved turbulent particle fluxes with the disjunct eddy covariance method and a~12-stage electrical low pressure impactor, Atmos. Chem. Phys., 8, 7405–7417, 2008. Seinfeld,~J H. and Pandis,~S N.: Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, Wiley, New York, 2006. Sievering,~H., Enders,~G., Kins,~L., Kramm,~G., Ruoss,~K., Roider,~G., Zelger,~M., Anderson,~L., and Dlugi,~R.: Nitric-acid, particulate nitrate and ammonium profiles at the Bayerischer Wald – Evidence for large deposition rates of total nitrate, Atmos. Environ., 28, 311–315, 1994. Sievering,~H., Kelly,~T., McConville,~G., Seibold,~C., and Turnipseed,~A.: Nitric acid dry deposition to conifer forests: Niwot Ridge spruce-fir-pine study, Atmos. Environ., 35, 3851–3859, 2001. Simpson,~I J., Thurtell,~G W., Neumann,~H H., Den Hartog,~G., and Edwards,~G C.: The validity of similarity theory in the roughness sublayer above forests, Bound.-Lay. Meteorol., 87, 69–99, 1998. Spindler,~G., Teichmann,~U., and Sutton,~M A.: Ammonia dry deposition over grassland – micrometeorological flux-gradient measurements and bidirectional flux calculations using an inferential model, Q. J. Roy. Meteorol. Soc., 127, 795–814, 2001. Staudt,~K. and Foken,~T.: Documentation of reference data for the experimental areas of the Bayreuth Center for Ecology and Environmental Research (BayCEER) at the Waldstein site, Dep. Micromet., University of Bayreuth, 2007. Stelson,~A W. and Seinfeld,~J H.: Relative-humidity and temperature-dependence of the ammonium-nitrate dissociation-constant, Atmos. Environ., 16, 983–992, 1982. Sutton,~M A., Nemitz,~E., Fowler,~D., Wyers,~G P., Otjes,~R P., Schjoerring,~J K., Husted,~S., Nielsen,~K H., San Jose,~R., Moreno,~J., Gallagher,~M W., and Gut,~A.: Fluxes of ammonia over oilseed rape – Overview of the EXAMINE experiment, Agr. Forest Meteorol., 105, 327–349, 2000a. Sutton,~M A., Nemitz,~E., Milford,~C., Fowler,~D., Moreno,~J., San Jose,~R., Wyers,~G P., Otjes,~R P., Harrison,~R., Husted,~S., and Schjoerring,~J K.: Micrometeorological measurements of net ammonia fluxes over oilseed rape during two vegetation periods, Agr. Forest Meteorol., 105, 351–369, 2000b. Sutton, M. A., Nemitz, E., Erisman, J. W., Beier, C., Bahl, K. B., Cellier, P., de Vries, W., Cotrufo, F., Skiba, U., Di Marco, C., Jones, S., Laville, P., Soussana, J. F., Loubet, B., Twigg, M., Famulari, D., Whitehead, J., Gallagher, M. W., Neftel, A., Flechard, C. R., Herrmann, B., Calanca, P. L., Schjoerring, J. K., Daemmgen, U., Horvath, L., Tang, Y. S., Emmett, B. A., Tietema, A., Penuelas, J., Kesik, M., Brueggemann, N., Pilegaard, K., Vesala, T., Campbell, C. L., Olesen, J. E., Dragosits, U., Theobald, M. R., Levy, P., Mobbs, D. C., Milne, R., Viovy, N., Vuichard, N., Smith, J. U., Smith, P., Bergamaschi, P., Fowler, D., and Reis, S.: Challenges in quantifying biosphere-atmosphere exchange of nitrogen species, Environmental Pollution, 150, 125–139, 2007. Thom,~A S., Stewart,~J B., Oliver,~H R., and Gash,~J H C.: Comparison of aerodynamic and energy budget estimates of fluxes over a~pine forest, Q. J. Roy. Meteorol. Soc., 101, 93–105, 1975. Thomas,~C. and Foken,~T.: Flux contribution of coherent structures and its implications for the exchange of energy and matter in a~tall spruce canopy, Bound.-Lay. Meteorol., 123, 317–337, 2007. Thomas,~R M., Trebs,~I., Otjes,~R., Jongejan,~P A C., Brink,~H T., Phillips,~G., Kortner,~M., Meixner,~F X., and Nemitz,~E.: An automated analyzer to measure surface-atmosphere exchange fluxes of water soluble inorganic aerosol compounds and reactive trace gases, Environ. Sci. Technol., 43, 1412–1418, doi:10.1021/es8019403, 2009. Trebs,~I., Meixner,~F X., Slanina,~J., Otjes,~R., Jongejan,~P., and Andreae,~M O.: Real-time measurements of ammonia, acidic trace gases and water-soluble inorganic aerosol species at a~rural site in the Amazon Basin, Atmos. Chem. Phys., 4, 967–987, 2004. Van Oss,~R., Duyzer,~J., and Wyers,~P.: The influence of gas-to-particle conversion on measurements of ammonia exchange over forest, Atmos. Environ., 32, 465–471, 1998. Vickers,~D. and Mahrt,~L.: Quality control and flux sampling problems for tower and aircraft data,~J. Atmos. Ocean. Technol., 14, 512–526, 1997. Wesely,~M L. and Hicks,~B B.: A~review of the current status of knowledge on dry deposition, Atmos. Environ., 34, 2261–2282, 2000. Wichura,~B., Ruppert,~J., Delany,~A C., Buchmann,~N., and Foken,~T.: Structure of carbon dioxide exchange processes above a~spruce forest, in: Biogeochemistry of Forested Catchments in a~Changing Environment: an German Case Study, Ecological Studies: Analysis and Synthesis, Springer-Verlag, Berlin, 161–176, 2004. Wyers,~G P., Vermeulen,~A T., and Slanina,~J.: Measurement of dry deposition of ammonia on a~forest, Environ. Pollut., 75, 25–28, 1992. Wyers,~G P., Oties,~R P., and Slanina,~J.: A~continuous-flow denuder for the measurement of ambient concentrations and surface-exchange fluxes of ammonia, Atmos. Environ. A Gen. Top., 27, 2085–2090, 1993. Wyers,~G P. and Duyzer,~J H.: Micrometeorological measurement of the dry deposition flux of sulphate and nitrate aerosols to coniferous forest, Atmos. Environ., 31, 333–343, 1997. Wyers,~G P. and Erisman,~J W.: Ammonia exchange over coniferous forest, Atmos. Environ., 32, 441–451, 1998. Zheng,~J., Zhang,~R., Fortner,~E C., Volkamer,~R M., Molina,~L., Aiken,~A C., Jimenez,~J L., Gaeggeler,~K., Dommen,~J., Dusanter,~S., Stevens,~P S., and Tie,~X.: Measurements of \chemHNO_3 and \chemN_2O_5 using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign, Atmos. Chem. Phys., 8, 6823–6838, 2008.