The offset between carbon dioxide (CO2) emissions and increase in ambient concentrations has led to scientific debate regarding the missing carbon (C) sink concentrated on the roles of temperate and boreal forests and nitrogen (N) fertilization in enhanced C sequestration. Meteorology and biology play key roles in determining CO2 fluxes, but water and nutrients interact to control plant productivity and thus C sequestration, and acute and chronic exposure to elevated levels of some air pollutants impacts plant health. Because of difficulties inherent in measuring and quantifying ambient concentrations and deposition of highly reactive N gases and speciated aerosol dry deposition, these components of the total flux are often omitted from observationally based studies. However, dry deposition represents a significant pathway for removal of pollutant species from the atmosphere and addition of nutrients to ecosystems. For example, data from the Combined Regional Experiment network indicate that up to 50% of nitrate (NO3-) was dry deposited.
This paper presents preliminary results of above canopy N measurements conducted during a field campaign in April 1998 on an AMERIFLUX flux tower (MMSF). The MMSF site is in the southern Ohio River Valley and is described in Schmid et al. (1998). Measurements from NADP/NTN network indicate the Midwest receives approximately 5-6 kg N ha-1 yr-1 from wet deposition of NO3 - and NH4+ alone. Currently no measurements of dry N deposition are available, however, dry deposition of N species at CASTNet agricultural sites in the MidWest were estimated to be equivalent or greater than wet deposition.
The measurement techniques employed at the MMSF during the intensive campaign and described in detail in this paper are as follows:
HNO3. 6-hour integrated measurements at two heights above canopy. Denuders, analysis by ion chromatography.
NH3. Continuous measurements at two heights above canopy. Diffusion scrubbers.
SO2. Continuous concentrations at one height above canopy. Flame photometric detection.
Particle size distribution (0.5-20 microns). Continuous measurements at one height above canopy. Aerodynamic aerosol sizer (TSI 3320APS).
Chemically speciated, size resolved aerosol measurements (0.056-18 microns). 6 hour integrated measurements at one height above the canopy. Ten stage Micro-Orifice Uniform Deposit Impactors using coated aluminum foils, compositional analysis by ion chromatography.
Methodological issues regarding measurement of N dry deposition will be documented and preliminary analyses of the in situ observations will be discussed.
References:
Clarke, J.F., Edgerton, E.G. and Martin, B.E. (1997): Atmospheric Environment, 31, 3667-3678.
Edgerton, E.G., Lavery, T.F. and Boksleitner, R.P. (1992): Environmental Pollution, 75, 145-156.
Schmid, H.P., Offerle, B., Shurpali, N. Grimmond, C.S.B., Barthelmie R.J. and Pryor S.C. (1998): Fluxes of latent heat, sensible heat and carbon dioxide above a Mixed Hardwood Forest in Southern Indiana. This conference.
Hidy, G.M. (1994): Atmospheric sulfur and nitrogen oxides. Academic Press, San Diego, 447pp.