(N) compounds in the lowest two
layers of the atmosphere are important in current environmental issues. The lowest
layer, the troposphere, extends from the earth’s surface up to about 10
kilometers. The next layer, the
stratosphere, extends from about 10 to about 50 kilometers above the
ground. Mixing between the two layers
is quite slow. Radionuclides that were
injected into the stratosphere during atmospheric testing of nuclear weapons
had a lifetime on the order of months to a few years in the stratosphere before
episodic mixing events would eventually bring the bomb debris into the
troposphere where it would have a lifetime of days to weeks before being
deposited onto the earth’s surface (Junge, 1963).
respect to the atmospheric N cycle (Graedel and Crutzen, 1993), inert molecular
nitrogen (N2) constitutes more than 99.9999% of the N
present in the atmosphere. Nitrous
oxide (N2O), making up more than 99% of the remainder of the N in
the atmosphere, is an important greenhouse gas and thus important for climate
change issues. N2O is
emitted from natural and industrial sources at the earth’s surface and is then
slowly mixed into the stratosphere where it has its sink, being slowly
destroyed by chemical breakdown (having a lifetime in the stratosphere of about
the troposphere the remaining trace N gases are important factors in several
current environmental issues, including air quality and impacts on water
quality resulting from the N deposition of the trace N gases and aerosols. These trace N compounds, such as ammonia (NH3)
and N oxides (NOx), are emitted into the atmosphere from natural and
anthropogenic sources in the biosphere, geosphere, and hydrosphere. Although the troposphere is about 10
kilometers deep, most of the trace N is in the 1-2 kilometers adjacent to the
earth’s surface. Compared to the other
spheres the atmosphere has large horizontal transport speeds and can thus
deposit trace N back onto the other spheres at distances of tens, hundreds, or
thousands of kilometers from where it was originally injected into the
injected into the lower troposphere, trace N gases are acted on by the
following processes: horizontal
transport (by winds), mixing vertically and laterally by turbulence, chemical
and physical transformation, and deposition from the atmosphere. These complex processes will be discussed
transformations in the troposphere occur between various gaseous constituents
(homogeneous chemical reactions) and between gases and liquid or solid
particles (heterogeneous chemical reactions).Homogeneous reactions between NOx and hydrocarbons result in ozone
formation, one of the primary ingredients in photochemical smog that was first
identified in the 1950's in the Los Angeles basin (Williamson, 1973). Solar radiation drives some of these
smog-producing reactions and they are referred to as photochemical reactions. High ambient air ozone concentrations are
important air quality concerns for both large urban areas as well as for large
regions such as the entire eastern USA.The atmospheric transformations convert some of the inorganic
trace N gases, emitted into the atmosphere, into organic-N gases. A combination of chemical and physical
transformations convert some of the trace N gases to suspended aerosol
particles of various sizes.
N2 in the atmosphere is not useful to most organisms until it is "fixed" or converted to a
form that can be chemically utilized by the organisms. Natural and anthropogenic processes
convert N2 to nitrogen oxides that are in turn converted in the atmosphere to nitrates and then are
ultimately deposited on the Earth's surface. Combustion in automobiles, power plants and other
industrial and mobile sources convert atmospheric N2 as well as N bound in the fuels into
nitrogen oxides. Soil denitrification is an important source of nitrogen oxides to the atmosphere.
The natural action of ionizing phenomena, such as lightning, converts N2 to nitrogen oxides.
One estimate (IPCC,1995) for the global sources of NO and NO2 (in units of millions of metric
tons/yr of N) gives a total value of 52.6 and the top four sources as: fossil fuel combustion (24),
soil release (12), biomass burning (8) and lightning (5). The estimate (IPCC,1995) for the global
sources of N2O is a total value of 14.7 and the top three sources are: oceans (3), wet soils in
tropical forests (3) and anthropogenic cultivated soils (3.5).
and turbulence at any location in the lower troposphere vary diurnally and from
day to day. Wind speed increases with
height so trace N gases emitted from tall smokestacks versus ground level
automobile tailpipes are transported more quickly downwind and must be mixed
down to the earth’s surface by turbulence before the N molecules can be
deposited by interacting with leaves, grass, soil, etc. Atmospheric transport and mixing brings
specific source emissions of the various trace N gases into close proximity
with trace gas emissions from hundreds of other sources to allow the complex
chemical and physical transformations to occur. For example NH3 from livestock sources in the plains
states can become commingled with automobile and industrial source emissions of
sulfur (S) and N oxides hundreds of miles downwind to produce ammonium
sulfate particles that are a major component of aerosols producing regional
haze events. Modeling has shown that a
substantial fraction of NH3 emitted by ground level sources is
deposited within 1-100 kilometers of the emission source; if the atmospheric NH3
reacts with S gases in the atmosphere to form sub-micrometer diameter
particles, then the NH3 is deposited much more slowly leading to
transport of 100s to 1000s of kilometers (Ferm, 1998).
deposition of N from the atmosphere contributes to the N cycles in the
geosphere, biosphere and hydrosphere and thus to environmental N issues related
to these spheres. It is important to
monitor the magnitude of these atmospheric deposition fluxes in time and
space. In addition it is important to
understand the major factors determining the variation of the deposition fluxes
in time and space in order to guide policy decisions that might be implemented
to reduce the magnitude of the fluxes.The deposition fluxes are divided into two components, wet and dry deposition. Wet deposition refers to N incorporated into
hydrometeors (liquid and solid water settling out of the atmosphere, such as
raindrops and snowflakes) or into cloud droplets and fog droplets that either
settle very slowly or are deposited by inertial impaction onto things such as
tree needles or leaves or other parts of the biosphere extending into the
atmosphere. Dry deposition refers to N
gases and particles directly interacting with and sticking to elements of the
geosphere, biosphere, and hydrosphere that are in contact with the atmosphere. The in-cloud and below-cloud components of
the wet deposition process can occur throughout the entire troposphere and into
the stratosphere. That is, large rain
clouds can extend vertically to the top of the troposphere and into the lower
portion of the stratosphere.Operationally wet deposition is measured by collecting precipitation
(rain, snow, hail, etc.) samples for chemical analysis. The wet deposition via cloud droplets is
seldom measured but can be a large flux in certain locations such as in high
elevation forests (Johnson and Lindberg, 1992). Dry deposition is usually
determined by multiplying measured air concentrations of trace N gases and N
aerosols by model derived deposition velocities. Dry deposition is much more difficult to monitor than wet
deposition and has greater measurement uncertainty. Dry deposition occurs all
the time. Wet deposition occurs during
only a small percentage of the hours of a year but is still a very important
removal process; as a first rough approximation we assume the annual wet and
dry deposition fluxes are equal in magnitude for N and S species. There remain many gaps in our knowledge of
the wet and dry deposition of N species. The tropospheric residence of time of
N molecules being measured in the wet and dry deposition ranges from minutes to
balance calculations are frequently done to elucidate aspects of N cycles. In this technique the mass fluxes of the
various chemical species into and out of a chosen volume are calculated. Depending on the study objectives one
selects a volume entirely within a sphere, or a volume that includes several
spheres. To study phenomena such as
hypoxia in rivers and lakes one might choose a volume that includes the
hydrosphere, geosphere, and biosphere, where the top surface of the volume is
the interface between these three spheres and the atmosphere. In such studies one requires calculations of
the fluxes of N from the atmosphere to these other three spheres. Thus, one requires estimates of wet and dry
deposition from the atmosphere to the earth's surface. For a specific research study, such as one
determining the atmospheric deposition to selected forest sites during a two
year period (Johnson and Lindberg, 1992), equipment is operated to measure the
fluxes. For deposition to larger areas,
such the Mississippi River Watershed (MRW), national network monitoring data
are used to estimate N deposition. With
the large number of wet deposition monitoring sites in the MRW, reliable
estimates of nitrate (NO3-) and ammonium(NH4+)
wet deposition can be made, however, it is known that the NH4+ data are
likely biased by an amount that is yet to be thoroughly quantified. These two species are the dominant inorganic
species in wet deposition. For dry
deposition to the MRW the number of monitoring sites is inadequate. Using the dry deposition network data
estimates of fluxes of nitric acid (HNO3), NO3- particles, and
NH4+ particles can be
made. A significant gap is that NH3
gas is not measured in the national network so dry deposition of this species
cannot be reliably made. For mass
balance studies for large areas such as the MRW one can avoid the need for
knowing the reduced N (NH4+and NH3 )
atmospheric fluxes by ignoring the volatilization of NH3 from
emission sources such as animal feedlots and commercial fertilizer application
for crop production. For small areas
(e.g. small watersheds) this approach is not valid. Other knowledge gaps relate to the fact that insoluble N and
organic-N are not measured in the national monitoring networks. The former may be relatively insignificant
but the latter is felt to be important and needs to be addressed with research
studies. Thus while estimates of dry
and wet deposition of atmospheric N fluxes can be made, there is a need for
considerable improvement in the monitoring techniques.
N species are measured in currently operating national deposition monitoring networks,
which include in Illinois six wet deposition sites and three dry deposition sites. From the wet
deposition database, average values for Illinois are about 3.4 kg N/ha-yr for N deposition from
NO3- and 3.3 kg N/ha-yr from NH4+. For dry deposition, in the same units, the average values
are about 0.1 from NO3- particles, 0.4 from NH4+ particles, and 2.4 from HNO3 vapor. These
dry deposition estimates apply to flat agricultural areas; in other parts of the state with less flat
terrain and more trees to increase turbulent mixing the dry deposition rates might be considerably
higher. NH3 is not measured at the dry deposition sites but some measurements in the late 1980's
indicated an average concentration of about .4 micrograms per cubic meter. Using a typical
literature dry deposition velocity for NH3 of 1 cm/sec gives a NH3 deposition value of 1.3. Some
unpublished NH3 measurements during the last year suggest that the current NH3 values might
be twice as high as the 1980's values. The sum of the six deposition values is about 11 kg N/h a-yr.
This estimated value for Illinois is for dissolved inorganic N (DIN) deposition; the national
networks do not provide any estimates of the dissolved organic N (DON) component. Prospero
et al (1996) review some research studies reporting DIN. The literature that they cite suggests
that DON my be about equal to DIN for the North Atlantic Ocean and for the global scale.
The concentrations of N species in the air and in precipitation vary seasonally, with
higher concentrations in the warmer part of the year. The spatial patterns of N concentration in
precipitation and N in wet deposition are well illustrated by the maps found on the NADP web
Ferm, M. 1998. Atmospheric ammonia and ammonium transport in Europe and critical loads:
a review. Nutr Cycling Agroecosystems 51: 5-17.
Gradel, T. E. and P. J. Crutzen. 1993. Atmospheric Change: An Earth System
Perspective. W.H. Freeman and Company, New York.
Intergovernmental Panel on Climate Change(IPCC). 1995. Climate Change 1994:
Radiative Forcing of Climate Change and an Evaluation of the IPCC IS92 Emission Scenarios.
Cambridge University Press, Cambridge, UK.
Johnson, D. W. and S. E. Lindberg. 1992. Atmospheric Deposition and Forest
Nutrient Cycling, A Synthesis of the Integrated Forest Study. Springer-Verlag, New York.
Junge, C. E., 1963: Air Chemistry and
Radioactivity. Academic Press, New York.
Prospero, J.M., K. Barrett, T. Church, F. Dentener, R.A. Duce, J.N. Galloway, H. Levy
II, J. Moody and P. Quinn. 1996. Atmospheric deposition of nutrients to the North Atlantic
Basin. Biogeochemistry 35:27-73.
Williamson, S. J. 1973. Fundamentals of Air Pollution. Addison-Wesley Publishing Co., Reading, MA.
Anthes, R.A., H.A. Panofsky, J.J. Cahir, and A. Rango.
1978. The Atmosphere 2nd Edition. Charles E. Merrill Pub. Co., Columbus, OH.
Bunce, Nigel, 1994. Environmental
Chemistry 2nd Edition. Wuerz Pub. Ltd., Winnipeg, Canada.
Carlson, T. N. 1991.Mid-Latitude Weather Systems. Harper Collins Academic, London.
Irving, P. M. (ed.). 1991. Acidic Deposition: State of Science and
Technology, Vol 1: Emissions, Atmospheric Processes, and Deposition. U.S. National Acid Precipitation Assessment
Program. U.S. Government Printing Office, Washington,DC.
Seinfeld, J. H. and S. N. Pandis. 1998. Atmospheric Chemistry and Physics, From
Air Pollution to Climate Change. John Wiley & Sons, Inc., New York.