Nitrogen (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).

With 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 170 years).

Within 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 (NO_x), 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 atmosphere.

Once 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 briefly below.

Chemical 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 NO_x 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).

Wind 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).

The 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 weeks.

Mass 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 site (nadp.isws.illinois.edu).