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For additional flux estimates not reported here refer to Jaffe (1992).

Fig. 12-8 NH3/ NOt and N20 sources and sinks. Where two numbers are given, the top value is the anthropogenic contribution and the lower number is the total flux (natural + anthropogenic).

due to a variety of human causes. Similar results are known for other regions of the globe (Vitousek et al., 1997b).

12.7.2 Climate: N20 and Tropospheric 03

Due to the imbalance of sources and sinks, atmospheric N20 is increasing by 3 Tg N/yr or 0.2%/yr. Figure 12-9 shows average N20 mixing ratios from four stations in the NOAA-CMDL network, Barrow, Mauna Loa, Samoa, and the South Pole (data are from the NOAA-CMDL and can be obtained from www.cmdl.noaa.gov). The most recent IPCC estimate gives a total N20 source of 16 Tg N, 7 Tg of which are a result of human activities (IPCC, 1997). The largest contribution to the anthropogenic N20 sources is 3 Tg N from

www.cmdl.noaa.gov."/>
Fig. 12-9 Global N20 concentrations based on NOAA-CMDL observations at BRW, MLO, SMC), and SPO. Data are from the NOAA-CMDL and can be obtained from www.cmdl.noaa.gov.

agricultural soils, mostly lost after applying fertilizer.

With respect to nitrogen fertilizers, the subject of gaseous losses due to denitrification has been extensively studied. Rolston (1981) presents a good review of this topic, and gives data suggesting that anywhere from 10-75% of fertilizer nitrogen may be lost by this process (typically in the range of 20%). Various crop management practices have been developed to counter this problem. For example, having large amounts of NOi , organic carbon and water will increase denitrification, whereas limiting one of these factors will decrease it.

Matson et al. (1998) describe an experiment on fertilizer usage and gaseous emissions on a wheat farm in Mexico. In this experiment, the authors used both a traditional/high fertilizer approach and a reduced fertilizer method, but where the fertilizer was applied in a manner that was more efficiently used by the growing plants. The results showed that when using a reduced fertilizer strategy crop yields per hectare were similar to the high fertilizer case, but gaseous N emissions (NO and N20) were about 10% of the base case. Since this strategy used much less fertilizer, this protocol also gave the best results with respect to the farmer's profits. This is important in that it shows that it is possible to reduce agricultural emissions of NO and N20 and improve the farmer's bottom line.

Tropospheric ozone is also radiatively active and there is good evidence that it is now about twice its pre-industrial concentration, at least in the northern hemisphere. This increase is due to increasing emissions of NO* from fossil fuel combustion, followed by photochemical ozone production (Crutzen, 1988; Logan, 1985; Volz and Kley, 1988). This human caused change contributes significantly to radiative forcing of climate (Marenco et al, 1994; IPCC, 1995). Logan (1994) conducted a detailed evaluation of free tropospheric ozone sonde data. In general, this analysis shows ozone trends of about 1-2% per year over the last two decades over the US, Europe, and Japan, with the higher trends observed over Japan and Europe. For the US and Europe the 03 concentrations appear to have approximately leveled out, whereas the Japanese stations continue to show increases. This is consistent with the fact that NO* emissions in the US and Europe are not increasing, whereas they are increasing rapidly (5%/year) in East Asia (Kato and Akimoto, 1992).

The increases in both N20 and tropospheric 03 are leading to increased radiative forcing. Based on the concentration changes between pre-industrial times, 1850 to 1992, C02, N20, and tropospheric 03 contribute 1.56, 0.14, and 0.4 W/m2 to global average radiative forcing (IPCC, 1995). However, due to its shorter lifetime (2-4 weeks), tropospheric ozone is quite inhomogeneous in the troposphere and so the uncertainty in calculating its contribution to radiative forcing is significantly greater than for the more long-lived species. Based on projected future trends of energy consumption and agricultural emissions, all of these are expected to continue to increase through the 21st century.

12.7.3 Stratospheric Chemistry

As mentioned previously, NzO plays an important role in stratospheric chemistry by providing the dominant source of NO* in the stratosphere (see Section 12.5). What is more difficult to predict is how stratospheric chemistry will change as a result of continued increases in the concentration of atmospheric N20. Early research suggested that increased N20 would lead to significant reductions in stratospheric 03. However, more current reports suggest that stratospheric NO* plays a key role in "protecting" stratospheric 03 from more significant losses from CFC-produced CI radicals (e.g., Solomon and Schoeberl, 1988; Toon and Turco, 1991). Exactly how future increases in N20 will impact stratospheric 03 is something of an open question at present.

An additional area of concern with respect to stratospheric ozone is possible direct emissions of NO* into the stratosphere by high-flying supersonic aircraft. This issue has come up repeatedly over the past 20 years, as air travel and pressure from commercial airlines has increased. However, despite substantial research effort to understand stratospheric chemistry, the question is complicated by the changing levels of stratospheric chlorine, first due to a rapid accumulation of tropospheric CFCs, followed by a rapid decline in CFC emissions due to the Montreal Protocol. To quote from the from the 1994 WMO/UN Scientific assessment of ozone depletion, executive summary (WMO 1995):

Atmospheric effects of supersonic aircraft depend on the number of aircraft, the altitude of operation, the exhaust emissions, and the background chlorine and aerosol loadings. Projected fleets of supersonic transports would lead to significant changes in trace-species concentrations, especially in the North-Atlantic flight corridor. Two-dimensional model calculations of the impact of a projected fleet (500 aircraft, each emitting 15 grams of NOv per kilogram of fuel burned at Mach 2.4) in a stratosphere with a chlorine loading of 3.7 ppb, imply additional (i.e., beyond those from halocar-bon losses) annual-average ozone column decreases of 0.3-1.8% for the Northern hemisphere. There are, however, important uncertainties in these model results, especially in the stratosphere below 25 km. The same models fail to reproduce the observed ozone trends in the stratosphere below 25 km between 1980 and 1990. Thus, these models may not be properly including mechanisms that are important in this crucial altitude range.

12.7.4 Photochemical Smog

Unhealthy concentrations of ozone due to photochemical production from nitrogen oxides are a daily occurrence for millions of people who live in large urban centers. This is especially true for inhabitants of large cities in the warmer climates, such as Los Angeles, Mexico City, Athens, and Beijing. For example, in the 1970s Los Angeles exceeded the US EPA 03 standard around 175-200 days per year (Lents and Kelly, 1993), however Los Angeles is now making progress. In the early 1990s, Los Angeles exceeded the 03 health standard on "only" 100-150 days per year and the peak concentrations also declined considerably. This change is a result of tightened vehicle emission standards, improved engine reliability and increased controls on non-vehicular sources, despite having more people driving nearly twice as many vehicle miles as in 1970. Nonetheless, there are serious concerns about whether cities such as Los Angeles can ever meet the existing ozone standard.

In 1997, the US Environmental Protection Agency tightened the 03 standard, changing it from 120 ppbv as a 1 h average, to 80 ppbv as an 8 h standard. This was done due to substantial new evidence that 03 health effects occur at this lower level. Since Los Angeles can not meet the current standard, it seems highly unlikely they will be able to meet this new standard. While for a long time Los Angeles could reasonably be called "the ozone capital of the world," it is now probably being exceeded by rapidly developing cities in other countries. For example, both Mexico City and Beijing have serious 03 smog problems that probably exceed the problems in Los Angeles.

Ozone also causes significant damage to vegetation. In some regions where intensive industry and agriculture coexist, there is the possibility for substantial impacts on food production due to ozone. This is because ozone damage to crops can occur at mixing ratios as low as 60 ppbv and also due to the fact that the application of nitrogen fertilizers will increase local NO emissions (as described above). Thus to some extent there is a "self-limiting" effect from adding additional fertilizer in that the increased NO emissions will result in decreased crop yields due to ozone damage. Based on the photochemical model of Chameides et al. (1994), crop yields in regions of China are already being impacted by a few percent. This impact will continue and worsen as China continues to industrialize.

While the subject of photochemical ozone production has been extensively studied, there are still some remaining uncertainties. The essential reactions have already been presented in Section 12.5, and will only be briefly discussed here. In all high-temperature combustion processes, particularly power plants and automobiles, NO is produced by the direct reaction of N2 + 02, and from nitrogen-containing fuels. This NO can then be oxidized by a variety of mechanisms to N02. In the presence of NOr and sunlight, the oxidation of CO, CH4, and other hydrocarbons results in ozone production. In an urban environment, the diurnal cycle of these trace species will generally exhibit a characteristic pattern of concentration maxima first in NO, then N02, followed by 03 around midday (National Academy of Sciences, 1977). Evidence indicates that natural hydrocarbons along with anthropogenically produced NOx are important precursors to urban and rural ozone (Liu et al., 1987,1988).

As seen in Table 12-2, global NO* production is dominated by anthropogenic sources. In an urban environment, virtually all NO* is from fossil fuel combustion.

12.7.5 Acid Rain

Acid precipitation, or acid rain, can causes significant impacts on freshwater, coastal, and forested ecosystems (e.g., Likens et al., 1996). Both NOi", from NO* emissions, and SO2 ~ from S02 emissions contribute significantly to acid rain. The relative ratio of SOiVNO, m precipitation will be substantially determined by the regional emissions of S02/N03. In developed countries, uncontrolled combustion of coal and high-sulfur fuel oil led to significant emissions of S02, relative to NO*. Due to strict control of smokestack S02 emissions in some regions and increasing NO* emissions from automobiles, the relative contribution of N03 is expected to increase (Sirois, 1993; Mayewski et al., 1990).

In remote ice cores, SOl~ and N03~ concentrations have increased due to anthropogenic emissions (Mayewski et al., 1986, 1990). This is due to the fact the precursor compounds (e.g.,

NO*) are exported from the source regions. For example, Honrath and Jaffe (1992) found elevated concentrations of nitrogen oxides at Barrow, Alaska during spring, as compared to summer. This is largely due to decreased removal processes during winter for some nitrogen oxides, such as peroxyacetyl nitrate.

12.7.6 Species Diversity

Every day the planet loses forever a large number of plant and animal species. This current rate of extinction exceeds anything in the past history of life on Earth. Most of this loss is due to loss in habitat, especially in the tropics. However, some species are lost due to changes in the nutrient balance and resulting changes in the ecosystem structure. Vitousek et al. (1997b,c) documents losses of plant diversity in several regions due to wet and dry deposition of anthropogenic NO^". This occurs because in regions with relatively high nitrogen deposition, a smaller number of species will flourish. According to Vitousek et al. (1997b) this reduced diversity makes the ecosystem less stable, as for example, during times of drought.

12.7.7 N Fertilization and the Global Carbon Cycle

Due to the fact that nitrogen is a limiting nutrient in many ecosystems, additions of fixed nitrogen can significantly increase plant growth. This is termed "nitrogen fertilization." In regions where anthropogenic nitrogen is deposited two results are possible: reduced vegetative growth due to acid precipitation (see above) or increased growth due to nitrogen fertilization. Typically, the initial nitrogen deposition will stimulate growth; however, later, a plant can become "nitrogen saturated" and no longer respond to additional nitrogen inputs (Mellilo et al, 1989; Vitousek, 1997b,c). To the extent that nitrogen fertilization causes increased plant growth, this will result in increased uptake of atmospheric C02 and increased global biomass.

One of the problems with understanding perturbations to the global carbon cycle has been the problem of the "missing sinks." This describes the fact that the relatively well-known anthropogenic sources of C02 significantly exceeded the annual atmospheric increase of C02. Some of the anthropogenic emissions are being taken up by the oceans, but based on a number of quantitative models, it is unlikely that the oceans are adsorbing all of the missing sink (IPCC, 1995).

Based on a variety of evidence, a number of researchers have now concluded that terrestrial biomass must be taking up a significant fraction of the annual anthropogenic carbon emissions. For example Tans et al. (1990) used C02 observations and a global model to calculate regional sources and sinks and concluded that a large carbon sink must be operative in the northern hemisphere. Schindler and Bayley (1993) used a biogeochemical approach to conclude that northern forests are storing an additional 1.02.3 Tg C/yr due to deposition of anthropogenic nitrogen. Hudson et al. (1994), using a global three-dimensional ocean-atmosphere-biosphere carbon model, reached a similar conclusion. Thus, it would appear that increased carbon uptake due to anthropogenic nitrogen fertilization is responsible for sequestering a large fraction of the 6 Tg of carbon emitted each year by human activities.

On the surface it would seem that global nitrogen fertilization is beneficial in that it reduces the concentration of C02 in the atmosphere and thus its radiative forcing. However, it raises the question of how long global biomass can continue to respond in this way. Should the northern forests switch over from nitrogen fertilization to nitrogen saturation, or if other nutri ents become limiting in these ecosystems, then the rate of rise of atmospheric C02 would increase (assuming emissions remained the same). Overall, this uncertainty is an important limitation on our ability to predict future concentrations of atmospheric C02.

12.7.8 The Future

Global population, fertilizer use, and fossil fuel combustion are all expected to continue to grow. Galloway et al. (1995) provided estimated N fluxes due to fertilizer production and fossil fuel combustion, by regions, for the present and the year 2020 (see Table 12-3). Based on these estimates, global fertilizer production will increase by more than 70% and fossil fuel emissions of NO, will increase by 115%! As can be seen from Table 12-3, most of this increase is predicted to occur in the developing world as their large populations attempt to reach the living standard and lifestyles of the developed world.

Questions

12-1 How would the nitrogen cycle change if life on Earth were suddenly absent? What would be the time scale for these changes?

12-2 If, as a result of anthropogenic activities, nitrogen is being removed from the atmospheric reservoir (as N2) to the oceanic reservoir (as NOi ), how long would it take to detect this change? Is this a thermodynamically favorable process?

Table 12-3 Current and estimated (for 2020) impacts on the global N cycle (after Galloway et al., 1995)

Emissions of NOv due to fossil fuel Fertilizer production and usage combustion emissions (Tg N/yr) (Tg N/yr)

Present 2020 Present 2020

Developed world 14 16 29 30

Less developed world 6 29 50 104

Asia (excluding Japan) 4 13 36 85

Global 21 46 78 134

12-3 How have agriculture and deforestation changed the global rates of nitrogen fixation and denitrification? How can increased agricultural productivity be sustained without using industrially produced fertilizers?

12-4 Discuss the importance of atmospheric N20. Why is it important to know something about its natural and anthropogenic sinks? What role might atmospheric N20 play in the control of planetary climate? (See e.g., Lovelock, 1979.)

12-5 Describe the trends in the ozone concentrations in the troposphere and stratosphere, and the total ozone column. What roles do nitrogen oxides play in these changes?

12-6 What are the key reactions that result in the formation of photochemical smog? How do increases in NO* emissions lead to lower peak ozone concentrations in some areas? Would you advocate the lowering of NO, emission standards in some areas? What strategies would you suggest for cities in developing countries to avoid becoming like Los Angeles?

12-7 Knowing that the average precipitation on Earth is approximately 1 m per year, calculate the global mean concentration of I\'03 in rainwater assuming that 50% of all NO, is removed in wet deposition as HN03. Assuming this were the only source of acidity, what would the pH be for this rainwater? Now redo this calculation using the NO, emissions for 2020.

12-8 From the data in Fig. 12-4 and Table 12-3, calculate the lifetime for atmospheric N20. Would you expect the atmospheric N20 growth rate (Fig. 12-9) to remain about the same, greater or slower in the year 2020 as compared to today? Explain.

12-9 Assuming the current emissions and sinks remain about the same, estimate the global atmospheric C02 mixing ratio in the year 2050. Now repeat this calculation, but this time assume that the terrestrial biosphere no longer continues to sequester some of this anthropogenic carbon.

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