Nitrogen Challenges

In both proposed feedback mechanisms, the dominant controUing factor is the surface ocean NO3 to PO ratio (see Fig. 1.16). Both feedbacks are expected to operate in situations of excess NO3 (i.e., high NOj to PO ratio, shown at the bottom of the feedback loop in Fig. 1.16), as well as in situations ofNO deficiency (i.e., low NO to PO ratio, shown at the top of the feedback loop), if this view is correct, these two feedbacks would provide a rather tight control of the marine nitrogen cycle, creating a well balanced homeostasis. A further consequence is that 

A first challenge to the PO control hypothesis emerges from the observation that the tropical and subtropical North Adantic are sites of high N2 fixation (Capone et ah, 2005 Carpenter and Romans, 1991), yet these waters have extremely low PO concentrations (Wu et ai, 2000). By contrast, there appear to be vast regions in the South Pacific that have substantial residual PO concentrations, yet there is litde evidence for enhanced N2 fixation (Carpenter, 1983). Clearly, other factors besides the NO to PO ratio must play a role in controlling N2 fixation. Karl et al. (2002) 

The marine Nitrogen-cycle, Carbon-cycle, and climate 

For example, nearly aU cUmate projections for the 21st century suggest that oceanic stratification wiU increase (Sarmiento et al., 1998), primarily as a result of the input of heat into the surface ocean, but in part also by a freshening of the upper ocean. This will likely cause a decrease in export production in the low latitudes, as it will restrict the upward transport of nutrients (Bopp et al., 2001 Matear and Hirst, 1999 Sarmiento et al., 1998 ). On the other hand, export production in the mid-to high-latitudes may actually increase due to lower fight stress in a shallower mixed layer (Bopp et al., 2001). 

This work was supported by the U.S. National Science Foundation (EAR-0404405) and the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG03-00ER63010. I thank Raleigh R. Flood and an anonymous reviewer for their helpful comments. 

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Borgmann, S. (2009) Electrochemical quantification of reactive oxygen and nitrogen challenges and opportunities. 

The challenge in these designs is to lower the NO without degradation in unit stability. In the combustion of fuels that do not contain nitrogen compounds, NOx compounds (primarily NO) are formed by two main mechanisms, thermal mechanism and the prompt mechanism. In the thermal mechanism, NO is formed by the oxidation of molecular nitrogen through the following reactions ... 

In addition, the production of fertilizers from molecular nitrogen is very energy-intensive. In the United States alone, hundreds of millions of barrels of oil are used every year to produce fertilizers. Finding ways to produce noncontaminating and inexpensive fertilizers without huge energy investment is a major challenge to today s chemical industry. 

In the USA, the Clean Air Act of 1970 established air-quality standards for six major pollutants particulate matter, sulfur oxides, carbon monoxide, nitrogen oxides, hydrocarbons, and photochemical oxidants. It also set standards for automobile emissions - the major source of carbon monoxide, hydrocarbons, and nitrogen oxides. An overview of the major standards is given in Tab. 10.2. The levels of, for example, the European Union (1996) are easily achieved with the present catalysts. The more challenging standards, up to those for the ultralow emission vehicle, are within reach, but zero-emission will probably only be attainable for a hydrogen-powered vehicle. 

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