SCIENCE 2.0 Website
I have posted the article below about PM2.5 pollution on Science 2.0. This website is a combined science magazine, blog and Facebook-type site for scientists and science educators. It has been in existence for about five years. Everything ever written for the site is accessible and free of charge. The articles are categorized under six subject areas: physical sciences, life sciences, earth sciences, medicine, social sciences and culture.
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Made in China: PM2.5 pollution from ammonia
Fill a glass with water and place it under a running tap. Water will flow out of the glass at the same rate that it flows into it. Although the same molecules are not in the glass at any given moment, we maintain the same volume of water. We have what's known as a steady state.
Nitrogen in the air is a steady state and is the most common gas in the atmosphere. In terms of molecules or volume — they are proportional to each other under the same conditions of pressure and temperature — about 78% of air is N2. Air's other main component, diatomic oxygen, has a bond-dissociation energy of 494 kJ/mol. But to break nitrogen's triple bond takes almost twice as much energy, 942 kJ/mol. This makes nitrogen unreactive within the atmosphere's normal temperature range.
This is a problem for plants. The nitrogen atom is not only needed for proteins, which include enzymes that speed up most of life's reactions, but are in the nucleotide bases of DNA and RNA. And nitrogen is also found in ATP. It's the molecule that is continuously produced either through photosynthesis or with the energy-releasing breakdown of glucose. It facilitates all sorts of reactions in life.
So to survive, plants rely on bacteria that decompose animal and plant waste into reactive nitrogen-containing ions, such as nitrate (NO3-) and ammonium (NH4+). Some plants have set up sophisticated partnerships with certain bacterial species involving ways of directly converting atmospheric nitrogen into NH4+. Other bacteria have the ability to return molecular nitrogen to the atmosphere by first converting ions that are unabsorbed by plants into nitrites. It's this last step that normally keeps the nitrogen in the air at steady state.
But we have further complicated the nitrogen cycle. With seven billion people on the planet, 1.3 billion in China alone, we have chosen to accelerate the growth of edible plants partly by manufacturing ammonia from nitrogen and hydrogen using high pressures and temperatures. Along with other forms of reactive nitrogen, these compounds are added to the soil. Whereas for decades so-called runoff of excess nitrogen (and phosphorus) fertilizer has drawn a lot of attention, the more subtle consequence of ammonia enrichment has been less obvious.
China uses 18.7 million tons of nitrogen fertilizer per year. There have been suggestions that government subsidies result in overuse, although presently China uses less than the U.S. on a per capita basis. When a densely populated country like China focuses its agriculture in only one geological area within its boundaries, the air’s ammonia concentration above these fertile lands increases. Ammonia goes on to react with the acids of oxides of sulfur and nitrogen generated by industrial and vehicle emissions. The ammonium compounds, one being ammonium sulfate ((NH4)2SO4), form very fine crystalline particles with diameters of 2.5 microns (2.5 x10-6 meters) or less. Particulate matter of these dimensions is referred to as PM2.5.
If you've ever had to use a fire extinguisher in your home, you may be aware that most of them contain monoammonium phosphate (NH4H2PO4) and/or ammonium sulfate. Cleaning up afterwards is not exactly one of life's pleasures. The dust is extremely fine, difficult to pick up and very irritating if inhaled. But those particles are probably still not as small as those that form in the atmosphere; those are 1/40th as wide as the average human hair. Their small size gives them a longer residence time in the air due to Brownian motion, and their size allows them to lodge deeply into the lung's air sacs. They aggravate most respiratory diseases and lead to premature deaths.
The conclusion from a joint study involving Health Canada, New York State University School of Medicine and The American Cancer Society was that each 10 μg/m3 elevation in fine particulate air pollution was associated with approximately a 4%, 6% and 8% increased risk of all-cause, cardiopulmonary and lung cancer mortality, respectively. Other studies have specifically shown that PM2.5 is far more worrisome than PM10 or sulfate or H+.
Further troublesome evidence was observed in January 2012 when the city of Beijing was covered with such thick smog that for two days almost 700 flights at airports were forced to be cancelled. In the same month, the city finally decided to report analyses of PM2.5, whereas previously all of China only reported PM10. This in itself is not going to ameliorate the emissions problem in China, but it's one small step in the right direction.
Sources
- Colorado State University Ammonia Best Management Practices
http://ammoniabmp.colostate.edu/link%20pages/impacts%20of%20ammonia.html - Current and Future Emissions of Ammonia in China
http://webarchive.iiasa.ac.at/rains/reports/Asia/China_nh3_klimont_
color_withref.pdf - http://www.sciencedirect.com/science/article/pii/S1352231001003016
- http://www.tandfonline.com/doi/abs/10.1080/02786820119445
- http://europepmc.org/abstract/MED/8875828/reload=0;jsessionid=4xMaEY2mISBUYJD2TRi1.110
- http://www.indexmundi.com/en/commodities/minerals/nitrogen/nitrogen_t5.html
