Nitrogen 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 per mole. But to break nitrogen's triple bond takes almost twice as much energy: 942 kJ per mole. 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. There are nitrogen atoms 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 and which facilitates all sorts of reactions in life.
So to survive, plants rely on bacteria which decompose animal and plant waste into reactive nitrogen-containing ions such as nitrates(NO3-) and ammonium(NH4+). Some plants have set up sophisticated partnerships with certain bacteria species that have evolved 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, a more subtle consequence of ammonia enrichment has been less noticed.
China uses 18.7 million tons of nitrogen fertilizer per year. There have been suggestions that because of government subsidies they overuse it, but presently they use less than the U.S. on a per capita basis. When a densely populated country like China has to focus its agriculture in only one area within its boundaries, the concentration of ammonia in the air above fertile lands increases. Ammonia then goes on to react with acids formed from industrial and vehicle emissions of sulfur and nitrogen oxides. 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, 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+.
For two days in January 2012, the city of Beijing was covered with thick smog, forcing the cancellation of almost 700 flights at airports. 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
Current and Future Emissions of Ammonia in China
http://www.sciencedirect.com/science/article/pii/S1352231001003016
http://www.tandfonline.com/doi/abs/10.1080/02786820119445
http://ukpmc.ac.uk/abstract/MED/8875828/reload=0;jsessionid=4xMaEY2mISBUYJD2TRi1.110
http://www.indexmundi.com/en/commodities/minerals/nitrogen/nitrogen_t5.html