Beijing has been battling an unwelcome, unrelenting and "very unhealthy" smog for many months, much of it made up of particulate nitrogen compounds suspended in the air. Nitrogen pollution in China has kept pace with the country's rapid growth. A study published in Nature last week finds that the rate of nitrogen pollution grew by more than half in the last 30 years.
Researchers from China Agricultural University analyzed data from across the country and found the amount of nitrogen expelled into its surroundings every year rose by 8 kilograms per hectare every year between 1980 and 2010. While most of the nitrogen still comes from waste fertilizer, the source of nitrogen pollution is shifting from agriculture to industry and transport.
One of the byproducts of this saturation is nitrous oxide, one of the most threatening global warmers along with carbon dioxide and methane.
The levels of nitrogen now seen in China are similar to the levels in Europe in the 1980s, before it implemented countermeasures to protect its environment. The deposition is also well in excess of all defined threshold limits and critical load levels, said Peter Vitousek, ecologist at Stanford University and co-author of the study.
Most of China's nitrogen pollution is in the form of ammonia compounds, the kind found in fertilizer. "In the highly productive regions of China, they are putting on often two or three times more fertilizer that a Midwestern American or European farmer would use. It is often in excess of what the crop can take up, and then that is lost to the environment," Vitousek said. "There isn't any way to use fertilizer without some loss to the environment, but in China the amount is far in excess of what's required."
Senior scientist at Woods Hole Research Center Eric Davidson said the excessive fertilizer use is inspired by the need for food security. "One of the biggest drivers of nitrogen pollution in China is that they are increasing their agricultural productivity."
'Astounding' levels of fertilization
"Food security is a very big issue in China, so the Chinese government has been subsidizing fertilizer costs, and as a result farmers in China are using fertilizer rates that are astounding," said Davidson, who is one of the authors of a recent U.N. Environment Programme report on global nutrient supply.
While ammonia compounds associated with farming were five times as prevalent as nitrogen oxides in the 1980s, they are now only twice as common in China's water, soil and air. The rise of coal plants, industrial production and motor vehicles spewing nitrogen oxides has tilted this balance. Coal consumption in China has risen more than threefold since the 1980s, and there are close to 21 times more wheels on China's roads.
The study, which divided the country into six economic regions, reveals that most areas that have the most factories and farms have nitrogen deposition rates higher than the national average. "The difference was really to separate the areas where economic development has been most rapid and where there's not," Vitousek said. "It is also to provide information to the Chinese authorities, who will have to take action regionally."
Both nitrous and ammoniacal pollutants are continuously and rapidly accumulating in China's environment and contribute to aerosol formation. Oxidized nitrogen becomes part of photochemical smog and ozone and is a major component of the infamous PM 2.5, particulate matter less than 2.5 microns in diameter that decreases visibility and is harmful when inhaled because it can penetrate deeply into the lungs.
A half-life of 100 years
"The problem with nitrous oxide is that its half-life in the atmosphere is more than 100 years. Even if we were to mitigate nitrous oxide now, we would still be paying the price for many generations to come," Davidson said.