A new study has implicated farms as a bigger source of fine-particulate air pollution than all other sources. This is no surprise in Europe, China, Russia and Europe, since food is the most important strategic resource everywhere.
And it's an easy enough problem to solve. As technology continues to improve, emissions will go down anyway, so fumes from nitrogen-rich fertilizers and animal waste that combine in the air with industrial emissions to form solid particles could double and particulate matter would still go down.
Agricultural air pollution comes mainly in the form of ammonia, which enters the air as a gas from heavily fertilized fields and livestock waste. It then combines with pollutants from combustion -- mainly nitrogen oxides and sulfates from vehicles, power plants and industrial processes -- to create tiny solid particles, or aerosols, no more than 2.5 micrometers across, about 1/30 the width of a human hair. The particles can penetrate deep into lungs, causing heart or pulmonary disease; a 2015 study in the journal Nature estimates they cause at least 3.3 million deaths each year globally.
The new study is not the first to flag agricultural pollution; many regional studies, especially in the United States, have shown it as a prime source of fine-particulate precursors. However, the study is perhaps the first to look at the phenomenon worldwide, and to project future trends. It shows that more than half the aerosol ingredients in much of the eastern and central United States come from farming. In Europe and China, the effect is even stronger. The aerosols form mainly downwind of farming areas, in densely populated places where farm emissions combine through a series of chemical reactions with those of cars, trucks and other sources.
"This is not against fertilizer -- there are many places, including Africa, that need more of it," said lead author Susanne Bauer, an atmospheric scientist at Columbia University's Center for Climate Systems Research and the NASA Goddard Institute for Space Studies. "We expect population to go up, and to produce more food, we will need more fertilizer."
The fact that agricultural emissions must combine with other pollutants to make aerosols "is good news," said Bauer. Most projections say that tighter regulation, cleaner sources of electricity and higher-mileage vehicles will cut industrial emissions enough by the end of this century that farm emissions will be starved of the other ingredients necessary to create aerosols. A study this January showed that global industrial nitrogen oxide emissions declined from 2005 to 2014, even as farm emissions boomed. (Fast-growing China and India are exceptions.)
Production of artificial fertilizers has skyrocketed from about 20 million tons in 1950 to nearly 190 million tons today--about a third of them nitrogen-based. Fertilizer production will almost certainly keep growing to keep pace with human population, but the amount of aerosols created as a result depends on many factors, including air temperature, precipitation, season, time of day, wind patterns and of course the other needed ingredients from industrial or natural sources. (In parts of Africa, Asia and the Middle East, aerosols or their precursors come mainly from desert dust, sea spray or wildfires.) The largest increases in farm emissions will probably be in Africa, while the slowest projected growth rates are in Europe, says the study.
The study appears this week in the journal Geophysical Research Letters.