Global
Study Shows How Agriculture Interacts With Industry
A new study says that emissions from farms outweigh all other
human sources of fine-particulate air pollution in much of the United States,
Europe, Russia and China.
The culprit: fumes from nitrogen-rich fertilizers and
animal waste that combine in the air with industrial emissions to form solid
particles—a huge source of disease and death.
The good news: if industrial
emissions decline in coming decades, as most projections say, fine-particle
pollution will go down even if fertilizer use doubles as expected. The study appears in the journal Geophysical
Research Letters.
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.
Fabien Paulot, an atmospheric chemist with Princeton University
and the National Oceanic and Atmospheric Administration who was not involved in
the study, said, “You might expect air quality would decline if ammonia
emissions go up, but this shows it won’t happen, provided the emissions from
combustion go down.” That means that pollutants other than ammonia should
probably be targeted for abatement, he said.
Johannes Lelieveld, lead author of the 2015 Nature study
disagreed. “The article underscores that all source categories should be
controlled,” he said.
“One should be cautious about suggesting that food
production could be increased” without increasing pollution, he said because
that “critically depends” on the assumption that societies will successfully
curb industrial emissions.
Lelieveld cited the recent scandal over Volkswagen’s
fraudulent auto-emissions controls, and pointed out that even with the recent
reductions in industrial pollution, most nations including the United States
still have large areas that exceed the World Meteorological Organization’s
recommended maximum of 10 micrograms of fine particulates per cubic meter.
Bauer says that if future industrial emissions go down, much
farm-produced ammonia will end up in earth’s troposphere, roughly 2 to 10
kilometers from the surface. There, lightning and other natural processes may
also help create fine particulates—“but it will be so high up, it won’t be a
problem for us,” she said. Most of these particles would be trapped by
raindrops and removed harmlessly, she says.
All this said, agricultural pollution raises other concerns.
Vast quantities of excess fertilizers wash off fields each year, polluting huge
watersheds; as just one example, each summer an oxygenless “dead zone” spreads
from the mouth of the Mississippi River, fueled by excess nitrogen from
upstream.
More careful fertilizer application would solve a lot of this, said
Bauer. On the other hand, industrial sulfates have been credited with reflecting
solar radiation and thus slightly mitigating ongoing global warming caused by
other fossil-fuel emissions.
“It’s all pollution, but in some sense, some of it
is good,” said Bauer. “We have to decide: do we want a small cooling effect, or
do we want clean air?”
Geophysical Research Letters is a
publication of the American
Geophysical Union. The other authors of the study are Kostas
Tsigaridis and Ron Miller, also both of Columbia University and NASA Goddard
Institute for Space Studies.