EARTH & ENVIRONMENT
When it’s hot, trees boost air pollution
Simulations show how much city shrubs add to ozone formation
BY ASHLEY YEAGER
Planting trees is often touted as a strategy to make cities greener, cleaner and
healthier. But during heat waves, trees
actually boost air pollution levels. When
temperatures rise, as much as 60 percent
of ground-level ozone is created with the
help of chemicals emitted by urban trees
and shrubbery, new simulations suggest.
While the findings seem counterintu-itive, “everything has multiple effects,”
says Robert Young, an urban planning
expert at the University of Texas at
Austin who wasn’t involved with the
study. The results, he cautions, don’t
mean that cities should stop planting
trees. Instead, more stringent measures
are needed to control other sources of
air pollution, such as vehicle emissions.
ATOM & COSMOS
TRAPPIST- 1’s seventh planet is a chilly world
When astronomers announced in February the discovery
of seven planets orbiting a supercool star, details about the
outermost planet were sketchy (SN: 3/18/17, p. 6). No more.
The seventh planet is chilly and definitely no place for life, the
international team reports May 22 in Nature Astronomy.
The seven-planet system TRAPPIST- 1 is 39 light-years from
Earth in the constellation Aquarius. Follow-up observations of
the system reveal that TRAPPIST-1h is about three-quarters the
size of Earth and orbits its star in just under 19 days. Although
TRAPPIS T-1h is much closer to its star than Earth is to the sun,
the exoplanet’s star has only 8 percent of the sun’s mass. As a result, TRAPPIS T-1h gets about as much starlight as the icy dwarf
planet Ceres, in the asteroid belt, gets from the sun.
Such limited light makes the planet too cold (–100° Celsius)
to harbor liquid water and therefore life as we know it, the
researchers report. Other TRAPPIST- 1 planets (see Page 18)
are more likely to be life-friendly. — Ashley Yeager
GENES & CELLS
Mouse sperm survive space to fertilize eggs
Mouse sperm could win awards for resilience. Sperm freeze-dried and sent into space for months of exposure to high levels of
solar radiation later produced healthy babies, researchers report
online May 22 in Proceedings of the National Academy of Sciences.
City trees help reduce storm water
runoff, provide cooling shade and store
carbon. But trees and shrubs also release
chemicals that can interact with their
surrounding environment to produce
polluted air. One chemical, isoprene,
can react with human-made compounds,
such as nitrogen oxides, to form ground-level ozone, a gas that can be hazardous
to human health. Monoterpenes and
sesquiterpenes also react with nitrogen
oxides, and when they do, tiny particles
similar to soot build up in the air. In cities, cars and trucks are major sources of
In the new study, reported online
May 17 in Environmental Science & Technology, Galina Churkina of Humboldt
University of Berlin and colleagues
If humans ever embark on long-term space flights, we’ll need
a way to reproduce. One potential hurdle (beyond the logistical challenges of microgravity) is the high amount of solar
radiation in space — radiation exposure is 100 times as high on
the International Space Station as on Earth. Those doses might
cause damaging genetic mutations in banked eggs and sperm.
To test this possibility, Japanese researchers sent freeze-dried
mouse sperm up to the space station, where the sperm spent
nine months. When rehydrated back on Earth, the sperm showed
some signs of DNA damage compared with earthly sperm.
But when the researchers used the space sperm to fertilize
eggs in the lab and then injected the eggs into female mice, the
mice birthed healthy pups that were able to have their own
offspring. The researchers suspect that some of the initial DNA
damage might have been repaired after fertilization.
If mouse sperm can survive a trip to space, perhaps human
sperm can, too. — Laurel Hamers
compared simulations of chemical concentrations emitted from plants in the
Berlin-Brandenburg metropolitan area.
The team looked at two summers: 2006,
when there was a heat wave, and 2014,
when temperatures were more typical.
At normal daily maximum summer
temperatures, roughly 25° Celsius on
average, plants’ chemical emissions
contributed to about 6 to 20 percent of
ozone formation in the simulations. At
the height of the 2006 heat wave, when
temperatures soared to over 30°, plant
emissions spiked, boosting their share
of ozone formation to up to 60 percent.
Churkina says she wasn’t surprised to
see the seemingly contrary relationship
between plants and pollution. “Its magni-
tude was, however, quite amazing.”
The results, she notes, suggest that
campaigns to add trees to urban spaces
can’t be done in isolation. Radical reduc-
tion of air pollution from various sources
is also needed, she says. s
NEWS IN BRIEF
TRAPPIST-1h, the outermost planet in its seven-planet system (orbital
paths shown), receives too little light from its star to support liquid water.