Fall 1999 Table
Scientific Whodunit: Identifying Sources of PAHs in Massachusetts
by Andrea Cohen, MIT Sea Grant
On a sunny July day
on the tip of Nahant, a few bucket-toting beachgoers are gathering
rocks and seashells. Above them, on a grassy hillock just downwind
of Logan Airport, Dan Golomb is doing his own collecting with a
pretty sophisticated system of buckets. For the past year, the UMass-Lowell
associate professor in the Department of Environmental, Earth and
Atmospheric Sciences and Ph.D. student George Fisher have been harvesting
polyaromatic hydrocarbons (PAHs), which are key ingredients in smog.
PAHs are formed from
the incomplete combustion of coal, oil, and gas. Black or blue smoke
coming from a tailpipe or smokestack, says Golomb, is one telltale
sign of PAH production. By measuring the amounts and identifying
the sources of PAHs found on Nahant, the researchers hope to learn
how these carcinogenic compounds arrive in Massachusetts Bay. That,
in turn, should help coastal managers develop plans for decreasing
the amount of harmful PAHs not just in Massachusetts Bay, but in
many similar urban coastal areas. The research is funded by MIT
Fisher and Dan Golomb with their novel system for collecting
PAHs. Photo: Andrea Cohen, MIT Sea Grant
PAHs are produced by
a variety of sources: power plants, incinerators, boilers and furnaces,
wood stoves, barbecues, lawn mowers, cars, trucks, and airplanes.
Many PAHs are confirmed carcinogens for laboratory animals and are
suspected carcinogens for humans. And, says, Golomb, "Bottom-living
fish and crustaceans are affected by lesions and tumors that most
likely are caused by the same PAHs."
Just how do the PAHs
get into coastal waters? Some come from spilled crude oil, leaching
from paved roads, and dumped tar and residue oil. But, the primary
source, says Golomb, is atmospheric deposition. That happens in
two manners: dry and wet.
In dry deposition,
airborne PAHs, in the form of vapor and particles, hit the ocean
surface and get absorbed. Explaining wet deposition, Golomb notes
that raindrops and snowflakes contain minute concentrations of PAHs.
"How do they get into raindrops and snow?" he asks. "As
rain or snow falls, it scavenges the PAHs that are airborne."
Those droplets then end up in coastal waters.
To simulate both
the wet and dry deposition of PAHs in the waters off Nahant, Golomb
and Fisher created a novel two-part device thats perched atop
a hill at Northeastern Universitys Marine Science Center.
On one side, the device contains a funnel to collect precipitation.
The other side (for dry collection) contains a small pool of water
on which particles can land-and that comprises the trickier
part of the project.
"We created a
water surface which we expose to this direct impaction of mole-
cules and particles," explains the researcher. "The difficulty
was in creating the continuous surface of water, maintaining it
through summer, when there is evaporation, and through winter, when
there is freezing, so that there is always a constant layer of water
on top of our collector, which absorbs the particles and the vapors."
In an earlier project,
Golomb made continuous measurements of PAHs in Nahant and in Truro,
Mass. That work provided data on the total amount of PAHs that comes
down per year per season. "But this project," says Golomb,
"is really a whodunit," aimed at identifying the areas
from which PAHs are streaming. Thus, the researchers are concentrating
on episodes: collecting rain from one storm or PAHs that arrive
when the wind is blowing from just one direction.
This is accomplished
via a wind vane next to the collection device and an innovative
software program written by Fisher-whose earlier career as
a meteorologist in the Air Force has come in handy. The program
receives information from the wind vane and, depending on its sampling
instructions, directs a motor to open or close a lid over the two
Fisher explains: "When
its not raining, we have eight different sectors of compass
we want to sample. If we want to collect from the north, the door
will open and only collect when its not raining and the wind
is coming from the north. The software allows us to change the sector
that we are sampling. That will help us determine perhaps where
the PAHs are coming from."
With the collection
device, wet precipitation trickles down a funnel into a cartridge
filled with an adsorbent that removes PAHs. On the dry side (which
collects PAHs on the pool of water), the water also runs down a
funnel with a similar adsorbent. The adsorbed PAHs are then flushed
out with a solvent so that they can be analyzed back at the lab
with a gas chromatograph-mass spectrometer, which provides
data on PAH content in grams per square meter per unit of time.
That work is being carried out by Eugene Barry, a professor in the
Department of Chemistry at UMass-Lowell.
Golomb and Fisher are
particularly interested in particles transported by northeast winds
because Logan Airport lies directly southwest of the collection
spot. According to Golomb, aircraft traveling to and from Logan
are a major source of PAHs, which can be seen as a hazy dark thread
in an otherwise blue sky.
However, the researcher
notes that the project doesnt aim to point fingers at one
airport or power plant, or even at a particular state or city. Instead,
it focuses on source areas, figuring out where the clouds transporting
PAHs originated. Using trajectory models published by NOAA, the
investigators are able to backtrack clouds for 24 or 48 hours, thereby
discerning "dirty clouds vs. clean clouds, or rain that contains
a lot of PAHs and rain that contains little," explains Golomb.
"Clouds that sweep
up the Washington/Boston corridor seem to bring more PAHs than clouds
that gather over the ocean and then precipitate here," he says.
"This is fairly logical. Clean clouds came from systems that
develop over the ocean and clouds from the southwest seem to bring
more bad deposition."
Eventually Golomb anticipates
that the findings from this research will more closely pinpoint
the culprits in PAH production. That, in turn, could help coastal
managers with their jobs. "Suppose we found out that incinerators
are a major source, then the managers would have to insist that
the incinerators be run with more controls to catch more smoke,"
Gazing from Nahant
to the airport, then to the Salem power plant in the northeast and
the Saugus incinerator to the west, Golomb concedes that coastal
pollution is inevitable with such urban sprawl. But by plucking
a few PAHs from the air now, he hopes to ensure that less pollution
winds up in Massachusetts Bay in the future.