Greetings again from 64° south.
As I write, persistent sea ice continues to stand between most of Palmer’s scientists and their objects of study. For access to the water and the islands around the station, scientists here rely on a small fleet of Zodiac inflatable small boats. Sturdy and versatile, Palmer’s black Zodiacs are very similar to craft the Navy uses to deliver its SEAL teams to hostile shores the world over. And yet, as I’ve learned over these past three weeks, no inflatable boat can contend with the foot and a half of refrozen brash ice that currently chokes the inlet here.
Without open water, LTER scientists studying the bacteria and phytoplankton in the sea around the station can’t even get get out to nearby stations “B” or “E” to collect water samples. (There are 20 or so predesignated LTER sampling locations around Arthur Harbor; I’ve posted a chart here.) And things aren’t much better for Palmer’s recently-arrived team of penguin researchers: Most of the penguin colonies Bil Fraser and his team have been studying for the past four decades or so are on islands even further offshore.
For two atmospheric scientists, however, the sea ice hasn’t been a major obstacle: Greg Roberts and Craig Corrigan, a team from the Scripps Institution of Oceanography, have been finding plenty of the small particles they came to study in the air and clouds above the station. Greg and Craig brought with them to the Antarctica an autonomous package of instruments to measure aerosols — the hundreds to tens of thousands of solid and liquid particles that are suspended in every cubic centimeter of the earth’s surface-layer air. (An aerosol technically refers to a suspension of particles in any gas; usually, and in this case, the gas is air.)
The two scientists were the inaugural speakers Oct. 31 in the 2013-2014 field season’s series of weekly “science talks.” (At Palmer, a member of the science party traditionally gives a presentation on Thursday nights to the other scientists and station crewmembers about his or her research, or on some other scientific topic.) As those assembled for the presentation on Thursday learned, aerosol particles play an incredibly important role in regulating our weather and our climate, even at very low concentrations: Without aerosol particles, for example, there would be no rain anywhere on earth.
Aerosols can be natural or anthropogenic: Clouds are aerosols in which billions of tiny water droplets have condensed around dust or other “seed” particles suspended in the air. (These particular aerosol particles are called cloud condensation nuclei, or CCN — one of the many parameters Greg and Craig are measuring here with their instrument.) Volcanic eruptions, anthropogenic exhaust emissions (from factories, power plants, ships, and cars, for example), and smoke from burning of biomass in forests also add to the number of particles in our air.
The ocean, it turns out, is also an important natural source of aerosol particles: When the wind drives the ocean’s waves to crest and break, sea spray (or spindrift) breaks into tiny droplets, sending particles of sea salt into the air. In addition, certain chemical compounds produced by phytoplankton — chief among them dimethylsulfide, or DMS — are emitted from the ocean into the surrounding air. DMS can then be oxidized to a number of other sulfur-containing compounds that serve as highly effective cloud condensation nuclei.
To identify the various sources of aerosols and obtain information about the particles’ physical properties, Greg and Craig have incorporated several different instruments into their Portable Aerosol Observing System, or PAEROS. The instruments within the package measure everything from the total number of particles in each volume of air to the size distribution of the particles (how many particles of what size are present) and whether those particles serve as cloud condensation nuclei, or CCN. The instrument also records several meteorological parameters, including rainfall and wind velocity and direction.
While PAEROS is autonomously measuring and recording data on the many terrestrial sources of aerosols in the air here — including those both natural and anthropogenic — Greg and Craig are equally as interested in capturing data on the various sea salt, sulfur, and organic compounds that come from the sea and are transformed into cloud condensation nuclei above the ocean. DMS, for example, is produced in the surface layer of the ocean through a complex series of metabolic processes in phytoplankton. Sea salt comes from breaking waves, and organic aerosol particles are thought to be generated by the bursting of bubbles at the ocean’s surface.
The relationship between DMS production and cloud formation was the basis for the “CLAW hypothesis,” a complex feedback loop described in 1987 through which phytoplankton could regulate their environment through increased DMS-based cloud formation. (The concept was formally described by Charlson et al. in a 1987 paper in the journal Nature; it takes its name from the first letters of each of the paper’s authors’ last names.)
Given their many sources, aerosol particles are ubiquitous: Air in a polluted city might contain concentrations as high as 50,000 particles per cm3, Greg and Craig’s data show that even the air in a place as devoid of human activity as Antarctica can contain concentrations of 100 to several thousand particles per cm3.
Because the “signal background” here in Antarctica is so low — i.e., there are normally very few natural or anthropogenic aerosols in the “clean” air above the Peninsula here — the PAEROS instruments can detect very subtle changes in the types and distribution of particles that might be missed if the instrument were deployed almost anywhere else in the world.
But even here in Antarctica, PAEROS can find evidence of anthropogenic material in the air: One of the instruments can discern particles containing “black carbon” — complex, dark-colored graphitic compounds that are produced through the incomplete combustion of fossil fuels and biomass — from other types of particles. Against the very low aerosol background here at Palmer, even the soot from a single passing ship’s smokestacks was enough to register a signal on the instrument. (Greg and Craig have seen almost no black carbon when winds are blowing clean air off the Antarctic continent to the south; when winds blow from the north, however, they can detect small anthropogenic signals in the air coming off South America.)
Greg and Craig will both depart Palmer in the next few weeks, but PAEROS will continue to record data on black carbon and other aerosol sources here until the field season ends in March. It looks increasingly likely that one of those other sources — phytoplankton DMS production — will elude the two scientists, at least personally. Both had hoped to be present to see what PAEROS was measuring when the first bloom of phytoplankton occurred this spring. With the sea ice still firmly in place, however, the season’s first bloom is still a few weeks or months away. Once the phytoplankton starting doing their thing, Greg and Craig will have to check in remotely from California to see exactly what PAEROS is saying.
Much more science to come from down here at Palmer.
A few images below from the past two weeks on station: