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Polarstern Cruise

The Observatory's Chief Scientist Alex Pszenny and former Staff Scientist Andy Wall participated in this research cruise on the German vessel Polarstern. The scientific expedition took Alex and Andy from Bremerhaven on the north coast of Germany, out into the Atlantic Ocean to the Canary Islands and then on to Capetown, South Africa. Follow their journey with daily journal updates, photos from onboard, and information from the ship's meteorological service.

Course plot IMAGE: Course plot

The Observatory's scientists, with funding from the National Science Foundation's Atmospheric Chemistry Program, were part of this international voyage specifically geared towards investigating air chemistry in the marine atmosphere. They worked with colleagues from the University of Virginia as well as other scientists from institutions in Germany and the United Kingdom. The research objectives for the Observatory's scientific team were to gain a better understanding of the global significance of halogen chemistry in marine air (learn more about this by following the links below). Throughout their journey Alex and Andy sampled air, analyzed samples, and enjoyed life at sea.

Objectives

The research team from the Observatory participated in a larger effort to study the chemistry of atmosphere over the Atlantic Ocean. There were many other groups of scientists aboard the Polarstern focusing on various aspects of atmospheric chemistry. The Observatory team's specific objectives were:


  1. To measure diel (day/night) variability of chemical species involved in cycling of halogens (chlorine, bromine and iodine) in the marine atmosphere along a north-south transect through the Atlantic Ocean.
  2. To characterize the acidity and associated chemical composition of near-surface marine aerosols as functions of size, time of day, location, and air-mass history.
  3. To collaborate with other participating scientists in assessing the importance of halogen "activation" chemistry and related influences on the cycling and lifetimes of tropospheric ozone and sulfur.

Background

Tropospheric ozone (O3) plays a central role in regulating Earth's environment. Photolysis of O3 in the presence of water vapor produces the extremely reactive hydroxyl radical (OH), which is the main chemical species responsible for removing for many important compounds from the atmosphere including methane, other hydrocarbons, carbon monoxide, and chlorofluorocarbon (freon) substitutes. At the Earth's surface, high concentrations of O3 can be toxic to humans and vegetation: it is one of the principal components of smog. In the middle and upper troposphere, O3 is a major "greenhouse" gas.

Simplified reaction scheme of marine bromine chemistry IMAGE: Simplified reaction scheme of marine bromine chemistry

Until the 1970s it was thought that tropospheric O3 was mainly supplied by transport from the stratosphere, and removed by deposition involving reactions with vegetation or seawater at Earth's surface. Research since then has shown that tropospheric O3 is in fact largely controlled by chemical production and loss within the troposphere.

Natural and anthropogenic aerosols also play key roles in atmospheric chemical processes and in the transmission of sunlight through the atmosphere. Aerosols influence gas phase chemistry by acting as sources or sinks of reactive chemical species and by decreasing or increasing the penetration of sunlight through the atmosphere. As such, they affect the oxidizing or cleansing capacity of the atmosphere, which determines the chemical lifetimes of many trace gases and of aerosols themselves. It is not possible to understand, let alone predict, the chemical state of the atmosphere without taking into account its multi-component and multiphase nature.

For technical background on halogen chemistry and its relationship to ozone and aerosols, see these two journal articles:


Equipment

Cascade Impactors
Cascade Impactors segregate and sample aerosols (particles) by size. They were attached to staging on the top deck of the Polarstern.

Aerosol sampler IMAGE: Aerosol sampler

Air was drawn through the Impactors and the particles fell on and stuck to specially-prepared plastic sheets that were later removed and stored for further analysis in the lab back in Bartlett, NH.

Among the chemical species measured were ammonium and sulfate, which usually comprise most of the mass of particles smaller than 1 micrometer (one millionth of a meter) in diameter. These tiny particles are mainly responsible for the haze that limits visibility in areas affected by air pollution. They also affect the penetration of light vertically through the atmosphere and can thus contribute to climate forcing.


Mist chamber

Mist Chamber

A Mist Chamber is a device that draws air through a fog-like mist of very clean water. Soluble gases become trapped in the water. The water is then analyzed using an Ion Chromatograph. Gases measured include ammonia, sulfur dioxide, and nitric acid, all of which are present in the atmosphere in trace amounts (a few parts per billion or less). These analyses were done aboard ship.


Ion Chromatograph
An Ion Chromatograph (IC) is an instrument that is used to analyze samples that come directly from the Mist Chambers and samples that are later processed from the Cascade Impactors.

We can quantify the amounts of cations (sodium, ammonium, potassium and others) and anions (chloride, bromide, nitrate, sulfate and others) were qualified using the IC. From the data, the concentrations of these various species in the air at the time of sampling can be calculated.

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