Mt. St. Helens dust, a nuisance on the ground, is a welcome natural "experiment" for atmospheric scientists. The sudden injection of strtospheric debris from a known volcanic source at known times provides valuable "before and after" data. It enables direct measuremen of the effect of stratospheric aerosols on Earth's solar energy income -- an effect which might influence climate.
M. Ackerman and C. Lippens of the Institut d'Aeronomie Spatiale de Belgique, together with French scientist M. Lechevallier, picked up the arrival of the Mt. St. Helens cloud over a Europe last June using a stratospheric aerosol-measuring balloon. Their photographs, published recently in Nature, show a striking contrast.
A pre-eruption view, taken toward the sun at an altitude of 37 kilometers May 7, shows a typical cloudscape with some haziness toward the horizon. A similar post-eruption view, taken 35 km high June 5, is virtually a washout. A large and sharply defined rise in haziness between 15.2 km and 15.6 km altitude obscures the scene.
Meanwhile, an American satellite with the awkward name Strtospheric Aerosol and Gas Experiment (SAGE), launched last year, is tracking Mt. St. Helens debris around the world. This fall satelite readings over North America have been supplemented by aircraft measurements, including Lidar (laser radar) probes. A research team from five universities is working on the project with support from the US National Aeronautics and Space Administration (NASA.)
Scientists expect stratospheric aerosols to affect climate by dimming the sunshine reaching the lower atmosphere and ground. An aerosol layer both absorbs some of the solar energy and scatters some of it back to space. The aerosols, which include sulfuric acid in the case of volcanic debris, also can interact chemically with stratospheric gases and change their energy transmission.
However, while this seems straightforward in theory, "our knowledge of the interaction is sketchy and speculative," notes James P. Friend, principal investigtor for the NASA-funded project. Now, he says, "I'm really excited about the prospects" of learning something directly about the atmospheric impact of volcanic aerosols. Also, the shape and makeup of the aerosol particles should tell something about the processes inside the volcano.
Ackerman, Lippens, and Lechevallier see equally intriguing prospects. A continuing series of balloon flights is gathering more data on how Mt. St. Helens debris scatters sunshine. Together with data on solar energy absorption, these should "help to evaluate more precisely the possible impact of stratospheric aerosol events," they say.
While there's little indication that Mr. St. Helens aerosols themselves will significantly affect climate, one benefit of the eruptions may be a better grasp of the role vulcanism can play in climatic change.