Fundamental surface chemistry gives new insight into the way acid rain attacks limestone.

The reaction of carbonate surfaces with the gaseous pollutant sulfur dioxide plays a role in atmospheric chemistry and in the accelerated degradation of limestone buildings and monuments.

To understand these processes on a molecular level, Vicki Grassian and colleagues at the University of Iowa, Iowa City, US, studied laboratory reactions of sulfur dioxide on calcium carbonate surfaces, using several different analytical techniques, in the presence and absence of water.

The researchers found that adsorbed water increased the amount of sulfur adsorbed onto the carbonate surface by up to thirteen times, because of increased mobility of the surface ions.

Sulfur was present on the surface as sulfite and sulfate ions. Initially, the two ions were present in equal measure, but adding water increased the ratio of sulfite to sulphate to five to one, by allowing islands of sulfite to form on the surface.

 

Acid rain leaves limestone buildings crumbling

 

Brian Mitchell, an astrochemist from the University of Rennes, France, said the findings are of great significance. 'In cities, buildings and statues are attacked by acid rain. This study serves to indicate the character of this attack and the conditions where it is accentuated,' said Mitchell.

'If atmospheric chemistry models are to accurately describe the chemistry of the troposphere, laboratory studies designed to understand the mechanisms of these reactions as well as determining accurate kinetics of these reactions are needed. We are very much interested in connecting fundamental molecular surface chemistry to global impacts,' said Grassian. 

Joanna Stevens