Sentence:  The rate of the gas phase exchange reaction appears to be proportional to the first power of the absorbed light intensity indicating that the radical intermediates are removed at the <WSD>walls</WSD> or by reaction with an impurity rather than by bimolecular radical combination reactions . 
Meaning of the word marked with <WSD> tags:  an architectural partition with a height and length greater than its thickness; used to divide or enclose an area or to support another structure