Sentence:  Mrs. Meyrick's house was not noisy: the front parlor looked on the river, and the back on gardens, so that though she was reading aloud to her daughters, the window could be left open to <WSD>freshen</WSD> the air of the small double room where a lamp and two candles were burning. 
Meaning of the word marked with <WSD> tags:  To remove or cover unpleasant qualities such as staleness, bad odour or taste (in air, breath, water, etc.).