The short answer is every living plant purifies the air. All plants remove carbon dioxide from the air and use it in their metabolic processes. They also use oxygen, but actually produce more oxygen -- as a by-product of photosynthesis -- than they consume.
In a now-famous series of experiments, Dr. Bill Wolverton, a NASA scientist, showed the effectiveness of several species of commonly grown indoor (mostly tropical) plants in removing volatile organic compounds, VOC's, from the air.
Many people believe indoor plants are very effective at reducing illness cause by breathing VOC's in the home and office. The illness is often referred to as "Sick Building Syndrome." It is likely that many plants that are not used indoors are just as effective at removing VOC's and other air impurities from the air outdoors.
Here is a list of plants shown to be effective in removing VOC's from indoor air included in a very nice article on the subject.
Growth on top of Echinacea purpurea (Eastern purple coneflower) July 03, 2012 - I grow purple coneflowers in my garden. ONE plant has something growing on the top of each cone. I would like to know what it is but I don't see how I can add a photo to this post. view the full question and answer
Changing the pH of the soil January 16, 2012 - Hi,
We have a job that has mostly Texas native plants on it. The architect is wanting to drop the pH levels of the soils to acidic levels that we don't feel is good for the plants and the area. ... view the full question and answer