Urban grime shown to reemit nitrogen into the air

BBC : Urban grime consists of a combination of chemicals, soot, and ash—emitted by exhaust pipes and chimneys—that settles on buildings and other surfaces. Although unsightly, it was thought to provide a permanent sink for harmful gases such as nitrogen oxide. A recent study shows, however, that exposure to sunlight can cause some of the grime’s nitrogen to be reemitted into the atmosphere. To test that theory, James Donaldson of the University of Toronto and colleagues conducted field experiments in Leipzig, Germany, in which they placed one shelf of window-glass beads in the Sun and one in the shade. They found that the beads exposed to the Sun showed a 10% drop in nitrogen levels compared with the ones in the shade. In addition, the nitrogen emitted appears to be in the form of nitrogen dioxide and nitrous acid, both of which are hazardous to human health. Therefore, the researchers say, cities with a lot of grime, such as London, may need to address this newfound source of pollution.