Another kind of urban aerosol particulate matter of considerable concern is acid fog, which may have pH values below 2 due to the presence of H2SO4 or HNO3. Acid fog formation covers a wide range of atmospheric chemical and physical phenomena. The gas-phase oxidation of SO2 and NOx produces strong acids, which form very small aerosol particles. These, in turn, act as condensation nuclei for water vapor. Acid-base phenomena occur in the droplets, and the droplets act as scavengers to remove ionic species from air. Because fog aerosol particles form in areas of intense acid gas pollution near the surface, the concentrations of acids and ionic species in fog aerosol droplets tend to be much higher than in cloud aerosol droplets at higher altitudes.