Photochemical smog

•Carbon, with an oxidation number (O.N.) of +4, as CO 2 or H 2 CO 3, or HC0 3- ;
• Sulfur, O.N. of +6, as SO 4 ;
• Nitrogen, O.N. of +5, as NO 3 ; and
• Iron, as Fe(III) , in the form of insoluble Fe(OH) 3.
Near the bottom, in the hypolimnium, the water is oxygen-depleted since it has no
contact with air and since 02 is consumed when biological material, such as the dead
algae that have sunk to these depths, decomposes. Under such anaerobic conditions,
elements exist in their most reduced forms:
• Carbon, with an O.N. of -4, as CH 4 ;
• Sulfur, O.N. of - 2, as H 2 S;
• Nitrogen, O.N. of -3, as NH 3 and NH 4 +; and
• Iron, as Fe(II), in the form of soluble Fe 2+ .
Anaerobic conditions usually do not last indefinitely. In the fall and winter, the top
layer of water is cooled by cold air passing over it, so that eventually the oxygen-rich
water at the top becomes more dense than that below it and gravity induces mixing
between the layers. Thus in the winter and early spring the environment near the
bottom of a lake usually is aerobic.