be photooxidized to S(VI) species on an aqueous suspension of
Fe 2 O 3 (20,26) as well as by means of oxidants produced in nature
by transition metal compounds ( 27 ).
B. NITROGENOXIDES
VOCs react readily with nitrogen oxides especially under
favorable atmospheric conditions (intensive sunlight, high
humidity, oxygenation, presence of transition metal species,
etc.). To date, only few of the reactions are recognized, but the
results of extensive studies focusing on the reactivity between
NOxand the VOC parent compounds in various biochemical sys-
tems are accessible and may be useful in solving environmental
problems; thus, the most relevant of these experiences are pres-
ented in this chapter.
Nitrogen oxides (NO, NO 2 ) belong to the most important spe-
cies active in tropospheric photochemistry, whereas N 2 O is not
degradable in the troposphere and undergoes photodegradation
only in higher atmospheric parts, where more energetic
(l<250 nm) UV radiation is available. Nitrogen monoxide plays
an important role in the formation of ground-level ozone in
highly populated areas, and in acid depositions. NO is emitted
by various anthropogenic and natural sources, such as fossil fuel
combustion, biomass burning, lightning, microorganism activity
in the soil. Nitric oxide (NO) is also naturally generated from
the amino acid arginine (C 6 H 14 O 2 N 4 ) in the reaction catalyzed
by an enzyme called NO synthase. In the atmosphere, NO is con-
verted fast to NO 2 by O 3 and peroxy radicals (HO 2 , RO 2 , where
RO 2 represents peroxy radicals derived from VOCs).
NO 2 dissociates to NO and O(^3 P) by exposure to solar radiation
(l420 nm):
NO 2!hv
NOþO^3 P
ð 7 ÞO(^3 P) combines rapidly with an oxygen molecule to form ozone
and in this way NOxis the most important precursor of O 3 in
the troposphere. NO and NO 2 interconvert in a few minutes. In
daytime, the lifetime of NOxin the lower part of troposphere is
typically 1–2 days and is determined by the oxidation of NO 2
by the OHradical to produce nitric acid:
NO 2 þOHþM!HNO 3 þM ð 8 Þ302 ZOFIA STASICKA