Exotic Molecules in
the Atmosphere:
Peroxyacyl Nitrates
Peroxyacyl nitrates, RC(O)OONO2,
have had the peculiar distinction of being unknown to chemists
until the simplest compound (peroxyacetyl nitrate, R-CH3,
hereafter PAN) was identified in urban photochemical smog in the
late 1950`s and early 1960`s. Peroxyacyl nitrates have no known
sources and are formed in-situ in the atmosphere in
reactions involving hydrocarbons and oxides of nitrogen. They
play an important role in the chemistry of the atmosphere - not
only in urban air but also in the unpolluted troposphere - and
have received attention for their toxicity to man and to plants
and for their importance in the long-range transport of reactive
nitrogen on regional and global scales.
The interesting properties of
peroxyacyl nitrates will be the topic of this lecture, with focus
on the chemistry (Kinetics and thermodynamics) involved in their
synthesis, the stability, and their fate and persistence in the
atmosphere. Measurement methods and ambient levels of PAN - from
Arctic air to the tropics - will be described using recent
examples including those of studies now being carried out in
Brazil. The chemical reactions that link the formation of PAN to
the use of ethanol as a vehicle fuel will be discussed with focus
to its relevance to urban pollution in Brazil. Higher molecular
weight peroxyacyl nitrates will be discussed, and recent
laboratory studies will serve as a basis to speculate on the
possible formation of as yet uncharacterized peroxyacyl nitrates
from biogenic emissions in tropical regions such as the Amazon.
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