A group of nitrogen-containing compounds have been defined as NOz in
addition to the terms NOx and NOy. Describe briefly the typical
compounds in NOz and the environmental impacts of this group.

NOx = NO + NO2
NOz = HNO3, HONO, NO3, N2O5, HNO4, PAN, RONO2, ROONO2 (eg products of
atmospheric oxidation of NOx)
NOy = NOx + NOz

NOx itself is involved in the ozone cycle.  And of course smog.  

Obviously, HNO3 (nitric acid) is a component of acid rain.  PAN
(peroxyacetyl nitrate) is involved in long-range transport of NOx (eg,
NOx reacts to form PAN which has a long lifetime - 100 days in cold
temperatures, and PAN degrades back to NOx), which leads to increasing
background ozone levels (not an important source for urban areas, but
for areas far away from other NOx sources this can be important)

-Marcus

(answer extracted from Seinfeld & Pandis, Atmospheric Chemistry and
Physics, one of the bibles of atmospheric chemists, as well as some of
my own knowledge)