Thanks to Joe Osborne from GASP for helping us with our continuing education on air pollution. Joe sends this information on ground level ozone and inversions:
Ozone is almost exclusively a summer pollutant because sunlight is
necessary for ozone to form.
Temperature inversions, which tend to trap locally-generated pollutants
in valleys, can occur year-round but are somewhat more frequent in the
summer as well. That said, inversions are more likely to result in
higher concentrations of particulate matter than ozone because:
1. particulate matter pollution is present year-round, significant
ground-level ozone concentrations are not. So even though winter
inversions are less common than summer ones, when they do occur, they
may well result in unhealthy particulate matter concentrations, but are
exceedingly unlikely to result in unhealthy ozone concentrations.
2. inversions tend to occur late at night or early in the morning--times
when the sun is not shining and ozone concentrations are low.
So in the summer we're likely to see elevated ozone and PM
concentrations. In the winter, elevated ozone concentrations are almost
unheard of (outside of some rather freakish examples like the wintertime
ozone exceedances associated with oil and gas development in Wyoming),
and elevated PM concentrations occur somewhat less frequently than in
the summer, but do still occur (sometimes aided by a winter inversion).
I do a somewhat better job of explaining this (and also illustrate with
some charts) at:
Joe Osborne, Legal Director
Group Against Smog and Pollution