When the leaves fall off the trees in autumn, how much does this
affect the O2 balance in the atmosphere in terms of quality of air?

Probably not significantly. Much of the O2 content of Earth's
atmosphere is delivered by the abundant tropical plant life, which
does so year round. This oxygen is rapidly distributed via global
weather patterns.

But, this would make for a good independent science project, measuring
the abundance of both O2 & CO2 as a function of both season &
latitude.

tellme: The most overlooked oxygen contributor on the planet is guess
what: Not trees, but oceanic algae. Some estimates run as high as 70%.
After all, algae are chlorophyll containing plants which produce
oxygen, just like trees. If the entire Amazon rainforest were to
disappear, it would be tragic, but the oxygen content of the
atmosphere would barely change.
Regards,
K22

Interesting comment, K22 ...

Me? I always thought algae was something that lived only in MY fish tanks.

Well, if it's so prolific, let's find a use for it ...

Food? Fuel?

Please realise, K22, that a suitable response from you could make us
both billionaires. (I know you are already.)

So, as denco always says ...

Looking forward

Bryan

You also have to remember that as leaves fall off the trees in the
northern hemisphere because it's autumn, leaves are starting to grow
on trees in the southern hemisphere, because it's spring!

This means that across the globe, "leaf density" remains pretty much
constant throughout the year.

probono and others:  Unless you have a private outdoor pond, as I do,
you can never truly appreciate the horrors of algae growth. It takes
years of experience to subdue it.
  Anyhow, there are some uses for oceanic algae. Seaweed is used in
some cuisines, but not mine for sure. Alginates are natural thickeners
used by the ton, in ice cream and other products. The name *alginate*
describes the source. No doubt there are other applications, but does
anyone really care?
K22

Hrmm. I was going to try and Officially answer this question. But it
seems google doesn't want accept any new answer people. So I will just
post it as a comment.

The levels of O2 are altered by the fall/winter in the northern
hemisphere, but not to a detectable level. Plants (both deciduous
(leafy) trees and many bushes and grass) do not perform photosynthesis
during the fall and winter months. This results in a cyclical
variation in Carbon Dioxide (CO2) levels. This season variation is
~5-6 ppmv: parts per million by volume. The total amount of CO2 is
approximately 380 ppmv. So the CO2 level cycles by ~1.5% annually.

O2 should change for the same reason, but the fraction of O2 to CO2 in
the atmosphere is 549:1 (by volume). Or O2 is 209,460 ppmv to CO2 ~380
ppmv. So the percentage variation of O2 is > 0.002%.

So as far as quality of air, it is non-significant.

Toufaroo-

While it is true that fall in the northern hemisphere is spring in the
southern, the southern hemisphere is not dominated by temperate
deciduous forests. They do exist in parts of New Zeland, Australia,
and to a lesser extent in southern south america. But most of the
forests of south america are tropical, and do not undergo seasonal
leaf loss. So there is a annual variation in the leaf density.

Water also stores oxygen. The colder the water, the more oxygen it can
store. This is why whales go to the arctic to feed, it can support
more life. Any gas in a liquid will also try to reach an equilibrium
with the air around it. Plants also consume O2 during the night and
give off CO2. There's just so much atmosphere, that it doesn't really
matter much. Increase the CO2 amount in the air, and plants can
perform photosynthesis quicker. I imagine if you took away all the
wind and jetstreams and such, and had stagnant air, then you could
possibly measure the change, however, even without 'wind' any
inbalance would try to seek equilibrium.