People generally say that they experience a "fresh" or "clean" smell
after opening the windows.  But what does this actually mean?  What is
that smell?

I would like to know what is that pleasant smell that fills my bedroom
after I open my windows, and how that smell comes to be.  The smell
becomes apparent quickly, within 15 minutes of so of opening my
windows (perhaps even more quickly when there is a breeze), and I
generally notice it when I leave the room and re-enter it.  In fact,
distinct smells develop when I open the windows in any part of my
home, smells that are unique to each room.

I know that it is not just the smell of the outdoors (the trees,
grass, etc.), because if I walk outdoors, I do not experience the same
smell.  And I cannot identify it as the smell of any specific item in
my room.  I also know that the overall nature of this smell seems to
have remained the same, across the different bedrooms in which I have
lived.

In your answer, please first explain what is happening as I open a
window in my bedroom.  (This may address changes in temperature,
humidity, scents, etc.)

Then please explain how this smell develops.  My hunch is that it has
to do with the exchange/mixing/reaction of the outdoor air with the
indoor air (which is comprised of all the things I have in my room -
my clothes, my personal care products, my blankets and sheets, etc.). 
If this is true, please describe the chemical/particulate composition
of the outdoor air vs. the indoor air to explain how this particular
smell might come to be when the two combine.  And please let me know
how it is that gases/particles/breezes interact to create this smell. 
But, if I am wrong and it does not have anything to do with the
chemistry of the air, then please do let me know what is actually
happening!

(By the way, I have a pretty far-flung hypothesis that this phenomenon
may have something to do with the oxidation that occurs when terpenes
settled on the surfaces and linens in my room react with ozone from
the outdoors.)

Thank you!  This is much appreciated.

An interesting question. I suspect that most of the smell is just the
general outdoors as you pointed out. i.e. the mixture of flora and
fauna and fresh air. You dont generally smell it when immersed in it
i.e. outdoors because your olfactory nerves are now overstimulated and
stop smelling it. The same reason why you dont smell the natural
(unpleasant smell) of your closed in bedroom. You smell your bedroom
stench when you go out and come back in. Like you do when entering
someone elses home there is a natural smell which eventually you dont
notice after entering for a while. So I think you are just smelling
the good outdoors but in small doses hence you notice the new smell.
Immerse yourself in it and you wont smell it anymore.