In Tales of a Fourth Grade Nothing according to Peter why don't leaves turn bright colors in the fall in New York City?

Tales of a Fourth Grade Nothing is a
hilarious book.  I find it funny for a variety of reasons, but each reason more or less
comes back to the fact that Peter is a hilarious narrator.  He moves from topic to topic
at a blinding speed, but it makes sense.  Maybe it makes sense because that is what my
own first grade child does.  It's complete stream of consciousness from my own kid, and
that's what Peter does do.  

For example, the answer to
your question can be found in chapter four.  The chapter begins with Peter talking about
mugging.  At one point, he admits that he doesn't think being mugged would be that bad,
because he would get to go to the police station and look at mug shots of criminals.
 Peter stays on topic for a bit and talks about how his dad has been mugged and why his
mom won't take the subway anymore.  Mugging is a topic on Peter's mind, because woven
through the mugging scenarios is Central Park.  What else is in central park other than
muggers?  Trees of course.  Peter rapidly shifts from mugging to his description that
New York's tree leaves just turn brown and fall off.  They don't change to red and
orange like country trees because of all of the pollution in the
city. 

In the
fall the leaves turn darker and drop off the trees. Sometimes there are big leaf piles
on the ground. It's fun to jump around in them. I never saw bright red, yellow, and
orange leaves until the day my father took us for a drive in the country. The reason the
leaves don't turn bright colors in New York is the air pollution. And that's too bad.
Because yellow and orange and red leaves really look
neat!