Re: what is the better time for walking up the hill for fresh air ?

Hi, 

Noticing that you are in HK, I think morning is still the better time to go 
uphill. Because human activities and pollution, which consume oxygen, are 
much heavier than that in morning, the air is far from fresh. 

The answer to your question would be yes if we were in an optimal condition 
with plants as the only variable. However, in natural conditions many 
limitations will generally limit photosynthesis. For example, pollution will 
inhibit photosynthesis and plant growth; high temperature will increase 
respiration of plant, which also consumes oxygen. Further, a large part 
(probably the majority) of photosynthetic productivity occurs in open 
oceans, mostly by oxygenic prokaryotes. So in a big city, the amount of 
oxygen produced by its trees is far less than the amount this city consumes.  
Therefore, noon is not a good time to go out and get fresh air. 

Reference:
Some websites are good for general public to understand photosynthesis:

 http://photoscience.la.asu.edu/photosyn/education/photointro.html
 http://www-new.life.uiuc.edu/govindjee/ptime/