carbondioxide absorption

Carbon takeup is not stored in the leaves, but in wood. Take a tree, look at its mass growth during a certain period of time (say 20 years) and multiply that amount by 44/24

The mass number of C = 12, O = 16, so total of CO2 = 44.

By photosynthesis the tree converts CO2 in C, storing 12 mass units in the wood while releasing O2, being 32 mass units, in the air. The carbon content of a tree is about half its mass, so that explains the figure of (44/12)/2 = 44/24

You might want to take a look at this page:

http://www.plant-trees.org/resources/Calculating%20CO2%20Sequestration%20by%20Trees.pdf

Indeed, trees give of CO2 at night, but as you already mentioned, there is a net uptake and it can be calculated as described above.

Unfortunately, leaves cannot be considered in the uptake calculation as these fall of during autumn and decompose, creating CO2 and other gasses like methane.

Adding to the cooling effect by the CO2 uptake is the efficiency of photosynthesis. About 20% of incoming sunlight is converted into sugars (and wood) and 80% is converted to heat, whereas on other materials like concrete the full 100% of solar energy uptake is converted to heat (and then I left out other effects as reflection etc etc). So there is both a direct and long-term effect of cooling using trees.

I'd recommend to start planting.