Why is mass number always a whole number?

Well, the mass no. = no. of proton+no. of neutron in an atom. So, There is not any atom which don't have any of them.

that's why it is always whole number

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.