one of the serious problems facing us is (that)of how to deal with the pollution problem.这里为...one of the serious problems facing us is (that)of how to deal with the pollution problem.这里为什么用that 不用the one或者是其他代词呀?

这里的that= the one. that表示特指,既可以代替单数的可数名词,也可以替代不可数名词; 其复数为those. 而one只能代替可数名词：
This tea is better than that (=that tea).
This book is dearer than that (= the one) on the desk.
These books are dearer that those (= the ones) on the desk.