Question

where is there lots of nitrogen in our atmosphere today?

Answer 1

Depends what you mean. About 78% of the atmosphere by volume is N2 gas, and that varies very little from place to place, so far as I know. There are very small amounts of various oxides of nitrogen (N2O,NO,NO2,N2O5, collectively known as NOx) which are created by combustion (air pollution) and lightning, as well as some reduced compounds like ammonia (NH3), which result from lightning and plant and animal decay. So the NOx would be more prevalent in, e.g. the lower atmosphere over big cities, and NH3 would be more prevalent over swamps and bogs.

If you meant to say "why" intead of "where," then the answer is that N2 is an extremely stable molecule, and once any nitrogen turns into N2, it tends to stay there and not do anything else. (As a contrast, O2, oxygen gas, is extremely reactive, and if it were not continually replenished by photosynthesis, in a few million years it would all react with rocks and minerals to produce solid oxides.)

Answer 2

If you're asking where all the nitrogen came from, it is synthesized in heavy stars from carbon and oxygen. When these stars blow up the nitrogen is ejected into the interstellar medium, and some millions of years later more stars (and planets) can condense, and they will contain nitrogen.