Nitrogen Dioxide comes with an unpaired electron on the nitrogen, hence it is in fact a radical. This should normally make it a very reactive compound, but it can dimerize to dinitrogen tetroxide. The downside then is that the bonded nitrogens are both positively charged, rendering it quite unstable again. Somehow, despite all this, the combination makes it a relatively(?) stable compound.
But what if H2O attacks this gas (nucleophilic attack)? Shouldn't we get H2NO3 as a result? And would this radical anion not be stabilized again by dimerization to N2O64-?
Also, NO has an unpaired electron as well. Can't it dimerize to N2O2, just like NO2 does? The Lewis structure looks more stable to me than the structure of N2O4.