How is dimerization done?
The electronic structure of NO2 leaves an unpaired electron on the nitrogen. This is one of the reasons it is quite reactive. But it is also the reason why it can easily dimerise to N2O4 as the two unpaired electrons in two molecules of the gas can pair up creating a weak bond. The dimer is favoured at low temperatures when the thermal energy isn't large enough to shake the dimer apart (I'm simplifying a bit). This is why the colour of the gas changes depending on temperature as the dimer is colourless but the NO2 is brown.