It all depends on how quickly you want this done. Earth's atmosphere is slowly leaking away each time an air molecule in the upper atmosphere reaches escape velocity (Earth's magnetic field and ionization of molecules in the upper atmosphere complicates things a little, see Atmospheric escape). The speed of air molecules is dependent on the square root of the absolute temperature so you need a rather large temperature increase to make the molecules go faster so the atmosphere will leak away more rapidly. The current mean speed of air molecules at room temperature (300K) ~ 347 m/s and speeds follow the Maxwell-Boltzmann distribution. The escape velocity is 11186 m/s. You only need to increase the fraction of air molecules with velocities greater than the escape velocity. 473
Increasing the temperature to 1500K will do the trick, although this will increase the pressure at first due to evaporation of volatile materials such as the oceans and the earth crust.
A good way to increase the temperature substantially is something like the Theia event wherein a Mars-sized planet collided with the proto Earth forming both the Earth and the Moon as we know them. As there are no Mars-sized bodies with a remote chance of a collision course with Earth to be found in the solar system at this point in time, it would have to be a rogue planet. For your purposes, this would have the added benefit of ripping away a substantial part of the atmosphere in the collision process. Still, it would take a few dozen megayears for cool down and evaporation.
An altogether different method is cooling down sufficiently such that the nitrogen and oxygen molecules freeze out of the air, which occurs 54.36K (the melting point of dioxygen). Moving Earth's orbit beyond Neptune would achieve this. Of course a collision with a rogue planet and as a result of momentum transfer, shifting the orbit of the resulting pair to a much wider, probably highly eccentric, orbit around the Sun will make all of this go a lot faster, but still a megayear for the end result. 208