If you wish for more UV light to reach the poles, you can deplete the ozone layer over them. This is exactly what happened on our world - ozone depleting chemicals reached the upper layer of the atmosphere and due to wind currents they concentrated over pole over the decades, specially the south pole.

The hole over the south pole is shown as the blue-ish hues below:

Source: https://earthobservatory.nasa.gov/images/38835/antarctic-ozone-hole-1979-to-2008

The ozone layer does not block all UV, and it is not the sole blocker of UV radiation. It does screen out most UV-C and UV-B radiation, which are the most harmful forms for life.

If you wish for a mechanism that does strip the poles of all gases, though, for a lower atmospheric pressure at the poles... I do not think such a thing would be feasible. The difference in pressure would mean wrath-of-god force winds at the polar boundaries. The pressures would either be equaled over relatively short geological time, or the permanent mega-hurricanes would make for a planet possibly devoid of life, with much different geological features due to erosion, most possibly a mix of both.

If you reduce the constraints to allow for planets similar to Earth during planet formation but different from current Earth: maybe a tectonic fault that causes the plates on the poles to collapse, for a hadean climate over them. This might be stable for hundreds of millions of years. The exposed mantle would heat up the air above it, causing the air to expand. It will be forced out of the polar regions with a lot of force, which may create a pressure boundary. Where it meets colder air it will go over and above, reducing its pressure over the pole while also maintaining a vortex over the region. Such a planet would be hellish at the poles, so the whole point of the other question about having UV-feeding algae would be lost.

If you wish for more UV light to reach the poles, you can deplete the ozone layer over them. This is exactly what happened on our world - ozone depleting chemicals reached the upper layer of the atmosphere and due to wind currents they concentrated over pole over the decades, specially the south pole.

The hole over the south pole is shown as the blue-ish hues below:

Source: https://earthobservatory.nasa.gov/images/38835/antarctic-ozone-hole-1979-to-2008

The ozone layer does not block all UV, and it is not the sole blocker of UV radiation. It does screen out most UV-C and UV-B radiation, which are the most harmful forms for life.

If you wish for a mechanism that does strip the poles of all gases, though, for a lower atmospheric pressure at the poles... I do not think such a thing would be feasible. The difference in pressure would mean wrath-of-god force winds at the polar boundaries. The pressures would either be equaled over relatively short geological time, or the permanent mega-hurricanes would make for a planet possibly devoid of life, with much different geological features due to erosion, most possibly a mix of both.

If you reduce the constraints to allow for planets similar to Earth during planet formation but different from current Earth: maybe a tectonic event that causes the plates on the poles to collapse, for a hadean climate over them. This might be stable for hundreds of millions of years. The exposed mantle would heat up the air above it, causing the air to expand. It will be forced out of the polar regions with a lot of force, which may create a pressure boundary. Where it meets colder air it will go over and above, reducing its pressure over the pole while also maintaining a vortex over the region. Such a planet would be hellish at the poles, so the whole point of the other question about having UV-feeding algae would be lost.