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believe the answer you're looking for has to do with the pressure-altitude-voltage derating related to corona discharge, which increases (gets worse) with higher altitude and peaks at around 80-100,000' Typically this is considered to occur at votlages above 300 V peak, but be sure you are accounting for all transients and internal voltages.
The effect of high altitude on nitrogen is essentially the same as it would be for air, since altitude affects all gases in a similar manner. If you filled a tire with nitrogen at sea level, then took that same tire to 8000 feet (assuming the air temperature was the same), the tire would read about 3.8 psi higher on the tire gauge.