Isotopes and their percentage existence in atmosphere When we talk about percentages of isotopes of an element in the atmosphere, is it percent by mass or volume or weight or something else?
Short answer: it's the percent by number of particles.

Longer answer: It's the proportion of particles of a particular mass for that element.  To chemists, a per particle basis is also commonly referred to as a per mol basis or a molar basis.

A way that it can be measured is by mass spectrometry - you pass the atoms through the apparatus which shows the mass to charge ratio. E.g. if you could pass $\ce{Cl}$ molecules, you'd see 2 peaks in the resulting graph - one is at 35 and the other at 37 marks which shows the mass/charge. First peak would be about 3 times larger (thus there were 3 times more particles). Thus chlorine would be about 75% isotope 35 and 25% isotope 37. So while this approach measures mass of each isotope, it also determines the relative molar amounts of each isotope.