Atmospheric O2 mole fraction is measured using a commercially available 'Oxzilla II' analyser (Sable Systems International Inc.), which employs two 'MAX‐250' lead fuel cell O2 sensors (Maxtec Inc.). The MAX‐250 is a galvanic type O2 sensor consisting of a lead anode, a gold oxygen cathode, and a weak acid electrolyte. A non‐porous Teflon membrane, which is bonded to the gold cathode, separates the air from the weak acid electrolyte. O2 from the air permeates the membrane and undergoes electrochemical reduction in the cell, which generates a current that is directly proportional to the partial pressure of O2 at the cell sensing surface.