Effect of altitude

Q: Is an analyser's calibration data affected by height differences between where it was calibrated (eg in the UK) and where it is used (eg 600 metres above sea level in a city like Madrid)?

A: There is no effect from changes in atmospheric pressure on electrochemical sensors as they operate on a volume per volume basis - a part per million is a part per million at any height. The only pressure effect on sensors is caused by pressure pulses/transients or back pressure from the exhaust.