<p>CiGas evaluates the NF labeling application and the conformity of the NF with respect to the MGs and SOPs.The NFs are visited by the units for audits with the objective to check the performance of the NF according to the MGs and SOP provided by the audit unit. Deviations from data quality objectives (DQOs) deserve special attention. During an audit, the following will be evaluated: (1) The sampling and instrument set-up, (2) the calibration and zero gas systems, (3) the QA/QC procedures implemented, (4) the training and instructions sessions, (5) data from calibration, zero gas, target gas, and standard addition, (6) the data delivery, (7) the results from intercomparison exercises, (8) the uncertainty evaluation, (9) the logbooks (instrument, measurements, station), (10) the scientific use of the data and (11) the overall equipment of the station.</p>