In addition to reference tests, data audits play an important role in the quality of CEMS data, as emphasized by many EPB staff we interviewed. HJ/T 75 provides the EPBs with guidance on when CEMS data are considered missing, abnormal, or invalid and the methods for handling those situations. For example, if the missing data period is less than 24 h, the missing emission data can be replaced with the average of the valid hourly measurements for the hour before and hour after the missing data period. If the missing data period is greater than 24 h, the missing emission data can be replaced with the highest valid hourly measurement observed within 720 h before the missing data period.