How should data be handled during instrument downtime or data outages in an ambient monitoring program?

• A. Flag downtime, document reason, and follow QA/QC plan for potential data substitution or exclusion; maintain downtime records.
• B. Ignore downtime and proceed as if data were valid.
• C. Replace downtime with random data.
• D. Downtime should be kept secret and not documented.

Answer: (A)

When an instrument is offline or data are unavailable, the essential practice is to preserve data quality by clearly signaling the gap and following the QA/QC protocol. Flag the downtime in the data set, document the reason for the outage, and apply the QA/QC plan to decide whether data should be substituted using approved methods or excluded from analysis. Maintain a complete downtime log with start and end times, the instrument involved, and the outage cause. This creates transparency for data users, supports regulatory and programmatic accountability, and ensures any data treatments are defensible. If the plan allows substitution, use only approved methods (such as imputing with data from a nearby or collocated monitor) and document the method used; if not, exclude the period to avoid introducing bias.