1. - Data record does not exist in the database (not filled)
2. - Data record exists in database, but there is no data
3. - Data record exists, with value, but invalid, and no null code
4. - Data record exists, with value, invalid, but null code has been set.

In the last case, the AQS value reports the null code as expected.

In the next to last case, wherethe data is invalid and no null code exists, ZZ is put into the position where a null code should be. This is simply to indicate to the user that these records are invalid records to which no null code has been applied. ZZ is not a valid null code. Because the data has been recorded by AirVision, the Method Code would be recorded (copied from the Site/Parameter record into the hourly record).

For the first two cases, the method code would not be reported, because no average has been recorded for the hour. Where the record exists (case 2), you would see the ZZ null code, to again represent a record that has isn't valid but does not have a null code set.

For the first case, there is no record to generate, and the AQS report would just skip that hour (hence the recommendation to always schedule the Fill Data Gaps task!).