Identical location & time'State_Air' v 'AirNow' measurements reporting different values of PM2.5

[RocketD0g]

A user reported that:

a) We're now getting duplicative station reporting from the 'State_Air' and "AirNow' adapters. It used to be the case that only 'State_Air' reported US diplomatic station AQ data. We should investigate whether 'AirNow' automatically picks up all US Dip stations and whether to deprecate the source.

b) At one of the stations - Kolkata - two different PM2.5 measurements were being reported at the same time. We need to figure out whether the two underlying data sources are truly reporting something different or if it is us. See below:

[sruti]

Looked into this and it turns out our adapter for the Airnow FTP server is wrong. According to this fact sheet, the data from Airnow is reported with a timestamp that indicates the beginning of the measurement period, but our adapter records it as the end. So the data is actually the same from both sources, just shifted by an hour. Should be a simple fix!

[RocketD0g]

This https://github.com/openaq/openaq-fetch/issues/675#issuecomment-585652756 may inform how this issue is fixed (e.g. if we deprecate getting non-US, non-MX, non-CA govt data from AirNow).

cc @sruti