It frustrates me that there are so many of these networks and they don't talk to each other. I deliberately went with a system that measures multiple values, but then I see other people going "we only measure one thing instead of nine so our units cost half as much".
I should try to script at least my systems so they write into as many of the open networks as I can manage.
Unfortunately, yes, the space is fragmented. A long while ago I chose https://luftdaten.info/en/ because sensor firmware, DB, and tools are truly open source (GPL, ODbL, MIT). Next to supporting many sensors, it also allows for your own back-ends (API, InfluxDB). The community is awesome and has hacker values.
The inexpensive part makes citizen science viable in sub-economic and hobbyist settings.
https://openaq.org/ is trying to consolidate air quality data, but I'm uncertain of their licensing and goals.
I should try to script at least my systems so they write into as many of the open networks as I can manage.
https://www.uradmonitor.com/