An air quality monitoring network built on a new generation of Internet of Things sensors. Using a new generation of laser particle counters to provide real-time measurement of PM1.0, PM2.5 and PM10. PurpleAir sensors are easy to install and only require a power outlet and WiFi. They use WiFi to report in real time to the PurpleAir map. Sells air quality sensors that are pretty expensive. Don't know if it'd be possible to upload data from other kinds of sensors.
Account creation requires authenticating with a Google account only.
They have a JSON API but it's read-only and just a straight dump from their database: https://www.purpleair.com/json
There is also a Thingspeak API: https://www2.purpleair.com/community/faq#hc-thingspeak-api
They also seem to have a REST API but you need an API key and ChannelID. Not a big deal, really.
I don't know if this service is suitable for my purposes.