So things have progressed with this project. I now have a working prototype that runs from the RavPower power bank mentioned here
This combined with solar input gives quite a good run time on WiFi
I have also got 3G connectivity using OpenVPN working. This allows me to tunnel the SQL data back to the home server where Grafana does its thing!
I have launched checkyourair.co.uk which has a live feed on PM2.5 stats and hopefully I will get to the point where there are multiple sensors in Preston.