After a few days of lower particulate pollution we are experiencing elevated PM2.5 levels again. Concentration peaked at just over 70ug/m³ overnight (AQI 158 "Unhealthy"). See the 24h graph below.

PM2.5 over the 24h up to 10.00 on 2018-01-11

The usual suspects for causes are low temperatures leading to increased boiler/heater activity, combined with low average wind speed (3mph).