Looking at the NOAA HMS Smoke and Fire Product (top left), the clumps of small red dots indicate masses of currently burning fires, with smoke plumes being indicated by the grey patches. The plume in North Dakota and Montana is most likely due to wildfires in the area, particularly the one in north central Montana yesterday. The MODIS Terra image (top right) has relatively normal AOD values in this area of smoke, although elevated AODs were seen in Texas, Mexico, and off of the East Coast.
The EPA AIRNow combined AQI loop (bottom left) illustrates good AQIs over much of the US with the exception of the Southeast, where moderate and even small periods of USG AQIs were seen. The NAAPS aerosol model's sulfate surface concentration image for 1800 UTC (bottom right) predicts increased sulfate levels on and off of the East Coast, which could help explain the increased AOD and AQI levels in the region.