The first image, courtesy NOAA HMS, shows the widespread fire and smoke activity in the Great Lakes and Plains regions of the U.S. These fires are mostly concentrated within these regions and are producing a large smoke plume over the regions. Theses smoke plumes are increasing the AOD in the regions, as can be seen in the next image, courtesy MODIS Aqua. There is also a large increase in AOD in the Southwest region, which is mainly due to dust.
The first image below, courtesy NRL NAAPS, shows the total optical depth for 18:00 Z. There appears to be strong dust activity in some of the same regions in the MODIS AOD plot above. The next image is from GEOS-Chem, which shows the total AOD at 550 nm. This shows strong amounts of Saharan dust being transported towards the U.S. which may have been the cause for the dust aloft over the western part of the country.