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Orange skies over San Francisco, September 9, 2020 (Photo credit: LE Brown)

As six of the largest 20 fires in California history poured smoke into the atmosphere in recent weeks, and new fires were sparked in Oregon and Washington, the skies over the Bay Area and beyond were blanketed in smoke and at times turned unnerving shades of orange. Reports of ash falling flooded social media as entire towns were evacuated due to the fires.

However — despite the eerie glow and blotting out of the sun by an unending blanket of smoke in the atmosphere — fine particulate matter (PM₂.₅) levels on the ground were less elevated than they had been in late August up until September 9. PM₂.₅ concentrations quickly increased the second week of September as the blanket of smoke above mixed into the air we breathe at the ground level. …



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