California, Bay Area Wildfires – Urban Soot Sampling

As of today (12th Sept 2020), wildfires in California and the rest of the west coast of the USA have consumed several million acres of forest cover. The national disaster took a dramatic turn when the skies turned into apocalyptic orange shades right after a heatwave during the labour day weekend. The skies from the Stanford campus looked as shown in pictures below on 9th Sept 2020. Since then, the shades have turned more normal but the air quality remains one of the worst in the world.

I didn’t mind the colour of the skies on the 9th too much as it was very similar to the sky colour during dust storms that I had experienced as a kid growing up in Rajasthan, India. But the real scare arrived when I was collecting my bike which was parked outside throughout the day. As I was about to sit on it, I noticed tons of tiny white specks. My immediate response was to scold myself to be more disciplined with cleaning up my bike regularly. But soon, I realized that the specks were everywhere, other bikes, cars, plants etc, which quickly brought me to hypothesize that the specks are actually ash generated from wildfires that was now depositing everywhere. This suddenly forced me to realize that the conditions are really grave, as we were all breathing these particles which might be accumulating in our lungs. Even a short 10 min bike ride to the lab caused coughing and irritation in my eyes.

After coming back home, I tried foldscoping some soot samples collected from my bike, and the images looked as shown below. A discussion with my colleague Ray revealed that the wildfires have been known to generate soot superaggregates, that are much larger than several mircons. In the sample I collected, I could see particles ranging in size from 10s to 100s of microns. My guess for these tremendously large particles is that there must be secondary aggregations happening as the ash settles back after being elevated to several hundreds of meters due to hot rising plumes and ocean drafts.

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