Although the fog burned off in many areas, a shallow lens of haze remained over much of the valley during the afternoon.
My thinking during the day was that this was probably predominantly natural haze as PM2.5 concentrations were quite low on Christmas Eve morning. However, the PM2.5 observations from Hawthorne Elementary show we reached levels unhealthy for sensitive groups Christmas evening. In fact, the increase from Christmas Eve morning is astounding. Normally one sees an increase in PM2.5 during a developing and persistent inversion of about 10 ug/m3 per day. What happened from Christmas Eve to Christmas was more than double that.
So, why was the PM2.5 so high? I'm not sure why, but there are a few possibilities. One is that in this case the inversion didn't develop aloft and descend slowly as is often the case when a ridge builds in. Instead, it developed as the warm front warmed temperatures aloft and precipitation cooled the near-surface airmass, resulting in a shallow cold pool right from the get go. Thus, any emissions were confined to a shallow layer. Indeed, the sounding collected in the late afternoon on Christmas shows a very shallow inversion based right at the surface, with a well mixed layer aloft. Little wonder the upper benches were so clear, with a different story in the valley.
I'm not entirely satisfied with that as an explanation, however, since the buildup in pollution seemed remarkably rapid. I wonder if the holiday "sparked" an increase in yule log burning, and that this, along with the shallow nature of the cold pool, resulted in the rapid increase in PM2.5. Having more wood fires with such a shallow cold pool would be a recipe for a rapid pollution rise. Being in unrestricted action, perhaps many more people than usual decided to go for the holiday fire.
A third possible factor is that the high humidity is causing somewhat inflated values, although the sampler used for the time series above I thought was somewhat immune to this effect (some samplers are not as water droplets inflate values).
Regardless, it's unfortunate that pollution along the valley floor was so high during this event.