Update: After a bit of searching I have found that the photo above actually depicts a supernumerary rainbow, not a true quadruple rainbow.
March was a particularly wet month in Big Sur with 8.81” of rain measured at my home, compared to the March average of 5.60” over the past 10 years measured at Big Sur station, just a mile or so away. The wet March triggered landslips and mudslides that closed Highway 1 and prevented me from getting home to record the pH during the height of the rains. (As of this writing, Highway 1 is still closed between Big Sur and Carmel with a projected opening date of early May.) However, I was able to record the pH of four rainfall events and the data are shown below.
The measured pH values are a clearly acidic, about 10 times more acidic than “unpolluted” rain. Of course, we still don’t know the source of the acidity. While atmospheric pollution does not seem to be the primary source of the acidity, it cannot be ruled out. Still, I strongly suspect that the acid rain is primarily biogenic, derived from gaseous emissions of oceanic phytoplankton.
For summaries of Big Sur pH measurements from previous years, please see the following posts: