During the 2009-2010 rainy season the precipitation gauge at my home here in Big Sur received a total of 36.24” of rain, which was more than the totals for any of the previous three years. It was a particularly long rainy season, lasting from October 13, 2009 to May 28, 2010.
With regards to rain pH there were 39 measurable events with a mean volume-weighted pH (± s.d.) of 4.80 ± 0.13. The lowest pH reading this past rain year was 4.37 recorded on December 27, 2009, and the highest pH reading was 5.65 recorded on May 18, 2010.
Season length: October 13, 2009 – May 28, 2010
Total rainfall: 36.24″
Mean volume-weighted pH (± s.d.) of 39 measured events: 4.80 ± 0.13
Data from previous years are summarized in last year’s post “Acid rain in Big Sur”.
The graph below shows the mean volume-weighted pH values recorded from Big Sur for the past 4 rain years, along with the mean volume-weighted pH values reported from six NADP sites along the Pacific coast, from southern California (Tanbark) to southeast Alaska (Juneau). Note that the NADP sites have data only through the 2008-2009 rain year. Data for the most recent rain year are not yet available from NADP.
As can be seen in the above graph all the NADP stations are reporting significantly higher values of precipitation pH than I’m finding here in Big Sur. Still, the NADP values do show an enhanced level of acidity in the rain. I believe the differences are due to the varying proximities of the stations to the ocean. All of the NADP stations are located 10 miles or more from the open ocean. If the ocean is a major source of the acidity in the rain (as I suspect it is), then the pH will be lower the nearer you are to the oceanic source.
I’m often asked if pollution from China could be traveling across the Pacific and contributing to the acidic rain here in Big Sur. The answer is that we don’t know and won’t know until someone takes the time to do a detailed chemical and isotopic study of the rain samples. However, SOL commenter “witsendnj” has pointed out a link to a modeling study suggesting that only about 12 percent of the pollution in the western US is coming from sources in Europe and Asia. I don’t put much faith in such simple models, but at least this study provides a starting point for future empirical studies to more accurately assess the sources of acidity coming from Asia.
The data from Big Sur are consistent with the hypothesis that biogenic emissions from oceanic phytoplankton are oxidizing in the atmosphere and producing sulfuric and organic acids that fall as acid rain. Below is a schematic of some of the proposed reactions. Please see the original paper  for more details.