Acid rain falling on Big Sur (photo by Lee Klinger)
Below are the rainfall amounts and pH results from Big Sur for February 2010. In general the pH values are a bit higher (less acidic) than average over the past four years (see here, here, and here).
Rainfall amounts and pH in Big Sur - February 2010
What these and other rainfall pH data show is that the average pH of rainfall events in Big Sur over the past four years fluctuates significantly, with values spanning an entire unit of pH. This means that some events are 10 times more acidic than other events. Who knew that the rainfall pH of individual weather systems varies to such a degree?
Could these fluctuations be due to sampling error? Almost certainly not. After each set of measurements I check the accuracy of the pH meter and sensor using the 4.00 and 7.00 standards used in the calibration. For the 29 sets of samples measured so far this rainy season, the 4.00 and 7.00 standards were measured at 3.99 ± 0.02 and 6.99 ± 0.01, respectively. That darn good accuracy in my opinion.
Sudden Oak Life 
Hi Lee, ‘acidity’ is all very well, but what would also be interesting is to know what kind of acidity. Are we looking at carbonic acid? Sulfuric / sulfurous acid? Hydrochloric acid? Nitric / nitrous acid? Once you have your sample of acid rainwater it should be possible to test for these different species. That will then provide a first level of insight as to where the acidity originates. Of course if you have a friend with a mass spectrometer we can do even better and look at the isotopic composition which may allow us to distinguish eg between S of fossil and marine origin.
Cheers, Oliver.
Hi Oliver, I agree it would be very interesting to have good data on the types and concentrations of acids in the rainfall. The NADP site in Hopland, CA reports some information on rainfall chemistry, but does not analyze for organic acids, only nitrate and sulfate. There appears to be similar levels of nitrate and sulfate in the rainwater, but unfortunately, these data do not really tell us much about the sources. You’re right that it would take a mass spec analysis to get at the bottom of this.
I would be happy to set up a lab and do this work, but realistically that is not likely to happen. Research funds are difficult to obtain if one is an independent scientist like myself. I am hoping that the data I’m collecting and reporting will catch the attention of a concerned researcher somewhere with adequate funding to address the problem.