“Having been in my progress so often misled by taking for granted the results of others, I have determined to write as little as possible but what I can attest by my own experience.” – John Dalton, Independent Scientist (1766-1844)
UPDATE (May 2, 2009): In the past 24 hours it has rained 0.93″ here in Big Sur with a pH of 4.56. I have amended the seasonal data accordingly.
UPDATE 2 (May 5, 2009): An additional 0.08″ was recorded, but the sample was too small to obtain an accurate pH reading. The seasonal data have been amended.
Anyone who thinks acid rain does not occur in pristine, unpolluted environments had better think again. Or better yet, go get some litmus paper and check it out for yourself.
Scientists studying Sudden Oak Death have dismissed acid rain as a relevant factor in oak mortality, pointing out that areas of severe decline are near the coast, upwind of the major sources of pollution, thus rainfall could not be acidic. Here along the coast of California storms blow in from the ocean where there are few sources of manmade pollutants. Rainfall pH, then, is not expected to be any more acidic than about 5.6, which is the theoretical pH of unpolluted rainwater in equilibrium with atmospheric CO2.
However, I have learned from past research that rainfall in pristine environments can sometimes be quite acidic. So for the past three years I have been recording rainfall at my home in Big Sur using a Stratus RG202 rain gauge. Readings of rainfall pH have been made with a high-precision Beckman (Model Φ250) pH meter using a standard two-point (4.00 & 7.00) calibration. The rain station is located 1.2 miles from the coast at 36° 16’N; 121° 49’W, and 922’ (281 m) elevation.
Here are the results:
Season length: Dec 17, 2006 (begin readings) – May 4, 2007 (partial season)
Partial season rainfall: 13.98”
Mean volume-weighted pH (± s.d.) of 24 measured events: 4.72 ± 0.16
Season length: Sep 22, 2007 – Apr 23, 2008
Total rainfall: 27.63”
Mean volume-weighted pH (± s.d.) of 24 measured events: 4.92 ± 0.15
Season length: Oct 13, 2008 – Apr. 8, 2009 May 2, 2009 May 5, 2009
Total rainfall: 31.13″ 32.06” 32.14″
Mean volume-weighted pH (± s.d.) of 23 24 measured events: 4.67 ± 0.21 4.66 ± 0.20
For comparison, the plot below shows field pH readings from Big Sur along with times series of lab pH readings from six NADP stations (Tanbark, So CA; Pinnacles, Central CA; Hopland, No CA; Hyslop, OR; Olympic, WA; & Juneau, AK) currently operating along the Pacific coast. None of these stations are truly “coastal” in that they are all 10 miles or more inland. Data were downloaded yesterday from the National Atmospheric Deposition Program website.
So what does all this mean?
First, it means that the rainfall here in Big Sur is notably more acidic than would be expected of unpolluted rainwater. Because the pH scale is logarithmic, the average rainfall pH of this past rainy season is about 10 times more acidic than “expected”.
Second, all of the sites shown in the plots above are reporting pH values that are slightly lower (more acidic) than might be expected, except for the Tanbark site, which is in the LA basin and is probably influenced by pollution.
Third, I do not know the kinds of acids in the Big Sur rainwater or their sources. But I do have some ideas, which I will share in later posts.
For now I would like hear from readers about their ideas on what is happening here? Why acid rain in Big Sur? What kinds of acids could be in the rain? Where are they coming from? How long has the rainfall been acidic? How might acid rain be affecting the oak ecosystem?