[Update: Another late season storm moved through Big Sur yesterday (June 28) depositing 0.42″ of rain with a mean pH of 4.74. I’ve adjusted the seasonal rainfall data below. These new data have no effect on the overall mean weighted pH for the 20010-2011 rain season.]
The 2010-2011 rain season in Big Sur seems to be over, finally, after a late season June storm. This season we received a total of
32.66” 33.08″ of rain, which is very near the average of the previous three years (32.0”). The rainy season lasted for 8 full well over 8 months, with the first rains arriving on October 6, 2010 and the last rains falling on June 6, June 28, 2011.
36 37 events, totaling 23.04” 23.46″, was measured for pH. The season volume-weighted average pH was 4.66 ± 0.10. This mean pH was lower than the average of the previous seasons (mean volume weighted pH 2007-2010 = 4.78).
Season length: October 6, 2010 to
June 6, June 28, 2011
Measured rainfall from
36 37 events: 23.04” 23.46″
Mean volume-weighted pH (± s.d.): 4.66 ± 0.10
The graph below shows the mean volume-weighted pH values recorded from Big Sur for the past 5 rain years, along with the mean volume-weighted pH values reported from six National Acid Deposition Program (NADP) sites along the Pacific coast, from southern California (Tanbark) to southeast Alaska (Juneau). Note that the NADP sites have data only through the 2009-2010 rain year. Data for the most recent rain year are not yet available from NADP.
The linear trends of the stations shown above are mixed, with Tanbark, Pinnacles, and Hyslop stations showing an increase pH over time, and Hopland, Olympic, Juneau, and Big Sur stations showing a decrease in pH. The reason for the differences in trends is not clear. The increase in the Tanbark pH may be due to the fact that it is influenced by pollution form the LA basin, and improved air quality may be contributing to the observed increase in pH at that station.
Note also that the mean pH from Big Sur is notably lower than the reported pH’s from the NADP sites. One reason for this difference may be related to the fact that at Big Sur I am reporting the “Field pH”, whereas the NADP sites are reporting the “Lab pH”. This is consistent with my observation that rainfall samples, as they age, tend to become less acidic. NADP sites report both “Lab pH” and “Field pH” values in the past, but stopped reporting the “Field pH” values several years ago. Another reason that Big Sur pH values are low is due to the close proximity of the open ocean compared to the NADP stations. If the ocean is the dominant source of acidity in the rain, then one would expect higher acidities (lower pH values) in locations nearer the source.