In this Chemistry question Is there any reason to fear personal exposure to rain with a pH of 3.1? I haven't gotten any answer, but there was some discussion about the reality of the measurement.

So I'll ask here, is a sustained rainfall with a pH of roughy 3.1 possible on Earth? Of course it would have to be related to some serious sources of pollution - perhaps intensive coal burning over a large geographic region. I say 'sustained' to rule out any fluke or highly unusual situations. Could I really fill a plastic bucket with a substantial amount of pH 3.1 water by putting it outdoors, under the sky, in the rain, in the right place on Earth?

Oh, for this question I'd like to exclude unusual situations related to volcanic eruptions as well.

  • $\begingroup$ One aspect is that rain is very poorly buffered because it is low in dissolved solids. That means that a small amount of hydrogen ion can have a big effect on the pH. $\endgroup$ – haresfur Oct 23 '16 at 23:47

A quick literature search seems to confirm Gordons estimation, even at the scale of a whole bucket:

[T]he annual mean pH, based upon samples collected weekly during 1970-1971 and weighted proportionally to the amount of water and pH during each period of precipitation, was 4.03 at the Hubbard Brook Experimental Forest, New Hampshire; 3.98 at Ithaca, New York; 3.91 at Aurora, New York; and 4.02 at Geneva, New York. Measurements on individual rainstorms frequently showed values between pH 3 and 4 at all of these locations. Data from the National Center for Atmospheric Research included precipitation pH values as low as 2.1 in the northeastern United States during November 1964.

Patel, C. K. N., E. G. Burkhardt, and C. A. Lambert. "Acid rain: a serious regional environmental problem." Science 184 (1974): 1176-1179.

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    $\begingroup$ nadp.isws.illinois.edu/data/AIRMoN/… is a data explorer for one site in the AIRMoN network. December 1992 shows a pH value of 3.67. I would definitely believe that if you go back to the 1970s, you could get pH values as low as 3. $\endgroup$ – Jareth Holt Oct 23 '16 at 19:59
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    $\begingroup$ Wow! that's incredible - those numbers are stunning! Thank you for finding this numerical data. $\endgroup$ – uhoh Oct 23 '16 at 22:50
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    $\begingroup$ Not only that, but the raInwater gets even more acidic if it falls in pine needles in the leaf litter, through H+ release by ion exchange. $\endgroup$ – Gordon Stanger Oct 23 '16 at 23:31

Raindrops gain a small amount of acidity as they fall through carbon dioxide in the air, but that's not what this question is about. Raindrops commonly start off as ice crystals which have to nucleate around something, usually an aerosol particle such as soot, clay, bacteria, sulphur dioxide , dimethyl sulphide, etc. If a small raindrop nucleates around an acidic core, then a pH of 3.1 is certainly possible. Much depends upon the raindrop size. With a larghe raindrop - up to about 4 mm maximum (it breaks up if any larger) there is likely to be huge dilution, whereas in a very small raindrop, as in a fine mist, there can be minimal dilution, and maximal acidity.

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    $\begingroup$ OK, but what about filling the bucket? $\endgroup$ – farrenthorpe Oct 23 '16 at 17:56
  • $\begingroup$ @farrenthorpe "...with a substantial amount" doesn't have to be completely full of course, but you are right, a light mist is good to know about, but I'm asking about rain you'd get drenched in. (hopefully not dissolved as well). $\endgroup$ – uhoh Oct 23 '16 at 22:44
  • $\begingroup$ This is a really helpful - and interesting - answer. I hadn't thought about the actual process of droplet formation, or that droplet size could correlate with final pH at the surface! $\endgroup$ – uhoh Oct 23 '16 at 22:45
  • $\begingroup$ Would it eventually dissolve an iron bucket? pH 3 is the sort of acidity where it starts to corrode railway lines. $\endgroup$ – Gordon Stanger Oct 23 '16 at 23:33

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