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In my textbook (Olmsted and Williams 4th ed) it is given that pH of pure water is 7, pH of unpolluted rain water is between 5-6 and pH of acid rain is between 4-5.

It is obvious that pH for acid rain would be low. But how is it acidic, even slightly, for unpolluted rain water? Shouldn't unpolluted mean without any solutes or dust in water? Shouldn't it be similar to pH of pure water (7)?

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At first thought, it might seem that rainwater should be clean like distilled water, as it evaporated, condensed, then fell back to earth. It sounds like a condenser or distillation apparatus in a laboratory. But in reality, the process is very different.

To start with, every raindrop must have a particle to condense upon, called a cloud condensation nucleus. According to this Wikipedia article:

Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100th the size of a cloud droplet on which water vapor condenses. Water requires a non-gaseous surface to make the transition from a vapor to a liquid; this process is called condensation. In the atmosphere, this surface presents itself as tiny solid or liquid particles called CCNs.

Furthermore, the composition of CCN is frequently very acidic, having formed from sulfuric acid or sometimes weak organic acids. These types of CCN form in both polluted as well as pristine environments. Both natural and man-made sources of sulfur emissions result in the photochemical oxidation to sulfuric acid, which forms new CCN which are very hygroscopic (water absorbing).

The type of CCN described above are most commonly formed over land. In marine environments, there are natural sources of sulfur gases, but most CCN form from evaporated sea spray, which is of course much more pH neutral.

However, regardless of how the cloud drop was originally formed, they all have another source of acidity from the atmosphere; they absorb carbon dioxide, which is rapidly converted to carbonic acid when dissolved in a cloud droplet. It is this absorption of naturally occurring, acidic carbon dioxide that is the primary reason that essentially all rain is somewhat acidic.

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    $\begingroup$ So the term "unpolluted" is not equivalent to pure $\endgroup$ – YAHB Mar 30 '17 at 17:22
  • $\begingroup$ @YAHB , exactly. The natural atmosphere is a pretty complex system, and even in unpolluted environments the composition of a simple raindrop can very greatly, but they will always be somewhat acidic due to CO2 absorption in addition to the other natural compounds discussed above. There is also wash-out of dust particles during the early part of a rainfall event that make natural rain even "dirtier"! $\endgroup$ – airhuff Mar 30 '17 at 17:29
  • $\begingroup$ @airhuff - The pH of "unpolluted" rainwater is governed by 2 equilibria constants, that for the Henry's law partitioning of carbon dioxide between air and water, and the other for the dissociation of carbonic acid to proton and bicarbonate. Using p$\ce{CO2}$ of $10^{-3.5}$ and given K$_{H} = 10^{-1.5}$, and pK$_{\rm a} = 10^{-6.3}$, we thus get $[\ce{H+}] = 10^{-5.65}$ or pH = 5.65 for rainwater. You might include these details to expand your answer to address the mechanisms controlling the pH more precisely than you have. $\endgroup$ – Todd Minehardt Mar 30 '17 at 17:41
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    $\begingroup$ @ToddMinehardt , thanks much for your input and work on your calculations. I shied away from further focus on CO2 because though I believe my highlighted statement is very true, the quantitative aspect is much more complex, as even pristine environments can have rain pH much lower than 5 due to natural emissions of sulfur gases, nitric acid formation from lightning, etc. See here. It would be nice to have some more quantitative calculation, but per my examples above, it is probably too complex for a comprehensive treatment. I think. $\endgroup$ – airhuff Mar 30 '17 at 18:26
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The source of the lowering of pH observed in rainwater is frequently presented as simply the dissolution of acid gases. This is displayed, for example, here, which cites CO2, SO2 and NO2. Further, to quote:

The sulfuric and nitric acids formed from gaseous pollutants can easily make their way into the tiny cloud water droplets. These sulfuric acid droplets are one component of the summertime haze in the eastern U.S.

However, this educational source also cites an alternate explanation:

Contrary View: The benchmark of "natural rain" is 5.6. Acid precipitation in the range of 4.2-5.0 has been recorded in most of the Eastern United States and Canada. EPRI (Electric Power Research Institute).... EPRI also contends that pH 5.6 may or may not be a valid reference point. It should not be considered the "background" or "natural" acidity of precipitation.

Even without man-made influences, there are natural sources of sulfur oxides, nitrogen oxides, and other species important to determining the precipitation acidity at any given time. Hence, trying to quantify man's contribution to the natural condition will never be possible, since the "natural background" condition cannot be known.

In the forest areas of Brazil at the headlands of the Amazon River, an area remote from civilization, the monthly average of 100 rain events in the 1960s ranged from pH 4.3 to pH 5.2, with the median value of pH 4.6 and one reading as low as pH 3.6.

To explain why the rainforest, not a normally cited source of pollutant or volcanic acid gases, is associated with a median pH of 4.6 requires a more complex explanation. Given the effect of sunlight in equatorial regions, I concur with the above comments by AirHuff on photochemical oxidation paths. I suggest the following source 'Participation of the Halogens in Photochemical Reactions in Natural and Treated Waters' by Yi Yang and Joseph J. Pignatello, available online. To quote, in parts:

Abstract: Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes... While inert to solar wavelengths, halides can be converted into radical and non-radical reactive halogen species (RHS) by sensitized photolysis and by reactions with secondary reactive oxygen species (ROS) produced through sunlight-initiated reactions in water and atmospheric aerosols, such as hydroxyl radical, ozone, and nitrate radical. In photochemical advanced oxidation processes for water treatment, RHS can be generated by UV photolysis and by reactions of halides with hydroxyl radicals, sulfate radicals, ozone, and other ROS... Recent studies indicate that halides, or the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of dissolved natural organic matter (DOM); alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, DOM, and pollutants; and give rise to certain halogen oxides of concern as water contaminants.

In the current context, aerosols rich in halide salts, transition metal oxides and photo-sensitive organic acids are likely also instrumental in understanding the mechanics of the pH effect.

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