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:
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.