Dear Si Abdul,

Acid in rainfall has a variety of natural sources. Carbon dioxide in the air combines with water to form carbonic acid. Volcanoes and forest fires add sulfur compounds to the atmosphere that form sulfuric acid. Although acid rain has many natural causes, their combined effect is relatively small.

The strong levels of acid rain observed in modern times are primarily the result of human activities. Over 90% of the sulfur dioxide released into the atmosphere today comes from the combustion of fossil fuels. Coal-burning power plants represent the largest source of sulfur dioxide entering the atmosphere. The sulfur dioxide combines with water to produce the sulfate ions that are present in acid rain.

With my best regards

Prof. Bachir ACHOUR

pH values measured in rain water above remote areas of the ocean are often acidic (4 < pH < 5) so that a rainwater pH below 5 is not always an indication of anthropogenic emissions.

The dominant sulphur compound emitted from the world's oceans is dimethyl sulphide (DMS). It originates from the decomposition of dimethyl sulphoniopropionate (DMSP) which is present in high concentrations in many marine algae, and functions as an osmoregulant. Following the death of the phytoplankton cells, DMS is released into the ambient seawater following the enzymatic breakdown of DMSP. Actually, DMS concentrations in the surface ocean are greatly in excess of those that would be in equilibrium with atmospheric values. The result is a net flux of DMS from the ocean to the atmosphere. After transfer across the sea-air interface into the atmosphere, much of the DMS is oxidised to SO2 and SO3, and eventually H2SO4.

Sulfate ions in rainwater from sea come via two ways. One is primary sulfate ion from sea-salt and it does not contribute to acidity. Another is secondary sulfate that comes following the pathway suggested by Francois Muller (above reply), which can contribute to acidity. How much it can contribute would depend upon relative contribution of sea-derived and pollution derived SO4. However, it relevant to state here that it is [H+] that decides acidity and not [SO42-]. It means if Sulfate is present as sulfuric acid, it will contribute but if it is present as some salt, it won't contribute. Here also, if this salt is with weak base (like NH3), water would slightly acidic but if the salt is with strong base (N+, K+, etc), it won't be acidic. Therefore, it is not a good idea to estimate pH based only on sulfate and nitrate concentrations. It is better to estimate free [H+] to calculate acidity.

 The salt in the ocean comes primarily from the weathering of rocks on land. When rocks are exposed at the surface of the Earth, minerals are weathered chemically to produce various solid compounds as well as free ions such as calcium (Ca2+), potassium (K+), and sodium (Na+). Volcanic eruptions produce CO2, hydrochloric acid (HCl), and sulfur dioxide (SO2), all of which are soluble in water. These solutes are brought to the ocean by precipitation and by river transport. Evaporation of water concentrates these materials, bringing the salinity of the oceans well above the salinity found in rivers and lakes. Some materials (such as calcium and potassium) are gradually removed from the ocean by organisms that use them as nutrients. Other materials are lost through sediment interactions: the sediments at the ocean’s bottom behave much like a water filter, adsorbing various ions associated with salinity and locking them away. These processes

of adding salt to the oceans, concentrating it through evaporation, and removing it

through various chemical and biochemical processes, have been active on Earth for billions of years; the average salinity of the ocean is therefore a function of the rates of these various processes, and has changed little through Earth’s history. 

Minimum amount of sulphur present in the salt water may  contribute  the acidity of rain.

All ions present in a water sample contribute to the acid-base balance. You can estimated the marine sulfate contribution using chloride concentration (assuming there is no other source of Cl in your water samples, which is not always the case) and a standard composition of marine waters. This is also useful for estimating Na and K marine contribution.  

From the strong ion balance and the CO2 atmospheric concentration you can also approximate  pH. 

You can find a quick introduction and references to water acidification in our book chapter, which researchgate-link I provide below: 

https://www.researchgate.net/publication/279627940_Chemical_composition_of_lakes_in_crystalline_basins_a_combination_of_atmospheric_deposition_geologic_background_biological_activity_and_human_action

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