Burning fossil fuels releases vast amounts of nitrous oxides, ammonia and sulphur dioxide into the atmosphere as well as carbon dioxide. Coastal areas are particularly at risk due to the short lifespan of these chemicals (up to 7 days)- most are deposited on land. In the ocean, the sulphur and nitrogen takes the form of dissociated products of nitric and sulphuric acid. These acids are strong and so dissociate completely in the seawater, lowering its pH. However, the overall process of acidification is more complicated than this as there are many chemicals dissolved in the oceans, each of which effects its own change when these new chemicals are added in. Although the changes in the sea due to nitrogen and sulphur compounds are only a fraction of the amount caused by carbon dioxide, the effects are compounded in coastal areas with 10-50% of the change due to these chemicals.
Doney SC, Mahowald N, Lima I, Feely RA, Mackenzie FT, Lamarque J-F & Rasch PJ. Impact of anthropogenic atmospheric nitrogen and sulphur deposition on ocean acidifiaction and the inorganic carbon system. PNAS; 2011: 104(37): 14580-14585