Weathering release occurs of Ca and other ions into surface waters. Rainfall is acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide, SO, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to the rocks on which it
21st-century fossil fuel regulations in the United States are subject to New Source Review (NSR) and Prevention of Significant Deterioration (PSD). Emissions from these plants have the largest contributions of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere. Facilities in the electric sector are restricted to emission standards defined by the Clean Air Act (United States). While monitoring and controlling environmental releases of plants has been difficult, the EPA has been enforcing their policies regarding air quality over the last decade under President Barack Obama. While the Clean Air Act postulates regulations and rules limiting the emissions of greenhouse gases, other laws are concerned with standards