Back in the 1970's and 80's, one of the big environmental challenges of the time was acid rain.
 
"Acid rain" refers to precipitation that includes high levels of nitric and sulfuric acids. While volcanoes and decaying vegetation contribute to levels of these compounds, they have been given a mighty boost by human activity, primarily emissions resulting from fossil fuel combustion. 
 
Acid rain negatively impacts on forests, waterways and soil; to the point of killing insects and aquatic organisms and even eating away at buildings and statues.
 
Various legislation and controls were put in place to decrease sulfur emissions; which certainly helped; but it seems the problem is popping up again; mainly as a result of from nitrogen emissions mixing with rain to create nitric acid.
 
According to an article on Scientific American, the U.S.'s Clean Air Act Amendments of 1990 set reduction targets for both sulfur dioxide (SO2) and nitrogen oxides (NOx). While sulfur dioxide emissions have decreased almost 70 percent from 1990 to 2008, emissions of nitrogen dioxide (NO2) have only decreased 35 percent and amended targets are yet to be implemented.
 
Nitric acid rain is the result of coal fired power generation, automobiles and even fertilizer production and use. In terms of automobiles, while measures such catalytic converters have decreased the amount of pollution per vehicle, the number of vehicles on the road is offsetting any gains made.
 
The threat posed by acid rain is yet another reason (as if we needed more) we need to make a more rapid shift to renewable energy technologies such as solar power, cut our gas consumption and to make more efficient use of natural fertilizers rather than relying on synthetics.
 
Even if we can't afford to get solar panels on our own rooftops, we can buy green power or just implement some simple and cheap strategies for saving electricity in our own homes and workplaces. Every little bit helps.