Acid rain seems to be more evident these days upon the faces of old statues, or lack thereof. But what is it from, and do we still have cases of acid rain falling upon us from the sky?
How acid rain forms
I remember hearing a lot about acid rain as a child in the 80's, but it's nothing new. It was first discovered in England in 1872, but later rediscovered in North America in 1963 by scientists at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. It became the environmental issue of the 1970's and eventually led to specific amendments in the United States Clean Air Act to be made in 1990 that specified on the avoidance of the direct cause of acid rain.
Acid rain forms when high concentrations of sulfur dioxide and nitrogen oxides are released into the atmosphere and transferred by wind currents to other regions. Once there, these gases mix with water molecules, oxygen, and other chemicals that develops into sulfuric and nitric acids and, with added precipitation, fall to the Earth. Acid rain is not always in the form of rain, which is why the preferred name is acid deposition. Sometimes, acid deposition forms as snow, fog, hail, or dust, as well as rain. The main cause of acid deposition is the burning of fossil fuels with two thirds coming from electric power generators. Now, the term 'acid rain' is a bit redundant because rain itself is already acidic; with acidity derived from natural gases in our atmosphere, including sulfur dioxide and nitrogen oxide. However, the greenhouse gases released from fossil fuel combustion add more than what's "natural" and can be very harmful to the environment it falls down onto. And because these gases travel with wind currents, they can be carried to great distances and come back down to the earth in the most wild of places, or the most urban of ones.
As harmful and intimidating as the name, acid rain sounds, it's actually not too far off. When acid rain falls onto the ground, it leaches aluminum from the soil and carries it to nearby streams, rivers, and lakes. Increased acidity in watery ecosystems can collapse them entirely. Many times, fish eggs will not hatch if the pH drops down to just 5 and adult fish can die if the acidity increases from there. Some aquatic animals can tolerate higher acidity, but many times the insects and plants they eat cannot and so, higher acidity can instead, starve them. Some areas, however, can tolerate higher acidity due to a buffering quality of its soil- other areas, though, are not as fortunate and are more vulnerable. Areas that do not have this buffer zone can be suddenly stressed when melting acid snowfall or a sudden downpour of acid rain drops its pH dramatically in a short period of time, resulting in sudden die-offs.
Not only does acid rain harm plants and animals from its leached aluminum, but it also absorbs minerals and nutrients from soil; depleting the surrounding vegetation. Acid fog will strip the nutrients from trees' leaves and needles, causing them to die and fall from the limbs that are then left bare. This prevents the trees from absorbing sunlight and causes them to be weak and unable to withstand freezing temperatures. This also can occur from acid dust that settles itself on leaves or on statues and architectural details. Many times, this is why the faces on statues look worn and faded. This is from the sulfuric and nitric acids that corrode metal and speed up the deterioration process of stones and paint.
In contrast, higher acidity in rain does the same thing to humans as normal acidity in rain. That said, having higher concentrations of sulfur dioxide and nitrogen oxide in the atmosphere that creates acid rain, does have harmful effects on our cardiovascular and respiratory systems and can lead to lung disease, heart disease, and asthma.
Acid rain today
Thanks to legal regulations put forth in both North America and Europe, acid deposition has been greatly reduced. In 1993, for example, there were 21,773 tons of sulfur dioxide and 27,080 tons of nitrogen oxide in our atmosphere. In 2017, there was 2,815 tons of sulfur dioxide and 10,776 tons of nitrogen oxide. That said, the damage has been done; the acid-neutralizing capacity of soils have been greatly reduced in sensitive regions that were exposed to acid rain for a long duration and thus, recovery is slow. Furthermore, these damaged ecosystems are even more susceptible to further harm done by almost any concentrations of sulfur dioxide and nitrogen oxide; even the quantities in normal rain. It's like when drinking too much soda strips away the protective enamel on your teeth. And even though we have greatly reduced these gases from entering our atmosphere, we are still creating acid rain today in certain areas. Vermont's rainwater, for example, has a pH level of 4.2-4.4 on any given day. By looking at the pH scale above, you can see that normal, healthy rainwater has a pH of 5.6 and so, Vermont's rainwater is 10 times the acidity of normalcy and thus, still harmful. And what happens in Vermont can still happen in many other places of the world, as well. The only way acid deposition can be reduced any further is by the collective alternative decisions in place of using fossil fuels.