Acid Rain 101

As raindrops hit my face on this morning’s bike ride, the refreshing effect outweighed the annoyance of being slightly damp when I dismounted. Rain symbolizes renewal for me, the cleanse and growth of springtime.

Later in the day my American environmental history course reminded me that polluted rain can bring destruction rather than renewal. In 1979, precipitation fell on Wheeling, West Virginia that was more acidic than lemon juice or even gastric acid. Those terrifying drops had a pH of 1.5, the lowest ever recorded in the U.S.

If you’re an American in your 20s (like me), you’ve probably never experienced the devastating effects of acid rain and might not know, beyond the general “pollution causes acid rain” explanation, why atmospheric water droplets can become intimidatingly acidic. During the 1980s, acid rain posed serious hazard to U. S. cities but has declined sharply in the last 23 years and atmospheric concentrations of compounds that cause acid rain continue to decline. It’s worth reviewing the processes that end in acid raining onto our homes, cars, and gardens, in case you don’t remember, or (like me) never fully understood.

When you think acid rain, two main elements should spring to mind: sulfur and nitrogen. It’s not the elemental forms of concern here, but they are good memory tags to attach to the phenomena. Though there are several points of entry for sulfur and nitrogen into the atmosphere, I will focus on burning fossil fuels for the sake of simplification. For a more nuanced explanation, see the acid rain section of Atmospheric Science for Environmental Scientists available online.

When we burn fossil fuels, particularly coal, the resulting smoke contains sulfur dioxide and nitrogen oxides. Those sulfur and nitrogen compounds enter the atmosphere and dissolve into water droplets. The crucial thing to note about sulfur and nitrogen oxides dissolving into water, is they are no longer simply oxides, they become acids: sulfuric and nitric acid.

The more oxides released into the air by burning fossil fuels like coal, the more acidic rain becomes. Also important to note is that coal from different locations has different amounts of sulfur, leading to different sulfur dioxide emissions. Low sulfur coal is mainly mined in western states while high sulfur coal is mined in the midwestern states. Using low sulfur coals helps keep acidic compounds in clouds from reaching worrisome levels. Also, “scrubbing” emissions from burned fossil fuel is helpful. Scrubbers placed on smoke stacks use crushed limestone to absorb sulfur as smoke exits.

In the decades of increased industrial production and very little air pollution regulation, acid rain began to dissolve buildings, stifle water organisms, and diminish vegetative health. Those very visible and undesirable outcomes were legislated against by the Clean Air Act and subsequent amendments in the 1970s and 1980s.

The Environmental Protection Agency’s Clean Air Status and Trends Network (CASTNET) published a report in February 2011 on the change in atmospheric concentration of both sulfur dioxide and nitrogen oxides from 1990-2009, noting that both dropped significantly. Nitrogen oxide concentrations in the air decreased 30 percent while sulfur dioxide decreased 56 percent. The report said that since CASTNET began measuring, air quality has never been higher. This was a surprisingly positive statement when on the whole emissions continue to rise. It’s nice to note that, though emissions may be pumping greenhouse gases into the atmosphere, at least we were able to curb the acid rain-producing oxides.

Thankfully, clean air legislation and I came of age simultaneously and even if spring weather makes my morning a bit damper, it doesn’t do the same for the vibrancy of the renewed life accompanying the season. The sun dried my bike before I got back on to ride home this evening and I expect tomorrow morning’s commute will reveal a world that is slightly greener from today’s added moisture.


One thought on “Acid Rain 101

  1. Pingback: Acid Rain Projects | CS Eagles

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