It's not like it is going to rain and the acid will damage you or your umbrella, or take down your house, but it is there and it's a problem.
But what is acid rain and how it is formed?
To understand this first we have to understand what “acid” stands for, in Chemistry the pH measures the acidity or basicity of an aqueous solution.
It’s a scale from 0 to 14, being 0 the acidic point and 14 the basic or alkaline point.
Pure water has a pH of around 7, but its common to have tap water with pH ranges of 6.8 to 7.2, this is due treatments to the water before is distributed.
pH represents the amount of H+ in the solution, its molar concentration (mol/L) in logarithmic ranges, although the equation involves the activity of the H+ and not the concentration, for understanding purposes we can simplify it to this:
pH = - Log [H+]
What does the “log” function does to a number? (10 base)
To be simple:
Log (100) = 2
Log (10) = 1
Log (1) = 0
Log (0.1) = -1
Log (0.01) = -2
So the “bigger” the number the bigger the log function, numbers below 1 have negative log functions.
Low concentrations of H+ give pH from 7 to 14
High concentrations of H+ have pH from 0 to 7
Rain is considered “pure” water and should have a pH of 7, but there are a lot of other compounds in the air besides Oxygen or Nitrogen, there are CO2, CO, NO, SO2 and more that come from pollution, although CO2 is formed naturally by animals and plants nowadays its concentration keeps raising.
In NORMAL conditions (that means if there weren’t humans that pollute the air) CO2 reacts with rain forming Carbonic Acid, making the pH of the rain drop some points (around 5.9 or 6), nothing to be alarmed.
But in a POLLUTED world there are more compounds in the air besides CO2, the 2 main contributions to ACID RAIN are SO2 and NOx (NO2, NO). These 2 in contact of water form Sulfuric Acid (from SO2) and Nitric Acid (mainly from NO2 that is formed from NO and Oxygen), making the pH of rain dropping to alarming points that go from 4 to 5 in some parts of the world.
But you could say “what is so alarming if it only dropped 1 or 2 points?, when we can wait till it drops to 1 to be alarmed”
The difference between 6 and 5 is 1 (basic math right?), in logarithmic terms 1 point is a 10 times difference. So that means that pH 5 is 10 TIMES more acidic than 6 so pH 4 would be A HUNDRED TIMES more acidic than the normal pH 6, I think you should be worried now Shouldn’t you?.
But these dangerous pH values start to worrying your heart?? I’m sure you are thinking on your roof?, Your toys in the backyard?, Your car?! Oh man you should bring it into the garage when it is raining from now on.
This is not why acid rain means trouble! To hell all of that; acid rain destroys entire forests, it destroys the soil so nothing can grow there again. This means fewer trees to take care of the CO2 in the air.
If you are still worried about your personal stuff, i can assure you that only if you have MARMOL statues outside they will fall hard and will become dust.
Acid rain won’t have ANY impact on humans, as it is too dilute to be considered a danger.
I guess that is the reason why we are still considering world pollution a secondary issue (although SO2 CO CO2 NO2 particles are the main cause of lung and asthma problems).