what does Acid rain form?

Rainwater with a pH value lower than 5.6 (the pH value of unpolluted rainwater) is known as acid rain. pH 低於5.6（未受污染雨水的pH值）的雨水，稱為酸雨。

Sulphur dioxide is the primary cause of acid rain. Sulphur dioxide causes acid rain because it produces dilute sulphuric acid with rainwater. 二氧化硫是最基本的酸雨成因。二氧化硫與雨水生成稀硫(VI)酸，導致酸雨。
2SO2(g) + O2(g) → 2SO3(g) (slow reaction 緩慢反應)
SO3(g) + H2O(l) → H2SO4(aq)
The actual pathways are more complex. 實際的途徑較以上的複雜。

Sulphurous acid is also formed when SO2 dissolves in rainwater. SO2 溶於水，亦會生成稀硫(VI)酸。
SO2(g) + H2O(l) → H2SO3(aq)

Nitrogen oxides also cause the formation of acid rain. When released to the atmosphere, nitrogen monoxide reacts with atmospheric oxygen at room temperature to form nitrogen dioxide. 氮氧化物亦導致形成酸雨。釋出到大氣中的一氧化氮，在常溫下與大氣中的氧反應，生成二氧化氮。
2NO(g) + O2(g) → 2NO2(g)

Nitrogen dioxide causes acid rain because it produces dilute nitric(V) acid with rainwater and atmospheric oxygen. 二氧化氮與雨水和大氣中的氧反應，生成稀硝(V)酸，導致酸雨。
4NO2(g) + 2H2O(g) + O2(g) → 4HNO3(aq)The actual pathway is more complex. 實際途徑較以上的複雜。

Carbon dioxide is not the source for acid rain since in air there is CO2 presence just as O2 & N2 . The carbonic acid is a weak acid too , it can be neutralized by any alkaline easily . However SO2 or NO2 will form much stronger acid when dissolve in water . We regard acid rain are from these gases particularly surphur dioxide . SO2 & NO2 can be produced from factories or from volcano erruptions or from thunderstorms .

"Acid rain" is a popular term referring to the deposition of wet (rain, snow, sleet, fog and cloudwater, dew) and dry (acidifying particles and gases) acidic components. A more accurate term is “acid deposition”. Distilled water, once carbon dioxide is removed, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals.[1]

H2O (l) + CO2 (g) → H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:

2 H2O (l) + H2CO3 (aq)
圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Equilibrium.svg/15px-Equilibrium.svg.png
CO32− (aq) + 2 H3O+ (aq)
Acid deposition as an environmental issue would include additional acids to H2CO3.