This refers to pollution that doesn't respect international boundaries - like acid rain. In simple words, it's pollution that can travel across borders, from one country to another.

Acid rain is rainfall that's more acidic than normal due to human activities. Let's break this down:

• Normal rain has a pH between 5 and 6, which is slightly acidic. This is because rain absorbs carbon dioxide in the atmosphere and forms weak carbonic acid.

• If the pH of rain is less than 5.65, we call it 'acid rain'. This pH scale is logarithmic, meaning a decrease of one pH unit shows a tenfold increase in acidity. For example, pH 4 is 10 times more acidic than pH 5. In severe cases, acid rain can have a pH as low as 3!

• Human activities, like burning fossil fuels (coal, oil, gas), release pollutants like sulphur dioxide and nitrogen oxides into the atmosphere. These pollutants combine with moisture and form weak sulphuric and nitric acids. Voila! We get acid rain.

• Coal-fired power stations and vehicles are the main villains here. Power stations produce lots of sulphur dioxide, while vehicles release most of the nitrogen oxides.

• Note: Natural gas is cleaner, causing less pollution because it contains little or no sulphur.

Pollutants can be deposited in two ways - dry or wet

• Dry Deposition: Happens near the source of emission and damages nearby buildings and structures.

• Wet Deposition: Happens when pollutants dissolve in precipitation. This can occur far from the source and is the main cause of TBP.

For example, rainfall in the northeastern USA has been recorded with pH values as low as 2.1, and this rain can fall at great distances from the source of the pollution.