Originally Posted by meteparozzi
A large part of the cause is 2-stroke scooters and general traffic. However, there are emission regulations, but they are pretty lax, and its nearly impossible to reduce any 2-stroke engine, as the oil is added directly to the engine (think lawnmower). Another massive addition would be oil refineries (which are plentiful due to relaxed industrial emissions laws) and coal-fired power plants. All of these in a small area.
Yes, it can be very difficult unless you use a catalytic converter like cars have. I worked on a project several years ago (late 90's) for manufacturing small converters suitable for scooters and motorcycles. We did have a workable product, but the company decided to abandon the business before taking it to market. One of the reoccuring problems was the incremental cost for the scooter. In Taiwan (at least) people are very cost conscious. Scooters retail for maybe $25,000NT to $45,000NT (about $780US to $1,400US).Going from memory, a converter would have added maybe $1,600NT (about $50US) to the price of the scooter. They also didn't have a sufficient supply of unleaded gas at that time. Even if the scooter was sold with a converter, it would have been easy enough to saw it out of the exhaust system and replace it with an empty tube. Given how many laws are enforced (helmets are required, but at any time you see maybe 20% of the people without them) in Taiwan, I don't think anyone would notice if your converter had been removed.