Yes there is a device that acts like a filter already on cars. It is called a catalytic converter. It blocks out around 90% of the harmful parts of exhaust. A good web site for you to look at is http://www.aqmd.gov/kidspage/ , http://www.aqmd.gov/monthly/jul97cov.html, and http://www.ns.ec.gc.ca/aeb/ssd/smog/smogfaq.html

The exhaust coming from a tailpipe contains many harmful things.One is an odorless, colorless gas called carbon monoxide. It fills up something in your red blood cells that is supposed to carry oxygen (look up "hemoglobin"). Other components are sulfur dioxide, nitrogen oxides, and carbon dioxide. The nitrogen and sulfur compounds are irritants that can damage tissue. They can also contribute to acid rain. For an overview of this problem and some solutions, see http://www.epa.gov/OMSWWW/05-autos.htm.

One way to reduce pollution and make our current emission control systems work better is to reduce the amount of sulfur in gasoline before it reaches our cars (http://www.rapca.org/comments/sulfur1.htm).
Cutting car emissions would certainly help reduce smog; people in the US drive as much as the rest of the world combined! Here's a site on some of California's efforts to reduce and measure car pollution:
http://www.webgarage.com/ci/EmissionsSurvey.htm

Thanks for asking.

Read the answers to the previous question first. Car exhaust is the major source of smog forming compounds. And the catalytic converter in your car engine is a mechanism of filtering out many of the smog forming compounds. However, many still get out via the exhaust. These compounds are formed from incomplete combustion of gasoline. Nitrogen oxides and hydrocarbons are released in the car exhaust. Ever since cars were invented engineers and scientists have been working to make the car engine more effecient, which means for every amount of gas the engine burns you get the most power and the least byproduct from incomplete combustion of fuel. However, this is a very challenging problem. The problem is that if you make it so an engine utilizes gas very effeciently, in that there is the least amount of incomplete combustion possible, you decrease the power you get out of the car. This is why really fast cars like Ferraris and Lamborghinis are super fast but have the worst gas mileage (~6-8miles/gallon of gas vs. a regular car 25 miles/gallon). So it's a trade off. People who buy cars want a fast car but one that doesn't need to have a fill-up every other day. So instead of trying to filter out the bad stuff, engineers try to design engines so that they don't produce as much of the bad stuff to begin with. If you are interested in cars you could lots more info on how car engines work to learn more about this problem in detail. Look on the web or library (or ask car guy/gal)!

Yes, and California has been a leader in the country for forcing its cars to have such filters. I think they are called catalitic converters. Every two years every person who owns a car in California has to have the exhaust checked at a "Smog Check" place. There they can measure what kinds of gases are emitted by the tail pipe. There are some things, though, that these filters cannot take out, like carbon dioxide. Most scientists believe that carbon dioxide is responsible for global warming.