According to a sign on the bus, about 65% of the Carbon Dioxide in Missoula's air is caused by cars. Aside from that there are some pretty obvious causes, to find which you just have follow your nose. Ick. The pulp mill, the road kill processing plant, and the bacon factory.

One of our main pollutants is small particulate matter in our desert. These are very fine sand particles. Most of it gets into the air as a result of the desert floor being disturbed by construction projects. Abatement is done by requiring construction firms to spray water or some concoction on the disturbed soil. The other pollution is the car pollution from LA that comes in through the mountain pass. It comes in especially when prevailing winds switch to drive the pollution in. We are protected by the fact that our high pressure tends to keep the winds from flowing in as much as it might.

mainly cars,heating,industrials

When we first conceived of this question, we meant what "air mass" or "regional weather front" brought the pollution. So New York gets it from Detroit; the Grand Canyon gets it from LA, etc. It's seasonal too. Sorry for the confusion between source (cars, etc.) and air masses.

Primarily pollution comes from cars and trucks, some from airplanes. Most of it is carbon dioxide and other gases, with dust raised by vehicles in motion.

Sigh. You're all silly people, looking for difficult answers when the simplest answer should suffice. The pollution in my air comes from upwind!

Automobiles, wood and pellet stoves, cement production facilities, farms and other manufacturing facilities.

Air pollution here comes from automobiles, forest fires, smelters in Butte, pulp plants west of us and the local cement plant.

Local coal-fired power plants.
Locally driven automobiles.
Local wind storms.
California.
China.

Dust, carbon monoxide, hydrocarbons, ozone make up the bulk of air pollution. What causes these things: automobile exhaust, generating plants, automobile tires, fireplace smoke, barbecue grills, large scale agriculture (pesticide spraying), trains (diesel engines), airplanes.

Air pollution mostly comes from combustion. I don't mean CO2 because that is usually naturally occuring in the air and that is just a matter of an equilibrium. What I consider air pollutents are harmefull substances which are mostly found as impurities in fossil fuels that are burned along with the rest of the fuel. These substances such as sulfur are not usually a present substance (in such large doses) in the air and therefore harm natural ecosystems (even our own!). There are also chemical substances known as fluorochlorocarbons which do not biodegrade and sometimes escape into the atmosphere ( ex: from an old fridge). These substances are also pollutents in the air but are found in much smaller quantities (thankfully) than the impurities found in fossil fuels.

Usually cars, buses, trucks etc.. but here our chief poisoner would be Ford's local foundry where they illegally re-smelt all their scrap into parts for new cars.

Mostly automobiles.

Pay attention!

Oh, and, agriculture also contributes a lot.

Most of it is from trucks and autos using highways 5 and 99, but we get some from the SF Bay ARea when the winds move their pollution through the Delta and then down to the "pocket" in the southern valley.

1. Ask the weather bureau about smog
2. Think about it - how many cars, what kinds of industry etc - go look (better yet, ride a bike and look)
3. Ask any primary school kid

Power plants in the midwest, and from cars in this area. Population density says localized pollution is without a doubt a large factor, but the jet stream blows pollution from the midwest here all the time. Sunsets are real pretty...