I live on the ground floor, on a main street (4 lanes of traffic), in front of a bus stop.
Police cars, fire trucks, ambulances also use this route daily. I am constantly coughing.
Can you tell me is an air purfier is the answer to keeping the apartment free of fumes?

Unfortunately this is not a good solution for your fume problem. The only solution would be for you to relocate. I know you don't want to hear that, but if you live at a bus stop and the buses stop and go then just where are all the fumes from the diesel going to be headed? Into your apartment whether you have the windows shut or not. This is a most unhealthy situation you have on your hands. Your coughing will only get worse due to the diesel. When I lived in NYC I could not breathe properly after a bus passed me by with the diesel fumes. Until I moved to Florida I had very bad health respiratory wise.

You could buy an air filter but the kind that has the throw away HEPA filters are a waste of money. I have an air filter in my home that I use a washable HEPA type filter in and I wash it out weekly. Makes a big difference in the dirt/dust collection around the house. But it is loud so get used to a "white noise" machine 24/7. It only took me a few days to adjust to the noise and I now don't hear it. Just turn the TV up a bit louder.

You could try the air filter but as I said the HEPA filters that are throw aways after 6 months are useless and actually are not effective after a few weeks. I had those HEPA filters that would not last and kept spending lots of $ on brand new ones. You need to get a good air filter system with one of the HEPA type washable filters and then keep it on all the time. Be sure to wash out the filter weekly as well. A washable HEPA filter cost me around $45 with shipping on Ebay. They custom cut the filter to the proper size for your air filter machine so it really does not matter which machine you buy as you won't be using their filters anyway. I have 2 Hunter air filters in my home with 2 custom fit filters. I even have the custom fit HEPA type filter on my ac/furnace as well. Huge difference in the air quality in my house now. I stumbled onto these filters back in Oct when I noticed that the replaceable/throw away filters would have cost me about $35 each every 2-3 months (when the instructions said change every 6 months - in their dreams). Now I'm not squandering my money on filters. I found the washable ones remove cooking odors quite well even though they don't have the carbon mat in them. Just another gimmick to make you think you're getting a better filter if you ask me.

It's worth a try to see if the HEPA washable filters will improve your coughing problem instead of moving.

To twinkiedooter: In case you don't get my above reply because I don't understand the reputation thing, I want to say many,many thanks for your reply.