A summary of the three papers is here: http://media.utoronto.ca/media-releases/traffic-emissions-ma... They just set up some measurement tools by the side of the road and waited for cars to drive by. They didn't check the "tune" of each car or test whether any kind of tune-up would change anything.

Edit: more links!

Naomi Zimmerman's webpage: http://naomizimmerman.com/publications/

Abstract of the relevant paper: http://atmos-meas-tech-discuss.net/8/2881/2015/amtd-8-2881-2...

This PDF is labelled "supplemental" but it looks like the full text of the paper. http://atmos-meas-tech-discuss.net/8/2881/2015/amtd-8-2881-2...

So that's a pretty significant conclusion to draw without any hard data on the "tuned-ness" of a car.

I wonder how much could be attributed to old cars instead. That's something that would be easily measured by identifying the cars. But that may open a different can of worms if they determine people with the oldest cars are the biggest polluters* because it could be seen as an attack on the poor.

* Odds are the old cars are the biggest polluters.. less efficient to start with, more likely to be out of tune, more worn parts, etc. But I don't have any data to state that as a conclusion.

Thanks for your interest in our work! (Lead author on the ultrafine particle mapping paper).

The SOCAAR facility has operated at this roadside location since 2006 and is one of the largest air quality research lab in Canada.

Over the years, we have routinely seen peaks in CO2 as vehicles pass our monitoring station. However, when some vehicles pass, we also see NOx, black carbon or ultrafine particle peak(s).

While we have video footage of the road, it can be extremely difficult to link the measurements to a single passing vehicle. Identifying these vehicles and why their emissions are different from the rest of the vehicle fleet is where the research our heading.

In our old car, the catalytic converter wasn't effective, and the cost of fixing it was more than the car than the car was worth. The check engine light was on constantly as a result, but it was easy to ignore. Judging by the smell of that car's exhaust, it was definitely in the 25%.

Bad air-fuel ratios also cause lots of pollution due to unburned fuel being sent out the tailpipe. I would think that this is less common with fuel-injected vehicles, but there are still plenty of old carburated vehicles on the road, especially where road salt isn't used.

Also, diesel engines have problems with carbon buildup over time, which reduces the combustion efficiency and causes the notorious black clouds of soot. (http://www.bellperformance.com/bell-performs-blog/blog/carbo...).

Badly tuned generally refers to incorrect AFR (air-fuel ratios). These can not only cause lower power, but increase the amount of unburned fuel and other chemicals in the exhaust.

On some high performance cars you can even go as far as using the fuel's injection to cool combustion and decrease knock in the combustion cycle of the engine...sometimes this can lead to a richer AFR (more fuel in exhaust gases) which causes pollution.

Most (if not all) cars decrease the air/fuel ratio on wide open throttle, which ensures lack of air to burn pistons, etc., as well as cooling the charge to allow greater volumetric efficiency. Carburetors had a "power valve" to accomplish this, fuel injected vehicles usually have a vacuum hose attached to the fuel pressure regulator to increase fuel pressure temporarily. A fouled O2 sensor, mass air flow sensor or manifold air pressure sensor could throw off the ratios during normal operation. I'd venture to guess that "badly tuned" implies a misfire (probably a fouled spark plug) causing an entire cylinders worth of raw gas dumped into the atmosphere. This excessive fuel will melt down the catalytic converter which exacerbates the problem.

Indeed. That is why I specifically stated on high performance cars. High performance turbo cars, for example, at WOT generally aim for 11.2-11.8 (pending tuner preference etc etc, but ballpark) AFRs.

GDI cars have the capability of going extremely lean under highway speeds to drastically increase mpg and decrease emissions. This is the beauty of GDI.

These "badly tuned" cars could also have worn out valve seals/piston rings which can cause excess amounts of oil to burn during combustion, adding to the chemicals spewing out the exhaust.

(Worn seals & rings leads to oil burning, not excess fuel consumption)

Indeed. That is what I meant, but alas that is not what my brain wanted to type.

The common term for such vehicles is "gross polluter". Here's a study on them (pdf warning)

http://www.csus.edu/indiv/s/shafizadehk/pubs/UT2004.pdf

It seems there's more to it than just badly tuned - it seems how to drive also effected the results:

"The most surprising thing we found was how broad the range of emissions was," says Evans. "As we looked at the exhaust coming out of individual vehicles, we saw so many variations. How you drive, hard acceleration, age of the vehicle, how the car is maintained – these are things we can influence that can all have an effect on pollution."

Not sure how they can tell "how a car is maintained" or "how you drive" from spot checking 100K with a roadside probe... anyone have thoughts?

They are speculating , but saying it isn't just the car's "genetics" (make model and production year)

"Tuned" is a holdover term from the days of carbureted fuel systems that doesn't have a lot relevance anymore.

Modern cars don't need routine "tune-ups." Yes, if a sensor fails or something like that it can cause an emissions problem, but a properly working engine computer tunes itself, and if it detects anything isn't what it should be, it activates the "check engine" light on the dashboard.

"badly tuned" for minimal emissions, I assume; tuning for maximum power is somewhat different than for minimum emissions.