Enlarge / A thoughtful soul decides to poison the air as he drives through the infield at Daytona International Raceway during the 2016 Rolex 24 race. (credit: Jonathan Gitlin)

I remember the first time someone rolled coal on me. It was 2006, and I was driving to work at the University of Kentucky. It was a bright, sunny day in Lexington, and I had the roof down and was stopped in traffic behind a large pickup truck with decidedly non-standard exhaust pipes exiting straight up behind the cab. Whoever was driving the pickup evidently noticed the Miata in his mirror and enveloped me in a thick cloud of soot when the lights changed.

As automotive subcultures go, intentionally modifying your truck's diesel engine to make extra pollution is one of the more antisocial ones out there. According to the US Environmental Protection Agency, diesel trucks with disabled emissions controls are far more widespread than you might think and emit more pollution than the diesel engines that got Volkswagen such hefty fines.

In 2016, Volkswagen agreed to a pair of court settlements totaling nearly $16 billion after it was caught selling diesel vehicles fitted emissions defeat devices. In total, the VW scandal affected more than half a million cars and SUVs sold in the US, which produced up to 40 times the legal limit of nitrogen oxides (NO _x ) when in daily operation.

Enlarge / Smog is bad. (credit: Steven Buss )

Common arguments about how electric vehicles cause more emissions than traditional vehicles when the electric grid is coal-powered are actually wrong . Still, it’s certainly true that a cleaner grid is needed to fully realize the benefits for climate change. Beyond the climate, part of the appeal of EVs is also the improved air quality, of course, and here the grid can be even more important. In the wrong situation, switching to an EV just moves the air pollution from the street to the power plant.

Building models

A team led by Northwestern’s Daniel Peters decided to have a particularly detailed look at this issue, examining several scenarios of grid generation and EV adoption in the US. The results show that even with today’s grid, switching to EVs produces significant benefits.

The researchers used simulated hourly air pollution data from vehicles around the country, along with emissions data for power plants. This went into a model of weather over the course of a year (2014, as it happens), which also simulated important chemical reactions and natural emissions of compounds that interact with pollutants. The resulting air quality simulations were applied to an EPA population health model to show the expected impact on human health.

Enlarge / Cars congest Intersate 10 in Los Angeles late October 2006. That year, the state sued several US and Japanese automakers for their alleged contribution to global warming. (credit: GABRIEL BOUYS/AFP via Getty Images)

On Tuesday, five automakers signed agreements with California's Air Resources Board to implement cleaner emissions standards over the next few years. BMW , Ford , Honda , Volkswagen Group , and Volvo will reduce vehicle emissions between model years 2021 and 2026.

Unlike Europe's rules, which fine automakers if they exceed a blanket fleet average for the amount of CO ₂ emitted per km, CARB has different targets for cars and light trucks based on their relative footprint. But each of the five OEMs has agreed to cut the amount of CO ₂ its vehicles produce per mile by about 17 percent by MY2026. The new agreement is broadly similar to one announced last year , although with a revised timeline that now runs through 2026.

Specifically, CO ₂ emissions from small cars would drop from 157g/mile in 2021 to 130g/mile in 2026, large cars from 215g/mile to 178g/mile, small light trucks from 195g/mile to 162g/mile, and large light trucks from 335g/mile to 278g/mile, with a formula to adjust vehicles that fall in between the small and large footprint areas. For context, the EU's new fleet-wide average, which came into effect in 2020, heavily fines any automaker whose fleet average exceeds 152g/mile (95g/km).