Authors’ note: The CURE100 Carbon Tracker tool, available here, now has full support in the latest version 288 for PM2.5 (Particulate Matter 2.5 microns or smaller) emissions and implements the models described in this article for light duty vehicles. The CURE100 BEST (Bus Electrification for Student Transportation) tool, available here, similarly has full support in the latest version 1.13 for bus PM2.5 emissions. In both cases, emissions from exhaust or electricity generation, tire wear and brake wear are included.

Summary

Electrifying transportation sharply reduces carbon emissions and is widely recognized as an essential step in fighting climate change. However, our investigation reveals that, in many cases, it increases particulate emissions, and also changes the geographical distribution of these emissions. Is this really true? What are the implications?

Introduction

If we had a nickel for every time an environmental activist said “Win, win, win” we would be rich! “A win for the pocketbook, a win for human health and a win for the environment” is an oft-repeated phrase. So, when we came across a seeming exception, it both caught us by surprise and motivated us to write this blog to seek validation from our readership.

Carbon reduction

We consider two cases: a sedan that drives 13,500 miles per year with a nationwide average fuel efficiency of 25 miles per gallon, and a large school bus that travels 10,000 miles per year at 6 miles per gallon. We consider gas and diesel variants of each. In each case, we consider replacement by an electric vehicle that requires 0.3 kWh and 2 kWh of charge per mile, respectively. We assume the vehicles charge in the “NYCW” region as defined in the eGRID (Department of Energy) database, which includes all of Westchester County, NY, and we use the corresponding figures for the carbon and PM2.5 emissions for electricity generation. If a school bus is charged with New York Power Authority (NYPA) power which is more than 80% clean, carbon emissions are even lower. The annual carbon impact is as follows, with detailed calculations and citations shown in Appendix A.

Case	Fossil fuel vehicle carbon (tons of CO2e/year)	Electric vehicle carbon (tons of CO2e/year)	Reduction
Gasoline car	4.8	1.5	68.8%
Diesel car	5.5	1.5	72.7%
Gasoline school bus	14.8	7.4 -> 1.5 with NYPA power	50.0% -> 89.9%
Diesel school bus	17.0	7.4 -> 1.5 with NYPA power	56.3% -> 91.2%

The above results are satisfying and intuitive, particularly given that electric motors are 3 to 4 times as efficient as their fossil fuel counterparts.

What happens to PM2.5?

Is what’s good for the goose also good for the gander? PM2.5 stands for particulate matter that is 2.5 microns or less in diameter. Particulate pollution is produced whenever fossil fuels are burned, whether in a car, a bus, a furnace or a power plant. PM2.5 is also emitted from vehicles due to tire wear and brake wear. PM2.5 is harmful to human health because particles settle in human lungs and cause asthma, emphysema, other respiratory disorders and even cancer. Unlike the global nature of the greenhouse gas problem, PM2.5 is a local phenomenon. Wouldn’t it be nice if PM2.5 reduces when transportation is electrified so that we could say, “Good for the pocketbook, good for health, good for the environment?” Unfortunately, the conclusion is, “Not always, it depends on how the vehicle is charged.”

We repeated analysis of the four cases above, this time focusing on PM2.5. Details and citations are in Appendix 2 and summarized in the table below. Once again, charging a school bus with NYPA power provides significant PM2.5 reduction.

Case	Fossil fuel vehicle PM2.5 (grams/year)	Electric vehicle PM2.5 (grams/year)	Reduction
Gasoline car	94.5	156.4	–65.5%
Diesel car	81.0	156.4	–93.1%
Gasoline school bus	270.0	726.8 -> 279.8 with NYPA power	–169.2% -> –3.6%
Diesel school bus	1,650.0	726.8 -> 279.8 with NYPA power	56.0% -> 83.0%

Results at a glance

The results are presented in the lead graphic above. Carbon numbers are in relation to the left axis, and PM2.5 values in relation to the right-hand secondary vertical axis.

For light duty vehicles, PM2.5 almost doubles. While there is an impressive reduction of PM2.5 for diesel buses, there is a huge increase for gas buses unless charging with clean electricity. Despite the superior efficiency of electric motors, the PM2.5 required to generate the electricity required to drive the same distance ends up being more than the emissions of a fossil fuel vehicle. Note that we have assumed “general” grid electricity in Westchester County unless the school bus is eligible to use NYPA power. In all cases, we have taken into account PM2.5 emissions from exhaust or electricity generation, tire wear and brake wear.

We find the increase of PM2.5 in electric cars and gas buses counter-intuitive, and invite comments from our readers, with a promise to publish relevant updates in this forum.

What are the implications?

We postulate the following conclusions:

1. Electrification of transportation generally leads to significant reductions in carbon. As the grid adopts a higher percentage of renewable energy sources, the gain gets bigger. As such, without electrifying transportation at scale, there is no hope of fighting the climate crisis.
2. However, electrifying transportation increases PM2.5, particularly for all light-duty and gasoline heavy-duty vehicles. Of course, it depends on where in the country the vehicle is charged because there are different PM2.5 levels of pollution per MWhr of electricity generation in different regions.
3. Electrifying transportation moves the emission of both greenhouse gases and the exhaust component of PM2.5 from the tailpipe of the vehicle (and hence the geographic locale in which the vehicle is driven) to a possibly remote location where electricity is produced at least in part from fossil fuels. This is a blessing for school children and school bus drivers.
4. However, the change in location of PM2.5 is an important environmental justice issue since most fossil fuel power plants are located in disadvantaged communities, which will bear the brunt of the additional pollution as transportation electrifies. On the other hand, higher PM2.5 emissions in a remote and unpopulated location can possibly do less harm than in a more populated region where the vehicles operate – while acknowledging that any harm is too much.
5. The faster we can decarbonize the grid, the faster we can truly achieve the “Win, win, win” outlined in the introduction, because electrification of transportation will lead to 100% reduction of greenhouse gases as well as reduction of the biggest component of PM2.5 once the greening of the grid is complete. Of course, PM2.5 emissions from brake and tire wear will be unchanged, but those are smaller impacts.
6. As a policy matter and a matter of environmental justice, we feel the urgency to both electrify transportation and green the grid – these have to proceed in a symbiotic fashion on a rapid and widespread scale in parallel.
7. Most public schools in New York get their electricity from the New York Power Authority (NYPA), which is greater than 80% clean and would significantly cut both carbon and PM2.5 emissions for electric vehicles. Therefore, electrification of school buses at least in New York state should proceed at the most rapid pace possible, without any hesitation due to PM2.5.
8. Unfortunately, NYPA power is not available to private EV drivers or private bus and truck companies. Our conclusion in these cases is also to move full steam ahead. As the grid is cleaned up, both greenhouse gas and PM2.5 reductions will automatically follow.

Appendix 1: Carbon

Gasoline car

Miles driven per year = national average of 13,500 miles [1]
Fuel efficiency = national average of 25 mpg
CO₂e per gallon of gasoline = 0.00887 metric tons [2]
Total annual CO₂e = 13,500 / 25 * 0.00887 = 4.8 metric tons

Diesel car

Miles driven per year = national average of 13,500 miles [1]
Fuel efficiency = national average of 25 mpg
CO₂e per gallon of diesel = 0.01018 metric tons [2]
Total annual CO₂e = 13,500 / 25 * 0.01018 = 5.5 metric tons

Electric car

Fuel efficiency = 3.3333 miles/kWh
Carbon content of electricity in NYCW region = 0.000370985031 tons/kWh [3]
Total annual CO₂e = 13,500 / 3.3333 * 0.000370985031 = 1.5 metric tons

Gasoline school bus

Miles driven per year = 10,000
Fuel efficiency = 6 mpg
CO₂e per gallon of gasoline = 0.00887 metric tons [2]
Total annual CO₂e = 10,000 / 6 * 0.00887 = 14.8 metric tons

Diesel school bus

Miles driven per year = 10,000
Fuel efficiency = 6 mpg
CO₂e per gallon of diesel = 0.01018 metric tons [2]
Total annual CO₂e = 10,000 / 6 * 0.010180 = 17.0 metric tons

Electric school bus

Fuel efficiency = 0.5 miles/kWh
Carbon content of electricity in NYCW region = 0.000370985031 tons/kWh [3]
Total annual CO₂e = 10,000 / 0.5 * 0.000370985031 = 7.4 metric tons

Electric school bus charged with NYPA power

Fuel efficiency = 0.5 miles/kWh
Carbon content of electricity in NYCW region = 0.000370985031 tons/kWh [3]
Fraction of clean power from NYPA = 0.8 [7]
Total annual CO₂e = 10,000 / 0.5 * 0.000370985031 * ( 1 – 0.8 ) = 1.5 metric tons

Appendix 2: PM2.5

Gasoline car

Miles driven per year = national average of 13,500 miles [1]
PM2.5 per mile driven due to exhaust = 0.003 grams @ 25 mpg for gasoline [4]
PM2.5 per mile driven due to tire wear = 0.003 grams [4]
PM2.5 per mile driven due to brake wear = 0.001 grams [4]
Total annual PM2.5 = 13,500 * ( 0.003 + 0.003 + 0.001 ) = 94.5 grams

Diesel car

Miles driven per year = national average of 13,500 miles [1]
PM2.5 per mile driven due to exhaust = 0.002 grams @ 25 mpg for diesel [4]
PM2.5 per mile driven due to tire wear = 0.003 grams [4]
PM2.5 per mile driven due to brake wear = 0.001 grams [4]
Total annual PM2.5 = 13,500 * ( 0.002 + 0.003 + 0.001 ) = 81.0 grams

Electric car

PM2.5 per MWhr of electricity in NYCW region = 0.0616 pounds [5]
Grams per pound = 453.592
Fuel efficiency of electric sedan = 3.3333 miles/kWh
PM2.5 per mile driven due to exhaust = 0 grams
PM2.5 per mile driven due to tire wear = 0.003 grams [4]
PM2.5 per mile driven due to brake wear = 0.001 grams [4]
Derating factor for brake wear due to regenerative braking = 0.2
MWhrs used = 13,500 / 3.3333 / 1000 = 4.05
Total annual PM2.5 = 4.05 * 0.0616 * 453.592 + 13,500 * ( 0.003 + 0.001*0.2 ) = 156.4 grams

Gasoline bus

Miles driven per year = 10,000
PM2.5 per mile driven due to exhaust = 0.019 grams/mile for gasoline [6]
PM2.5 per mile driven due to tire wear = 0.006 grams/mile [6]
PM2.5 per mile driven due to brake wear = 0.002 grams/mile [6]
Total annual PM2.5 = 10,000 * ( 0.019 + 0.006 + 0.002 ) = 270 grams

Diesel bus

Miles driven per year = 10,000
PM2.5 per mile driven due to exhaust = 0.148 grams/mile for diesel [6]
PM2.5 per mile driven due to tire wear = 0.014 grams/mile [6]
PM2.5 per mile driven due to brake wear = 0.003 grams/mile [6]
Total annual PM2.5 = 10,000 * ( 0.148 + 0.014 + 0.003 ) = 1,650 grams

Electric bus

PM2.5 per MWhr of electricity in NYCW region = 0.0616 pounds [5]
Grams per pound = 453.592
Fuel efficiency = 0.5 miles/kWh, courtesy of Highland Electric (bus company)
PM2.5 per mile driven due to exhaust = 0
PM2.5 per mile driven due to tire wear = 0.016 grams
PM2.5 per mile driven due to brake wear = 0.004 grams
Derating factor for brake wear due to regenerative braking = 0.2
MWhrs used = 10,000 / 0.5 / 1,000 = 20.0
Total annual PM2.5 = 20.0 * 0.0616 * 453.592 + 10,000 * ( 0.016 + 0.004*0.2 ) = 726.8 grams

Electric bus charged with NYPA power

PM2.5 per MWhr of electricity in NYCW region = 0.0616 pounds [5]
Grams per pound = 453.592
Fuel efficiency = 0.5 miles/kWh, courtesy of Highland Electric (bus company)
PM2.5 per mile driven due to exhaust = 0
PM2.5 per mile driven due to tire wear = 0.016 grams
PM2.5 per mile driven due to brake wear = 0.004 grams
Derating factor for brake wear due to regenerative braking = 0.2
Fraction of clean power from NYPA = 0.8 [7]
MWhrs of “dirty” electricity used = 10,000 / 0.5 / 1,000 * ( 1 – 0.8 ) = 4.0
Total annual PM2.5 = 4.0 * 0.0616 * 453.592 + 10,000 * ( 0.016 + 0.004*0.2 ) = 279.8 grams

References

[1]           Car and Driver, February 24, 2023, https://www.caranddriver.com/auto-
loans/a32880477/average-mileage-per-year/.

[2]           U.S. Environmental Protection Agency, https://www.epa.gov/greenvehicles/greenhouse-
gas-emissions-typical-passenger-vehicle.

[3]           U.S. Department of Energy, eGRID (Emissions and Generation Resources Integrated Database), https://www.epa.gov/egrid.

[4]           Our Carbon Tracker uses Emission Factors (EFs) for transportation from the EPA MOVES
3 model https://www.bts.gov/content/estimated-national-average-vehicle-emissions-rates-vehicle-vehicle-type-using-gasoline-and. We total up emissions from fuel combustion, tire wear and brake wear.

[5]           Our Carbon Tracker uses Emission Factors (EFs) for electricity generation emissions from eGRID (Emissions and Generation Resources Integrated Database),  https://www.epa.gov/egrid/egrid-related-materials.

[6]           Our BEST tool uses Emission Factors (EFs) for buses from the EPA MOVES 4 model https://github.com/USEPA/EPA_MOVES_Model/blob/master/docs/MOVES4CheatsheetOnroad.pdf. We total up emissions from fuel combustion, tire wear and brake wear.

[7]           “More than 80 percent of the electricity NYPA produces is clean renewable hydropower,” from https://www.governor.ny.gov/news/governor-hochul-announces-116-million-develop-clean-energy-industry-workforce.