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Texas study finds PHEV use could increase ozone at night, decrease ozone during the day

A study by researchers at the University of Texas found that in general, use of plug-in hybrid electric vehicles (PHEVs) can lead to an increase in ozone during nighttime hours (due to decreased scavenging from both vehicles and EGU stacks) and a decrease in ozone during daytime hours. In addition, the study, published in the IOP journal Environmental Research Letters, found that charging plug-in vehicles at night could slightly reduce, on average, ozone.

Ozone forms as hydrocarbons and nitrogen oxides, emitted into the air, react with sunlight. Two of the largest emitters of these pollutants are vehicles and electricity generating units (EGUs), with some of the most densely populated regions in the US still failing to meet the National Ambient Air Quality Standards after 30 years of regulation.

The team modeled the air quality impacts of replacing approximately 20% of the gasoline-powered light duty vehicle miles traveled (VMT) with electric VMT by the year 2018 for four major cities in Texas: Dallas/Ft Worth, Houston, Austin, and San Antonio. Plug-in hybrid electric vehicle (PHEV) charging was assumed to occur on the electric grid controlled by the Electricity Reliability Council of Texas (ERCOT), and three charging scenarios were examined:

  • Charging the car at off-peak times in the night.
  • Charging to maximize battery life (charging just before use and only the amount of charge needed to complete the trip).
  • Charging the battery when it was a convenient time for the driver (typically just after vehicle use).

The team modeled a subset of electricity generating units (EGUs) in Texas contributing the majority of the electricity generation needed to charge PHEVs at the times of day associated with each scenario with a regional photochemical model (CAMx). The net impacts of the PHEVs on the emissions of precursors to the formation of ozone included an increase in NOx emissions from EGUs during times of day when the vehicle is charging, and a decrease in NOx from mobile emissions.

The changes in maximum daily 8 h ozone concentrations and average exposure potential at twelve air quality monitors in Texas were predicted on the basis of these changes in NOx emissions.

Among the findings of the study:

  • The potential air quality impact of PHEVs is dominated by the impact of the NOx decreases from mobile sources. The general trend is to see decreases in 8 h averaged ozone concentrations during daytime hours, and increases in 8 h average ozone concentrations during nighttime hours.

  • PHEVs are likely to positively impact air quality with regards to attainment of the 8 h ozone standard. On average across all 12 monitors, and all three scenarios, the 8 h maximum ozone concentration is predicted to decrease by approximately 0.15 ppb.

  • For all charging scenarios in San Antonio and Austin, potential exposure is reduced. The results are less clear in Dallas and Houston. However, nighttime charging is the charging scenario most likely to minimize the negative impact of PHEVs on attainment and exposure in all cases in those two cities.

The impact of PHEVs on ozone is largest on days forecast to have high ozone. This high ozone day impact is desirable for both attainment of regulatory standards and for exposure. Mobile source emissions decrease during nighttime hours often cause increases in nighttime ozone due to decreased scavenging of ozone by nighttime NOx. Nighttime increases in ozone are less likely to impact humans because fewer people are awake and outside and therefore fewer people are being exposed to higher ozone during nighttime hours. Thus, the switch of 20% of LDV VMT from gasoline to electric travel shifts ozone formation to a time period that is likely less harmful to humans.

This study has shown that while in most cases there is little difference in maximum ozone concentrations between the air quality impacts of the three charging scenarios, the ‘convenience’ charging scenario is most likely to cause increases in daytime ozone. In contrast, changes in ozone concentrations integrated over the entire episode, showed greater differences between scenarios, with nighttime charging showing the best performance.

While changes in greenhouse gas emissions have not been a focus of this analysis, it is worth noting that, using eGRID average carbon dioxide emissions factors and MOBILE6 CO2 emissions totals, CO2 emissions were estimated to decrease by about 17 000 tons day-1 due to mobile source decreases, and increase by 8000, 7000 and 7000 tons day-1 for the night, convenience and battery scenarios respectively. Therefore, the PHEV scenarios presented in this work would decrease CO2 emissions by over half regardless of when they are charged.

—Thompson et al.

The results in general show positive air quality results due to the use of PHEVs regardless of charging scenario with the nighttime charging scenario showing the best results on average by a small margin. This further supports efforts to develop regulation to encourage nighttime charging; an example would be variable electricity pricing. As more of the fleet switches over to PHEVs and a larger demand is placed on the electricity grid, it will become more important that we design and implement policy that will encourage charging behaviours that are positive for both air quality and grid reliability.

—Dr. Tammy Thompson, (now at MIT), lead author


  • Tammy M Thompson et al. (2011) Air quality impacts of plug-in hybrid electric vehicles in Texas: evaluating three battery charging scenarios. Environ. Res. Lett. 6 024004 doi: 10.1088/1748-9326/6/2/024004



"The results are less clear in Dallas and Houston"

Houston is the home of refineries, so it is unlikely that a few thousand EVs charging would make much difference. Texas is the home to oil wells, refineries and coal fired power plants, so it is not surprising this study comes from that state.

I know it says on balance PHEVs are a plus, but the impression it gives is that it is not as good as you might think. After Exxon, headquartered in Houston put millions of dollars into creating doubt about global warming, I have my doubts on motives from people that have other reasons to say what they do say.



This whole thing sounds rather shakey and like they are bunch of politicians using double speak to try and hide any meaningful facts.

Bob Wallace

Is Texas not the state that has surplus nighttime wind-electricity? Why would Texas burn fossil fuels to charge EVs when they often have fuel-free power on the grid?

Was there no mention of generating electricity by any method other than burning coal in this study?

Bob Wallace

And hot off the tubes...

"By now it’s common knowledge that Texas is poised to transition from the fossil fuel economy into wind power (to say nothing of solar power), so it shouldn’t come as a surprise that wind powered cars are making their first U.S. appearance in the Lone Star State, courtesy of the alternative energy leader Green Mountain Energy. The company has just introduced a new “electricity product” for residential use, which makes wind power available to anyone who pays a home electricity bill and owns an electric vehicle."


Here's the study behind the above article...


As you can see the study assumes that the Texas grid going forward will be like the Texas grid in 2009. There's no allowance for increased wind and solar coming to the Texas grid. Flawed study.


Most studies are flawed and normally lean towards whoever pays for it.
It is so easy to leave out those factors which would give unwanted results.

Nick Lyons

Texas certainly doesn't have the best case electricity generation mix for displacement of GHG/criteria emissions (although some wind generation is coming on line, I believe). Results would be much more positive for plugins on the west coast, not so great in the upper midwest and northeast, if my memory of the regions most dependent on coal-fired generation is correct.


Out west half is natural gas, in the east half or more is coal. That is one of the first arguments brought up, what fuel do you burn to make the electricity? Which brings up how we produce electricity in the first place.

If the 600 coal fired power plants were converted to IGCC and made fuels as well as electricity we would be ahead of the game. The power plant companies have been milking profits doing it the same old way for so long, there is no money left to do anything else, it all went out as dividends.

Stan Peterson

The EPA is trying desperately to justify their continued level of funding, when it's becoming no longer really necessary.

Notice that they said "...with 'some' of the most densely populated regions in the US still failing to meet the National Ambient Air Quality Standards after 30 years of regulation."

Under the standards set at Earth Day in 1970 when all Americans were only dreaming about clean air and water, while breathing dirty air, the entire United States now meets those dreamed for and wished for, air quality standards.

During the intervening time, the standards have been tightened, so the definition of what constitutes "clean air" is now even much cleaner then on the First Earth Day. The careful choice of the word "some" is meant to convey and leave the impression of many polluted places, but "some" actually turns out to mean just two areas in the entire USA.

These two areas are all that remain of the once many polluted places in America, and are still not clean by today's tougher criteria. These are Metro Houston with its refineries, and Metro Los Angeles with its peculiar and unique geography and air inversions. But both are much, much cleaner than on the first Earth Day.

The exhaust from the typical American car is cleaner than the ambient air in most locations, and the modern American ICE powered auto, not even an electric vehicle, is actually a "clean air factory". Every moment it is operating, it sucks in dirtier air, processes it, while generating power to move the car, and expels even cleaner air, out of the tailpipe.

The United States of America is the first nation in the world to have addressed all toxic pollution, of its Air, Waters, and Land and virtually completely cured it.

Since North America is the World's largest carbon dioxide sink, and emits no net CO2 from Man or Nature, another scientific fact carefully masked, and furthermore sequesters a large amount of CO2 generated on other continents, America is now clean. Our massive land set-asides for multi-use including bio-sequestration, that we also know as "Parks" and "Wilderness" have solved that problem. America still is and will continue to be getting even cleaner, everyday, as we naturally replace older vehicles and generating stations, homes, and factories, with newer, cleaner versions.

These are the scientific TRUTHs which these desperate attempts at obfuscation, are trying to mask and hide.

We should be planning a massive national Celebration and Holiday to declare our Victory for the Environment, a "V-E Day". The time is rapidly coming, even if it has not in fact already arrived, to thank the EPA and their state and local equivalent bureaucrats, for their 41 year effort on the War for the Environment.

We can declare a "Peace Dividend" as we demobilize our large EPA armies. The time has arrived when we can shrink the EPA as well as state and local equivalents down to the size needed to merely maintain what we have achieved. We can turn out attention to address other necessary things, as we celebrate unique American exceptionalism.


As soon as you shrink the EPA budget more polluting greed mongers will be trying to cut corners for higher profits by polluting even more. Everyone knows this, it is a force for evil that will never be defeated. Eternal vigilance is the only way to keep it from ruining everything.


The grid mix for Texas in 2006; http://www.window.state.tx.us/specialrpt/energy/uses/exhibits/exhibit27-6.php

As of 2010, wind generated 7.8 percent of the grid's power, up from 6.2 percent in 2009 and 4.9 percent in 2008. In April and November, when the breezes blew especially strongly, wind averaged 12.1 percent of the grid's generation.

Wind may have risen, but natural gas fell: Gas stood at 38.2 percent of electricity use on the Texas grid last year, compared with 42.1 percent in 2009. Coal usage rose too, surpassing gas generation for the first time since at least 2002: In 2010, 39.5 percent of Texas's grid's generation came from coal, up significantly from 36.6 percent a year earlier. However, over a longer time frame — going back to 2005, when coal was 39 percent of ERCOT's generation — coal's share has held more or less steady. That contrasts to the national trend, where coal has fallen from 49.5 percent of electric generation nationwide in 2005 to 44.6 percent in 2009.

The upper midwest and northeast may not be the best place to recharge with wind power NOW but the region has the best wind resources in America; http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap_80meters.pdf

A report from the American Wind Energy Association (AWEA) lists these 10 states (in order) as having the best wind energy resources.
1.North Dakota
4.South Dakota

In fact, it's been calculated that if you put wind turbines on just 2% of the land in the first four you could generate 100% of the electrical energy Amercia uses in a year. Of course that's not saying it would actually be generated WHEN you need it. For that you'd have to have a whole lot of BEVs plugged into the grid overnight to store it until you do.

Now that sounds like a plan to me.




From SAP 2.2’s new synthesis
of the North American carbon
• Terrestrial sinks (ca. 2003)
are collectively only 30% of
fossil fuel emissions.
The terrestrial sink is primarily
associated with regrowing forests
in the United States (≈ 50% of the
• The future of the North American
terrestrial sink is highly uncertain,
with the expectation that the
forest regrowth contribution will
decline as forests mature clouded
by uncertainty in ecosystem
response to CO2 and climate.


Conventional ICE vehicle have DC brush motors implemented as starters and generators. The high currents involved in the operation of both devices can actually generate ozone.
EVs on the other hand, are generally equipped with brushless AC motors either single- or triple-phased. DC brush motors, also ozone production prone, are rarely used in a drive train becuse of their low efficiency and not being maintenance free. In a properly engineered EV, matching the state of the art, there are no current gaps and subsequent sparkings that are the cause of ozone. So what is all this garbage about?

Nick Lyons

@Stan: "large EPA armies" ??? What alternate universe are you living in?

EPA as share of US budget: ~$10B out of a budget of $3.69T. In other words, under 0.3% of the total Federal Budget.


More at stan:
Even during good years, when there's enough growth in vegetation to make North America a net carbon sink, that only means the continent is taking in more CO2 than we are generating ON the continent. As a country with a huge trade deficit, America is responsible for a lot more carbon than that is made off continent;

Stan Peterson

Thanks for the confirmation guys.

I agree that bio-sequestration comes from growing plants. That is precisely what bio-sequestration is, and means.

The Amazon Rain forest is a largely mature forest with little net new growth, so its CO2 sequestration is marginal, despite eco-chondriac mutterings to "save the rain forest" which it turns out is NOT being cut down at all, on net. That makes South America only the much smaller second continent in natural net CO2 sinks.

To the degree the eco-chondriacs prevent agriculture and silviculture, the bio-sequestration should decline.

But most agriculture is annual, so the re-growing will happen every year. Lots of our forests are managed for harvesting lumber and paper production, and are regularly cut down and regrown. If not lightning and forest fires will ensure their eventual renewal.

The North American continent has massive amounts of land used for farming, ranching, forests, lumber and paper production; and it will continue to do so, "ad perpetua".

A truly astounding TRUTH that is also never mentioned, is that the size of our bio-sequestration land set-aside is truly enormous and continues to grow, as each new Presidents and other politicians create new Parks.

There is more Park, Wilderness and BLM land set-asides, than the area of the original 13 Colonies at the time of our revolutionary founding. I challenge you to look it up for yourselves, if you don't believe, as it would be an illuminating educational moment. I now live in a State larger than all the New England States of my birth combined, in which only 17% of the land is open to human beings. My state is not untypical of most western States too.


Somehow creating national parks is going to make up for the billions of tons of coal burned? I do not think so.


"Ozone forms as hydrocarbons and nitrogen oxides, emitted into the air, react with sunlight."

Ok, so at night, how is the ozone created?


What this is saying is all those coal fired power plants can run all night charging EVs and the pollutants will dilute somewhere down wind and become ozone the next day when the sun comes out. It is just someone else's problem 12 hours later.

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