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

Resources

  • 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


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