EPA researchers find widespread use of nano cerium diesel fuel additives could have measurable impact on air quality
|Predicted surface-level concentrations of cerium due to use of nCe diesel fuel additives. Credit: ACS, Erdakos et al. Click to enlarge.|
Results of a modeling study by researchers from the US Environmental Protection Agency (EPA) suggest that widespread use of nanoparticulate cerium (nCe) diesel fuel additives across the US would have a measurable effect on regional air quality.
The model calculations suggest modest decreases of average PM2.5 concentrations and relatively larger decreases in particulate elemental carbon (EC). On average, across the 14-day winter and summer periods modeled, the percent change in EC exceeds that of PM2.5 by a factor of 5 in urban areas. As EC is a short-lived climate forcer, the reduction in EC concentrations has potential policy implications.
The results also predict ozone levels would increase in urban areas of the Midwest and Northeast, in contrast to a region-wide O3 decrease in the Southeast. Although the slight decrease in PM2.5 concentrations is desirable, the simultaneous increase in O3 could offset that benefit in some non-attainment areas, the team suggested.
The results also predicted increased naphthalene and decreased ambient levels of acrolein. EPA classifies naphthalene as a possible human carcinogen; further, changes in ambient levels of naphthalene could have implications on the formation of secondary organic aerosol (SOA), thereby enhancing PM2.5 concentrations.
However, they also found that the total US emissions of fine-particulate cerium are estimated to increase 25-fold and result in elevated levels of airborne cerium (a domain-wide-average cerium concentration of 0.5 ng/m3 and a maximum value of 22 ng/m3), which might adversely impact human health and the environment.
Although PM2.5 and EC concentrations decrease, the use of nCe diesel fuel additives results in emission of cerium, likely as single and aggregated nanoparticles. While the predicted concentrations of cerium are orders of magnitude lower than the reference concentration (RfC) (200 ng/m3) for microscale cerium, no RfC has been established for nCe.
The increase in ultrafine DPM [diesel particulate matter] emissions, along with the observed increase in some HAP [hazardous air pollutants] emissions, indicates a potential degradation of near-road air quality.—Erdakos et al.
Background. Efforts to reduce diesel particulate emissions include the use of oxidation catalysts; diesel particulate filters (DPFs); low-sulfur diesel fuels; and fuel additives; some additives also improve fuel economy.
One class of diesel fuel additives gaining acceptance comprises engineered nanomaterials composed of cerium compounds (nCe). Three such referenced in the study are Eolys, from Rhodia Electronics & Catalysis; Envirox, from Energenics Ltd.; and Platinum Plus, manufactured by Clean Diesel Technologies, Inc.
These fuel additives also reduce fine particulate matter (PM2.5) emissions and alter the emissions of carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbon (HC) species, including several HAPs.
Whereas the reduction of DPM emissions is a clear benefit of nCe-based additives, the simultaneous increase in cerium emissions might offset that benefit. The human-health effects of inhaling cerium-laden soot are a subject of active investigation, while the risk to aquatic ecosystems and soil organisms is less of a concern. Beyond these first-order effects of adding cerium to the environment, numerous studies have demonstrated that nCe additives alter the magnitude of other pollutants emitted from diesel engines. Widespread use of these additives may thus have a significant impact on air quality. For example, changes in nitrogen oxides (NOx) and volatile organic compound (VOC) emissions resulting from nCe additive usage could affect ambient levels of ozone, PM, and hazardous air pollutants (HAPs).—Erdakos et al.
In their investigation of the effect of nCe on air quality, the EPA team used the Community Multiscale Air Quality (CMAQ) model with multi-pollutant capability to predict atmospheric concentrations of criteria air pollutants (CAPs) and HAPs for a hypothetical scenario in which all on-road and non-road diesel vehicles in the eastern US use nCe additives. The researchers compared simulations of a 1-month long period during winter and summer with addition of nCe to all diesel fuels with base case simulations that use standard emission inputs.
They compiled data from existing literature and computed relative changes in PM, carbon monoxide (CO), NOx, and total hydrocarbon (THC) emissions for each pair of measurements (with and without nCe additive).
Garnet B. Erdakos, Prakash V. Bhave, George A. Pouliot, Heather Simon, and Rohit Mathur (2014) “Predicting the Effects of Nanoscale Cerium Additives in Diesel Fuel on Regional-Scale Air Quality” Environmental Science & Technology doi: 10.1021/es504050g