Georgia Tech study finds link between sulfate, metallic particles from vehicles and adverse health impacts
Metals from brakes and other automotive systems are emitted into the air as fine particles, lingering over busy roadways. Now, researchers at Georgia Institute of Technology have shown how these vehicle-emitted metals—such as copper, iron and manganese—interact with acidic sulfate-rich particles already in the air to produce an aerosol that, when inhaled, is more likely to cause oxidative stress and impact respiratory health. Their study is published in the ACS journal Environmental Science & Technology.
The study, which was sponsored by the National Science Foundation and the US Environmental Protection Agency, showed how the metals are emitted mainly in an insoluble form but slowly become soluble after mixing with sulfate. In other words, the sulfate plays a key role in making metals soluble before they are inhaled, which could explain the association of sulfate with adverse health impacts, the researchers said.
Oxidative potential (OP), referred to as the ability of particles to generate ROS [reactive oxygen species] by consumption of antioxidants and/or generation of oxidants, has been used as a health-based exposure measure of PM in several recent studies. … To mitigate adverse health effects, ambient particle OP sources, and atmospheric transformations that alter OP, need to be known.
A number of aerosol components have been found to correlate with aerosol OPDTT [dithiothreitol]. These include bulk water-soluble organic carbon (WSOC), humic-like substances (HULIS) and highly oxygenated organic aerosols, and more specific aerosol components, such as polycyclic aromatic hydrocarbons (PAHs), quinones, and water-soluble transition metals (e.g., manganese (Mn) and copper (Cu)). Source apportionment points to incomplete combustion from biomass and fossil fuels (gas and diesel engines), and sources associated with transition metals, such as mineral dust and resuspended road dust from tire or brake wear.
In contrast to OPDTT, correlations suggest that transition metals (i.e., Cu) are the main aerosol component contributing to OPAA [ascorbic acid], with road-traffic a major source.—Fang et al.
For the study, the team collected size-segregated ambient PM from a road-side and representative urban site in Atlanta, GA. They then measured elemental and organic carbon, ions, total and water-soluble metals, and water-soluble OP (using a high-throughput analytical system that simulates the toxic response that such a mix would have on cellular organisms).
Sulfate was spatially uniform and found mainly in the fine mode, whereas total metals and mineral dust cations were highest at the road-side site and in the coarse mode, resulting in a fine mode pH < 2 and near neutral coarse mode.
The team found that soluble metals and OP peaked at the intersection of these modes; the researchers concluded that this demonstrates that sulfate plays a key role in producing highly acidic fine aerosols capable of dissolving primary transition metals that contribute to aerosol OP. Such sulfate-driven metals dissolution may account for sulfate-health associations reported in past studies, they suggested.
That’s the smoking gun. The sulfate essentially dissolves those metals; when you breathe in those particles, the metals could be absorbed directly into the blood stream and cause problems throughout the body. For the first time, a mechanism emerges to explain why small amounts of acidic sulfate can adversely affect health.—Athanasios Nenes, a professor and Georgia Power Scholar in the School of Earth & Atmospheric Sciences and the School of Chemical & Biomolecular Engineering
While the sample taken from the testing site located farther away from the highway had less particulate metal, there was still enough to cause an increase in the oxidative potential, showing that roadway pollution could travel through the air and potentially cause problems in surrounding areas as well.Dust from brakes and tires isn’t the only source of metals in the air. Incinerators and other forms of combustion also produce mineral dust and metallic particles, which could mix with sulfate to trigger a similar reaction.
The researchers noted that while the amount of particulate sulfate in the southeastern United States has decreased during the past 15 years as sulfur dioxide emissions from power plants have fallen, there’s still enough acidic sulfate in the air to keep the pH of particles very low, in the range of 0 to 2, transforming insoluble ambient metals to a soluble form.
There’s a chain reaction happening in the air above busy highways. Acidic sulfate in the atmosphere comes into contact with those metals emitted from traffic and changes their solubility, making them more likely to cause oxidative stress when inhaled.
Vehicle tailpipe emissions are going down, but these kinds of emissions from braking will remain to some extent, even if you drive an electric car. Therefore, this kind of process will continue to play out in the future and will be an important consideration when we look at the health effects of particulate matter.—Rodney Weber, a professor in Georgia Tech’s School of Earth & Atmospheric Sciences and corresponding author
This material is based upon work supported by the National Science Foundation under Grant No. 1360730 and the U.S. Environmental Protection Agency under Grant No. RD834799. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the U.S. Environmental Protection Agency.
Ting Fang, Hongyu Guo, Linghan Zeng, Vishal Verma, Athanasios Nenes and Rodney J. Weber (2017) “Highly acidic ambient particles, soluble metals and oxidative potential: A link between sulfate and aerosol toxicity,” Environmental Science & Technology doi: 10.1021/acs.est.6b06151