Total VOC emission factors for end uses of petrochemical sources in the study. Shown in the bottom row are sales data of fuels for mobile sources (from Fig. 1A) and sales data of volatile chemical products. The green symbol and dashed arrow illustrate the large reductions in tailpipe VOC emission factors as precatalyst on-road gasoline vehicles were replaced by present-day vehicle fleets. Error bars reflect the 95% confidence interval of the mean or expert judgment. McDonald et al. Click to enlarge.

Recent research suggests that adverse human health effects occur below current US standards for both PM_2.5 and O₃. Thus, it is important to identify and quantify the most important human-produced sources of VOC emissions to effectively mitigate air pollution and improve human health, the authors said.

Automotive emissions of VOCs have decreased steadily from efforts to control tailpipe emissions in the United States and Europe. As a result, other sources of VOC emissions are likely growing in relative importance. Transportation emissions of NO_x and VOCs have long been considered major contributors to formation of O₃ and SOA in urban areas, although recent studies have suggested the importance of nonvehicular sources as major contributors. Emissions from the use of chemical products have been difficult to constrain in models or from ambient measurements. One challenge has been the lack of available atmospheric measurements of oxygenated volatile organic compounds (OVOCs) common in everyday household products.

Here, we focus on volatile chemical products (VCPs), including pesticides, coatings, printing inks, adhesives, cleaning agents, and personal care products. These products contain organic solvents, which lead to substantial emissions of VOCs to the atmosphere.

We show that success in controlling air pollution has changed the proportions of sources of anthropogenic VOC emissions in the United States, decreasing the relative contribution from transportation fuels and increasing the contribution from VCPs. We consider four key pieces of evidence to support this finding: (i) energy and chemical production statistics; (ii) near-roadway measurements of transportation emissions, together with laboratory testing of chemical products; (iii) ambient air measurements away from roads; and (iv) indoor air measurements.

—McDonald et al.

People use a lot more fuel than they do petroleum-based compounds in chemical products—about 15 times more by weight, according to the new assessment. Even so, lotions, paints and other products contribute about as much to air pollution as does the transportation sector. In the case of PM_2.5, particle-forming emissions from chemical products are about twice as high as those from the transportation sector, the team found.

As transportation gets cleaner, those other sources become more and more important.

—Brian McDonald, corresponding author

The scientists concluded that in the US, the amount of VOCs emitted by consumer and industrial products is actually two or three times greater than estimated by current air pollution inventories, which also overestimate vehicular sources.

Contributors to ambient air pollution in Los Angeles. (A to D) Distribution of (A) petrochemical product use, (B) VOC emissions, (C) VOC reactivity with OH, and (D) SOA formation potential across petrochemical sources only. Contributions from nonfossil sources are not shown. Uncertainties in source apportionment were determined by Monte Carlo analysis. McDonald et al.Click to enlarge.

For example, the Environmental Protection Agency estimates that about 75% of VOCs emissions (by weight) come from vehicular sources, and about 25% from chemical products. The new study, with its detailed assessment of up-to-date chemical use statistics and previously unavailable atmospheric data, puts the split closer to 50-50.

The disproportionate air quality impact of chemical product emissions is partly because of a fundamental difference between those products and fuels.

Gasoline is stored in closed, hopefully airtight, containers and the VOCs in gasoline are burned for energy. But volatile chemical products used in common solvents and personal care products are literally designed to evaporate. You wear perfume or use scented products so that you or your neighbor can enjoy the aroma. You don’t do this with gasoline.

—NOAA atmospheric scientist Jessica Gilman

Christopher Cappa, professor of civil and environmental engineering at UC Davis, said that there has been a persistent gap between levels of fine particles measured in urban air and predictions from models. The new work has the potential to close this gap, he said.

The team found that they simply could not reproduce the levels of particles or ozone they measured in the Los Angeles area without including emissions from volatile chemical products. They also determined that people are exposed to very high concentrations of volatile compounds indoors.

Additional authors on the study were: Joost A. de Gouw, Stuart A. McKeen, Yu Yan Cui, Si-Wan Kim, CIRES and NOAA Chemical Sciences Division; Gregory J. Frost, James M. Roberts, Thomas B. Ryerson, and Michael Trainer, NOAA Chemical Sciences Division; Shantanu H. Jathar, Colorado State University and UC Davis; Ali Akherati, Colorado State University; Jose L. Jimenez, CIRES and CU Boulder; Julia Lee-Taylor, CIRES and National Center for Atmospheric Research; Patrick L. Hayes, Université de Montréal; Drew R. Gentner, Yale University; Gabriel Isaacman, NCAR; VanWertz, Virginia Tech; Allen H. Goldstein and Robert A. Harley, UC Berkeley.

This research was supported by NOAA, the CIRES Visiting Fellowship Program, Aerodyne Research, Inc., the National Science Foundation and the Sloan Foundation. CIRES is a partnership of NOAA and the University of Colorado Boulder.

Resources

• Brian C. McDonald, Joost A. De Gouw, Jessica B. Gilman, Shantanu H. Jathar, Ali Akherati, Christopher D. Cappa, Jose L. Jimenez, Julia Lee-Taylor, Patrick L. Hayes, Stuart A. McKeen, Yu Yan Cui, Si-Wan Kim, Drew R. Gentner, Gabriel Isaacman-Vanwertz, Allen H. Goldstein, Robert A. Harley, Gregory J. Frost, James M. Roberts, Thomas B. Ryerson, Michael Trainer (2018) “Volatile chemical products emerging as largest petrochemical source of urban organic emissions” Science Vol. 359, Issue 6377, pp. 760-764 doi: 10.1126/science.aaq0524