Georgia Tech’s School of Chemical & Biomolecular Engineering is known for the far-reaching impact of the research conducted by our faculty. In this Q&A feature, we spotlight research by Associate Professor Nga Lee (Sally) Ng.

In 2014, Ng published the paper, “Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern U.S.,” in Proceedings of the National Academy of Sciences. Since that time, the study has amassed 167 citations in other journal articles (Web of Science).

In the study, Ng found that certain emissions from cars and coal-fired power plants promote processes that transform naturally occurring emissions from trees into organic aerosols that can affect climate, air quality, and human health.

The researchers, including Georgia Tech Professors Athanasios Nenes and Rodney Weber, found that sulfur dioxide and nitrogen oxides directly and substantially mediate the formation of aerosols from the volatile organic compounds produced by trees.

“This finding is good news for pollution control. If we are able to further reduce sulfur dioxide and nitrogen oxide emissions, we will decrease not only sulfate aerosols, but also organic aerosols,” Ng explains.

Q&A

• Why do you think this paper has been cited so frequently?

Atmospheric aerosols (tiny particles in the air) have substantial impacts on human health and global climate. It has long been established that organic carbon compounds given off by trees further react in the atmosphere to form organic aerosols. However, little is known regarding the extent to which human activities affect these natural processes and reactions. This has been a topic of intense study in our aerosol community.

Possible mechanisms have been proposed in prior studies, but a coherent picture on how human and natural emissions interact in the ambient environment has not emerged. In this study, we provided direct evidence on the magnitude of the influence of manmade pollutants (e.g., sulfur dioxide and nitrogen oxides from power plants and vehicles exhausts) on processes that transform naturally occurring emissions from trees into organic aerosols. We achieved this by combining ambient measurements from sites in and around Atlanta and rural Alabama, as well as laboratory studies.

The publication of our work in a high impact journal (PNAS) provided an excellent platform for increasing the visibility of this study in our immediate research community and beyond. There have been many complimentary studies since then on how sulfur dioxide and nitrogen oxides affect the formation of aerosols from tree emissions in the atmosphere.

• Do you continue to research this area? What is some related research you’ve published?

Yes, how manmade pollutants alter the reactions of natural emissions from trees in the atmosphere has been a major research focus of my group.

For instance, we have demonstrated how the very different levels of sulfur dioxide emissions from two power plants in the southeastern US (Harllee Branch power plant and Scherer power plant) resulted in differing amount of organic aerosols formed downwind (Xu et al., JGR, 2016).

We have also conducted ambient studies (Xu et al., ACP, 2015) and laboratory studies to investigate how nitrogen oxides and compounds from their further reactions in the atmosphere control organic aerosol production and transformation from tree emissions during nighttime, e.g., Boyd et al., ACP, 2015, Nah et al., ES&T, 2016, and Boyd et al., ES&T, 2017. Together with other researchers in the field, we have also published a detailed review article on this topic, Ng et al., ACP, 2017.

The success of pollution control strategies in the U.S. has resulted in decreased emissions of sulfur dioxide and nitrogen oxides over the years. How changing future emissions influence the formation of aerosols from trees remains a topic of strong interest for the research community. We have co-organized a series of workshop/symposium/town hall on relevant research topics at Georgia Tech (“Workshop on Nitrate Radicals and Biogenic Volatile Organic Compounds: Oxidation, Mechanisms and Organic Aerosol”), the American Association for Aerosol Research Annual Conference (“Effects of NO_x and SO₂ on BVOC Oxidation and Organic Aerosol Formation”), and the American Geophysical Union Fall Meeting (“Nitrate Radicals and Biogenic Volatile Organic Compounds”).