Latest Research ~ Alex Guenther

You’re telling me that our landscaping can contribute to air pollution?  Who knew!?!  Alex Guenther, that’s who!

Alex has spent the past 40 years studying plant emissions known as Biogenic Volatile Organic Compounds (BVOCs) and their role in air pollution and climate.  He and his team recently published a new study in the Proceedings of the National Academy of Sciences (PNAS) that helps to quantify the impact of BVOC emissions, especially in periods of extreme heat and drought.

Unlike the protective ozone layer in the stratosphere, tropospheric (ground-level) ozone is a major air pollutant that can cause or contribute to major health conditions and can also damage ecosystems.  It is also a greenhouse gas that contributes to climate change.  Ozone forms when organic molecules, including BVOCs, react in the presence of sunlight with nitrogen oxides generated from combustion of fossil fuels.  The reactions of BVOCs with atmospheric oxidants also generates airborne particles known as secondary organic aerosol (SOA) particles.  These make up a significant portion of airborne particulate matter that can penetrate deep into the lungs, exacerbating health conditions such as asthma or heart disease, and they are even associated with neurodegenerative diseases.

It is important to note that BVOCs on their own do not contribute to air pollution; they need the pollutants emitted from vehicles and power plants to form ozone and particles.  This means that understanding BVOC emissions and how they interact with anthropogenic emissions is very important for developing cost-effective and health-protective control policies.

This was very much a team effort for the Guenther group.  Their work clearly demonstrates how the release of BVOCs from plants is affected by climate conditions.  As human-driven climate change intensifies and extreme weather events occur more frequently, plant emissions of BVOCs increase with adverse effects on air quality and other climate factors.  AirUCI postdocs Hui Wang and Sanjeevi Naralingam and graduate student Allison Welch were key contributors to this study, demonstrating how heat waves and droughts driven by climate change can cause typically low-emitting plants to become substantial contributors to air pollution.  These extreme conditions can dramatically alter the way plants emit BVOCs that promote the formation of ozone.  

Hui Wang’s studies found that BVOCs are an important link among air quality, atmospheric chemistry, and the climate system.  Once they are released in the atmosphere, especially under extreme heat and drought conditions, BVOCs can influence the atmosphere in indirect ways.  This intensified release of BVOCs can further worsen air quality and intensify climate change.  As human-driven environmental change intensifies and extreme weather events occur more frequently, plant emissions of BVOCs increase with adverse effects on air quality and other climate factors. 

Hui studied Arctic sedges as well as Carex praegracilis (a sedge native to California commonly planted as a water-wise alternative to turf grass).  He found that sedges show very strong temperature response patterns in their BVOC emissions, and such a strong response suggests that current models may be underestimating Arctic isoprene emissions.

Sanjeevi Naralingam’s work supports the findings of Hui’s studies.  He examined the composition of BVOCs which includes molecules like isoprene, a compound naturally released by many trees including cypress, poplars, and oaks (of which many species are native to California).  Isoprene is highly reactive, playing a key role in the formation of tropospheric (ground-level) ozone as well as secondary organic aerosols (SOA), which as discussed above, influence air quality and climate.  

It is well known that BVOC emissions play a significant role in driving climate trends and that SOAs, which BVOCs help create, can scatter sunlight and promote cloud formation.  Clouds can cool the atmosphere by reducing the amount of solar radiation reaching the Earth's surface, but their overall impact is less understood compared to long-lived greenhouse gases such as carbon dioxide.  “The role of BVOC emissions in influencing air pollution and climate change is not well understood,” said Sanjeevi.  “Some plants usually emit very low levels of BVOCs, but during stressful events like heat waves, these plants become super-high emitters.”

Alex’s team plans to study more types of urban plants to help determine which ones are most affected by heatwaves with the goal of identifying species that may worsen air quality, particularly as communities strive to encourage greener local environments.  “We need to consider the potential for including urban BVOC emission controls in air quality management plans,” he says.  His team’s research findings are providing a clearer picture of how BVOC emissions respond to extreme conditions, and are already sparking efforts toward improving air quality and climate models. 

Content inspired by an article written by Ph.D. student Lurui Niu from the UC Irvine Department of Earth System Science. Niu is a 2024-2025 UC Irvine School of Physical Sciences Science Communication Fellow.  Thank you, Lurui!