Competition not concentration matters when forming cloud-influencing aerosols

April 14, 2015
Researchers are seeking to understand the chemical processes leading to secondary organic aerosol formation from isoprene compounds emitted by many species of trees including eucalyptus (shown here). Credit: Elizabeth Donoghue (Flickr) via a Creative Commons License

Tiny aerosols greatly influence cloud formation and sunlight's scattering or absorption. Yet, the factors that influence the formation of these aerosols, known as secondary organic aerosol or SOA, are often assumed. Researchers determined that previous assumptions about nitrogen oxide levels were overly simplistic; the amount of SOA produced from isoprene released by trees as well as the SOA volatility are more accurately tied to interactions with electron-rich, carbon-based chemicals, known as organic peroxy radicals, that compete with nitrogen oxides in reactions.

Results of this study indicate that nitrogen oxides cannot be assumed to have a linear effect on SOA formation. The complexities of the relationship need to be included in the next generation of models to accurately predict the dynamics of SOA formation and composition in different environments.

Isoprene, an organic compound produced and emitted into the atmosphere by many tree species, plays an important role in tropospheric ozone chemistry and in formation of SOA particles. The particles can affect Earth's radiation balance directly through scattering or absorption of sunlight and indirectly through forming cloud condensation nuclei. Accurately simulating the impacts of SOA on climate requires understanding the chemical processes that lead to SOA formation in the atmosphere under a wide variety of environmental conditions.

Previous laboratory studies have demonstrated different SOA oxidation yields and properties at "low" (near zero) and "high" levels of (NOx). Current models use a linear combination of these extreme conditions to predict altered SOA formation in the presence of anthropogenic emissions. Unfortunately, data from recent laboratory and field studies have not been consistent with this simple model. To help address this challenge, a team of U.S. Department of Energy researchers conducted laboratory experiments in an environmental chamber to investigate the effects of NOx on the volatility and chemical composition of SOA generated by isoprene photooxidation.

Volatility is a key property of organic aerosol because it determines the partitioning between the gas and particle phases, and thus the formation of SOA particles. Researchers found that the volatility and oxidation state of isoprene SOA are sensitive to, and exhibit a nonlinear dependence on, NOx levels. The dependence likely arises from gas-phase chemistry of organic peroxy radicals, which compete with NOx in reactions, and succeeding particle-phase reactions. This observation helps reconcile the seemingly contradictory observations of the NOx effect on SOA reported in previous literature studies.

Explore further: Scientists find missing piece of air particle equation hiding in the walls

More information: "Effects of NOx on the volatility of secondary organic aerosol from isoprene photooxidation." Environmental Science & Technology 48(4), 2253–2262 (2014). DOI: 10.1021/es404842g

Related Stories

Brown carbon works both sides of the climate equation

February 4, 2014

There is an atmospheric particle not satisfied with only a single role in the climate. The ambitious culprit? Brown carbon aerosol steps outside the box and acts to both warm and cool the climate. A brown secondary organic ...

Clean smell doesn't always mean clean air

October 29, 2014

Some of the same chemical reactions that occur in the atmosphere as a result of smog and ozone are actually taking place in your house while you are cleaning. A researcher in Drexel's College of Engineering is taking a closer ...

A new method for measuring the viscosity of nanoparticles

March 28, 2013

For the first time, scientists measured the chemical diffusivity and viscosity of atmospheric organic particles, thanks to a new approach from scientists at Pacific Northwest National Laboratory, University of Washington, ...

Recommended for you

Heavy nitrogen molecules reveal planetary-scale tug-of-war

November 17, 2017

Nature whispers its stories in a faint molecular language, and Rice University scientist Laurence Yeung and colleagues can finally tell one of those stories this week, thanks to a one-of-a-kind instrument that allowed them ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.