Standardizing Air Chemistry

A recent United Nations report on global warming includes more accurate and reliable climate predictions than ever before, and its authors give credit to recent improvements in the fundamental understanding of climate processes (C&EN, Oct. 7, page 13). The noted improvements rely in part on standards that ensure the accuracy of climate models, and the International Union of Pure & Applied Chemistry plays a key role in keeping these standards up to date.

A critical component of climate prediction is atmospheric chemistry and researchers’ ability to model it. An IUPAC committee has issued six studies on atmospheric chemistry over the past decade, each one focusing on different sets of reactions that occur in the atmosphere. The latest effort looks at heterogeneous processes involving airborne liquid particles (Atmos. Chem. Phys. 2013, DOI: 10.5194/acp-13-8045-2013).

Atmospheric modeling is important not just for regional and global climate prediction but also for understanding phenomena such as air quality and acid rain. Nothing about this is simple. Modelers’ predictions are as good as their inputs, and they require accurate kinetic parameters for a number of reactions. Those reactions might occur in the gas phase or on particles, be initiated or altered by light, and occur in conditions ranging from a hot and humid 40 °C at Earth’s surface to a cold and dry –80 °C in the stratosphere.

For individual atmospheric chemistry modelers, keeping up with the latest research to stay current on the best parameters for every reaction would be a daunting task. So for the past several decades international committees have assessed kinetic and photochemical data to recommend consensus parameters. The current incarnation of the IUPAC committee that does that, the organization’s Task Group on Atmospheric Chemical Kinetic Data Evaluation, has published several sets of recommendations since 2004.

The committee also annually reviews newly published experimental data for reactions it has already covered to revise parameters as necessary. Updated data sheets are freely available on the group’s website, iupac.pole-ether.fr.

“I think I can say without reservation that unless you are an expert in the field, you don’t have a chance of going into the literature and figuring out for yourself what is and is not reliable,” says Sasha Madronich, a senior scientist at the National Center for Atmospheric Research (NCAR). Madronich uses the IUPAC parameters in his modeling of chemical processes in the atmosphere.

The field is complex, Madronich says, and lab experiments have many different complications and idiosyncrasies. A lot of hands-on experience goes into developing good judgment, he adds.

Modelers are often reluctant to change computer code in their atmospheric-modeling programs, yet they do when the IUPAC group releases updated recommendations. Madronich points to that as an indication of the respect the modelers have for the committee members. Other efforts to pinpoint kinetic parameters for atmospheric reactions exist, Madronich notes, but the IUPAC effort stands out for its scope and up-to-date values.

Settling on consensus parameters for atmospheric reactions is a task that is “pretty complicated,” says Timothy J. Wallington, the senior technical leader for environmental sciences at Ford Motor Co. and current chair of the IUPAC group. The committee started a decade ago by establishing consensus parameters for gas-phase reactions of O_x, HO_x, NO_x, and SO_x species, then moved through gas-phase reactions of organic and halogenated species before tackling heterogeneous reactions on solids and liquids in the past few years.

People might think that parameters for simple gas-phase reactions were long settled, but that is not always the case, Wallington says. He notes in particular the reaction of hydroxyl radical with NO₂, which is considered to be among the most important atmospheric reactions. Both hydroxyl radical and nitrogen dioxide can participate in a complex cycle of reactions that form health-harming ozone at Earth’s surface. If they react with each other, however, they form nitric or peroxynitrous acid, cutting off ozone production. The IUPAC group first published kinetic parameters for these reactions in 2004, but the team revisited the reactions and revised the values in 2012 on the basis of recent experimental and theoretical work (Science 2010, DOI:10.1126/science.1193030; J. Phys. Chem. A 2012, DOI: 10.1021/jp212095n).

In its most recent evaluation of new reactions, those of heterogeneous reactions on solid and liquid particles, the committee began to address one of the big sources of uncertainty in atmospheric and climate modeling: the role of aerosols. Airborne aerosol particles can be emitted directly into the atmosphere, such as from soot or sea spray, or can form in the air through condensation of airborne compounds. Either way, chemical reactions involving aerosol particles play key roles in air quality and climate.

Among the common classes of aerosols are those composed of sulfuric acid, which forms when sulfur dioxide emissions react with water. High in the atmosphere, the reaction of hydrochloric acid with hypochlorous acid on sulfuric acid particles is critical for activating chlorine to destroy protective ozone.

In developing a recommended rate constant for this reaction, the IUPAC committee had to assess the reliability of experimental data. Committee members considered how well experiments simulate atmospheric conditions, whether detection methods are sensitive enough to accurately analyze species at low concentrations, and how diffusion may affect the observed kinetics, says IUPAC group member Markus Ammann. Ammann leads the surface chemistry group in the Laboratory of Radiochemistry & Environmental Chemistry at the Paul Scherrer Institute, in Switzerland. In some cases, committee members had to reanalyze data from older kinetic experiments using more recently determined solution properties, such as the solubility of hydrochloric and hypochlorous acid in sulfuric acid under atmospheric conditions.

In putting together its report on heterogeneous reactions on liquids, the committee prioritized the reactions believed to be most important to the atmosphere. There were some reactions that committee members would have liked to include but didn’t, Ammann says. Hydroperoxyl radical, for example, plays a key role in the network of atmospheric radical reactions, but “very few and conflicting data sets have made it impossible for us to provide conclusive recommendations,” he says.

“We hope that the lack of recommendations in some of the important systems will lead to further laboratory efforts to better understand the kinetics of key species,” Ammann adds.

The IUPAC committee’s next effort will likely focus on aromatic compounds, including cresols, nitrophenols, and furans. These chemicals are primarily emitted from motor vehicles and contribute to ozone and aerosol formation. “Accurate kinetic parameters for their oxidation processes are needed for a better development of their reaction mechanisms for urban and regional atmospheric models,” says IUPAC team member Abdelwahid Mellouki, who directs the atmospheric reactivity research group at France’s National Center for Scientific Research.

“Looking to the future, as computer models and computational power get more substantial, that allows us to put more and more chemistry detail into models,” Wallington says. IUPAC’s standards-setting efforts will help to ensure that the chemistry is as accurate as possible, allowing researchers to put even more confidence into air quality and climate predictions, he notes.