Laboratory Studies Breakout Session Summary

DOE Atmospheric Chemistry Annual Meeting, Nov. 1996

The laboratory group met on the afternoon of Tuesday, November 19 as a group, with the combined field experiments and modelling groups meeting on Wednesday, November 20 and with the modelling and theoretical group on Thursday, November 21.

Those in attendance included scientists with current ACP support: Paul Davidovits (Boston College), Barbara Finlayson-Pitts (University of California, Irvine), Jeff Gaffney (Argonne), Yin-Nan Lee (Brookhaven), Nancy Marley (Argonne), Peter McMurry (University of Minnesota) and Doug Worsnop (Aerodyne).

In addition, several scientists from DOE laboratories and visitors from Universities and government agencies participated. These included: Jonathon Abbatt (University of Chicago), Steven Colson (Pacific Northwest Laboratory), Bruce Garrett (Pacific Northwest Laboratory), Vernon Morris (Howard University ), Scot Martin (University of North Carolina) and Ron Patterson (U.S. EPA) representing the NARSTO Management Coordinator's Office.

Successes and Deliverables

During the past year, the laboratory studies group has made significant contributions towards the DOE Atmospheric Chemistry Program goals of understanding the impacts of energy usage on the atmosphere in both polluted and remote areas. These successes can be divided into two overall areas: (1) the development and application of new techniques for the measurement of key species in air, and (2) greatly improved understanding of the fundamental mechanisms and kinetics of important atmospheric reactions in such a way that the results can be used in ACP field and modelling efforts. A brief summary follows. Details can be found in the abstracts of posters presented at this meeting.

Development and Application of New Methods for the Measurement of Energy-Related Atmospheric Species:

• Techniques to measure particles down to 3 nm in size have been developed in Professor McMurry's laboratory. This greatly enhances our ability to probe the mechanisms of particle formation in air, which is important for understanding the effects of energy usage on visibility reduction, health effects and climate.
• A method of identifying and measuring hydroxyacetone, a product of the reaction of the biogenically emitted organic isoprene with OH, has been developed in Dr. Lee's laboratory. This provides a means of assessing the contribution of isoprene to ozone formation and hence understanding the relative contributions of anthropogenic versus biogenic VOCs. Clearly such research is central to developing cost-effective control strategies for ozone and associated species.
• Techniques to separate, identify and measure PANs up to four carbons in length have been developed by Drs. Gaffney and Marley. Because these PANs are closely associated with the formation of ozone, and in themselves have known effects on plants at ambient levels, their quantification is essential to understanding the VOC-NO_x chemistry which ultimately leads to the formation of ozone.
• Techniques to measure total nitrates (sum of gaseous HNO₃ and aerosol nitrate) have been developed in Dr. Lee's laboratory. An ability to quantify total nitrate, along with NO_x, is important in assessing the cycling of NO_x and helps to identify the dominant radical sink process, i.e., RO₂(HO₂) RO₂(HO₂) vs OH-NO₂, which intimately governs the efficiency with which NO_x produces O₃. Furthermore, the measurement of total nitrates represents a necessary step toward understanding the current quandaries associated with the so-called NO_y measurement, of which total nitrates should account for a significant fraction.
• Progress has been made by Drs. Gaffney and Marley towards modifying the ozone-alkene chemiluminescence detector for the measurement of total reactive organics. Because of the extreme difficulty in individually identifying and measuring the hundreds (or thousands) of organics found in air, such an instrument will be a tremendous asset in field studies directed to assessing the relative importance of VOC and NO_x in ozone formation.
• A product which is unique to the chlorine atom-isoprene reaction, 1-chloro-3-methyl-3-butene-2-one, has been identified in laboratory studies of this reaction by Professor Finlayson-Pitts' research group. Since recent studies indicate that phytoplankton in the ocean produce isoprene, field measurements of this product would provide confirmation of the contribution of chlorine atoms to VOC-NO_x chemistry in coastal regions. Methyl vinyl ketone and methacrolein that are formed in the OH-isoprene reaction are not major products of the chlorine atom reaction. Thus the role of OH versus Cl in the formation and fate of ozone can be separated in future studies by measuring these products.
  

Kinetics and Mechanisms of Energy-Related Atmospheric Reactions

• Kinetic techniques have been developed in Dr. Lee's laboratory to study very fast aqueous phase reactions, and these have been applied to such reactions as that of O₃ with I^-. This will facilitate the assessment of fast reactions in aerosol particles, fogs and clouds as well as the ocean surface, and provides crucial input to models which include aqueous phase processes.
• The uptake of formaldehyde into highly acidic droplets containing H₂SO₄ and HNO₃ was shown by Dr. Worsnop and Professor Davidovits to lead ultimately to the formation of nitrous acid. While the atmospheric implications need to be explored fully, this observation suggests that aerosols may play a key role in the formation of HONO, an important source of OH under many conditions. Understanding such processes is critical to provide an accurate evaluation of various ozone control strategies.
• PAN and NO₂ were shown by Drs. Gaffney and Marley to react heterogeneously on soot, soils and other surfaces. Understanding such processes is very important in modelling the formation of ozone and other photochemical oxidants in that NO₂ produces O₃ via photolysis. On the other hand, PAN ties up the NO_x so that it does not contribute to ozone formation. If it is recycled back to NO₂ by heterogeneous processes, however, it may not be the sink for NO₂ which has been assumed in the past.
• The kinetics of oxidation of SO₂ in concentrated sulfuric acid solutions similar to those found in some atmospheric aerosol particles by O₃ was shown by Professor Davidovits and Dr. Worsnop to be slower than in aqueous solutions. Such data are critical for quantifying the contribution of SO₂ to acid deposition, and to understanding the role of SO₂ in particle formation in the atmosphere.
• The existence of strongly surface bound water on salts such as NaCl and its central role in heterogeneous reactions with gases such as HNO₃ have been discovered in Professor Finlayson-Pitts' laboratory. Such data are critical for accurate modelling of the chemistry of sea salt particles in the marine boundary layer as well as in unique situations such as the plumes from oil well burning in Kuwait which contained substantial salt concentrations.
• The reactions of synthetic sea salt with NO₂ and HNO₃ were shown in Professor Finlayson-Pitts' laboratory to be dominated largely by the crystalline hydrates such as MgCl₂.6H₂O. Understanding the contributions of such components of sea salt to the overall reactivity and potential formation of atomic chlorine is essential to the accurate assessment of the role of chlorine atoms relative to other oxidants such as OH and NO₃ to the formation and fate of ozone in coastal regions.
• The uptake of ammonia, the only gaseous base found in the atmosphere, on water and sulfuric acid solutions has been quantified by Professor Davidovits and Dr. Worsnop. These data are critical for an assessment of acid deposition and the nitrogen cycle, as well as for understanding the formation and fate of aerosol particles.
• An upper limit to the formation of N₂O from the reaction of CO₃* with N₂. was established by kinetic studies in Professor Finlayson-Pitts' laboratory. These data are essential to understanding the chemistry of the stratosphere, since N₂O is the major "natural" source of oxides of nitrogen in this region.
• Aerosol residence times have been probed by Drs. Marley and Gaffney using isotopic tracers. These data indicate very long lifetimes for small particles, which has important implications for their potential to contribute to tropospheric chemistry. Clearly, models of ozone formation and the role of particles on visibility reduction, health effects and climate must take into account such data.

In short, over the past year the laboratory studies group have not only contributed to improved understanding of the fundamental kinetics and mechanisms of atmospheric reactions of key importance to evaluating the impacts of energy production and use, but also to the measurement in field studies of key species. In addition, critical data have been provided for use in state-of-the-art models.

Future Plans

The laboratory studies group discussed in some detail where the greatest uncertainties currently lie in evaluating the impacts associated with energy production and use. The general consensus of the group was that the role of aerosol particles in a wide variety of energy-related atmospheric phenomena represented by far the greatest gap in our current understanding. Not only do these particles play a central role in visibility reduction and radiative forcing, but they have also been intimately linked in recent years to effects on human health, including increased mortality. In addition, they are directly associated with diesel energy sources. While they are known to be inextricably linked to all of these issues, a great deal remains to be learned about the exact nature of the interrelationships. Last, but certainly not least, virtually nothing is known about the relationship between the chemistry of aerosol particles and the formation and fate of ozone and other oxidants.

Areas of future focus recommended by this group were subdivided into reactions in the condensed phase, interactions involving both gases and condensed phases and processes which are unique to the gas phase but which also have important implications for the atmospheric formation and fates of condensed phases as well. Specific problems which the group highlighted are summarized below.

Aerosol-Condensed Phase

• Chemical characterization of particles not only in bulk particle size ranges, but more importantly, single particle analysis.
• Characterization of the chemical nature of the interface, particularly in light of laboratory studies over the last few years which have provided tantalizing "hints" that there are unique species at the gas-aqueous phase interface which may play critical roles in the uptake of gases and in their reactions.
• Characterization of the uptake of aerosol particles into aqueous systems and the chemistry associated with these solvation processes. This has important implications not only for the behaviour and impact of particles in the atmosphere, but also for health effects when particles are inhaled.
• Photochemistry in condensed phase mixtures characteristic of the atmosphere.
• Characterization of the optical properties of the condensed phase, including contributions to both scattering and absorption of light.

The general consensus was that while some data are available which bear on these issues, most of it involves the inorganic particle components. Relatively little is known about organics of various kinds, despite the fact that these may play a critical role in determining the physical and chemical properties of aerosol particles and their roles in energy related issues.

Gas-Aerosol Interactions

• Potential interconversions of NO_x and NO_y on aerosol particles.
• Nucleation from low volatility gas phase species, growth of particles, and phase changes in particles.
• Role of chemisorbed species at interfaces in the exchange between the gas and condensed phase, and on the chemistry.
• Effects of gas phase organics, especially biogenics such as terpenes, on the formation of particles.
• Effects of aerosol particles either as sinks for O₃, or to form O₃, e.g., via the production of NO_x or HONO.

Gas Phase Processes

• Formation, fate, and measurement techniques for multifunctional organics, including oxygenates and nitrates, many of which may be taken up into aerosol particles.
• Identification of potential components of "missing NO_y" such as organic nitrates.

In summary, the consensus was that our fundamental understanding of the formation, composition, chemistry and photochemistry, and interfacial and optical properties of atmospheric aerosol particles is quite rudimentary at the present time, especially with respect to organics. Furthermore, very little is known about their role in a wide range of atmospheric phenomena which are central to the effects of energy production and usage.

Given the relatively small laboratory studies group, it was recommended that our immediate future efforts in this area be focussed on the issue of the interrelationships between the chemistry of oxidants and aerosols. This ties in, with and supports, both the field experiments as well as with the efforts of the modeling and theoretical group.

Collaborations with Field Experiments

As detailed above under "Successes and Deliverables", a significant portion of the laboratory groups efforts over the past year have involved supporting field experiments through the development and application of new measurement techniques, and the identification of species in laboratory studies which are prime candidates for field explorations. Numerous collaborative efforts have been developed and their success hinges on continuing and expanding these successful collaborations between the laboratory and field groups, as well as developing new ones in the future.

The consensus of the laboratory studies group was that the current state of understanding of the issues involving aerosol particles described above under "Future Plans" is quite rudimentary. Hence a great deal of laboratory research needs to be done in this area in order to be able to provide tangible guidance to, and forge collaborative efforts with, the field experiments group in this area.

Collaborations with the Modeling and Theoretical Group

Collaborations are already in place between the laboratory studies group and the modeling and theoretical studies group, as well as the field experiments group. For example, Elaine Chapman (PNL) and Jeff Shorter (Mission Research) have been collaborating on sensitivity studies of chlorine atom chemistry in the marine boundary layer using the laboratory measurements of the Finlayson-Pitts group and the TAGA field experiments of Chet Spicer and colleagues (Battelle Columbus). In addition, results from laboratory studies are made available to the modeling and theoretical group on an ongoing basis.

The possibility of the laboratory studies and modeling and theoretical groups holding a workshop tightly focussed on a limited number of issues was discussed. It was agreed that topics most likely to result in fruitful collaborations will be discussed via e-mail over the coming months, with particular focus on the aerosols issue. If a suitable focus is developed, a workshop may be held prior to the next ACP meeting to develop collaborations and new directions in more detail.