The ACP researchers who met and discussed the recent successes, possible future research directions, and areas for collaborative research within the ACP program were:
Paul Davidovits, Boston College
Barbara Finlayson-Pitts, University of California, Irvine
Jeff Gaffney, Argonne National Laboratory
Bruce Garrett, Pacific Northwest National Laboratory
Jeremy Hales, ENVAIR
Yin-Nan Lee, Brookhaven National Laboratory
Nancy Marley, Argonne National Laboratory
Chester W. Spicer, Battelle, Columbus, Laboratory
Ignatius Tang, Brookhaven National Laboratory
Marv Wesely, Argonne National Laboratory
This group met on the morning and afternoon of Wednesday, December 6, 1995 in order to share recent successes in laboratory research sponsored by the DOE ACP program, identify future areas of mutual research interest and importance to ACP needs, and to determine courses for improved collaboration between other ACP researchers within the laboratory research breakout group and the other ACP breakout groups (i.e., Modelling, UV-B and ozone, Aerosols, Field Studies and Aircraft Instrumentation). The following is a summary of those discussions.
Laboratory studies within the ACP program are obtaining fundamental information on the chemical and physical properties of trace gas and aerosol species of importance in the atmosphere (troposphere and stratosphere). The laboratory programs perform an important role in supplying the modelers with both kinetic and mechanistic information concerning atmospheric chemical processes as well as physical parameters such as ultraviolet or infrared cross-sections needed for radiative and chemical modelling efforts. As well, the laboratory research is involved in improving or developing needed instrumentation for field studies to obtain better data sets for model evaluation and validation. Laboratory studies can also lead to identification of chemical pathways that may be important in atmospheric chemistry, e.g., heterogeneous chemistries on aerosol interfaces. Once identified laboratory studies can aid in model development and field validation to determine the importance of various chemical pathways.
Some examples of the recent successes identified during the laboratory - breakout group discussions included:
€ Development of new techniques for exotic atmospheric species for future field efforts. These included recent improvements in the use of luminol-based chemiluminescent detection of nitrogen dioxide and peroxyacyl nitrates (PANs) which should allow low part per trillion detection of these species with 2-3 minute analysis times, the use of the trace atmospheric gas analyzer (TAGA) system for measurement of nitrous acid and chlorine species in ambient air, and the use of aldehyde adducts to improve detection for these compounds using HPLC techniques.
€ Laboratory studies examining the gaseous reaction products produced in formaldehyde/nitric acid/sulfuric acid systems have observed HONO formation. This work indicates that their may be a number of heterogeneous pathways for production of this photochemical source of OH.
€ Aerosol studies have shown using laser Raman spectroscopy that metastable states may occur in inorganic salts, and that the homogeneous nucleation of sulfuric acid occurs much more rapidly than expected. This work points to the need for a better understanding of the roles that these aerosol species play in the formation of cloud condensation nuclei and in aerosol transformation chemistries.
€ Examination of halogen chemistry reactions with isoprene have found that the reactions in the presence of oxygen will lead to chlorinated products such as HOCl and larger chlorinated and oxygenated organics, as well as to methyl vinyl ketone and methacrolein. To the extent that HOCl photolyzes to form atomic chlorine, this reaction will not be a sink of Cl atoms, but rather part of a chain process.
€ Using the TAGA system, measurable levels of chlorine gas were observed in recent field studies indicating that chlorine radical initiated chemistries may play important roles under certain tropospheric conditions in the oxidation of organics and in ozone and oxidant chemistries.
€ The formation of the radical anion intermediates [Cl...NO2]- and [Br...NO2]- was observed using electron spin resonance in the NO2 reactions with NaCl and NaBr, respectively. These intermediates are very stable at room temperature, and may be the species responsible for synergistic effects on the deep lung which were observed some years ago when rats were exposed to NaCl aerosols and NO2 simultaneously. Thus even though a reaction such as the NO2-NaCl, NaBr reaction is relatively slow, it may still be important in terms of impacts on human health.
€ A number of studies were reported that indicate that chemistry at the interface of aerosol and liquid water droplets can occur at increased reaction rates and also lead to unique thermochemical and photochemical processes. These studies all indicate that interfacial chemical and physical processes could be playing a larger role in atmospheric chemistry than previously appreciated.
A number of topic areas were discussed which examined possible areas for future ACP focus. In general, it was thought that gas phase studies of more exotic compounds, particularly organics and NOy species along with the general study of aerosol physics and chemistry were areas where the laboratory efforts could make solid contributions in future efforts. Areas discussed included:
€ Nitrous acid and Halogen chemistry in the troposphere. This topic area was chosen due to its potential importance as a source of OH radicals and Cl radicals which can initiate oxidative chain reactions. Discussions were focused around possible heterogeneous production processes and sinks, the need for field analytical tools, its possible importance on regional scales, and its nighttime chemistry.
€ NOy identification and chemical studies. A number of questions have arisen due to unexplained observations of high NOy in regional field studies. These studies indicate that a number of possible sources of not well characterized nitrogen oxide containing compounds are present in the troposphere. The discussions on this topic examined possible sources and outlined some laboratory work that could aid in resolving these discrepancies.
€ Interfacial chemistry-heterogeneous reactions on surfaces. The discussions in this area centered around the possible enhancement or retardation processes on interfaces as compared to bulk chemical or physical processes, and the needs for novel tools to probe heterogeneous reactions on the surfaces of aerosol particles without perturbing the interface.
€ Organic chemistry in the troposphere. Organic oxygenates and nitrates as well as multifunctional compounds are not well understood with regard to their photo- or thermochemistry. Photooxidation of organics under low NO conditions which are expected on regional and global scales was thought to be an area where ACP researchers could make strong contributions to improving our understanding of tropospheric chemistry on these scales.
€ Collaborative Efforts within ACP. During these discussions of laboratory work, areas where the studies could be used to aid in field study planning and interpretation of field observations were noted. As well, particular attention was given to the role of the laboratory research efforts in helping to resolve modelling uncertainties and in developing or improving the analytical capabilities within ACP.
Specific details of the discussions in these topic areas follow:
NITROUS ACID
Nitrous acid is a source of OH in the troposphere due to its high photochemical absorption cross section leading to direct formation of this radical and nitric oxide. As well, a number of recent studies indicate that nitrous acid may have deleterious health effects due to its acidity and due to its ability to react with amino compounds in the body leading to nitrosamine formation which are known mutagens and carcinogens. Recently, a few measurements of HONO have been reported using denuder techniques which indicate that higher steady-state levels of this compound may exist in the troposphere during the daytime. One possible reason for these observations are interferences with the denuder methods used for these measurements. Laboratory studies could be carried out examining likely interferences such as peroxyacylnitrates (PANs), peroxynitrates, organic nitrites, etc. As well, the denuder methods could be used in field studies in conjunction with other more direct methods for HONO determination. The use of the TAGA, tunable diode lasers, and or differential optical absorbance spectroscopy (DOAS) in conjunction with denuder or other wet-chemical methods would allow these discrepancies to be resolved. It would also be useful for new less-expensive methods (i.e., derivitization-flourescence techniques) to be validated using the more expensive and equipment intensive TAGA or tunable diode laser systems.
Although the gas phase sources of HONO may be limited, there are a number of indications that heterogeneous reactions of nitrogen dioxide on surfaces (smog chamber studies, etc.) could be an important source of HONO in the troposphere. Indeed, preliminary work performed by ACP researchers at Boston College have observed nitrous acid formed when nitric acid, sulfuric acid, and formaldehyde are mixed together. Further work is needed to determine the rates of this reaction to evaluate its importance in tropospheric chemistry. Other possible sources that deserve examination include the reaction of peroxyacyl nitrates (PANs) and peroxynitrates with ammonium nitrate, since PAN is known to react with ammonia in the gas phase to form nitrous acid.
Two other areas where nitrous acid chemistries need to be explored include nighttime sinks involving interfacial aerosol reactions and reactions in fogs and clouds (i.e., aqueous chemistry). The photochemistry of nitrous acid on surfaces and in aqueous phases in the presence of organics is another area where little work has been done. The question of the existence of hydrated HONO species in the gas phase has arisen from time to time, it was suggested that the TAGA or the tunable diode laser systems at Battelle could be used in a few experiments to determine the potential for HONO/water cluster formation by varying the relative humidity in a HONO calibration gas stream.
NOy CHEMISTRIES
As well as HONO, other NOy species are not well characterized or understood particularly with regard to their potential roles in aerosol and other heterogeneous chemistries. Some specific examples of compounds and classes of compounds that need to be studied included, peroxynitric acid, nitric acid, PANs, organic nitrates, organic nitrites, nitro-aromatics, etc. The photochemistry of these compounds need to be examined with regard to their potential tropospheric impacts particularly in the UV-B and UV-A regions of the solar spectrum. The studies should examine gas phase properties as well as reactions on surfaces and in aqueous solutions in the presence of organic species similar to the studies outlined for HONO. For example, recent studies of the photochemistry of nitrate on the surface of alkali halides indicate that the photochemistry is quite different than that for bulk sodium nitrate, forming gas phase NO2 rather than solid nitrite. This suggests that reactions on such surfaces may play a role in converting NOy back to NOx. The possible photosensitization of these compounds to photochemical reactions by organics (i.e., ketones, aldehydes) and organics (i.e., nitrogen dioxide) via energy transfer should not be ignored.
It was noted that the fundamental determination of the ultraviolet cross sections for these species as well as many organic oxygenates were needed by the ACP modelers to adequately assess the impacts of increased UV-B radiation in the troposphere due to stratospheric ozone depletion. The possibility of heterogeneous conversion of dissolved nitrate species to form nitrogen dioxide needs to be examined as this may be a source of photochemically active NOx important for regional and global scale modelling. The temperature effects of these processes should also be determined, particularly for peroxynitrates and peroxynitric acid aqueous chemistries since they have strongly dependent equilibrium reactions.
Novel analytical methods that can specifically measure these species are needed to better evaluate the various organic nitrate contributions to NOy. Chemiluminescent methods as well as a variety of spectroscopic techniques are showing promise for improving the analytical capabilities of ACP in this area.
HALOGEN CHEMISTRIES
Sources of molecular chlorine, bromine, and iodine along with mixed halide gases need to be determined in the troposphere to evaluate their role in tropospheric chemistry. A number of potential heterogeneous sources including the reactions of N2O5 and ClONO2. Reactions with NaCl aerosols (sea salt) are being examined by ACP researchers. Aqueous phase chemical reactions on wet aerosols, fogs, and clouds should also be explored.
The halogen atoms formed by photolysis of the chlorine, bromine, or iodine gases will likely react with organics or ozone in the troposphere. Reactions of chlorine with organics such as isoprene can lead to the formation of HCOCl which in turn can photolyze to yield chlorine radicals. Thus, it is possible that chlorine can act via chain reactions to increase their role in the oxidation of organics in the troposphere, similar to OH.
Detection and measurement techniques for ClNO2, ClNO, HOCl, are needed. Likely candidates are tunable diode lasers and mass spectroscopic methods that make use of the chlorine isotopic signature to eliminate possible interferences.
Aqueous phase reactions of chlorine containing compounds may be important sources of organochlorine species. In particular, surface reactions on aerosols and gas phase chemistries of chlorine radicals with isoprene and other natural hydrocarbons.
INTERFACIAL STUDIES - QUASI LIQUIDS - SINGLE PARTICLES
The chemical and physical processes occurring on single particle interfaces and on the surface of aerosol and cloud droplets were felt to be an important area for future research. There is a strong likelihood that interfaces in lab systems and in the atmosphere have unique characteristics which could be important in aerosol chemistry and physics. In addition, there is increasing evidence that water previously considered to be physically adsorbed on solids is better described as a two dimensional "quasi-liquid layer" under many conditions. Thus for both liquid aerosols and solid particles in the atmosphere, the nature of the interface at a molecular level needs to be much better understood in order to characterize uptake and reactions at these interfaces, and how to parameterize them in atmospheric models.
There is definite need for new approaches to these difficult areas for experimental work if we are to accurately apply lab results to atmospheric systems. Techniques will be needed that are sensitive and specific with regard to the molecular area that they probe, and be performed under conditions as close as possible to atmospheric in order to not alter the physical or chemical nature of the surfaces. Laser based techniques and spectroscopies are showing increasing applicability to this area.
Single particle mass spectrometry is also finding increasing use and may also shed some light on interfacial processes. For example, changes in composition with particle size may be useful in assessing the importance of unique interfacial reactions, as large changes in surface to volume ratios are accessible using very small particles, i.e., less than about 0.1 micron.
ORGANICS
The tropospheric chemical processes of organics need to be further understood and identified, particularly for the oxygenated, nitrated, and multifunctional compounds expected from the photooxidation of natural and anthropogenic hydrocarbon emissions. Recent studies done by ACP and other researchers have observed that oxygenated organics can account for an appreciable if not the major reservoir of carbon in the non-methane hydrocarbons (NMHC) present in the troposphere.
Although the models predict the formation of a number of multifunctional oxygenated organics from OH and ozone oxidation little is known regarding these species physical and chemical properties. Measurement techniques which will not decompose them during sample and analysis are needed since many of these compounds are surface sensitive and thermally labile.
To evaluate the oxidation of natural organics, we need to develop measurement methods for specific marker oxygenated hydrocarbons such as those developed for methyl vinyl ketone and methacrolein which are known products of isoprene oxidation. Other signature compounds such as vanillin can be used to act to determine biomass burning since this compound is produced in large quantities when lignin is cracked.
Several questions need to be answered concerning the roles of oxygenated organics on tropospheric chemistry. What are the effects of the photochemical reactions of mixed functional oxygenated and nitrated organics on the free radical chemistry in the troposphere? How readily are they taken up by aerosols, clouds and fogs? Do the oxygenates affect aqueous phase chemistry?
There are also many questions that are now arising concerning the sources of carboxylic acids observed in the gas and aerosol phases in field studies. Organic peracids are predicted to be produced from the decomposition of PANs and from the photochemical oxidation of aldehydes and ketones under low NO conditions. Measurements of their ultraviolet absorption cross sections are needed by the UV-B modelers. As well, methods that can determine their levels in the troposphere need to be developed. The aqueous reactions of peracids and organic peroxides need to be examined since they are known to be strong oxidizing agents.
As previously mentioned, organic nitrates are a potentially important part of the "missing" NOy. They can also form multifunctional species particularly if they are formed from the reaction of nitrate radical with monoterpenes during nighttime chemistry. Measurement methods are needed for their determination in the troposphere and for laboratory kinetic studies. Their aqueous chemistries and surface reactions need to be better characterized to determine whether they are contributing to HONO signals as interferences and/or are acting as a heterogeneous source of HONO in the atmosphere. Their role in the NOy balance and in the carbon balance of isoprene, monoterpene, aromatic, and large organic oxidations need to be better determined. The potential reaction of organic nitrates and peroxynitrates in aqueous systems and on aerosol surfaces need to be examined to determine if they can act to effectively convert nitrates to NOx.
SOME EXAMPLES OF COLLABORATIONS AND INTERACTIONS
Based on our discussions, it was clear that the ACP laboratory researchers are actively interacting with each other in a number of areas. Some examples of these interactions included:
€ Studies of the field detection of Cl2 and Br2 and of halogen chemistry between ACP researchers at Battelle, Columbus, PNL, and UC Irvine.
€ Development of improved time response and sensitivity for NO2 and PANs for aircraft field measurements involving ANL, BNL, and PNL.
€ Studies of oxygenated organics, aldehyde, acids, and peracids being performed by ANL and BNL, along with associated analytical method development.
Interactions between the ACP laboratory researchers and the ACP field and modelling activities were also noted. Examples being the measurement of UV-B cross sections for exotic organics (e.g., peracids, etc.) and the measurement of HONO, chlorine, and organics in ACP field studies. It is strongly suggested that in future ACP annual workshops that some time is specifically set aside for further interactions between the modelers, the field investigators, and the ACP laboratory researchers to aid in better targeting the laboratory studies to ACP needs.