Participants included Jim Cowin (PNNL), Steve Colson (PNNL), Bob Disselkamp (PNNL), Barbara Finlayson-Pitts (Univ. of Calif., Irvine), Bruce Garrett (PNNL), Vicki Grassian (Univ. of Iowa), Dan Imre (BNL), Nels Laulainen (PNNL), Yin-Nan Lee (BNL), Nancy Marley (ANL), Bob McGraw (BNL), Pete McMurray (Univ. of Minn.), Leonard Newman (BNL), Tica Novakov (LBNL), Ray Wells (USAF Res. Lab), Doug Worsnop (Aerodyne)

Laboratory efforts are a key part of the ACP as they include the study of basic chemical properties (e.g., reaction rates, products, solubilities, etc.) that are needed for accurate model development and updating. Laboratory research also allows specific gas-phase and heterogeneous tropospheric reactions to be identified and evaluated with regard to their potential importance in urban, regional, or global-scale tropospheric chemistry. Indeed, laboratory studies can serve as useful tools for field effort planning when coupled to the modeling efforts in ACP by identifying the most likely field sites where a specific chemical process can be verified and quantified. Laboratory research is also used in the development of new or improved instrumentation for the measurement of key air pollutant species in both laboratory and field with high selectivity and sensitivity. Thus, the ACP laboratory research efforts act to fill a central role between modeling and field efforts by improving our knowledge base and measurement capabilities.

During this breakout session, laboratory scientists from the ACP university and national laboratories identified successes in the areas of gaseous phase chemistry, halogen chemistry and sources, homogeneous aqueous phase chemistry and heterogeneous chemistry, condensation nuclei and cloud condensation studies, and field instrumentation development.

Gaseous Phase Chemistry

Unique, chlorine-containing organic products from the reactions of chlorine atoms with isoprene and 1,3-butadiene have been measured and may serve as "markers" of this chemistry in coastal marine regions. (Finlayson-Pitts, Univ. Calif., Irvine)

 Peroxybenzoyl nitrate synthesis has been successfully accomplished using the strong acid nitration of peroxybenzoic acid. This will make the examination of the properties of this potentially important peroxyacyl nitrate possible. (Gaffney and Marley, ANL)

Halogen Chemistry and Sources

Ozone in the presence of radiation with a wavelength of 254 nm has been shown to react with wet sea salt particles above the deliquescence point to form molecular chlorine Cl₂. If photolysis of O₃ in either the gas or liquid phase to generate OH is the initiating step, a similar process is expected in the actinic region from 290-320 nm in the troposphere. This could be a significant source of Cl₂ and hence chlorine atoms in the marine boundary layer. (Finlayson-Pitts, Univ. of Calif., Irvine)

Ozone reacts in the dark with a frozen seawater surface, generating Br₂. This reaction might be the initiating step for the observed loss of ground level ozone in the Arctic at polar sunrise, and may also occur in mid-latitudes in winter. (Finlayson-Pitts, Univ. of Calif., Irvine)

Homogeneous Aqueous Phase Chemistry and Heterogeneous Chemistry

Interactions of SO₂, H₂S, CO₂ with OH^- have been studied at 291 K with the following rates determined: SO₂: k₂(OH^-) = 3.0x109 M^-1s^-1; H₂S: k₂(OH^-) = 1.8x109 M^-1s^-1; CO₂: k₂(OH^-) = 1.3x103 M^-1s^-1. (Davidovits, Boston College)

Even "dry" synthetic sea salt was shown to hold a great deal of water, probably due to the presence of crystalline hydrates. This water controls the uptake and reactivity of HNO₃, and likely other gases as well. (Finlayson-Pitts, Univ. of Calif., Irvine)

Studies of the temperature dependency of ammonia sulfate hydration have found significant differences at temperatures approaching the freezing point of water. Significant changes in hydration are observed at 5 C as compared to 25 C; a liquid-liquid phase transition has been proposed to explain the observations. (Imre, Tang, and McGraw, BNL)

The hydration of a mixed organic-inorganic system has been successfully studied and the results reported for the first time. (Tang and Imre, BNL)

Nitrogen dioxide (NO₂) surface adsorption has been observed on water. Preliminary results have found this to be reversible with an approximate lifetime of 0.01 s for the absorbed state. (Davidovits, Boston College; Worsnop, Aerodyne)

Mass accommodation coefficients and surface reactions of NH₃ on aqueous and concentrated sulfuric acid droplets have been determined. In basic solution, (pH > 10) uptake is limited by physical solubility (Henry's law constant, H~100 M atm^-1), but enhanced uptake is observed at time scales less than 0.01 s, indicating the formation of a surface complex. In the region of pH about equal to 1.0, uptake reflects the mass accommodation coefficient (~0.08 at 283 K). The mass accommodation coefficient exhibits a negative temperature dependence consistent with a cluster nucleation model. Uptake increases with increasing acidity as NH₃ reacts with H⁺ in solution. In concentrated sulfuric acid solutions, the NH₃ uptake coefficient increases to near unity, independent of temperature, indicating the effect surface reactivity of NH₃ with H⁺. The surface reaction of NH₃ with H⁺ have a cross section of about 10^-14 cm². Results from co-deposition studies in which an aqueous surface, initially at a pH of 4.0, was simultaneously exposed to both gas phase NH₃ and SO₂ are in accord with NH₃ and SO₂ bulk phase chemistry. (Swartz, Shi, and Davidovits, Boston College; Jayne, Worsnop, and Kolb, Aerodyne)

Extended uptake studies of SO₂ were completed. At low pH, the uptake rates are consistent with those found in earlier studies, confirming the existence of a surface complex. The uptake rises at higher pH, most likely due to a direct reaction, i.e., SO₂ + OH^- Æ HSO₃. The second order reaction rate is k₂ = 3x109 M^-1 s^-1. As the pH increases, uptake rises toward the value of the mass accommodation coefficient. The mass accommodation coefficient for SO2 has been measured and is consistent with the clustering model and can be expressed as (1 - a) = exp(-DG/RT), with DH = -5.1 kcal mol^-1 and DS = 20.3 cal mol^-1 K^-1, At -10 C, a = 0.36. (Swartz, Shi, and Davidovits ,Boston College; Jayne, Worsnop , and Kolb, Aerodyne)

The Boston College-Aerodyne horizontal bubble train apparatus was used to investigate the uptake of gaseous SO₂ by sulfuric acid solution over a wide range of acidity (pH = 2 to 70% wt) at 293 K and in co-deposition with H₂O₂ and O₃. The two most likely paths of SO₂ oxidation involving H₂O₂ and O₃ do not exhibit significant reaction enhancement at the gas-liquid interface and are not likely to be the cause of enhanced sulfuric acid observed in jet plane exhaust. Rattigan. Boniface, Swartz, and Davidovits, Boston College; Jayne, Worsnop, and Kolb Aerodyne).

The uptake of alcohol in aqueous systems has been modeled in collaborative studies with the Boston College/Aerodyne Group. (Garrett, PNNL)

The results of isotope exchange experiments on gas-phase ethanols and acetic acid at aqueous interfaces indicate that isotope exchange occurs with unit probability in the interaction of acetic acid isotope with water. In the case of the alcohols, on the other hand, the isotope exchange occurs mainly with the ions (H⁺ or OH^-; D⁺ or OD^-). Further, the functional dependence of the results on pH suggests that the alcohol molecule forms a transient surface complex with the ion and that D-H exchange occurs principally via the flow of an electron from deuterium to hydrogen. This transfer is not a one-time process, but is a rapid interchange. That is, an electron transfers rapidly back and forth between the deuterium and hydrogen resulting in two possible surface configurations for this interaction. The lifetime of the complex is estimated to be 10^-8 s. (Davidovits, Boston College; Jayne, Worsnop, and Kolb, Aerodyne)

The reaction of peroxyacetyl nitrate (PAN) and NO₂ on the surfaces of carbonaceous soots and humic acids has been observed in preliminary studies using luminol chemiluminescent detection and a flow reactor. These results indicate that these species are likely to react on carbonaceous soot particles in the troposphere, particularly in urban polluted environments. (Gaffney and Marley, ANL)

Ozone reactions with the surfaces of surrogate organics have been explored as models for ozone deposition to plant and other organic surfaces. Sulfhydryl groups have been found to be particularly reactive surfaces. (Lee, BNL)

Condensation Nuclei and Cloud Condensation Studies

Sulfuric acid/water nucleation has been observed to be much faster than a simple binary system would have predicted. These results indicate that this system is much more complicated than currently modeled. (McMurray, Univ. Minn.)

The role of organics as potential cloud condensation nuclei has been examined. If the organics have a reasonable aqueous solubility they have been found to be effective CCN, and therefore cannot be neglected when considering CCN sources (Novakov, LBNL)

Calcium carbonate seed particles have been found to effect nucleation rates. (Imre and McGraw, BNL)

Field Instrumentation Development

Formaldehyde measurements can now be accomplished in the field using a modified DNPH/HPLC method in three minutes. (Lee, BNL)

Luminol detection with fast gas capillary gas chromatography can be used to monitor nitrogen dioxide and peroxyacyl nitrates (PANs) with one-minute time resolution at low ppt levels. (Gaffney and Marley, ANL)

A reactive hydrocarbon instrument under development is based upon the temperature dependence of ozone chemiluminescent reactions with the organics. Isoprene and monoterpene hydrocarbons have been found to react via addition at low temperature and addition and abstraction at higher temperatures, indicating a high sensitivity for detection exceeding conventional flame ionization detection. (Gaffney and Marley, BNL)
Instrumentation for the measurement of sulfuric acid, CNC, and fine aerosols on the G-1 aircraft has been developed and tested. (McMurray, Univ. of Minn).

A flow reactor-aerosol mass spectrometer (AMS) for studies of tropospheric aerosol nucleation and growth kinetics is under development that can simultaneously measure the size, particle number density, and size resolved chemical composition of the particles in the flow reactor. AMS detection of solid inorganic and organics has been demonstrated with a 0.1-mm-diameter single-particle detection limit. (Jayne, Worsnop, and Kolb, Aerodyne; Leard and Davidovits, Boston College).