A51E-0825 0800h
Measurements of HO$_{2}$ Uptake to Sulfuric Acid and Ammonium Sulfate Aerosol: Mass Accommodation Coefficients and Net Reactive Loss
We present room temperature measurements of HO$_{2}$ uptake to aqueous submicron H$_{2}$SO$_{4}$ and (NH$_{4}$)$_{2}$SO$_{4}$ aerosol particles at 35% and 40% relative humidity, respectively, obtained with an entrained aerosol flow tube coupled to a chemical ionization mass spectrometer. By doping the aerosol with Cu(II) ($\sim$ 0.1 M in aerosol) to create an efficient aerosol sink, we determine lower limits to the mass accommodation coefficient to be 0.8 $\pm$ 0.3 for H$_{2}$SO$_{4}$ and 0.5 $\pm$ 0.1 for (NH$_{4}$)$_{2}$SO$_{4}$ particles. In the absence of Cu(II), net reactive loss of HO$_{2}$ on H$_{2}$SO$_{4}$ aerosol was slow, and we measured a reaction probability less than 0.01. In contrast, loss of HO$_{2}$ to (NH$_{4}$)$_{2}$SO$_{4}$ aerosol, buffered to pH = 5.1 but without Cu(II), was efficient in our experiment. For this system, aerosol-induced loss of gas-phase HO$_{2}$ was observed to follow second-order kinetics and we infer from our measurements a second-order aqueous phase reaction rate coefficient of 1 $\pm$ 0.25 x 10$^{7}$ M$^{-1}$ s$^{-1}$ in good agreement with an estimate of 2 $\pm$ 1 x 10$^{7}$ M$^{-1}$ s$^{-1}$ based on literature values of HO$_{2}$ aqueous-phase chemical parameters. These results imply that heterogeneous loss of HO$_{2}$ to submicron aqueous sulfate aerosol will be strongly temperature dependent with negligible contribution to odd hydrogen radical (HO$_{x}$) removal rates at temperatures warmer than $\sim$ 270 K, but potentially large contributions to HO$_{x}$ loss (e.g., $>$ 50% of total HO$_{x}$ loss) in colder regions of the troposphere depending on the available aqueous aerosol volume. We discuss these results and the importance of heterogeneous chemistry on tropospheric HO$_{x}$ budgets.
A51E-0826 0800h
Permeability of Organic Films at the Air-Aqueous Interface
Organics represent a significant percentage of the composition of atmospheric aerosols. It has been determined in recent field studies that amphiphilic organics, such as fatty acids, are a large constituent of the organic fraction. Fatty acids have the unique ability to partition to and remain at the air-aqueous interface. Their presence at the interface alters many interesting physical, chemical, and morphological properties that are discussed. In this set of experiments, we address the permeability of gaseous molecules, either water or small organics, across the hydrophobic barrier created by a fatty acid monolayer. The evaporation rate of the organically coated water subphase is investigated as a function of the fatty acid carbon number, from C12 to C24. Subsequently, the permeability of the hydrocarbon barrier to small organics is monitored. The relevance of these findings as they relate to the properties of organic aerosols as well as recent surface-sensitive aerosol field measurements is discussed.
A51E-0827 0800h
Uptake of C$_{1}$ $-$ C$_{5}$ Monocarboxylic Acids on Ammonium Nitrate as a Function of Temperature and Relative Humidity.
The uptake of C$_{1}$ $-$ C$_{5}$ monocarboxylic acids on ammonium nitrate (AN) has been investigated as a function of temperature and relative humidity using a Knudsen cell flow reactor coupled with FTIR-Reflection Absorption Spectroscopy (FTIR-RAS). Initial uptake coefficients ($\gamma$'s) and coverages ($\Theta$'s) were determined over the temperature range 200 to 240 K. The uptake of the C$_{1}$ $-$ C$_{4}$ acids was quite efficient but temperature dependent, with larger coverages and uptake coefficients being observed at lower temperatures. The values of $\gamma$ and $\Theta$ were also dependent on the carbon chain length, with the shorter chain acids displaying larger coverages but smaller uptake coefficients at a given temperature. Valeric acid (C$_{5}$) did not adsorb onto ammonium nitrate under any of the conditions studied. Infrared spectra revealed that the acids ionized on the surface, despite the fact that the ammonium nitrate films were effloresced. The initial uptake coefficients were analyzed using a precursor-mediated adsorption model to determine $\Delta$H$_{ads}$ and $\Delta$S$_{ads}$ values for each species. Adding small amounts of water vapor (4% RH) to the chamber resulted in unlimited uptake and dramatically increased values of $\gamma$ and $\Theta$. Higher relative humidity resulted in further increases of both $\gamma$ and $\Theta$. The IR spectra revealed that the organic acids ionized on the surface. Furthermore, the IR spectra revealed that a liquid layer is formed when many of the acids adsorbed onto the film at RH $>$ 10%. These liquid water features were not observed at a similar relative humidity in the absence of the acids. These studies suggest that uptake of organic acids on AN dramatically increases the water uptake properties of the inorganic salt.
A51E-0828 0800h
Thermodynamics of Carbonates and Hydrates
Carbonates are widely present in mineral aerosol and may interact with gas-phase species through reactions of the form: XCO$_{3}$ + 2HY = X(Y)$_{2}$ + CO$_{2}$ + H$_{2}$O (R1), where X = Ca or Mg and Y = NO$_{3}$ or Cl. Laboratory investigations of R1 (X = Ca, Y = NO$_{3}$) indicated that, in idealized N$_{2}$ environments, HNO$_{3}$ is irreversibly taken up by CaCO$_{3}$ to form Ca(NO$_{3}$)$_{2}$. The idealized experimental environments did not contain CO$_{2}$, which is present in the atmosphere and would tend to drive R1 in the reverse direction. In the bulk, the X(Y)$_{2}$ salts are known to exist in stable hydrated forms under conditions relevant to the atmosphere. Although the phase state of the particular X(Y)$_{2}$ product influences the equilibrium disposition of R1 and is important to a number of air quality issues, an equation capturing the temperature (T) dependence of the deliquescence relative humidity (DRH) of hydrated salts has not been reported to date. One goal of this work is to determine the thermodynamically preferred state governed by R1 under conditions relevant to the atmosphere. A related objective is to derive an equation for predicting DRH(T) for hydrated forms of X(Y)$_{2}$. Using principles of thermodynamics, equilibrium concentrations of HNO$_{3}$ and HCl as functions of RH and T were determined for reactions of the form of R1 (X = Ca or Mg, Y = NO$_{3}$ or Cl). These concentrations were compared with ambient measurements of HNO$_{3}$ and HCl to determine the possibility of R1 proceeding in the reverse direction under atmospheric conditions. An equation for DRH(T) for hydrated salts was derived by building on previous work for anhydrous salts. Predictions of DRH(T) for hydrates agree well with available measurements and indicate that DRH(T) is markedly different for the hydrated and anhydrous forms of some salts. For cases of R1 with X(Y)$_{2}$ in the stable hydrated form, the forward direction of the reaction is thermodynamically preferred for atmospheric conditions. For cases of R1 with anhydrous X(Y)$_{2}$, the reverse direction is preferred under some low RH scenarios. Our work suggests that hydrated states of X(Y)$_{2}$ should be considered when modeling heterogeneous reactions of the form of R1 in low RH situations.
A51E-0829 0800h
Surface Reactions of Dry Alkali Halides Salts With Ozone and the Influence of Water Vapor
Heterogeneous reactions of sea-salt aerosol particles and sea ice have been implicated in the chemistry and composition of the marine boundary layer. For example, reactions of important atmospheric oxidants (e.g., hydroxyl radical and ozone) with seasalt halides lead to the formation of reactive halogen radicals that can significantly affect tropospheric ozone concentrations and the deposition of mercury. A number of previous laboratory experiments have investigated the reactivity of ozone with salts by measuring the reactive loss and/or formation of gas-phase species. Results from these studies indicate that reactions at the interface can play an important role in the heterogeneous chemistry of aerosols. However, the efficacy of the surface chemistry of ozone with alkali halide salts has yet to be elucidated with surface spectroscopy studies. Using X-ray photoelectron spectroscopy (XPS), we have investigated changes in the surface composition of various alkali halide salts in our ultra-high vacuum (UHV) instrument as they are exposed to ozone. This was done by monitoring the O(1s), Cl(2p), Br(3d) and I(3d) photoelectron peaks. Peak binding energies (BE) provide species identification and integrated areas provide a quantitative analysis of the surface. Salt samples were either freshly cleaved single crystals or high grade salt crystals pressed into pellets. In order to identify the products formed at the surface of the oxidized salts, reference spectra were obtained from commercially available salts of NaClO$_{x}$ (x = 1 - 4), NaBrO$_{x}$ (x = 3) and KIO$<sup><small>a</small></sup>_{x}$ (x = 3, 4). The in-vacuo exposure of ozone to dry alkali halides led to an uptake of oxygen on the surface. The exposure of ozone to NaCl and NaBr single crystals leads to a relatively small uptake of oxygen, with O(1s) BE's consistent with the reference spectra of NaClO$_{3}$ and NaBrO$_{3}$, respectively. However, due to the small amount of oxygen uptake, there was no evidence of chloride or bromide oxidation in the Cl(2p) and Br(3d) spectra, respectively. The reaction of ozone with the surface of a KI pellet led to the rapid formation of KIO$_{3}$ on the surface. This was evidenced in both the build up of oxygen on the surface (O(1s) spectra) and the oxidation of iodide to iodate (I(3d$_{5/2}$) spectra). Water vapor exposure to KI below the deliquescence point, both before and after exposure to ozone, changes the surface chemistry. For example, a dry KI surface was exposed to ozone to generate a surface composition containing a mixture of iodide and iodate. Upon exposing this oxidized surface to water vapor below the deliquescence of KI, the iodate disappeared. This water induced chemistry is consistent with the oxidation of iodide by iodate, leading to the liberation of molecular iodine (Dushman reaction). Thus, although KI was exposed water vapor below the deliquescence point, the chemistry at the salt surface appears to be aqueous-like. Unlike the case with ozone reacting with a "dry" KI surface, the availability of surface adsorbed water appears to allow for the evolution of iodine into the gas phase. Similar experiments are being performed on mixed NaCl/NaClO$_{3}$ and NaBr/NaBrO$_{3}$ surfaces. The effects of water vapor exposure to these oxidized salt surfaces will be discussed.
A51E-0830 0800h
Surfactant Control of HCl and HBr Uptake Into Supercooled Sulfuric Acid
Surfactant molecules on sulfuric acid droplets potentially alter the rates of heterogeneous reactions in the upper troposphere and lower stratosphere by blocking gas molecules from entering the acid. We perform molecular beam experiments with deuterated sulfuric acid solutions (56-68 wt % D$_{2}$SO$_{4}$/D$_{2}$O at 213 K) with varying concentrations of surfactants including 1-butanol and 1-hexanol, which segregate to the surface to form a nearly complete monolayer. We direct a beam of a protic gas HX (X = Cl or Br) at a continuously renewed film of supercooled D$_{2}$SO$_{4}$/D$_{2}$O in vacuum and measure the fraction of thermalized HX which undergo HX \rightarrow$ DX exchange. We have further shown that this HX \rightarrow$ DX exchange fraction is approximately equal to the probability of HX entering the acid. Our results appear to contradict the notion that surfactants generally impede gas transport. The presence of surface butanol does not alter the rate of D$_{2}$O evaporation from the liquid surface, whereas surface hexanol slightly does. Our most striking result is that the surface butanol molecules actually increase the HX \rightarrow$ DX exchange fraction, implying that HX dissociates more readily at the interface when butanol is present. This enhancement may be caused by the dilution of the acid near the surface by segregated butanol molecules, which provide additional OH groups for protonation by HX. For hexanol, we observe an enhancement in the HX \rightarrow$ DX exchange fraction at higher acidity and a small reduction at lower acidity.
A51E-0831 0800h
Hygroscopicity of Biomass Burning Derived Organic Species Particles
Organic species derived from biomass burning can potentially affect the hygroscopicity and cloud condensation activities of aerosols. The hygroscopicity of particles of 3 organic species most commonly detected in biomass burning aerosols (levoglucosan, mannosan, and galactosan) were investigated. The hygroscopic measurements were performed by levitating single particles of roughly 10 microns in diameter in an electrodynamic balance by electric fields and equilibrating the particles at different RH for in situ relative mass determination. Levoglucosan, mannosan, and galactosan particles did not crystallize nor deliquesce. They existed as highly concentrated liquid droplets at low RH, suggesting that biomass burning aerosols may retain water at low RH. These particles show very similar hygroscopic growth and the RH dependence of the Gf, (the ratio of particle diameter equilibrated at a given RH to that at the reference RH (<10%)) can be modeled by Gf = (1-RH/100)^-0.10. Field study has shown that the light scattering coefficient of aged smoke particles was smaller than the young smoke particles. This suggests that the aged smoke particles were less hygroscopic than young smoke particles. This observation may due to the formation of spherical, amorphous carbonaceous particles and the rapid conversion of more hygroscopic potassium chloride in young smoke particles to less hygroscopic potassium sulfate and potassium nitrate in aged smoke particles. In addition, it is possible that levoglucosan can be further converted into more simple organic acids such as C2-C5 dicarboxylic acids, which are generally less hygroscopic with Gf < 1.10 at 85%RH during the initial aging of smoke particle. It is noted that, although malonic acid has Gf = 1.40 at 85%RH, oxalic acid is usually the most abundant dicarboxylic acids detected in biomass burning aerosols and has a Gf = 1.08, which is less hygroscopic than levoglucosan (Gf = 1.21). These hygroscopic results provide additional evidence that aged smoke particles may be less hygroscopic than the fresh smoke particles.
A51E-0832 0800h
Visibility, Aerosol Concentrations, and Relative Humidity
Transported aerosols from populated and industrial areas result in regional haze that causes visibility degradation in areas valued for their scenic beauty, such as the National Parks. These areas are designated as Class I Areas in the United States, and there are specific visibility goals put forth to ultimately return these areas to natural conditions. It is important to understand the role of meteorological factors on the extinction of visible light to achieve this goal. The objective of this research is to explore the contributory role played by relative humidity on visibility degradation using the field data collected by us in Central California from the intensive ambient aerosol sampling campaign conducted from 2000 summer-2001-2002 winter1. The data include PM-2.5 nitrate, sulfate, and carbon mass concentrations, as well as simultaneous measurements on light scattering, light absorption, ambient temperature and relative humidity. The dataset is highly time-resolved, allowing the affect of temporal variations of particle chemical composition and meteorological features to be considered. The effect of relative humidity on visibility reduction has been explored by statistical analysis of this data. The final results provide response curves that allow calculation of light scattering and absorption, given aerosol concentrations and relative humidity. Our results are compared with those obtained using formulae suggested for analysis of IMPROVE data collected under conditions of low temporal resolution to understand the affects of temporal resolution on the characteristics of the relationships among extinction, aerosol loading and type, and relative humidity. IMPROVE is a monitoring network established with the goal of monitoring regional haze in many of the Class I areas. References 1. Lunden, M.M., T.L. Thatcher, S.V. Hering, and N.J. Brown (2003). The Use of Time- and Chemically-Resolved Particulate Data to Characterize the Infiltration of Outdoor PM-2.5 into a Residence in the San Joaquin Valley. Environmental Science and Technology 37, pp 4724-4732. 2. Malm, W.C., "IMPROVE, Interagency Monitoring of Protected Visual Environments," ISSN: 0737-5352-47, Colorado State University, May 2000.
A51E-0833 0800h
Aerosol Radiative Properties and Their Dependence on Relative Humidity and Wavelengths during ICARTT
Aerosol optical and hygroscopic properties were measured onboard the NOAA RV Ronald H. Brown as part of the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) in the Gulf of Maine during July and August of 2004. A scanning relative humidity (RH) nephelometry system was used to measure the total light scattering and backscattering coefficients by particles (\sigma$_{sp}$ and \sigma $_{bsp}$, respectively) at controlled RH with three wavelengths of scattered light ($\lambda$). Controlled RH ranged from 40$%$ to 85$%$, while the scattering coefficients were measured at 450 nm, 550 nm, and 700 nm. Measurements were completed 90$%$ of the entire field campaign. The dependence \sigma$_{sp}$ and \sigma $_{bsp}$, hemispheric backscatter fraction (b), $\AA$ngstr$ {o}$m exponent ($\aa$), and single scattering albedo ($\omega$) upon $\lambda$ and RH will be reported. The dependence of aerosol hygroscopicity on air mass type and aerosol chemical composition will be discussed along with meteorology and origin of the sampled air masses. The ambient aerosol hydration state (i.e., on the upper branch, lower branch, or intermediate to the two branches of the hysteresis loop) will also be described. These results are important to better understand the hygroscopic properties of ambient aerosol and to parameterize ambient aerosol optical properties for use in large-scale climate models.
A51E-0834 0800h
The Deliquescence Behavior of Internally Mixed NaCl/Dicarboxylic Acid Aerosols in the Kelvin Regime
Organic compounds have been shown to contribute significantly to the mass fraction ( 10-70%\ ) of fine particulate matter in the troposphere. Low molecular weight dicarboxylic acids, formed by oxidative processes in the atmosphere, constitute a substantial portion of the organic component. Investigations have shown the organic component often modifies the hygroscopic behavior and cloud nucleating ability of an inorganic aerosol, thus having implications for both direct and indirect radiative forcing. Most laboratory studies of internally mixed inorganic/organic aerosols have focused on dicarboxylic acids. The deliquescence behavior of these mixed aerosols has been suggested to be primarily dependent upon the organic component's water solubility. The hygroscopic behavior of 5 nm NaCl and three internally mixed NaCl/dicarboxylic acid aerosols (NaCl/oxalic acid, NaCl/malonic acid, and NaCl/succinic acid) were observed. The motivation for this work was to investigate the thermodynamic properties of aerosols at the beginning of their life cycle. A limited number of studies have been reported for aerosols with diameters less than 30nm where the Kelvin effect is expected to be significant. In this work, an increase in the deliquescence relative humidity relative to bulk measurements was observed for 5 nm NaCl aerosols. The presence of a dicarboxylic acid was shown to modify the deliquescence behavior of pure NaCl in all three cases. The deliquescence behavior of these internally mixed particles will be compared to the behavior observed and modeled for the corresponding bulk systems.
A51E-0835 0800h
Size Resolved CCN Spectra Measured During the FACE2004 Field Experiment in Central Germany
The FACE 2004 field experiment took place at the Taunus Observatory (Kleiner Feldberg, central Germany) in July and August 2004. As part of this project we measured cloud condensation nuclei (CCN) in various air mass conditions, including recent pollution from the industrialized Rhein - Main area, aged pollution from Eastern Europe, and relatively clean air of marine origin. We measured CCN efficiencies (i.e., CCN divided by total particle concentration) as a function of supersaturation (S) at different particle diameters, which were selected by a differential mobility analyzer upstream of the CCN counter. The method of measuring size dependent CCN spectra gives more detailed information than measurements without size resolution. It simplifies closure studies with particle chemical composition, which we determined at the same particle diameters by a Quadrupole Aerosol Mass Spectrometer. By combining these size dependent CCN spectra with aerosol number size distributions, the size distributions of CCN can be derived for a given S. We will present first results on the FACE 2004 CCN measurements, namely a comparison of CCN efficiencies and CCN size distributions in different air mass conditions and the relationship of particle chemistry to the CCN efficiencies of each selected size fraction.
A51E-0836 0800h
Evaluating the Contribution of Carbonaceous Aerosols to CCN with a Global Aerosol Microphysics Model
A two-moment sectional model of aerosol microphysics is included in the GISS GCM in order predict tropospheric cloud condensation nuclei (CCN) concentrations. The model includes sulfate, sea-salt, elemental carbon, organic carbon, and mineral dust aerosols. The aerosol size distribution is simulated explicitly by modeling the rates of nucleation, condensation, and coagulation. Here we use the model to examine the contribution of carbonaceous aerosols to tropospheric CCN levels. Using currently available emissions estimates, the model predicts that primary organic carbon (OC) aerosols make a dominating contribution. This contribution includes the effect of ultrafine primary OC particles that must grow to CCN sizes as well as OC particles that are already large enough when emitted. The sensitivity of CCN levels to uncertainties in our knowledge of aging will be discussed.
A51E-0837 0800h
Observations of the Enhancement of Particulate Nitrate by Clouds
An airborne in-situ sampling study was conducted out of Cleveland, Ohio from 21 July - 18 August, 2004 to investigate the role of clouds in the transport and transformation of pollutants. This study was part of the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) 2004 field intensive. Measurements of trace gases (including SO$_{2}$, HNO$_{3}$, NH$_{3}$ and H$_{2}$O$_{2}$), particle size distribution and chemistry, and cloud microphysics were made from the National Research Council of Canada Convair 580 aircraft. Cumulus and towering cumulus were sampled at multiple levels on several flights. Cloud droplet residuals from a counterflow virtual impactor (CVI) were sampled with an Aerodyne aerosol mass spectrometer (AMS), a TSI condensation particle counter, and a TSI differential mobility analyzer. Preliminary observations show that the ratio of nitrate to sulphate, determined from the AMS measurements, was often significantly higher in the cloud droplet residuals compared with the clear-air aerosol sampled below the cloud bases. For example, on one afternoon flight over central Ohio, the average NO$_{3}$/SO$_{4}$ value in cloud was 1.5, and in clear air below cloud was 0.09. One possible mechanism is that the increase in nitrate resulted from the scavenging of HNO$_{3}$ by the cloud droplets; although this would likely require some buffering of the acid for the nitrate to remain in the particle phase after the droplet is evaporated in the CVI. This and other possible explanations to account for the enhanced in-cloud nitrate are considered.
http://www.msc-smc.ec.gc.ca/research/icartt/index_e.html
A51E-0838 0800h
Study of the Aerosol Indirect Effect by Large-Eddy Simulation of Marine Stratocumulus
A total of 73 3D LES simulations of marine stratocumulus clouds covering both nighttime and daytime conditions were performed to explore the response of cloud optical depth ($\tau$) to various aerosol number concentration ($N_a$ = 50--2500 cm$^{-3}$) and the co-varying meteorological conditions (large-scale subsidence rate and SST). The idealized FIRE and the ASTEX Lagrangian 1 sounding profiles were used to represent the lightly and heavily drizzling cases, respectively. The first and second aerosol indirect effects are identified. Through statistical analysis, $\tau$ is found be to both positively correlated with $N_a$ and cloud liquid water path (LWP) with a higher correlation associated with LWP, which is predominantly regulated by large-scale subsidence and SST. Clouds with high LWP occur under low SST or weak large-scale subsidence. $\tau$ as a function of $N_a$ and SST (or large-scale subsidence rate) is also derived statistically, which can aid in isolating the cloud optical depth change due to the variation in CCN from the LWP variability associated with different meteorological conditions. Introduction of a small amount of giant sea salt aerosol into the simulation lowers the number of cloud droplets activated, results in larger cloud droplets, and initiates precipitation for non-drizzling polluted clouds. However, giant sea salt aerosol is found to have a negligible effect on $\tau$ for lightly precipitating cases, while resulting in a relative reduction of $\tau$ of 2%-66% (increasing with $N_a \geq$ 1000 cm$^{-3}$) for heavily precipitating cases, suggesting the impact of giant sea salt is only important for moist and potentially convective clouds. Finally, a regression analysis of the simulations shows that the second indirect effect is more evident in clean than polluted cases. The second indirect effect is found to enhance (reduce) the overall aerosol indirect effect for heavily (lightly) drizzling clouds, that is $\tau$ is larger (smaller) for the same relative change in $N_a$ than considering the Twomey (first indirect) effect alone. The aerosol indirect effect is lessened in daytime conditions and is dominated by the Twomey effect. This study suggests that concurrent observation of cloud LWP and $N_a$ is needed for assessing the aerosol indirect effect.
A51E-0839 0800h
The Impact of Mixed Soot / Sulfate Aerosols on Cloud Formation
Internally mixed soot / sulfate aerosols are ubiquitous in marine atmospheres, found both in polluted and quite pristine environments. Soot particles become more hydrophilic as they age in the atmosphere, both due to alteration of their surface properties via chemical reaction and to their mixing with inorganics. Were these aerosols to significantly impact aerosol activation during cloud formation, they could be important contributors to indirect radiative forcing because they are so widespread. We use recent observations that quantify the mixing state of soot and sulfate during several field campaigns (e.g., P\'osfai et al., 1999) and measurements of the contact angle of soot with water (Zuberi, 2003) to constrain a model of condensation onto mixed soot sulfate aerosols (Gorbunov and Hamilton, 1997). We use this model in conjunction with a detailed aerosol microphysics model and a constant-speed model of a cloud updraft to consider the impact of these internally mixed aerosols on aerosol activation during cloud activation. Data from the ACE-1, ACE-2, and INDOEX field campaigns are used to form representative aerosol populations, which are used as inputs to the model. We find that soot inclusions increase the fraction of aerosols that activate in the extremely polluted environments observed during the INDOEX campaign, and have little impact in the less polluted or clean environments observed during the ACE-1 and ACE-2 campaigns.
A51E-0840 0800h
Laboratory Study of the Physical Chemistry of (NH$_{4}$)$_{2}$SO$_{4}$/H$_{2}$SO$_{4}$/H$_{2}$O: Implications for Cirrus Cloud Formation
Aqueous atmospheric aerosols have been known to contain ammonium and sulfate ions in varying ratios for some time. These three components constitute a ternary system whose molecular makeup consists of ammonium sulfate (high ammonium content), sulfuric acid (high sulfate content) and water. We have studied the ammonium sulfate/sulfuric acid/water system using differential scanning calorimetry (DSC) and infrared spectroscopy of thin films at low temperatures. Previous work on this system explored the phase diagram from 273 K to 373 K, leaving the low-temperature portion of the phase diagram undiscovered. Some work has focussed on only one slice of this system: ammonium bisulfate/water. We have constructed the temperature-contour ternary phase diagram for ammonium sulfate/sulfuric acid/water. From our results, it is clear there are several binary subsystems: letovicite/water and ammonium bisulfate/SAT, among others. However, we have clear evidence that ammonium bisulfate/water is not a binary system that exists. Rather it is a slice through the ternary system and thus is much more complex, and can not be represented by a simple ammonium bisulfate/water binary phase diagram. Implications of our results on understanding cirrus cloud formation will be discussed.
A51E-0841 0800h
Heterogeneous Nucleation of Ice by Long Chain Alcohols: Perspectives on the Structure of Water and Theories of Nucleation
At temperatures greater than about -30 degrees C, ice cloud formation is only possible due to the microphysical effects that aerosol particles have on the water or solution droplets with which they are in contact. Though modeling studies have indicated that ice formed in this fashion is sufficiently prevalent as to have a significant impact on the radiative properties of cirrus clouds, the link between aerosol particles and the specific subset which catalyze freezing transitions is tenuous. The most frequently used model is classical nucleation theory, which predicts freezing efficacy for various substances based on knowledge of some basic physical properties of the substances (i.e. lattice match to bulk ice). As recent work has implied that there are likely some limits to the classical theory, we have used infrared spectroscopy to probe thin films of water covered with monolayers of long chain alcohols as the water undergoes a freezing transition. Long chain alcohols were chosen because of their suspected role in atmospheric ice production and their previously observed anomalous nucleating behavior. Interestingly, the spectra show a continuous transition from a spectrum characteristic of supercooled water to that of ice. We interpret our results in light of predictions from classical nucleation theory and in light of structural theories of water. The relevance of nonclassical nucleation theories is also discussed.
A51E-0842 0800h
Laboratory Investigations of Immersion Freezing in Concentrated Aqueous Solutions: the Role of Water Activity
Cirrus ice clouds have a large influence on the global radiative forcing and the dehydration of the upper troposphere and lower stratosphere. Efficient dehydration might occur through sedimentation of a few large ice particles initially formed by heterogeneous ice nucleation. At present, only a few types of ice nuclei have been investigated in laboratory studies at upper tropospheric conditions. In this study, immersion freezing in aqueous solution droplets was studied experimentally using a variety of ice nuclei. The investigated nuclei include inorganic amorphous or crystalline materials (AgI and SiO2 nano-particles), organic crystallites (dicarboxylic acids), as well as organic surfactants (nonadecanol). The nuclei were either suspended in the droplets or prepared as a monolayer at the droplet surface. Droplet sizes varied between ~5-1100 micrometer. Freezing was investigated in a temperature range of 180-273 K using either a differential scanning calorimeter or a cooling stage equipped with a CCD camera. The experiments revealed that nonadecanol monolayers and AgI nano-particles were effective ice nuclei in aqueous solutions. In contrast, SiO2 nano-particles led only to a very small increase of freezing temperatures when compared to the situation without added nuclei. Depending on dicarboxylic acid only a weak or no ice nucleation activation was found. The effect of solutes on the heterogeneous freezing points was very similar for all investigated nuclei, i.e., decreasing freezing temperatures with increasing solute concentration. Furthermore, the data were interpreted within a water activity based approach to ice nucleation. The analysis shows that, apart from nuclei specific parameters, water activity is an important parameter for describing heterogeneous ice nucleation in supercooled solution droplets. This shows the water activity based approach to be useful for parameterizing heterogeneous freezing in atmospheric applications.
A51E-0843 0800h
Heterogeneous Ice Nucleation on Soot
The indirect aerosol effect of soot on Earth's climate is poorly understood partially because it is not clear if soot is an effective ice nucleus. Using an optical microscope coupled to a flow cell we determined the ice nucleating properties of soot. These particles were deposited on the bottom hydrophobic surface of the flow cell and the relative humidity was controlled by holding the vapour pressure constant while decreasing the temperature at a steady rate. As the temperature of the cell was reduced, the onset of ice formation was observed with digital optical microscopy. The ice nucleating properties of n-hexane soots and lamp black were studied between 258 to 238K. Also, lamp black was oxidized by ozone, and the ice nucleating properties of this soot were investigated. In all cases soot was found to be a poor ice nucleus. These results suggest that soot found in the troposphere is not an important ice nucleus at temperatures above 238K. Work is underway to establish if soot is a better ice nucleus at temperatures lower than 238K.
A51E-0844 0800h
Investigations of the Physical and Chemical Properties of Free Tropospheric Ice Nuclei
In spring of 2004 researchers from several institutions participated in INSPECT-II, a field campaign aimed at determining the physical and chemical properties of aerosols activating as ice nuclei (IN) under conditions present in mixed-phase and cold clouds. Studies were conducted at Storm Peak Laboratory located atop Mt. Werner (3210 m above MSL) in the northern Colorado Rocky Mountains. Emphasis of the study was on determining the impact of Asian dust on the concentrations of IN, but contributions from other aerosol sources were also expected. In this study, a continuous-flow diffusion chamber (CFDC) was used to activate IN. A counter-flow virtual impactor (CVI) located immediately downstream of the CFDC separated activated ice nuclei from inactive particles. Flow from the CVI was split between two aerosol mass spectrometers (Aerodyne's Aerosol Mass Spectrometer (AMS) and NOAA's Particle Analysis by Laser Mass Spectrometry (PALMS)) which evaluated the composition of the residual IN. These mass spectrometers also measured ambient particle composition. Additional measurements were made of the ambient aerosol population and included particle size distributions from 0.004 to 20 $\mu$m, particle hygroscopicity, cloud condensation nuclei activity and composition of bulk collections. This paper describes the experimental methodologies employed and provides some preliminary IN concentration data. These data are compared with data from INSPECT-I and with some of the measurements of the ambient aerosol population. These comparisons are used to draw inferences regarding the properties of the fraction of particles serving as ice nuclei.
A51E-0845 0800h
Ice nucleation by cloud particle residues: Chemical compositions and inferences regarding ice initiation in clouds
Assessments of the impact of changing atmospheric aerosol particle concentrations and compositions on cold cloud processes depends on, 1) understanding the sources and behaviors of particles that serve as ice nuclei and, 2) demonstrating a direct connection between the ice nuclei and ice formation in clouds. Toward this end, this paper describes the combined use of two techniques to reveal the properties and influence of aerosol particles that act as heterogeneous ice nuclei (IN) in clouds. The role of IN in different cloud types is also assessed through comparison to cloud ice particle measurements. A counterflow virtual impactor was used for selectively sampling cloud particles during aircraft measurements of clouds. The concentrations, sizes and overall chemical compositions (by electron microscopy) of the evaporated residual aerosol particles were determined and these residual particles were then re-processed to determine their ice nucleating behavior for conditions of relevance to the clouds. A continuous flow ice-thermal diffusion chamber was used to process IN. Nucleated ice crystals were collected and IN residues were analyzed using electron microscopy. This method permitted comparing overall cloud particle residual chemistry to ice nuclei chemistry. The methodology is demonstrated through examples of studies in two cloud types. In the first case, we examine IN found in sub-tropical anvil cirrus clouds during the NASA CRYSTAL-FACE experiment. The variable contribution of heterogeneous IN to the overall concentrations of ice in anvil cirrus is emphasized, which was especially linked to the presence of Saharan dust. In no cases were heterogeneous ice nuclei the dominant contributor to ice present in anvils. We also examine the relation of IN to the presence of ice in mixed-phase clouds in the late Fall over the Northeast U.S. and Eastern Canada. Clear correlation between IN and ice in clouds was demonstrated on a few occasions. IN composition was dominated in both situations by markers for mineral dust particles, but with some apparent industrial contributions.
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Sources of Biogenic and Anthropogenic Semi Volatile Organic Carbonyls and Their Effects on the Air Quality in Suburban and Remote Area
Measurements for biogenic and anthropogenic semi volatile organic carbonyl compounds in gas and particle phases have been conducted using an annular denuder sampling system in Tokyo Metropolitan University (TMU), Minami-Ohsawa, Tokyo, Japan (suburban area, May 12-15, 2004) and Boardman, OR, U.S. (remote area, June 9-17, 2004). Number of the samples collected at TMU and Boardman were 20 and 62, respectively, and 3 dicarbonyls (C$_{2}$-C$_{5}$), 5 aliphatic aldehydes (C$_{8}$-C$_{12}$), 2 hydroxy carbonyls (C$_{2}$ and C$_{3}$) and one unsaturated carbonyl were detected in both gas and particle phases in both sites. Concentrations of NO$_{x}$, ozone, CO, VOCs and OH lifetime were also measured in the suburban site. Gaseous concentrations ranged from detection limit (approx 1 pptv) to 520 pptv (methylglyoxal). 15-75% of these compounds existed as aerosols in their total concentrations, in average. The concentrations showed a diurnal variation with a maximum at noontime, and the composition suggested some of the compounds derived from both biogenic and anthropogenic sources. We found that these semi volatile carbonyls had approx 10% of contribution as OH sinks among the gaseous organics in the site. In the remote site, measurements were conducted in a hybrid poplar forest (plantation) and three different sites outside of the forest, and the forest is surrounded mainly by deserts. Suggesting that C$_{8}$-C$_{12}$ aliphatic aldehydes derived from poplar plant and/or soil in the forest, in general, gaseous concentrations of aliphatic aldehydes in down wind site of the forest exceeded those in the forest. In this presentation, we evaluate about the importance of the biogenic and anthropogenic semi volatile carbonyls as OH sinks and as precursors of tropospheric ozone. We also discuss about the importance of the compounds as aerosol components and their sources in the suburban and remote air.