A54B-01 INVITED 16:00h
Hygroscopicity of organic-containing particles and implications for warm and cold cloud formation
The hygroscopic growth of ambient and laboratory-generated aerosols exposed to varying relative humidities has been commonly investigated using the Humidified Tandem Differential Mobility Analyzer (HTDMA). A methodology is presented that uses particle growth factors, obtained from HTDMA data, to derive solution water activities as functions of composition. The method is applied to HTDMA data for selected inorganic and organic species. Water activities derived in this manner generally compare well with previously-published data obtained by other techniques, although discrepancies were found for some of the investigated dicarboxylic acids. The derived water activity data are then used to predict critical supersaturations for drop formation and initiation of homogeneous freezing, and compared with published experimental data. Except for malonic acid, predicted critical supersaturations for drop formation are well within experimental uncertainities. Relative humidities required for initiation of homogeneous freezing at cirrus temperatures are not as well represented; possible reasons for observed differences, including experimental limitations, are discussed.
A54B-02 16:15h
Deliquescence and Crystallization of Ammonium Sulfate Particles Internally Mixed With Water-Soluble Organic Compounds
Deliquescence and crystallization relative humidities of particles containing ammonium sulfate internally mixed with water-soluble organic material have been measured. The organic material included malonic acid, glycerol, levoglucosan, and Suwannee River fulvic acid. Results for deliquescence of systems with malonic and fulvic acids are in agreement with existing literature values. The glycerol system undergoes deliquescence at slightly lower relative humidity than previous measurements. The levoglucosan results are the first of this kind. Deliquescence relative humidities for the different systems are the same within the uncertainty of the measurements when the organic mole fraction is less than 0.35. The maximum deviation of deliquescence relative humidities across the systems is approximately 10 % relative humidity at an organic mole fraction of 0.6. The crystallization relative humidity (CRH) of ammonium sulfate with malonic acid, glycerol, or levoglucosan, decreases significantly from the CRH of pure ammonium sulfate when the organic mole fraction is greater than 0.25. This is in contrast to our previous study with glutaric acid where the CRH remained close to the CRH of pure ammonium sulfate up to a glutaric acid mole fraction of 0.4. In terms of atmospheric implications, we estimate that organics, on average, are only a minor perturbation on the deliquescence relative humidity of the pure inorganic particles; however, the organics, on average, may decrease the CRH of pure inorganic particles significantly and this effect depends on the type of organic material.
A54B-03 16:30h
Deliquescence investigated by environmental transmission electron microscopy
Although the deliquescence relative humidities of particles such as sodium chloride or ammonium sulfate are well known when their diameters are larger than 100 nm, there have been few studies on the deliquescence of particles having diameters between 1 and 100 nm, which are abundant in the troposphere. These previous studies do, however, show different behaviors of nanoparticles as they interact with water vapor. Environmental transmission electron microscopy (ETEM) allows high resolution, in-situ observation of nanoparticles as relative humidity is cycled in an environmental cell located in the electron microscope column. In order to utilize ETEM to study deliquescence of nanoparticles, the conditions the particles experience under the electron beam need to be determined. We have used a variety of sub-micron salt particles to determine water vapor pressure and particle temperature in the environmental cell of the ETEM. With this knowledge, we plan to extend the capabilities of ETEM to study phase changes of atmospherically relevant nanoparticles.
A54B-04 INVITED 16:45h
Aerosol effects on cloud activation
The effects of aerosols on clouds are recognized as one of the largest sources of uncertainty in assessments of anthropogenic climate change. This talk will address key sources of uncertainty that exist in current assessments of the aerosol indirect effect, and provide a theoretical/modeling perspective to how details on aerosols properties and processes can be incorporated into models up to the global scale.
A54B-05 17:00h
The influence of aerosol composition on cloud drop number concentration
The effect of aerosol composition on cloud drop concentration is explored, with a focus on water soluble organic carbon. Studies published to date yield conflicting results regarding the magnitude, and even the sign of the effect, compared to well-characterized inorganics. We present a systematic investigation of the reasons for these discrepancies by examining the ranges of physico-chemical properties of water soluble organics that most influence drop formation. We show that when considered individually, composition parameters such as increased molecular weight and surface tension suppression can lead to significant effects on droplet concentration. When considered together, these effects tend to counteract one another and produce much smaller changes. In addition, an assessment of the published literature suggests that estimates of composition effects on drop concentration based on equilibrium assumptions can be much larger than similar estimates under non-equilibrium conditions.
A54B-06 17:15h
Recent Field Studies and an Improved Understanding of Aerosol / Ice Cloud Interactions
It has been known for several decades that aerosol particles can nucleate the ice phase, and thus form clouds, via two distinct processes: homogeneous freezing of an aqueous medium and heterogeneous action by solid components known as ice nuclei. In the past several years laboratory studies of the freezing properties of aerosols representative of those found in the troposphere have been conducted and cloud residue analyses have described some of the attributes of ice nuclei. A recent emphasis has been placed on conducting field missions to better understand aerosol / ice cloud interactions. These studies, including CRYSTAL-FACE and INSPECT, have greatly increased our understanding of the composition of ice forming particles, and the mechanisms by which they freeze. For example, single particle instruments have shown that ice nuclei originate from a small subset of atmospheric particles and that aerosol composition, namely organic content, affects homogeneous freezing. Episodic events, such as dust storms, have been shown to have a significant effect on cloud properties and may change the mechanism by which freezing occurs. Ultimately, these data may improve our ability to model aerosol / cloud interactions and better predict the effect of anthropogenic perturbations.
http://lamar.colostate.edu/~pdemott/spl/Splproj.htm
A54B-07 17:30h
Heterogeneous Nucleation of Ice on Mineral Dust Studied With a New Laboratory Technique
Mineral dust particles may contribute significantly to the indirect aerosol effect by inducing the formation of cirrus ice clouds. Recently, the NASA-CRYSTALFACE campaign has shown that mineral dust particles emerging from the Saharian desert can act as heterogeneous ice nuclei. Using a new experimental technique, we present the results of a study of the heterogeneous nucleation of ice for the following particle systems: mineral dust particles, effloresced (NH$_4$)$_2$SO$_4$/H$_2$O-particles containing mineral dust particles, pure solid (NH$_4$)$_2$SO$_4$/H$_2$O-particles, and H$_2$SO$_4$-coated mineral dust particles.\The new experimental apparatus allows us to control the temperature of the particles between 190--300 K and to adjust the relative humidity (RH) between 0 % and 100 % with respect to water. The setup is designed in a way so that supersaturation with respect to ice can be achieved. The instrument is calibrated by observing known phase transitions and melting points of crystals. The phase changes of the aerosol particles are observed by optical microscopy and Raman spectroscopy.\The experiments show that the investigated particle systems are very efficient ice nuclei, inducing ice formation between 100--115 % RH with respect to ice in a temperature range of 197--240 K. Below 240 K only deposition mode freezing is observed, i.\ e.\ the formation of ice directly from the gas phase. \The experiments show that mineral dust particles can be preactivated, i.\ e.\ nucleation of ice is facilitated if the particle was previously involved in ice nucleation. Mineral dust particles in the preactivated state nucleate ice at 10--30 % lower RH compared to the non-preactivated ones.\The experimental results suggest another nucleation pathway for the formation of cirrus ice clouds. These findings may explain the low RH-values observed for the onset of upper tropospheric cloud formation in field studies.
A54B-08 17:45h
Laboratory Studies of Ice Nucleation on Organic Aerosol Surfaces
Ice clouds play a key role in governing the Earth's radiative balance and in regulating the climate system. However, the chemistry and microphysics of ice surfaces and microphysical mechanisms in ice clouds are still unexplored, despite their dominant role in global climate change. Now, it has become apparent that atmospheric aerosols contain significant amounts of organic material and substantially impact the formation of natural clouds. It is believed that clouds formed in supersaturated environments may incorporate a heterogeneous component of particle nature, alternatively may freeze homogeneously on pre-existing supercooled liquid droplets. In this paper, we provide an experimental approach to ice nucleation by supercooled aqueous solutions of dicarboxylic acids at higher temperatures than that required for homogenous freezing thresholds. Present results suggest that low molecular weight dicarboxylic acids may initiate ice nucleation in the atmosphere thereby may play an important role in the formation of cloud particles. Thus, the possibility of ice formation by these compounds can not be ruled out, which in turn may influence the radiative forcing by atmospheric aerosols, however, this data is currently lacking in climate models.