A13B-1166
Statistical Analysis of Cumulonimbus Anvil and Surrounding Aerosol Properties in MODIS Retrievals Over Subtropical and Tropical Regions
In a contribution to a study of the factors controlling cumulonimbus anvil crystal size, we performed a statistical analysis of MODIS Version 5 retrievals of cloud and aerosol properties. Geographical regions were chosen to coincide with the sites of the 2002 CRYSTAL-FACE field campaign (southern Florida) and the 2006 TWP-ICE field campaign (Darwin, Australia), and comparison was also made to larger continental and marine regions (central Amazon and central Pacific Ocean). The MODIS data were processed with algorithms developed in IDL (Interactive Data Language) that extract retrieved cloud optical thickness, aerosol optical depth, brightness temperature, effective particle radius, and cloud mask. Overall, our analysis indicates differences in anvil ice effective radius of only about 15% between the land and ocean regions examined, consistent with previous analyses. Correlation coefficients between anvil-top effective particle radius and locally retrieved aerosol optical depth are statistically significant but are very small except in the biomass burning region, where fires are expected to directly impact cloud properties (through pyrocumulus generation) and aerosol concentrations also reach exceptionally high values. We also demonstrate that the subset of anvil effective radius data analyzed (those for which local aerosol optical depth data were retrieved) are statistically representative of all such regional clouds.
A13B-1167
The Comparison among different cloud classification schemes using Satellite Imagery
Using GOES-11 satellite imagery data, two different classification techniques, namely Artificial Neural Network (ANN) and Support Vector Machine (SVM), are compared to evaluate the quality of cloud classification. The cloud data is classified into twelve types, stratus (St), Stratocumulus (Sc), Cumulus (Cu), Altocumulus (Ac), Altostratus (As), Cirrus (Ci), Cirrocumulus (Cc), Cirrostratus (Cs), Cumulus Congestus (CuC), Cs associated with convection (CsAn), Cumulonimbus (Cb), Clear (Clr), respectively. Data training and testing are verified based on the cloud classification technique in Naval Research Laboratory. Images are taken from west of the Pacific Ocean area, and training cases are built by randomly extraction of 1000, 5000 and 10000 samples. Limited improvement is achieved when a larger amount of samples is used. The attributes of data samples are extracted via Karhünen-Loöve transform. ANN was developed to mimic the neurophysiology of the human brain so as to detect the complex nonlinear relationship in the data. However, poor performance is observed when irrelevant attributes or small data sets exist. SVM is a newer statistical algorithm in machine learning, particularly suitable for pattern classification and nonlinear regression by minimizing the structural risk. It performs well for the existence of irrelevant attributes data and even small data set. Both classification methods show consistent results with overall accuracy larger than 80%. The accuracy of cloud classification using SVM is generally 3-8% better than that using ANN while the computational cost in prediction using SVM is significantly less than that using ANN.
A13B-1168
The Polarization Signature of Arctic Fog during the Polar Sunrise
This investigation reports the polarization Lidar signature in the near infrared (1.574 µm) of mixed phase and supercooled water fog collected during the polar sunrise period February-March 2007 at ambient temperatures from -33 C to -29 C respectively. A simple physical model and a scattering matrix simulation code for non spherical particles were combined to simulate the microphysical and optical properties of the measured fog under the environmental conditions of the experiment. A good agreement between the simulated and measured values of the lidar observable (i.e. linear depolarization fraction) was found. The measured volumetric linear depolarization ratio suggests negative temperature dependence.
A13B-1169
Observations of Size-resolved Drizzle Rates in Marine Stratocumulus
While it is widely acknowledged that drizzle is a key process in the stratocumulus-topped marine boundary layer (MBL), there remain many outstanding questions regarding its quantitative impacts on the system. Here, we report in situ aircraft measurements of the size-resolved drizzle rate in marine stratocumulus using the Artium Flight Phase Doppler Interferometer (F/PDI) during the Marine Stratus Experiment in July 2005. One of the advantages of the F/PDI relative to previous instruments is accurate cloud drop size distribution measurements across a wide range of sizes, 4 to 150 μm diameter. The lower bound of drop size that is often considered drizzle varies substantially, with typical values ~50 μm diameter. However, Nicholls (QJRMS 1984) reports observations that suggest that the contribution of drops smaller than 50 μm to the drizzle rate can be very substantial, particularly at cloud top, although the resolution of the instrumentation allowed for only a coarse analysis. The size distribution of drizzle is relevant to a number of processes. For example, the rate of drop evaporation after it falls below cloud base into the sub-cloud layer depends on drop size. In turn, such evaporation can be an important for the dynamics within the boundary layer and, via feedbacks, impact the cloud layer itself. Also, drop- drop interactions (such as collision-coalescence) within the cloud are strongly dependent on drop size, and therefore the development and evolution of drizzle itself is size-dependent. We utilize these new high resolution F/PDI measurements to examine the following questions: a. What is the size distribution of drizzle? Preliminary results suggest that there are periods of significant drizzle (> 1 mm per day) where drops smaller than 50 μm dominate. b. How and why does this vary in space, both horizontally and vertically? c. What are the impacts of the size distribution on sub-cloud evaporation and, hence, latent heat transport? What are the implications for the dynamics of the cloud-topped MBL?
A13B-1170
Fine-scale modeling of entrainment and mixing of cloudy and clear air
The EMPM (Explicit Mixing Parcel Model) predicts the evolving in-cloud variability due to entrainment and finite-rate turbulent mixing using a 1D representation of a rising cloudy parcel. The 1D formulation allows the model to resolve fine-scale variability down to the smallest turbulent scales (about 1 mm). The EMPM calculates the growth of thousands of individual cloud droplets based on each droplet's local environment. Our analyses of EMPM results address two fundamental difficulties that the large-eddy simulation (LES) approach faces when attempting to represent the effects of entrainment and mixing on droplet microphysics. One is representing the subgrid-scale (SGS) variability of subsaturation and its impact on droplet size distribution (DSD) evolution. Another is accounting for the finite rate of SGS mixing and therefore of droplet evaporation. We have used EMPM results to characterize the evolution of the DSD due to a single isobaric entrainment and mixing event for a range of conditions. We have also used the EMPM to quantify the dependence of the relative humidity time scale on droplet evaporation, turbulent mixing, and droplet sedimentation time scales.
A13B-1171
The Vapor Pressure of Supercooled Water Measured Using Infrared Spectroscopy
We present measurements of the vapor pressure of supercooled water utilizing infrared spectroscopy, which enables us to verify unambiguously the fact that our data correspond to the vapor pressure of liquid water, not a mixture of liquid water and ice. Our values of the vapor pressure and derived value of the latent heat of vaporization are in agreement with previous work. Below -13 °C, the water film which we monitor to determine coexistence of liquid water (at one temperature) and ice (at another, higher, temperature) de-wets from the hydrophilic silicon prism employed in our apparatus. The de-wetting transition indicates a quantitative change in the structure of the supercooled liquid.
A13B-1172
High-Resolution ice Nucleation Spectra of Sea-Ice Bacteria: Implications for Cloud Formation and Life in Frozen Environments
Even though studies of Arctic ice forming particles suggest that a bacterial or viral source derived from open leads could be important for cloud formation in the Arctic (Bigg and Leck, 2002), the ice nucleation potential of most polar marine psychrophiles or viruses has not been examined under conditions more closely resembling those in the atmosphere. In this paper, we examined the ice nucleation activity (INA) of several representative Arctic and Antarctic sea-ice bacterial isolates and a polar Colwellia phage virus. High-resolution ice nucleation spectra were obtained for droplets containing bacterial cells or virus particles using a free-fall freezing tube technique. The fraction of frozen droplets at a particular droplet temperature was determined by measuring the depolarized light scattering intensity from solution droplets in free-fall. Our experiments revealed that all sea-ice isolates and the virus nucleated ice at temperatures very close to the homogeneous nucleation temperature for the nucleation medium -- which for artificial seawater was - 42.2 degC (standdev. 0.3 degC). Our results indicated that these marine psychro-active bacteria and viruses are not important for heterogeneous ice nucleation processes in sea ice or polar clouds. These results also suggested that avoidance of ice formation in close proximity to cell surfaces might be one of the cold-adaptation and survival strategies for sea-ice bacteria. The fact that INA occurs at such low temperature could constitute one factor that explains the persistence of metabolic activities at temperatures far below the freezing point of seawater (Junge et al., 2006).
A13B-1173
Heterogeneous nucleation of ice catalyzed by high molecular weight organic compounds, before and after ozonolysis, using octadecene as a model
High altitude clouds may be affected by the products of biomass burning, which can be lofted into the upper troposphere through deep convection. To further complicate the picture, once in the atmosphere, organic compounds may be transformed through oxidation, possibly changing their characteristics as freezing catalysts. Using 1- and 9-octadecene as a model for unsaturated, non-polar high molecular weight organic compounds, we will show that exposure to ozone does not change the characteristic temperature at which a coating of octadecene catalyzes heterogeneous ice nucleation. In addition, the phase of the octadecene (liquid or crystalline) when ozonolysis takes place does not affect the characteristic freezing temperature. Results from studies of the same system using infrared spectroscopy will also be presented.
A13B-1174
Comparison of Fe(II) Photo-Formation Characteristics Between Aqueous Humic Acid Solutions and Aqueous Extracts of Atmospheric Aerosols Collected at Okinawa Island, Japan
Photochemical cycles of Fe(III)-Fe(II) affects the oxidation and the reduction of transient species such as active oxygen species and various transition metals in the atmospheric condensed phases. Although the importance of organic ligands to iron cycling (e.g. ligand-to-metal charge transfer) is becoming clearer, the mechanism by which photochemical reduction of Fe(III) to Fe(II) are not well understood. Humic acid (HA) is considered as an important organic ligand for Fe(III) complexes in the environment. HA is a collection of organic compounds that exist in nature but whose structures are not well known. Commercially available HAs as received from the manufacturers contain trace amount of iron. Using this residual Fe, we investigated the photochemical formation of Fe(II) in aqueous HA solutions to elucidate the photochemical cycles of Fe(III)-Fe(II) in the atmospheric water drops. We purchased HAs from several different suppliers. We investigated the effects of pH and wavelengths on Fe(II) photo-formation using monochromatic radiations at 313, 334, 366, and 405 nm. Concentrations of photochemically formed Fe(II) were determined by ferrozine-HPLC technique, and the apparent quantum yields were determined based on the total absorbance of the HA solutions. Fe(II) photo-formation characteristics of the aqueous humic acid solutions purchased from different suppliers showed slightly different wavelength dependence. Furthermore, we compared Fe(II) photoformation characteristics observed in aqueous HA solutions with those in the aqueous extracts of atmospheric aerosols collected in Okinawa, Japan. The results showed that the apparent quantum yields of the aerosol extracts were 5-10 times higher than those of the HA solutions. Wavelength-dependence of Fe(II) photo-formation observed in the aqueous extracts of aerosols was similar to that seen in the aqueous HA solutions.
A13B-1175
Photochemical Formation of Fe(II) in the Aqueous Solutions of Fe(III)- Dicarboxylates
Although there have been many studies reporting the photochemical formation of Fe(II) in various aqueous-phase such as rain, cloud waters, seawater and aerosols, the detailed formation mechanisms are not well understood. To better understand the mechanisms of Fe(II) formation, we attempted to determine the molar absorptivity and the quantum yield of Fe(II) photoformation for individual Fe(III)-dicarboxylate species. The concentrations of Fe(II) and total dissolved Fe were measured by a Ferrozine-HPLC method. The Visual MINTEQ computer program was used to calculate the equilibrium concentrations of chemical species in the solutions of Fe(III)-dicarboxylate complexes. The molar absorptivity and the product of the quantum yield and the molar absorptivity of Fe(III)- dicarboxylate complex can be analysed by UV-VIS spectrophotometer and photochemical experiments, and these experimental data were combined with the calculated equilibrium Fe(III) speciation to determine individual molar absorptivity and quantum yield of Fe(II) photoformation for a specific Fe(III)-dicarboxylate complex. Preliminary results, using an oxalate whose quantum yield has been previously reported, indicate that this approach gives lower quantum yield values in air saturated solutions than previously reported.
A13B-1176
Control Factors for PM10 Management in Seoul
In January 2005 the Special Act on Metropolitan Air Quality Improvement was set into force. In July 2006 the new Mayor of Seoul who pushes the Special Plan for Clean Seoul 2010 as a key project was inaugurated. Many new provisions have been implemented during the past few years. However, reduction of PM10 and control of vehicle emissions become a key word of the present management plans to the general public. This is mainly because high concentration of PM10 is easily recognized without intricate devices and vehicles are a core element of urban life. How much a tight control of vehicle emissions will contribute to the management of PM10 in Seoul has come to an issue along with local emission vs. long-range transport, vehicle emission vs. fugitive dust, mass concentration vs. hazardous substances, and so on. In fact, lots of published and measured data indicate that the contribution of vehicle emission, especially that of primary emission of particulate matter from vehicles, to PM10 (mass concentration) is only limited. In this paper the status of PM10 in Seoul was assessed by comparing its concentration and composition to those of other major cities in the world. Temporal and spatial variations in PM2.5 measured between 2002 and 2005 were analyzed in comparison with those in PM10. Smog and health risk were suggested as an issue for managing PM10 as well as PM10 mass concentration. Control factors for the corresponding issues were discussed.
A13B-1177
Study for PM2.5 composition and variations at Ieodo Ocean Research Station, Korea
PM2.5 has been collected since June 2004 at Ieodo Ocean Research Station (IORS), which is located in the middle of China and South Korea. For 3 years from June 2004 to June 2007, average mass concentrations were 20.97±16.86 μg/m3 and concentrations were the highest in spring (29.32μg/m3) and lowest in summer (17.00μg/m3). Water soluble ions were determined during December 2004 to September 2005. SO42- (32.2%) and NH4+ (14.2%) were the most abundant species. In winter, SO42- accounted for 42% of PM2.5 means, which was higher than that in spring (26%). Nitrate was thought to be lost through evaporation. The cluster analysis of backward trajectories for 5 days was performed to examine the possible aerosol sources. High mass concentrations were observed in air masses originating from China inland (26.93μg/m3). Also, the seasonal PM2.5 mass concentrations were well correlated with the frequency of western wind. Compared with PM2.5 measurements at Gosan during the ABC- EAREX2005 (March 2005), PM2.5 mass and major ionic concentrations were higher at IORS while the variation pattern was similar in two stations. These results implied that PM2.5 mass and its major ionic species at IROS were greatly influenced by outflows from China. http://atmos.korea.ac.kr
A13B-1178
Mass flux and ionic composition of foam droplets generated from natural and artificial seawaters
In the remote marine boundary layer (MBL), sea salt aerosol (SSA) particles are an abundant and climatologically important class of particles. Wind stress on the ocean surface produces whitecap foams, which are the main source of SSA particles. Laboratory foams designed to mimic oceanic whitecaps were generated using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. We have reported on the fluxes of submicron particles by number; here we report mass and composition of sub- and supermicron SSA. Foam droplets were conditioned to 80% relative humidity and collected using a microorifice impactor. These substrates were analyzed by ion chromatography and atomic absorption spectroscopy. Sufficient aerosol matter was collected to accurately characterized mass distributions in the diameter range Da,80 = 0.56 - 5.6 μm. The ionic composition of seawater was conserved during the foam bubble bursting process. Enrichment factors (EF) calculated relative to Na+ for Cl-, SO2-4, Mg2+, Ca2+, and K+ were all indistinguishable from 1 with a precision of approximately 20%. The mass flux of submicron SSA was consistent with our earlier measurement of number flux. The mass flux of supermicron SSA (Da,80 = 1.0 - 5.6 μm) was comparable to that for the submicron particles. In contrast to submicron SSA, which was approximately unaffected by the aqueous media composition, supermicon SSA mean diameters and fluxes varied with aqueous media composition. Filtered and unfiltered natural seawater exhibited a distinct peak in the range Da,80 = 1.8 - 2.5 μm. The present results support the hypothesis that seawater organic matter affects the mass and size of supermicron SSA particles.
A13B-1179
Evaluation Of Sensitivity Of Mass-independent Oxygen Isotopes In Aerosol Nitrate To Environmental Factors Using A Photochemical Box Model
An existing photochemical box model for use in polluted marine boundary layers was modified to allow for the explicit tracking of the mass-independent isotopic composition of oxygen in aerosol nitrate as well as other atmospheric species such as OH and H2O2. This modified model was then used to study the sensitivity of the mass-independent isotopic composition of atmospheric nitrate(HNO3) to variables such as relative humidity, temperature ozone and NOx concentrations. Here we present the results of these studies and compare model predictions of the mass-independent oxygen isotopic composition of aerosol nitrate to measurements taken in fine (<1micron) and coarse (>1 micron) aerosol samples taken in a variety of locations, from coastal urban environments, the tropics (Ecuador), inland California (Riverside), and Antarctica. Regarding Antarctica, we comment on the isotopic composition of OH there and the ramifications of these findings for the isotopic composition of other oxygen bearing compounds in the Antarctic atmosphere.
A13B-1180
Analysis of Aerosol Mass Spectrometer Data in Pittsburgh, Mexico City, and Blodgett Forest by Positive Matrix Factorization
Mass spectrometric measurements of ambient aerosols yield organic spectra that are a mix of nucleated particles, freshly emitted particles from many sources, and particles which have undergone some amount of processing (condensation, oxidative reaction, cloud processing, etc.). Further understanding of the important sources and processes for organic aerosols requires deconvolution of the organic fraction of ambient aerosols. A well-known source apportionment technique, Positive Matrix Factorization (PMF), has been applied to Aerodyne aerosol mass spectrometer (Q-AMS and HR-ToF-AMS) datasets acquired in Pittsburgh (2002), Mexico City (2003), Blodgett Forest (2007) and other sites. Sensitivity analysis of the Pittsburgh case is performed with synthetic datasets and characterizes the behaviour of PMF with AMS datasets. PMF is a least-squares fitting method for source apportionment commonly applied to speciated aerosol datasets (Paatero, Chemomet. Intell. Lab. Sys. 1997, 37, 23-35). The structure and precision of the AMS dataset are significantly different from datasets compiled from previous measurements of multiple aerosol components (metals, organic and elemental carbon, ions, etc.), are very large (~300 points per sample, with ~3000-10,000 samples for these campaigns), and fragmentation of molecules during ionization gives each mass spectrum strongly interrelated data. Additionally, because the Aerodyne AMS does not detect elemental carbon or metals (common and important in most PMF analyses), application of this technique to AMS data yields significantly different source profiles than in previous PMF studies. Each site has a hydrocarbon-like organic aerosol (HOA) factor that is likely primary OA and multiple oxygenated organic aerosol (OOA) factors that are likely secondary OA, and biomass burning organic aerosol (BBOA) can be separated in some locations. The OOA factors always include one highly-oxygenated factor (OOA-I) that resembles the spectrum of fulvic acid and less-oxygenated factors that resemble spectra from chamber studies of SOA. Assignment of factor spectra to these categories is aided by comparison to spectra collected by many research groups using the Aerodyne AMS to measure individual compounds, reactions in smog chambers, and primary emission sources. These spectra have been compiled in a publicly-available database (http://cires.colorado.edu/jimenez-group/AMSsd/spectra.html).