A53B-01
Fundamental Interaction Between Atmospheric Radicals and an Aerosol Surface: A Molecular Picture of the Interface
How gas phase materials become incorporated with aerosol has been an intriguing subject for decades, and considerable work has been done to understand the interactions between closed shell molecules and liquid water. Currently, atmospheric chemists are developing an understanding of the importance of radical chemistry in the atmosphere, and an understanding of the interactions between open-shelled radical species and liquid phase aerosols is now necessary for this field. To probe these interactions, we have used quantum chemistry optimizations to predict the energetics of the hydroperoxy radical in the presence of a spherical water cage. Optimizations were initialized by placing the radical both on the inside and outside of the cage. Our calculations show that it is energetically favorable for the radical to bind to the outside of the cage. The fact that the energy difference between the internal and external configurations is small, however, suggests that there may be a partitioning of radicals between the two states. Chemistry on the surface of an aerosol will be reported. This may have important ramifications for our understanding of radical chemistry and may lend new insight into the role that aerosols play in atmospheric chemical processes.
A53B-02
Surface Partitioning and Stability of Mixed Films of Fluorinated Alcohols and Acids at the Air- Water Interface
The production of fluorinated compounds over the past 50 years has had numerous industrial applications. For example, perfluorinated carboxylic acids are used in the synthesis of polymers and fire retardants, perfluoroalkyl sulfonates act as surface protectors, and fluorotelomer alcohols are incorporated into products such as paints, coatings, polymers, and adhesives. Fluorotelomer alcohols (FTOHs) are linear polyfluorinated alcohols with the formula CF3(CF2)nCH2CH2OH (n=1,3,5,…). They have been suggested as possible precursors for perfluorinated carboxylic acids and detected in the troposphere over several North American sites. Perfluorocarboxylic acids have even been detected in the arctic food chain, human blood, tissues of animals and environmental waters. We report the surface activity of fluorotelomer alcohols and perfluorinated carboxylic acids at the air-water interface by using a Langmuir trough. Isotherms of the pure compounds along with mixed films with other organic carboxylic acids were collected. The main objective of these experiments was to understand their heterogeneous chemistry by characterizing the pure and mixed films, which serves as a representative model for organic films on atmospheric surfaces such as those found on oceans and aqueous aerosols. Film properties and behavior, notably stabilization, evaporation from the subphase, and miscibility in the single-component mixtures as well as in the mixed films will be discussed. An important consequence of FTOHs and perfluorocarboxylic acids being found to partition to the air-water interface is the possibility of their transport and widespread distribution and deposition using atmospheric aerosols.
A53B-03
Sensitivity of AOT calculation to relative humidity
The importance of relative humidity (RH) on aerosol optical thickness (AOT) is strongly amplified in a nearly water vapor-saturated atmosphere because mass extinction efficiency (MEE) of hygroscopic aerosols increases very rapidly under these conditions. The RH-MEE relationship presents a challenge for AOT calculation because at high RH (>90%) there is a large range of MEE possible for a given RH, and a small change in RH creates very large uncertainty in MEE. In this work, we present a sensitivity experiment that explores the AOT uncertainty in a global chemistry transport model simulation that is derived from the representation of RH used in the AOT calculation. We will particularly focus on the following questions: 1. What is the optimal time step for the AOT calculation in terms of RH variation over high RH regions? 2. Where and when is the AOT most sensitive to changes in RH?
A53B-04 INVITED
Photochemical Aging of Organic Aerosol Particles
Secondary Organic Aerosol (SOA) particles are produced in the atmosphere as a result of oxidation of volatile
organic compounds (VOC). Primary Organic Aerosol (POA) particles are directly emitted in the atmosphere by
their sources. This research focuses on the mechanisms of direct photochemical processes taking place in
model SOA and POA particles, the role of such processes in aging of organic aerosol particles, and the effect of
photochemistry on particles' physicochemical properties.
To address these questions, artificial SOA and POA particles are investigated with several laboratory-based
approaches relying on cavity ring-down spectroscopy and mass-spectrometry. SOA particles generated by dark
oxidation of d-Limonene, alpha-Pinene, and beta-Pinene by ozone are all found to absorb radiation in the
tropospheric actinic window. The UV absorption photolyzes SOA constituents resulting in a release of small VOC
molecules back in the gas-phase, and considerable change in SOA chemical composition. For terpenes
featuring a terminal double bond, the main SOA photolysis products are invariably found to be formaldehyde and
formic acid. Similar observations are obtained for products of ozonolysis of thin films of unsaturated fatty acids
and self-assembled monolayers of unsaturated alkenes. For the case of fatty acids, a very detailed mechanism of
ozonolysis and subsequent photolysis is proposed. The photolytic activity is primarily attributed to organic
peroxides and aldehydes.
These results convincingly demonstrate that photochemical processes occurring inside SOA and POA particles
age the particles on time scales that are shorter than typical lifetimes of aerosol particles in the atmosphere.
http:aerosol.chem.uci.edu/
A53B-05
Stratospheric and upper tropospheric aerosol retrieval from Limb Scatter signals
Solar occultation observations made by the SAGE family of space instruments have provided a record of global stratospheric and upper tropospheric aerosols that extends over 20 years. Since the demise of SAGE II and SAGE III however, there are presently no space instruments devoted to continuing this aerosol data set. The paper will show that aerosol extinction profile, together with a moment of the size distribution, can be accurately retrieved from limb scatter measurements. The methodology will be described, and retrieval examples will be presented using data from two limb scatter instruments, namely SAGE III and OSIRIS. The retrieved extinction profiles will be compared with SAGE II and SAGE III occultation aerosol products for a series of wavelengths. It will be shown that the retrieval accuracy is good (less than 5%), with precision on the order of 25%. Once operational, the retrieval method will be applied to the data collected by the still-operating limb scatter instruments (namely OSIRIS and SCIAMACHY) in order to extend the aerosol data record into the present time. In the future, the OMPS Limb Profiler instrument, which is presently manifested on NPP (launch date = September 2009) will be used to update the global aerosol record.
A53B-06
The atmospheric photooxidation of gaseous PAHs on water films
The processing of organic chemicals such as PAHs in fog can lead to more harmful oxy- and nitro-PAHs. Hence, the photo-transformation of gas phase PAHs (e.g. Naphthalene and phenanthrene) is important to understand in the atmospheric environment. This work summarizes our work on the uptake and UV-photo-oxidation of these gas phase PAHs in thin water films such as occurring in fogs and aerosols. A flow tube reactor was used to carry out these reactions. The rate of photo-oxidation was substantially higher in a thin film as compared to a bulk phase reaction1. Several products were identified in the water films and the mechanism of photo-oxidation was assessed. The presence of a natural surfactant in the water led to multiple effects on the rate of reaction. These were characterized via a dual mechanism of self-sensitized and surfactant-sensitized pathways for reaction. The atmospheric implications of these observations will be explored in this paper.
A53B-07
A Computationally-Efficient Kinetic Approach for Gas/Particle Mass Transfer Treatments: Development, Testing, and 3-D Application
The Weather Research and Forecast/Chemistry Model (WRF/Chem) that simulates chemistry simultaneously with meteorology has recently been developed for real-time forecasting by the U.S. National Center for Atmospheric Research (NCAR) and National Oceanic & Atmospheric Administration (NOAA). As one of the six air quality models, WRF/Chem with a modal aerosol module has been applied for ozone and PM2.5 ensemble forecasts over eastern North America as part of the 2004 New England Air Quality Study (NEAQS) program (NEAQS-2004). Significant differences exist in the partitioning of volatile species (e.g., ammonium and nitrate) simulated by the six models. Model biases are partially attributed to the equilibrium assumption used in the gas/particles mass transfer approach in some models. Development of a more accurate, yet computationally- efficient gas/particle mass transfer approach for three-dimensional (3-D) applications, in particular, real-time forecasting, is therefore warranted. Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution (MADRID) has been implemented into WRF/Chem (referred to as WRF/Chem-MADRID). WRF/Chem-MADRID offers three gas/particle partitioning treatments: equilibrium, kinetic, and hybrid approaches. The equilibrium approach is computationally-efficient and commonly used in 3-D air quality models but less accurate under certain conditions (e.g., in the presence of coarse, reactive particles such as PM containing sea-salts in the coastal areas). The kinetic approach is accurate but computationally-expensive, limiting its 3-D applications. The hybrid approach attempts to provide a compromise between merits and drawbacks of the two approaches by treating fine PM (typically < ~ 1 μm) with the equilibrium approach and coarse PM with the kinetic approach. A computationally-efficient kinetic gas/particle mass transfer approach in MADRID has recently been developed for 3-D applications based on an Analytical Predictor of Condensation (referred to as kinetic/APC). In this study, WRF/Chem-MADRID with the kinetic/APC approach will be further evaluated along with the equilibrium and hybrid approaches using a 19-day NEAQS-2004 episode (July 3-21 2004) over eastern North America. The NEAQS- 2004 episode provides an excellent testbed for WRF/Chem-MADRID with different gas/particle mass transfer treatments for several reasons. First, this region typically suffers a poor air quality with high ozone PM2.5 episodes and large nitrogen deposition. Second, this region is characterized with complex topography (e.g., land vs. sea), meteorology (e.g., large-scale regional transport vs. local-scale sea-breeze), emissions (e.g., urban vs. natural), and co-existence of major PM species (e.g., sulfate/nitarte vs. sea-salt). Third, extensive gas and aerosol measurements are available from International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field study. The model outputs will be evaluated using observations from ICARTT and other routine monitoring networks such as Aerometric Information Retrieval Now (AIRNow) and Speciation Trends Network (STN). The effect of different gas/particle mass transfer approaches on simulated gas and aerosol concentrations will be examined along with a comparison of their computational costs. The gas/particle mass transfer approach that provides the best compromise between numerical accuracy and computational efficiency will be recommended for 3-D research-grade and real-time forecasting applications.
A53B-08
Fogwater Chemistry and Air Quality in the Texas-Louisiana Gulf Coast Corridor
The presence of fog water in polluted atmosphere can influence atmospheric chemistry and air quality. The study of interactions between fog water and atmospheric gases and aerosols are very important in understanding the atmospheric fate of the pollutants. In this Study several air samples and fogwater samples were collected in the heavily industrialized area of Gulf Coast corridor( Houston, TX and Baton Rouge, LA). A total of 32 fogwater samples were collected, comprising of nine fog events in Baton Rouge (Nov 2004 to Feb 2005) and two fog events in Houston (Feb, 2006), during the fog sampling campaigns. These samples were analyzed for pH, total and dissolved carbon, major inorganic ions, organic acids, and aromatics, aldehydes, VOCs, and linear alkanes organic compounds. Fogwater samples collected in Houston show clear influence of marine and anthropogenic environment, while Baton Rouge samples reveal a relatively less polluted environment. Also, a time series observation of air samples indicated that fog event at the monitoring site impacted the air concentrations of the pollutants. This is attributed to presence of surface active organic matter in fog water.