Objectives
The purpose of this research project is to construct and test data analysis techniques and mathematical models that relate the emissions of gases and primary particles to atmospheric particulate matter concentrations. In both 1996 and again in 1997, field experiments were conducted in Southern California in which the evolution of the size distribution and chemical composition of the urban aerosol complex was observed using methods that focus on the evolution of the individual aerosol particles. The data from the 1996 and 1997 experiments will be used as a model evaluation data set.
Background
During September and October of 1996 and again in August through November 1997, field experimental programs were conducted in the Los Angeles area in which the evolution of the size distribution and chemical composition of the urban aerosol complex was observed using methods that focus on the evolution of the individual aerosol particles. In 1996, experiments were conducted in which the background marine aerosol was first characterized as it flows across the Pacific coastline in Southern California. Lagrangian air parcels were sampled as they were transported across the urban Los Angeles area from Long Beach to Fullerton to Riverside, CA, in the presence of direct emissions from urban pollution sources and as the aerosol is modified by gas-to-particle conversion processes. In 1997, air parcels were studied as they were transported from the high traffic area of central Los Angeles to a downwind site at Azusa, and nitrate aerosol formation was studied as air parcels were transported across the large ammonia source in the Chino dairy area. Both organic and inorganic aerosol species were sampled simultaneously, (1) by aerosol time-of-flight mass spectrometers that view single particle size and composition, (2) by MOUDI cascade impactors from which particle chemical composition was sampled as a function of particle size, (3) by filter-based fine particle samplers and (4) by electrical aerosol analyzers and optical particle counters that measure particle size distributions directly and continuously.
The purpose of the present project is to apply atmospheric models and other advanced data analysis methods to the study of atmospheric aerosol processes using the data base from the 1996 and 1997 Southern California field studies.
Aerosol processes air quality models have been developed that seek to compute how the size distribution of the chemical composition of the aerosol evolves as the source emissions undergo atmospheric transport and transformation. Nearly all of these models developed to date have assumed that the aerosol is "internally mixed", that is that all particles of the same size have the same chemical composition. We know both from examination of atmospheric particles via electron microscopy with X-ray detection and from Professor Prather's aerosol time of flight mass spectrometry data that this really is not true, but the internal mixture assumption persists because it renders an extremely difficult modeling calculation more tractable and because in the past there were no atmospheric data bases on single particle composition that could be used to check the ability to model single particle chemical characteristics in any case.
Within the last two years both of these barriers to further progress in modeling particle properties at more nearly the single particle level have been relaxed. First, from the successful conclusion of the 1996 and 1997 Southern California field experiments described above, we now have for the first time data on single particle size and composition collected within the format of trajectory-oriented field studies that are suitable for use in studying particle evolution over time as air masses are transported across successive air monitoring sites. Such data can be used as an air quality model verification data set. Second, a new aerosol processes trajectory model has been developed that represents the atmospheric particles as a "source-oriented external mixture" in which particles from the different major source types in an urban area evolve separately from each other, thereby allowing the model to track particle-to-particle differences in chemical composition for particles of the same size in the atmosphere. That model also retains a description of individual particle composition, and can identify those particles that evolve by incorporation of water and heterogeneous liquid-phase chemistry separately from hydrophobic particles that do not easily grow upon humidification, for example.
The experiments conducted during the 1996 and 1997 field studies are ideally suited to providing the atmospheric data on single particle chemical composition needed to test this Lagrangian particle air quality model. The data also are well suited to the development and testing of the individual modules that form the building blocks within comprehensive aerosol processes air quality models in order to describe gas/particle partitioning and light scattering, for example. The objective of this project is to pursue such model evaluation studies, and in the process to learn more about the processes that govern particle evolution in the atmosphere.
Method of Approach
The experimental design followed during the 1996 and 1997 field experiments was oriented toward following air parcel trajectories. Data analyses and model testing thus will be cast into a trajectory model format. Air quality models will be applied to explain the relationship between emissions and air quality for both organic and inorganic particulate matter at nearly the single particle level. The air quality models used eventually will include both the aerosol processes trajectory model of Kleeman et al. As well as novel chemical mass balance receptor models that relate source contributions to ambient particulate matter concentrations.
Summary of Progress and Accomplishments
During the first year of this project, an air quality model that follows the evolution of single particles in the atmosphere was developed. That model was then combined with new emissions measurements and used to predict the size distribution and chemical composition of the airborne fine particle mixture observed at Long Beach, Fullerton, and Riverside, CA, during September, 1996. Comparison of air quality model predictions to both filter-based and cascade impactor-based measurements of particle size and chemical composition showed good agreement at all three air monitoring sites. Source contributions to fine particle air quality calculated by this model as a function of particle size and chemical composition provide very useful insights into the likely effect of future emissions control programs. That successful model development effort was completed through publication of a journal article on our findings during the year 1999 (see Kleeman et al., 1999 cited below).
The air quality model just described is capable of predicting the detailed composition of individual particles in the atmosphere for the case of an externally mixed aerosol. In the past, it has been impossible to evaluate the predictions of the model that are made at the single particle level because of the lack of a corresponding set of data on single particle size and chemical composition in the atmosphere. During both the1996 and 1997 field studies mentioned above, measurements of single particle size and chemical composition were made by Professor Kimberly Prather's research group at the University of California at Riverside working in collaboration with our research group. During 1999, our research centered on merging Professor Prather's single particle data with our own data taken by filter samplers, cascade impactors and electronic particle size distribution monitors during the 1996 and 1997 experiments in order to produce the most accurate picture that can be obtained of single particle chemical composition for future comparison to the air quality model results. In the course of that effort, a method for determining the particle counting efficiency of the aerosol time of flight mass spectrometer (ATOFMS) instruments that make the single particle measurements has been developed and published (see Allen et al., 2000 cited below). Using these counting efficiency calibration curves, the particles measured by the ATOFMS instruments during the field studies were adjusted to reflect the number of particles actually present in the atmosphere according to broad particle classes (ammonium nitrate-containing particles; carbon-containing particles, soil dust particles, sea salt particles, and combinations of these types). The data were them organized in a way that particle evolution during transport across the Los Angeles basin could be visualized, and a journal article describing the methods and findings was prepared (see Hughes et al., in press, cited below). It was seen that sea salt particles are converted progressively to sodium nitrate, and that ammonium nitrate and organic compounds accumulate on particles that become progressively more complex chemically over longer and longer travel times downwind. These findings are in qualitative agreement with the air quality model results. As the research project nears the end of its programmed life, efforts center on organizing data from the 1997 field experiments so that they too can be compared to model predictions
Interactions with other CAO Projects
This research is a continuation of the 1996 field experimental program supported by the Center from June 1996 to May 1998, and as such is directly related to a previous CAO project. This work on air quality model development and testing incorporates the gas/particle partitioning approach to secondary organic aerosol formation that results from ongoing Center-sponsored research by Professors Seinfeld and Flagan. The modeling efforts also benefit from ongoing source testing and emissions characterization work supported previously by the California Air Resources Board and by contributions from industry to Caltech's Center for Air Quality Analysis, and which are presently supported by the USDOD and USEPA under other research agreements.
Status of Original Plan
As the progress reported above indicates, we have produced a significant number of publications that attest to our accomplishments under this research project. However, even in the presence of this success, modifications to the scope of work were necessary near the start of this project, for the following reason. In our original proposal, we noted that additional support for air quality model development and testing was then being sought and that if additional funds were awarded from other sources then the scope of work under the present research program would be modified to avoid any duplication of effort. In mid-1998, we received an award from the USEPA as part of a special 12-investigator program designed to speed the construction and testing of combined air quality models for ozone and fine particles. We therefore separated the work under the ozone and fine particles grant from the work being done under the CAO-sponsored program. The trajectory modeling study described above was completed quickly by employing CAO funds for set-up of the model evaluation data, and ozone and fine particle grant funds for model execution, and will be reported to both sponsors as a result. In order to capitalize on the success of this approach, we decided to extend the modeling studies to include trajectory modeling of the 1997 Southern California experiments described in the (revised) statement of project objectives (in the original CAO proposal only the 1996 data were to be studied). Again we proposed to use CAO funds for model evaluation data base creation, and ozone and fine particles grant funds for model development and execution. With that division of effort in mind, Jon Allen and Lara Hughes have concentrated on setting up the 1997 data so that they can be compared to the air quality model, while Prakash Bhave and Michael Kleeman working under ozone and fine particle center grant support have concentrated on air quality model improvement and execution (not funded by CAO).
Future Plans
Handling of QA/QC
This is a purely computational and theoretical research program, and as such does not involve quality control procedures in the laboratory. Care will be taken not to make mathematical errors in the present work.
Key Personnel
Graduate Student:
Lara Hughes
Post-Doctoral Fellow:
Jonathan Allen
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