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1997 Progress Report: Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
EPA Grant Number: R824970C010Subproject: this is subproject number 010 , established and managed by the Center Director under grant R824970
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: EERC - Center for Airborne Organics (MIT)
Center Director: Seinfeld, John
Title: Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
Investigators: Cass, Glen
Institution: California Institute of Technology
EPA Project Officer: Shapiro, Paul
Project Period:
Project Period Covered by this Report: January 1, 1996 through May 31, 1997
Project Amount: Refer to main center abstract for funding details.
RFA: Center on Airborne Organics (1993)
Research Category: Targeted Research
Description:
Objective:The purpose of this research project is to conduct a field experimental program in which the evolution of the size distribution and chemical composition of the urban aerosol complex is observed using methods that focus on the evolution of the individual aerosol particles. The goal is to directly observe how primary particles emitted from the many air pollution sources in a major city are modified by gas-to-particle conversion processes in the atmosphere.
Rationale: The US Environmental Protection Agency recently has established a new ambient air quality standard that limits fine particle concentrations in the atmosphere. In order to devise emissions control strategies that can be relied upon to achieve specified improvements in fine particle air quality, it is necessary to understand both the relative contributions that important primary particle emissions sources make to ambient particle levels and also how those particles are modified over time in the atmosphere by gas-to-particle conversion processes.
Recently, two developments have occurred that make it possible to examine the transformation and fate of single particles in the atmosphere. First, aerosol time-of-flight mass spectrometers (ATOFMS) have been designed and are being tested that can determine both the size and the chemical composition of individual aerosol particles at the rate of hundreds of thousands of particles per day. Both organic and inorganic composition can be examined. By sampling with ATOFMS systems at multiple sites located along single air parcel trajectories, it should be possible to observe in detail the nature of particle transformations as particles age in the atmosphere due to atmospheric chemical reactions. Second, in a separate development a Lagrangian particle air quality model has been developed at Caltech that tracks the evolution of aerosol particles in the atmosphere in a way that it is possible to account for the particle-to-particle differences in chemical composition for particles of the same size that is evident in the ATOFMS data. Versions of that model are under development that retain information on the original sources from which the various primary seed particles were emitted (which is particularity useful for preliminary emission control strategy evaluation). Experiments are proposed here that will provide atmospheric data on single particle chemical composition in the Southern California atmosphere that in the short term can be used to observe and describe the transformations of individual particles by chemical reaction in the atmosphere and that in the future could be used to test air quality models against data on single particle composition in the atmosphere.
Completion of the research proposed here on the evolution of the Southern California aerosol as it is transported across the Los Angeles urban area will meet several needs. First these experiments will serve to determine how the single particle data base generated by time-of-flight mass spectrometers can be aggregated to recreate the bulk aerosol size distribution and chemical composition as measured by cascade impactors and electronic size distribution analyzers. Second, the results will describe the evolution of the Southern California aerosol as it is transported and transformed in the atmosphere. Since Rubidoux near the proposed trajectory end points at Riverside probably has the highest fine particle concentrations in the nation, detailed information on how that aerosol is created is expected to advance our understanding of how such severe air quality problems can be controlled. Third, the experiments will provide a model verification data set for later testing the predictions of Lagrangian particle aerosol processes air quality models that are needed for use as design tools during the control strategy testing phase of the state implementation planning process for airborne particulate matter.
Approach: Experiments will be conducted in which the background marine aerosol first is characterized based on measurements made at Santa Catalina Island which is located upwind of the Los Angeles area in the summer. Then Lagrangian air parcels will be sampled as they are transported across the urban Los Angeles area 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. Both organic and inorganic aerosol species will be sampled simultaneously, (1) by time-of-flight mass spectrometers that view single particle size and composition, (2) by cascade impactors from which particle chemical composition can be measured as a function of particle size, (3) by filter-based samplers and (4) by electronic instruments that measure particle size distributions directly and continuously.
Status: During September and October of 1996, a field experimental program was 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. 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. Both organic and inorganic aerosol species were sampled simultaneously using the methods discussed above.
There were three principal objectives of the 1996 field experiments. These include (1) calibration of the ATOFMS units under field operating conditions in order that they can be used to represent the atmospheric particle size and composition distribution accurately, (2) description of the evolution of the atmospheric particle mixture as it is transformed by passage across Southern California, and (3) organization of a data base that could be used at a later date to test the predictions of atmospheric models for particle formation and transport.
In response to the first objective, these experiments were designed to determine how the single particle data base generated by time-of-flight mass spectrometers could be aggregated to recreate the bulk aerosol size distribution and chemical composition as measured by cascade impactors and electronic size distribution analyzers. That objective has been met. Through comparison of particle size distribution data collected by MOUDI impactors, electrical aerosol analyzers, and optical particle counters to the particle counts as a function of particle size measured by the aerosol time of flight mass spectrometers (ATOFMS) it has been possible to determine the counting efficiency of the ATOFMSs. As shown in Figure 1, the counting efficiency of an ATOFMS declines rapidly as particle size is reduced, but the counting efficiency performance is stable and reproducible during the experiments. Next, calibration curves that relate the ATOFMSs peak heights for individual chemical species to the absolute quantity of the chemical species in the atmosphere have been constructed. The particle counting efficiency correction curves described above first are applied to the ATOFMS output, then the appropriate ion counts that represent the important chemical species in the particles are compared to the concentrations of these species as a function of particle size measured from the MOUDI impactor substrates. Examples of the ATOFMS calibration curves for nitrates and ammonium ion as a function of particle size are shown in Figures 2 and 3. Analogous calibration curves for organics and for metals are under development at present. By scaling the original particle counts taken by the ATOFMSs according to the calibration curves, it has been possible to convert the ATOFMS from a qualitative to a quantitative instrument for measurement of particle size and composition at the single particle level. This is important because the calibration experiments are very labor intensive and can be performed only periodically, while the ATOFMSs are able to take data continuously for weeks on end. Through the application of the calibration curves (which effectively replicate the particles in proportion to the degree to which they are undercounted), the ATOFMS has been converted into a continuous air monitoring instrument at the single particle level whose output can be interpreted in terms of absolute concentrations as a function of particle size.
The second objective of the 1996 experiments was to describe the evolution of the Southern California aerosol as it is transported and transformed in the atmosphere. This work on descriptive analysis of the data collected is underway at present. The experiments were designed such that a single air parcel (hopefully) could be followed as it moved progressively over the Long Beach, Fullerton, and Riverside air monitoring sites. As seen in Figure 4, that experimental design succeeded, and we now have raw data on essentially the same air mass as it passes over each site in turn. Those data are presently being examined in order to describe the particle transformations that are observed empirically as the air masses examined are aged over time in the presence of effluents from the urban emissions sources. Descriptive analysis of the experimental data should be completed by June of 1998.
The third objective of the experiments was to provide a model verification data set that could be used in the future to test the predictions of aerosol processes air quality models that seek to predict the compositional differences between particles in the atmosphere at nearly the single particle level. By June of 1998, that data base will have been constructed following full application of the calibration data to all of the ATOFMS data, and following the merger of the roughly 5000 chemical species concentration measurements made via manual samplers into a consolidated array of the data.
Future Plans: The remaining research to be conducted involves data analysis and preparation of journal article manuscripts that describe: the experimental design and particle evolution as seen by bulk chemical measurements made along the air parcel trajectories, the calibration of the particle counting efficiency of the aerosol time of flight mass spectrometers, the calibration of the chemical species sensitivity of the aerosol time of flight mass spectrometers, and a description of the chemical evolution over time of the Los Angeles area particulate air pollution as seen by the aerosol time of flight mass spectrometers.
Key Personnel
Post Doctoral Scholar: Jonathan
Allen
Graduate Students: Lara Hughes and Michael Kleeman
Undergraduate Student: Robert Johnson
Ecosystem Protection/Environmental Exposure & Risk, Air, Geographic Area, Scientific Discipline, Waste, RFA, Atmospheric Sciences, Fate & Transport, particulate matter, Environmental Chemistry, Monitoring/Modeling, Environmental Monitoring, State, aerosols, atmospheric transformation, California (CA), urban environment, contaminant transport models, chemical characteristics, fate and transport, urban air, aerosol time-of-flight mass spectrometry (ATOFMS), emissions, real-time monitoring, urban air , chemical kinetics, size distribution, particulates, atmospheric organic aerosols, atmospheric chemistry, gas to particle conversion, organic contaminants, ambient aerosol, organics
Progress and Final Reports:
Original Abstract
Main Center Abstract and Reports:
R824970 EERC - Center for Airborne Organics (MIT)
Subprojects under this Center:
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R824970C001 Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion
from Spark-ignition Engines
R824970C002 Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions
R824970C003 Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase
Photooxidation and Gas-to-Particle Conversion
R824970C004 Mathematical Models of the Transport and Fate of Airborne Organics
R824970C005 Elementary Reaction Mechanism and Pathways for Atmospheric Reactions
of Aromatics - Benzene and Toluene
R824970C006 Simultaneous Removal of Soot and NOx from the Exhaust of Diesel Powered
Vehicles
R824970C007 Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic
Aromatic Compounds
R824970C008 Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons
as Alternate Motor Fuels and Additives
R824970C009 Markers for Emissions from Combustion Sources
R824970C010 Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
R824970C011 Microengineered Mass Spectrometer for in-situ Measurement of Airborne
Contaminants