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1997 Progress Report: Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion

EPA Grant Number: R824970C003
Subproject: this is subproject number 003 , 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: Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion
Investigators: Seinfeld, John , Flagan, Richard
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 objective of this project is to gain a better fundamental understanding of the atmospheric oxidation of volatile organic compounds (VOCs) important in urban and regional air quality.  specific aims are to determine the gas-phase mechanisms of reaction of important VOCs with the hydroxyl radical, the atmosphere's most ubiquitous oxidizing species, and to elucidate the mechanisms of formation of organic aerosols from the atmospheric oxidation of VOCs. 

Rationale: Gas-to-particle conversion is a ubiquitous process in the atmosphere, determining the size and composition of particles from the polluted urban atmosphere to the remote marine boundary layer.  Understanding the detailed chemistry and physics of atmospheric gas-to-particle conversion will allow us to predict the effects of primary gaseous and particulate emissions changes on airborne particulate matter, in the urban and regional setting, and the effects of sulfur and other species on the generation of cloud condensation nuclei in the remote atmosphere.  A principal goal of the research program is the development of comprehensive air quality models based on the most complete description of atmospheric chemistry and physics.  These models are forerunners of those that will eventually be used in the regulatory process.  This research is aimed at developing the organic portion of advanced gas-aerosol models, and to advance the current state of understanding of molecular processes.  The component of the proposed research on gas-phase photooxidation chemistry has the goals of adding to the body of kinetic and mechanistic data for atmospheric organics, with particular emphasis on those VOCs that are potential aerosol precursors. 

Approach:  The integrated research program to determine the mechanisms of photooxidation and secondary organic aerosol formation in the atmosphere for a number of important anthropogenic and biogenic hydrocarbons is carried out in the Caltech outdoor reactor and via ab initio molecular simulation.  Experiments in the indoor reactor are used to probe chemical mechanisms.  The large outdoor smog chamber is employed to study the integrated gas-phase and gas-to-particle conversion dynamics. 

Status:  A. Ab Initio Studies of Atmospheric Reaction Mechanisms 

Atmospheric reactions of the butoxy radical (C4H100) has been the subject of numerous investigations in recent years.  A key intermediate in the photooxidation of n-butane in the troposphere, the 1-butoxy radical is the simplest alkoxy radical that can undergo isomerization via 1,5-H shift to create a d-hydroxyalkyl radical forming numerous subsequent products.  1-Butoxy and 2-butoxy are thus representative of a large class of alkoxy radicals that are produced by the photooxidation of >C4 aliphatic hydrocarbons in the troposphere and whose fate determines the ultimate end products of these reactions. 

The atmospheric oxidation mechanism of n-butane has been investigated by means of density functional theory and ab initio calculations.  Calculation of energies of reactants, transition states, and stable intermediates predicts the detailed pathways leading to experimentally observed products of n-butane oxidation.  Also serving as a  model system for the oxidation of larger alkanes, quantitative information is obtained for elementary reaction steps that heretofore have been subject to speculation.  Complete basis set model chemistries CBS-4 and CBS-q were used with B3LYP/6-31G(d,p) optimized geometries to calculate energies of over 70 stable species and transition states.  Energies based on density functional theory were obtained at the B3LYP/6-311+G-(3df,2p)//B3LYP/6-31G(d,p) level of theory.  The principal pathway following formation of the 1-butyl radical from hydroxyl (OH) attack on n-butane is found to be 1,5-H shift of the 1-butoxy radical.  After conversion to the d-hydroxy-1-butoxy radical, another 1,5-H shift is expected to be the primary route to 4-hydroxy-1-butanal.  4-Hydroperoxy-1-butanal can be formed after 1,6-H shift in chemically activated 4-hydroxy-1-butylperoxy radicals.  Whereas b-scission in 1-butoxy is an endothermic process, fragmentation of 2-butoxy into C2H5 and CH3CHO is predicted to be the major degradation pathway of the secondary butyl radicals. 

B. Secondary Organic Aerosol Formation 

A series of experiments was performed in an outdoor, photochemical smog chamber in order to investigate the aerosol forming potentials of biogenic hydrocarbons.  One class of these compounds, monoterpenes (C10H16), can be classified according to one of three structure types:  bicyclic monoolefins, cyclic diolefins, and acyclic triolefins.  A number of compounds of each type were investigated.  In addition, two sesquiterpenes (C15H24) and the two oxygenated monoterpenes were examined.  Data were used to explore the relationship between the noted structural differences in these biogenic compounds and their secondary organic aerosol yields.  A series of experiments was also run in a dark system in the presence of ozone or nitrate radicals in order to identify the primary oxidant responsible for secondary organic aerosol formation. 

Future Plans:  Plans include continuing ab initio studies of atmospheric reaction mechanisms, interacting with the group of Joseph Bozzelli where appropriate.  Also, outdoor smog chambers studies of organic aerosol formation from biogenic hydrocarbons and data analysis with respect to molecular characteristics of the hydrocarbons will be completed, and application of three-dimensional modeling of gas/aerosol behavior in South Coast Air Basin will continue. 

Key Personnel 
Postdoctoral Fellow:  Tim Jungkamp
Graduate Students:  Jay Odum, Robert Griffin, and David Cocker 
CAO Collaborators:  Joe Bozzelli and Larry Lay (NJIT) 

Supplemental Keywords:

Ecosystem Protection/Environmental Exposure & Risk, Air, Scientific Discipline, Waste, RFA, Physics, Atmospheric Sciences, Fate & Transport, particulate matter, Environmental Chemistry, atmospheric transformation, ambient air quality, chemical characteristics, fate and transport, Volatile Organic Compounds (VOCs), chemical kinetics, air quality models, particulates, gas to particle conversion, atmospheric transport, particle transport

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

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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