Grantee Research Project Results
2008 Progress Report: Production of Secondary Organic Aerosol from Multiphase Terpene Photooxidation
EPA Grant Number: R833750Title: Production of Secondary Organic Aerosol from Multiphase Terpene Photooxidation
Investigators: Shepson, Paul
Institution: Purdue University
EPA Project Officer: Chung, Serena
Project Period: November 1, 2007 through October 31, 2010 (Extended to October 31, 2011)
Project Period Covered by this Report: November 1, 2007 through October 31,2008
Project Amount: $333,397
RFA: Sources and Atmospheric Formation of Organic Particulate Matter (2007) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Particulate Matter , Air
Objective:
The objective of this research is to improve our quantitative and mechanistic understanding of the production of secondary organic aerosol, an important type of air pollutant, from the atmospheric oxidation of α- and β-pinene. We aim to determine product yields for major gas phase OH- and O3-induced oxidation products with much smaller uncertainty bounds than have previously been reported, thereby improving the capability of air quality models that simulate aerosol production from BVOCs. We will study the oligomerization of aerosol phase species, and study the extent to which photochemistry in aerosols and in cloud water contributes to secondary organic aerosol production. The information produced from the linked laboratory and field studies will be used to develop improved computer model modules that describe secondary organic aerosol from these important terpenes.Progress Summary:
1. Development and Testing of the Proton Transfer Reaction Linear Ion Trap (PTRLIT). An objective of this work is to employ a newly developed technology, the PTRLIT, to studies of terpene oxidation and the nature of oxidation products that may undergo gas-to-particle conversion. While the PTRLIT was developed with methyl vinyl ketone (MVK) and methacrolein (MACR) as test cases, it had not been previously evaluated for terpenes and their oxidation products. We have completed a series of experiments in which we evaluated the ability to trap and selectively detect terpene isomers through collision-induced-dissociation (CID) experiments. The PTRLIT was intercompared with a conventional triple quadrupole mass spectrometer. We find that the CID spectra are very similar in the two cases. However, the PTRLIT enables ion chemistry in the trap for pursuit of improved selectivity. These results are described in our manuscript, Müller et al., 2009.
During the summer of 2008, we conducted measurements of a variety of VOCs in the atmosphere above the mixed deciduous/coniferous forest at the University of Michigan Biological Station (UMBS), using the PTRLIT. Although the data are still being evaluated and analyzed, it is clear that we will produce quantitative measurements at high time resolution (~15 min.) for isoprene, MVK, MACR, acetone, toluene, and the terpenes. These data will establish the PTRLIT as a new method that takes the PTRMS instrument to the next level in selectivity and sensitivity.
2. Photochemical reaction chamber studies of α-pinene oxidation. We have been conducting a series of experiments in which α-pinene oxidation is studied in our 5500 liter all-PFA photochemical reaction chamber. Thus far, these have involved traditional VOC/NOx irradiations, during which NO, NO2, NOy, total PANs, and total organic nitrates have been measured using our newly developed chemiluminescence-based NOx instrument, that enables measurement of total PANs and total organic nitrates using a thermal decomposition (TD) inlet that converts the nitrate species to NO2, followed by photolysis to produce NO, which is then detected. O3 is measured with traditional UV absorption instruments, and particle size distributions and number are determined using our Scanning Mobility Particle Spectrometer (SMPS), purchased through this grant. Thus far, we have been focused on determination of the terpene nitrate yields (RONO2), as many recent publications have reported evidence of nitrate functionalities in ambient aerosol from forest-impacted air masses. We have synthesized the “terpene nitrates” (TNs) that originate from OH radical addition across the double bond, so that these (two) specific isomers can be quantitatively determined, using GC/ECD methods. However, a significant fraction of OH reaction with α-pinene occurs via H-atom abstraction from saturated carbon atoms; the total RONO2’s can be determined with our TD method, while the TNs can be determined by GC/ECD. Thus those that arise from abstraction will be determined by difference. Preliminary indications are that the total yield is in the middle of the range of the two existing published reports. These irradiations are providing a starting point for the development of a computer model of the gas and aerosol-phase chemistry.
We have also begun sampling the aerosol produced (mostly of which arises from ozonolysis) through filter samples, which are being analyzed using Desorption Electrospray Ionization Mass Spectrometry. Once specific products are identified, we will use these samples to compare to filters samples acquired from the UMBS site, to examine evidence for α-pinene impact on aerosol production in that environment.
3. Analysis of the mechanisms for methacrolein oligomerization in the presence of H2SO4. We have been studying the nature of the oligomer produced in the condensed phase reaction of methacrolein with itself, in the presence of H2SO4, as a simple model system involving a multifunctional atmospheric OVOC. DESI-MS analysis of the oligomeric material shows evidence for products arising from Diels-Alder cycloaddition reactions, which have not been previously reported for atmospheric SOA. In addition, H2SO4 acts to initiate oxidation and hydrolysis reactions that influence the composition of the aerosol. Interestingly, there are also H2 addition reactions (i.e. reduction) simultaneously occurring in the oligomeric mixture. Several structures and mechanisms have been identified/developed.
At the same time, we used DESI-MS and electrospray-MS to analyze cloud water samples acquired from our ALAR aircraft and cloud water collector. We found evidence for oligomeric material in the cloud water samples, and evidence for organo-sulfates in the oligomers. These findings are reported in the manuscript by grad. student Marc Fiddler in Fiddler et al., 2009.
4. Studies of aerosol production at the UMBS. During the summer of 2008, graduate student Nate Slade conducted aerosol measurements at UMBS, using the SMPS instrument. Number and Size distributions between 15nm and 750nm were measured from ~10meters above the forest canopy, from a small enclosure, and short inlet line, that enabled vertical profile measurements of aerosols above the forest canopy. Several instances of particle nucleation were observed at this site. These measurements were conducted simultaneously with other VOC and OVOC measurements, e.g. for terpenes, MVK and MACR, HCHO, and glyoxal (in collaboration with the Keutch group at U. Wisc.), as well as OH, HO2, NOy species, and meteorological variables (and CO2 fluxes). We are in the process of investigating the data, in pursuit of the hypothesis that terpene oxidation can contribute to nucleation events, in clean air conditions. A workshop was conducted at Purdue on Dec. 10, 2008, to begin the process of collaborative data analysis. All data from PROPHET2009 will soon be available on the PROPHET web site.
Future Activities:
1. Analysis of PROPHET2008 data from the UMBS site, and preparation of manuscripts.
2. Measurements at UMBS in the summer of 2009. Our plans for 2009 involve collection of aerosol samples on quartz fiber filters and cascade impactor stages for DESI-MS analysis, for comparison to smog chamber experiments with α-pinene.
3. Smog chamber experiments. These will include a comparison of the condensed phase analysis of aerosol produced in the presence and absence of NOx, to examine the importance of ozonolysis products, and organic nitrates. Detailed product yield measurements will continue.
4. Chemical mechanism formulation. We are in the process of formulation of an explicit and detailed mechanism for gas phase chemistry, and gas to particle conversion in the α-pinene system.
Journal Articles:
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Progress and Final Reports:
Original AbstractThe 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.