Grantee Research Project Results
2004 Progress Report: Atmospheric Processing of Organic Particulate Matter: Formation, Properties, Long Range Transport, and Removal
EPA Grant Number: R831081Title: Atmospheric Processing of Organic Particulate Matter: Formation, Properties, Long Range Transport, and Removal
Investigators: Donahue, Neil , Davidson, Cliff I. , Pandis, Spyros N. , Robinson, Allen , Adams, Peter
Institution: Carnegie Mellon University
EPA Project Officer: Chung, Serena
Project Period: September 1, 2003 through August 31, 2006
Project Period Covered by this Report: September 1, 2003 through August 31, 2004
Project Amount: $449,994
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter
Objective:
The objectives of this research project are to:
- determine yields of secondary organic aerosol (SOA) from biogenic and anthropogenic precursors under typical atmospheric conditions, including variations of temperature and volatile organic compound (VOC)/nitrogen oxides (NOx);
- develop and/or revise computationally efficient mechanisms capable of reproducing the observed SOA yields in air quality models;
- discover, using these new mechanisms, the relative contribution of biogenic and anthropogenic emissions to SOA yields in different regions of the United States;
- and using the results of these findings, test control strategies for PM 2.5 with particular emphasis on multi-objective behavior given observed VOC/NOx variations in SOA yields.
Progress Summary:
Work to date has focused on the first two objectives.
Objective 1: Determination of SOA Yields
Ongoing experiments have addressed SOA generation from ozone-terpene reactions including: a-pinene, b-pinene, d-limonene, a-humulene, and b-caryophyllene. Experiments have focused on variations in mass yield, measured with a scanning mobility particle size spectrometer in a 10-m3 Teflon environmental chamber. Parameters varied (independently) include: amount of oxidized precursor, oxidant (ozone) level, oxidation temperature, chamber temperature following oxidation, and NOx levels , with and without supplemental ultraviolet (UV) illumination.
The volatility of terpene SOA is lower than previously anticipated in two significant ways. The temperature dependence of SOA yields is shallow, consistent with an overall heat of vaporization approximately 30 kJ/mole rather than the typically assumed 110 kJ/mole, and the traditional 2-component models cannot simultaneously fit available data combining temperature and the amount of oxidized precursors. At least three constituents are needed, and one of these has a very low vapor pressure. The practical consequence of these findings is that SOA yields are larger at low total oxidized terpene levels than predicted in current SOA formation mechanisms and that changes with temperature are slight (true yields will be higher at high temperature than in current models but also lower at low temperature).
SOA yields from ozone-terpene reactions are reduced in the presence of UV-A light. The reduction is roughly constant at 3 percent of the total oxidized precursor (meaning that the fractional reduction in SOA yields is very large for small total precursor levels). There is a corresponding delay in particle nucleation in experiments conducted without an aerosol seed. Gas chromatography/mass spectrometry (GC/MS) samples reveal an altered product distribution, with higher volatility products favored in the presence of UV light. Our conclusion is that a key species is photolabile in the gas phase. This is either an intermediate lost to photolysis that changes the condensable product yields or some condensable product itself that is lost to photolysis. The practical consequence of these findings is that terpene SOA yields in the atmosphere may be significantly lower than yields based on parameterizations derived from experiments conducted in dark chambers.
SOA yields from terpenes depend strongly on VOC/NOx in experiments that isolate the ozone-terpene reaction, with a significant decrease in yields occurring as NOx levels increase beyond a VOC/NOx of approximately 8:1 (the traditional spine on the Eckma ozone isopleth graph). This effect is not in any current SOA mechanisms, which have a NOx dependence only from NO 3 and from reactions following OH-initiated oxidation. SOA generation may be completely eliminated for VOC/NOx less than 5:1. FTIR filter samples reveal substantial production of organic nitrates at high NOx, with features that are preferentially removed when the chamber is heated, showing that these nitrates are more volatile than the compounds they replace. The practical consequence of these findings is that NOx control strategies may have a dramatic effect on SOA arising from ozonolysis of biogenic precursors (monoterpenes).
Experiments have detailed the variation in SOA yields with initial oxidation temperature (as opposed to the post oxidation temperature dependence discussed above). Preliminary analysis shows that yields appear to be independent of the oxidation temperature above 25°C (after correction for volatility by returning the chamber temperature to 25°C). Below 25°C, yields appear to decrease sharply. Preliminary interpretation is that a bimolecular reaction becomes important at low temperatures, where unimolecular decomposition or isomerization of some intermediate becomes slow. The practical consequence of these findings remains unclear, as the putative bimolecular chemistry has not been proven and may or may not extend from the environmental chamber to atmospheric conditions. At a minimum, however, this will be important for interpretation of chamber experiments.
Two efficient OH radical sources have been developed. One is the traditional methyl nitrite photolysis source, whereas the other relies on ozonolysis of tetra-methyl ethylene (TME). The second method shows great promise as a steady, high-yield source capable of maintaining a steady OH production rate with control based on a steady, small flow of TME into the chamber as well as the ozone concentration. These two sources will enable experiments on OH-initiated oxidation of anthropogenic precursors.
Objective 2: Parameterizations for Air Quality Models
We have developed a new yield parameterization for SOA from the ozone and a-pinene reaction, incorporating both the extent of a-pinene oxidation and temperature. This requires a minimum of three model condensable species. The practical consequence of this new parameterization will likely be an increase in model SOA generation under typical ambient conditions.
We have developed a new SOA module for PMCAMx in which various precursors, including terpenes and antropogenic compounds such as xylenes are unlumped, allowing assessment of the contribution of individual precursors to total SOA. VOC/NOx dependence (from combined OH and ozone chemistry) is described in the parameterization. Model runs based on Los Angeles conditions show a large majority (95%) of SOA arising from anthropogenic precursors for typical Los Angeles conditions. The practical consequence of this work will be an improved ability to assess the relative contributions of anthropogenic and biogenic emissions to SOA production.
Future Activities:
In Year 2 of the project, we will focus on:
- Conducting SOA yield experiments initiated by OH oxidation of precursors typical of anthropogenic emissions. These experiments will include the parameter variations described above for ozone-initiated reactions.
- Incorporating the full range of experiments reported above into new SOA yield parameterizations.
- Conducting preliminary simulations for the East-coastal model domain developed for interpretation of eastern supersite findings.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 58 publications | 20 publications in selected types | All 20 journal articles |
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Type | Citation | ||
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Donahue NM, Huff Hartz KE, Chuong B, Presto AA, Stanier CO, Rosenhorn T, Robinson AL, Pandis SN. Critical factors determining the variation in SOA yields from terpene ozonolysis:a combined experimental and computational study. Faraday Discussions 2005;130:295-309. |
R831081 (2004) R831081 (2005) R831081 (Final) |
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Supplemental Keywords:
air quality modeling, analytical chemistry, atmospheric sciences, environmental monitoring, aerosol analyzers, aerosol particles, air sampling, air toxics, airborne particulate matter, atmospheric particulate matter, chemical speciation sampling, emissions, measurement methods, particle dispersion, PM 2.5, transport modeling, air quality,, RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, particulate matter, air toxics, Environmental Chemistry, Air Pollution Effects, Monitoring/Modeling, Environmental Monitoring, Engineering, Chemistry, & Physics, Environmental Engineering, carbon aerosols, air quality modeling, particle size, atmospheric particulate matter, health effects, particulate organic carbon, aerosol particles, atmospheric particles, chemical characteristics, PM 2.5, air modeling, air quality models, airborne particulate matter, air sampling, carbon particles, air quality model, emissions, particulate matter mass, ultrafine particulate matter, particle phase molecular markers, transport modeling, modeling studies, thermal properties, particle dispersion, aerosol analyzers, measurement methods, chemical speciation samplingRelevant Websites:
http://airquality.web.cmu.edu Exit
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.