Grantee Research Project Results
2005 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, 2004 through August 31,2005
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: Particulate Matter , Air Quality and Air Toxics , Air
Objective:
The objectives of this research project are to: (1) determine the yields of reaction products forming secondary organic aerosol (SOA) from biogenic and anthropogenic precursors under typical atmospheric conditions, including variations of temperature and volatile organic compounds (VOCs) and nitrogen oxides (NOx); (2) develop and/or revise computationally efficient mechanisms capable of reproducing the observed SOA production in air quality models; (3) discover, using these new mechanisms, the relative contribution of biogenic and anthropogenic emissions to SOA yields in different regions of the United States; and (4) test control strategies for fine particulate matter (PM2.5) in light of these findings, with particular emphasis on multiobjective behavior given observed VOC/NOx variations in SOA yields.
Progress Summary:
The past year has been transformative. Research on this project has led to a new understanding of organic particle behavior in the atmosphere. This advance has drawn on support from other projects, including Science To Achieve Results (STAR) research on primary emissions (Robinson, Donahue, and Adams) as well as National Science Foundation (NSF) support covering hygroscopicity of organic particles (Pandis and Donahue) and the oxidation of isoprene (Donahue, principle investigator [PI]). The end product is a theoretical framework for the formation and transformation of organic particles, coupled with the realization that essentially all organic particulate matter is semivolatile. Initially, this realization challenges the dual notion of secondary and primary organic aerosol. We have developed a framework to describe this behavior that facilitates understanding of semivolatile partitioning and also points to efficient modeling of the partitioning, including evolution of volatility because of progressive chemical transformation during long-range transport of semivolatile material. Extensive publication of both experimental and modeling results is in progress.
Objective 1: Ongoing Experiments Have Addressed SOA Generation From Ozone—Terpene Reactions Including: α-pinene, β-pinene, δ-limonene, α-humulene, β-caryophyllene
Experiments have focused on variations in the aerosol mass fraction (aerosol mass produced/precursor mass oxidized), measured with a scanning mobility particle size spectrometer in a 10 m3 Teflon environmental chamber. Experiments during the past year were substantially enhanced by an Aerodyne Aerosol Mass Spectrometer and an Ionocon Proton Transfer Mass Spectrometer (PTRMS) obtained with an NSF Major Research Instrumentation (MRI) grant. These instruments enabled specific measurements of aerosol parameters and continuous measurement of vapor-phase compounds, including removal of the precursor. Experimental parameters varied include (independently): amount of oxidized precursor, oxidant (ozone) level, oxidation temperature, chamber temperature following oxidation, and NOx levels (NO and NO2), with and without supplemental ultraviolet (UV) illumination. Results are summarized in the following paragraphs.
SOA product formation can be interpreted in terms of a “basis set” of product saturation concentrations spanning the full range of organic aerosol concentrations observed in the atmosphere (0.01 µg m-3 for a “nonvolatile” product to 1,000 µg m-3 for a “volatile” product, with intermediate volatility products separated by factors of 10). Semivolatile partitioning follows according to the well-established treatment of Pankow. The basis-set representation puts all organic aerosols on a common basis, thus permitting a complete representation of semivolatile partitioning with five to seven highly constrained parameters, as opposed to the traditional “two-product” formulation in which each precursor produces SOA products with four unconstrained degrees of freedom. This permits:
- Uniform representation of semivolatile organics in models.
- Consistent evaluation of experimental data.
- Easy separation of source fractions for source attribution.
- Direct representation of temperature effects with realistic ΔHvap.
- Conceptualization and formal representation of aging effects as a result of secondary chemistry in both the vapor and condensed phases.
This last item will transform thinking and the representation of organic aerosol, including both SOA (the subject of this project) and primary emissions (the subject of a second STAR project, Robinson, PI). In areas dominated by regional emissions, organic aerosol will not be represented by static “yields” or “emissions factors” but rather by emission of chemically active compounds whose products and thus whose partitioning evolves throughout long-range transport from the source to the receptor site.
Low-volatility, first-generation products revealed in the basis-set representation increase SOA mass production by about a factor of two over the current state-of-the-art two-product models. This is especially true for total organic aerosol of 5 µg m-3 and below, which is the typical average loading for regions in noncompliance with the 15 µg m-3 annual-average National Ambient Air Quality Standards. Also, it is germane to global modeling studies where models significantly underpredict observed organic aerosol levels. Chamber data using the Proton Transfer Reaction Mass Spectrometer (PTRMS) to measure terpene consumption in real time give us the first experimental results to constrain significantly the 0.1 – 10 µg m-3 region of semivolatile partitioning. The practical consequence of these findings will be increased SOA production in environments with relatively low organic aerosol concentrations.
SOA production from ozone-terpene reactions is reduced in the presence of UVA light. A basis-set analysis of experimental data extending to low COA shows a 0.03 reduction in the mass yield of a low-volatility product with a saturation concentration of 1 µg m-3 for low-NOx conditions only. The practical consequence of these findings is that terpene SOA production in the atmosphere may be significantly lower than suggested by product yields based on parameterizations derived from experiments conducted in dark chambers.
SOA production from terpenes depends strongly on VOC/NOx in experiments that isolate the ozone-terpene reaction, with a significant decrease in SOA production for α-pinene occurring as NOx levels increase beyond a VOC/NOx of approximately 8:1 (the traditional spine on the ECMA ozone isopleth graph). A basis-set analysis of these data is consistent with very low product yields with saturation concentrations less than 100 µg m-3 for high-NOx conditions. Experimental data can be reproduced accurately with a simple mixing model based on RO2 radical branching. The practical consequence of these findings is that NOx control strategies will have a dramatic effect on SOA arising from ozonolysis of biogenic precursors(monoterpenes).
Limonene shows much more complicated chemistry:
- Oxidation of both double bonds produces much lower vapor pressure products and consequently much more SOA than α-pinene. Oxidation of the internal double bond only yields products and SOA very similar to α-pinene.
- At low NOx, the initial gas-phase ozone reaction appears to be rate limiting. Evidence points to oxidation of the second double bond occurring in the condensed phase with a high ozone uptake coefficient. It is far from clear, however, that this second oxidation will occur so rapidly on actual atmospheric particles.
- SOA production increases once both double bonds are oxidized at high NOx but acts much like α-pinene with lowered SOA at high NOx when only the endocyclic double bond is oxidized. This appears to be because of competing effects—NOx products have higher vapor pressure than some of the organic acids produced at low NOx, but the added mass of the nitrate in organic nitrates increases the mass yield of products where low vapor pressures are obtained from oxidation at a different site (the other double bond). High NOx conditions also appear to dramatically slow the heterogeneous oxidation of the second double bond under chamber conditions.
- It still is unclear how oxidation of the second double bond will proceed in nature or how that will influence SOA production. It seems probable that a combination of homogeneous and heterogeneous processes, possibly dominated by OH radical, will be predominate. The practical consequence of these findings is that limonene may be a very potent SOA precursor, but that SOA may be formed with a substantial time delay following emission because of kinetic effects.
- The temperature dependence of SOA partitioning is modest at best. This can be explained within the basis-set formalism, which also shows that the temperature dependence of aged aerosol will be even lower than fresh SOA. The practical consequence of this finding is that models with only a few products and high enthalpies of vaporization will overestimate dramatically the temperature dependence of semivolatile partitioning.
- Two efficient OH radical sources have been developed. One is the traditional methyl nitrite photolysis source, whereas the other relies on ozonolysis of tetramethylethylene (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 in the coming 18 months on OH-initiated oxidation of anthropogenic precursors.
Objective 2: Parameterizations for Air Quality Models
The basis-set formalism described earlier will transform organic aerosol modeling. We are in the process of modifying the aerosol modules in the Particulate Matter Comprehensive Air Quality Model with extensions (PMCAMx) to suit the basis set (this is fairly straightforward) and also to permit both gas- and condensed-phase oxidation of the semivolatile species carried in the model.
Modeling shows that isoprene SOA may comprise a significant portion of the total SOA in the southeast.
We have developed a new SOA module for PMCAMx in which various precursors, including terpenes and anthropogenic compounds such as xylenes, are unlumped, allowing assessment of the contribution of individual precursors to total SOA. A VOC/NOx dependence (from combined OH and ozone chemistry) is described in the parameterization.
Objective 3: Elucidation of the Biogenic/Anthropogenic Contribution to SOA
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.
Consideration of SOA ultimately will require a full life-cycle consideration of aging. This task will extend well beyond the current project. A preliminary assessment, however, supports the “fallacy of the polluting tree.” Specifically, semivolatile partitioning of biogenic vapors only occurs in the presence of a substantial substrate of anthropogenic low vapor-pressure material. It is thus inappropriate to call biogenic semivolatile vapors “biogenic SOA.” The practical consequence of this finding is that reductions in primary organic aerosol also will reduce the SOA fraction by causing higher vapor pressure material to evaporate.
Six peer-reviewed papers were published during the reporting period, one more is in press, five are under review, and three are on the verge of submission. At least three further manuscripts will be submitted in the next few months, and six more should emerge from the final 6 months of work on the project, for a total of approximately 24 peer-reviewed publications for the project.
At least 15 presentations were given during the past year, including substantial elements derived from this project. These included invited talks by the PI in several international meetings, the Atmospheric Chemistry Gordon Conference, and a major environmental health conference in Pittsburgh.
Future Activities:
During Year 3 of the project, we will focus on the following objectives:
- Fully incorporating our basis-set representation of SOA formation and transformation into regional air quality models.
- Continuing experiments into SOA formation from OH-initiated reactions.
- Developing “one-stop shop” representations of SOA formation from major precursors and oxidants for modeling studies.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
| Other project views: | All 58 publications | 20 publications in selected types | All 20 journal articles |
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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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Donahue NM, Robinson AL, Stanier CO, Pandis SN. Coupled partitioning, dilution, and chemical aging of semivolatile organics. Environmental Science & Technology 2006;40(8):2635-2643. |
R831081 (2005) R831081 (Final) R832162 (2005) R832162 (2006) R832162 (Final) |
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Huff Hartz KE, Rosenorn T, Ferchak SR, Raymond TM, Bilde M, Donahue NM, Pandis SN. Cloud condensation nuclei activation of monoterpene and sesquiterpene secondary organic aerosol. Journal of Geophysical Research--Atmospheres 2005;110(D14):D14208 (8 pp.). |
R831081 (2005) R831081 (Final) |
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Pathak RK, Stanier CO, Donahue NM, Pandis SN. Ozonolysis of α-pinene at atmospherically relevant concentrations: temperature dependence of aerosol mass fractions (yields). Journal of Geophysical Research--Atmospheres 2007;112(D3):D03201 (8 pp.). |
R831081 (2005) R831081 (Final) |
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Presto AA, Huff Hartz KE, Donahue NM. Secondary organic aerosol production from terpene ozonolysis. 1. Effect of UV radiation. Environmental Science & Technology 2005;39(18):7036-7045. |
R831081 (2005) R831081 (Final) R832162 (Final) |
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Presto AA, Huff Hartz KE, Donahue NM. Secondary organic aerosol production from terpene ozonolysis. 2. Effect of NOx concentration. Environmental Science & Technology 2005;39(18):7046-7054. |
R831081 (2005) R831081 (Final) R832162 (Final) |
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Presto AA, Donahue NM. Investigation of α-pinene + ozone secondary organic aerosol formation at low total aerosol mass. Environmental Science & Technology 2006;40(11):3536-3543. |
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, particulate matter, PM2.5, transport modeling, air quality,, RFA, Air, Ecosystem Protection/Environmental Exposure & Risk, Scientific Discipline, particulate matter, Air Quality, Environmental Chemistry, Engineering, Chemistry, & Physics, Monitoring/Modeling, Air Pollution Effects, air toxics, Environmental Engineering, Environmental Monitoring, aerosol particles, health effects, particle size, air sampling, carbon particles, emissions, chemical characteristics, air quality modeling, measurement methods, airborne particulate matter, thermal properties, atmospheric particles, modeling studies, particulate matter mass, air quality model, particulate organic carbon, carbon aerosols, aerosol analyzers, transport modeling, air quality models, atmospheric particulate matter, particle phase molecular markers, ultrafine particulate matter, chemical speciation sampling, particle size measurement, particle dispersion, air modelingRelevant Websites:
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.