Development of Gas-Phase Atmospheric Chemical Mechanisms

1. Development of Gas-Phase Atmospheric Chemical Mechanisms Overview • Mechanism development objectives, evaluation, levels of detail • SAPRC mechanisms: components, versions, updates • Recommendations and thoughts on the future William P. L. Carter University of California, Riverside, CA October 27, 2014 Gas-Phase Atmospheric Mechanisms

2. Gas-Phase Atmospheric Chemical Mechanisms • Used to represent gas-phase reactions in air quality models to predict formation of secondary pollutants from emissions. • Such models needed for the development of efficient control strategies to reduce O3, and other secondary pollutants • Current gas-phase mechanisms were developed and tested primarily for predicting ozone and other oxidants. • Representing gas-phase processes leading to particle formation is now also a priority • Reactions of hundreds of emitted compounds and many thousands of intermediates and products need to be represented. • Condensation necessary for practical applications. • Mechanisms differ in condensation levels and approach • Mechanisms have many uncertainties, assumptions and estimates. Predictions need to be tested using observations. Gas-Phase Atmospheric Mechanisms

3. Mechanism Development Objectives • Predictive capability • First priority for mechanisms for regulatory models. • Requires evaluation of predictions against measurement data representing environments to be modeled • Consistency with accepted laboratory data and theories • First priority for research mechanisms • Necessary for scientific credibility • Reduces chance of compensating errors • Sometimes consistency with accepted data is in conflict with predictive capability for environmental chamber experiments • Appropriate condensation for the modeling application • Too much condensation limits utility and accuracy • Too much detail wastes resources and may give illusion of accuracy that does not exist Gas-Phase Atmospheric Mechanisms

4. Evaluating Predictive Capability • Conditions used to obtain the evaluation data must be less uncertain than the mechanism being evaluated • Environmental chamber data provide the most practical test of mechanisms without emissions and meteorological uncertainties • Has uncertainties and disadvantages: uncertain chamber effects, artificial conditions, etc. • Some important uncertain parameters in current mechanisms have to be adjusted to fit chamber data • Ideally, predictive mechanism development should be closely linked to conducting and using chamber experiments • Ambient measurements also provide important tests to predictive capability • But uncertainties in ambient conditions limit utility for mechanism development and adjustment Gas-Phase Atmospheric Mechanisms

5. What Is Adjusted to FitEnvironmental Chamber Data? • Mechanism adjustment is not a “curve fitting” exercise. Mechanisms are not really “derived” from chamber data. • If changing one parameter does not fit the O3 or NOx curves, usually it means that you have a missing or wrong reaction. • Sometimes poor fits caused new reactions to be proposed that are subsequently confirmed by laboratory studies • Chamber data can be used to chose between chemically reasonably alternatives • Usually the adjustment is made to improve fits to rates of NO oxidation and O3 formation. Usual adjustments: • RONO2 yield from RO2 + NO (VOCs with no radical sources) • Photoreactive product yields or kphot’s (aromatics) • Radical yields in O3, O3P reactions (alkenes) Gas-Phase Atmospheric Mechanisms

6. Cases where Predictive Capability and Accepted Data and Theories Conflict Historical examples (resolved by subsequent studies) • The need for a chamber radical source to model chamber experiments had no scientific credibility when first proposed • Subsequently found to be due to HONO offgasing • Publication of early Carter mechanisms were delayed because it chamber data not fit using the accepted HO2 + NO rate constant • Resolved when a more direct measurement that gave a higher rate constant was published in 1977 Unresolved Current examples: Chamber data indicate that: • Radical yields in O3 + 1-alkene reactions lower than indicated by laboratory studies • NOx levels where aromatic reactivity depend on NOx are much lower than indicated by product yield data Gas-Phase Atmospheric Mechanisms

7. Problem with Radical Yields inOzone + Alkene Reactions • Radical yields in O3 + 1-alkene reactions that fit chamber data are lower than accepted in current literature • Problem most evident with the C4+ 1-alkenes • Current SAPRC mechanisms use the lower yields that fit the chamber data • MCM uses higher yields that overpredict O3 formation rates in chamber experiments Gas-Phase Atmospheric Mechanisms

8. Plots of ([O3]‑[NO]) Model Errors vs. NOx for Benzene and Toluene Chamber Experiments NO2-dependent reaction not important at NO2 < ~1 ppm, consistent with laboratory data NO2-dependent reaction adjusted to be important at lower NO2 than consistent with laboratory data Benzene ([O3]‑[NO]) Model Error Toluene Initial NOx (ppb) Gas-Phase Atmospheric Mechanisms

9. Examples of Widely Used Gas-Phase Atmospheric Chemical Mechanisms Carbon Bond (CB) Mechanisms (Latest: CB05, CB05-TU, CB6) • Most widely used for regulatory modeling in U.S. • Relatively condensed for computational efficiency RADM and RACM Mechanisms (Latest: RACM, RACM2) • Developed for regional modeling in U.S. and Europe. • Less condensed than Carbon Bond SAPRC Mechanisms (Latest: SAPRC07, SAPRC11) • Used for regional modeling and reactivity scales • Varying condensation depending on application Master Chemical Mechanism (MCM) (Current: MCMv3.2) • Mostly used in Europe in trajectory models, but also in U.S. • Least condensed; semi-explicit (1000’s of species) Gas-Phase Atmospheric Mechanisms

10. Levels of Detail in Mechanisms • Fully or Semi-Explicit (e.g., MCM, Generated mechanisms) • Necessary for incorporating full sum of knowledge in models • Not necessarily advantageous for highly uncertain systems • Requires estimation methods for all possible reactions and database of known rate constants and branching ratios • Practicality requires at least some simplifications • Explicit VOC, Lumped product (e.g., detailed SAPRC) • Ideally based on explicit mechanisms where possible • More appropriate for VOCs with very uncertain mechanisms • Suited for deriving reactivity scales and lumped mechanisms • Lumped VOC (e.g., lumped SAPRC, CB’s, RACM, etc.) • Ideally based on more detailed mechanisms • Often best balance between modeling efficiency and detail • Most widely used in airshed models in the U.S. Gas-Phase Atmospheric Mechanisms

11. Condensed Mechanisms andPredictive Capability Condensation has consequences for model predictions • Limits ability to compare model with measurement data. • Lumping methods based on O3 impacts are usually not appropriate for SOA prediction. • Lumping slowly reacting compounds with different reaction rates affects predicted transport and multi-day effects. Cannot be adequately evaluated using chamber data • Cannot test for individual compounds. Evaluating with mixture experiments of limited utility Not all condensation approaches are compatible with current scientific understanding. Examples include: • “Lumped structure” method (early CB mechanisms) – other parts of molecules significantly affect products and reactions. • Use of smaller compounds as basis of lumped species can’t reflect effects of molecule size on reactivity and SOA. Not all condensation approaches are compatible with current scientific understanding. Examples include: Gas-Phase Atmospheric Mechanisms

12. SAPRC Mechanisms • The SAPRC* mechanisms provide examples of chemically consistent mechanisms with varying levels of chemical detail. • SAPRC mechanism development began in the mid-1980’s and is continuing today. Latest complete version is SAPRC-07, but updates underway (SAPRC-11 has updated aromatics). • Currently used for research and regulatory modeling in California and the U.S. • More detailed versions used for ozone reactivity scales. • More condensed versions used for regional modeling. • Because of its varying levels of detail, the current state of SAPRC can give an indication of the state of gas-phase mechanism development in general. • SAPRC originally stood for (California) Statewide Air Pollution Research Center, which no longer exists. Gas-Phase Atmospheric Mechanisms

13. Relationships Between Componentsof the Current SAPRC Mechanisms Base mechanism for inorganics and common organic products SAPRC mechanism generation system (explicit reactions for ~620 VOCs) Manually derived lumped mechanisms for ~85 aromatics and ~45 other VOCs Product lumping SAPRC-07 lumped product mechanisms for individual VOCs VOC lumping VOC Reactivity Scales SAPRC-07 Lumped Mechanisms Standard Ambient VOC Mixture(s) Simulations of test scenarios Use in Airshed Models CS07 Condensed Mechanisms Gas-Phase Atmospheric Mechanisms

14. SAPRC Mechanism Generation System • Generates full mechanisms for hydrocarbons and compounds with -O-, -OH, -CO‑, -CHO, -ONO2 , Cl, and -NH2 groups. • Mechanism generation software uses measured rate constants and branching where available, estimation methods where not. • “Lumping Rules” used to derive organic product model species for lumped product versions of SAPRC. • Limitations: • Not used for aromatics • Currently limited to generating mechanisms in presence of NOx. Peroxy + peroxy mechanisms not generated. • Cannot estimate reactions with radicals whose heats of formation cannot be estimated. • Does not understand steric effects. • Available online at http://mechgen.cert.ucr.edu Gas-Phase Atmospheric Mechanisms

15. Ozone Changes Caused by Condensations: SAPRC-07 to CSAPRC-07 Average of absolute change in O3 for scenarios at various ROG and NOx levels, relative to uncondensed SAPRC-07 CSAPRC-07 Condensation Levels (see table) Gas-Phase Atmospheric Mechanisms

16. Numbers of VOC Model SpeciesRequired For More Detailed Mechanisms • Greater chemical detail in models requires representing more compounds explicitly. • Point of diminishing returns reached if all compounds are explicit • Versions of updated SAPRC may have varying numbers of explicit species Fraction of Mass Represented Explicitly Numbers of Compounds Represented Explicitly in Mechanism • Mass fractions in total anthropogenic U.S. emissions mixture (2005 inventory) Gas-Phase Atmospheric Mechanisms

17. Effects of SAPRC-07 Lumping on Ozone Predictions for a Multi-Day Scenario (MIR conditions) Ozone (ppm) Mechanisms are even closer at higher ROG/NOx • 5-Day box model simulations with continuous emissions • SAPRC-07T represents propene, xylenes, and several other important compounds explicitly instead of using lumped model species. Gas-Phase Atmospheric Mechanisms

18. Updates to SAPRC-07:SAPRC-11 Aromatics Mechanism Gas-phase mechanism • Aromatic mechanism updated and revised to much give better fits to results of many new aromatic – NOx experiments at UCR • But many inconsistencies and problems remain • Same base mechanism and lumping methods as SAPRC-07 • Results published in Atmospheric Environment. Aromatic SOA Mechanism • Lumped SOA model species added to mechanism to predict SOA formation for 14 aromatic hydrocarbons and 3 phenols • Yields and partitioning coefficients adjusted to fit results of ~300 UCR aromatic – NOx and – H2O2 experiments • Fitting data required use of 4 SOA formation processes, with different dependences on reaction conditions • Fits to SOA formation has more scatter than fits to ozone. Gas-Phase Atmospheric Mechanisms

19. Aromatic – NOx Runs Aromatic – H2O2 Runs (Model – Experimental) / Average Experimental PM Volume (mm3/cm3) Model Fits for All Experiments used toDerive Aromatic SOA Parameters No correlations of model errors with initial reactant levels Gas-Phase Atmospheric Mechanisms

20. Distribution of Error Ranges in Ozone and PM Predictions for Aromatic Experiments PM OH adjusted to fit amounts of VOC Reacting O3 Ozone Fits shown on same scale for comparison No OH Adjustments Gas-Phase Atmospheric Mechanisms

21. SAPRC Update Projects Currently Underway: • Update and document mechanism generation system • Completely Update SAPRC mechanism • Complete update of base mechanism to current evaluations. • Improve VOC organic product lumping approach to improve predictions of NOx recycling and SOA precursors. • Document SAPRC modeling, mechanism evaluation software Desired: • Develop mechanisms for SOA precursors from other VOCs following the approach used for aromatics. • Use mechanism generation to develop MCM-like near-explicit SAPRC mechanisms that are consistent with condensed versions • Collaborate with other detailed mechanism development and estimation efforts Gas-Phase Atmospheric Mechanisms

22. Effects of Mechanism Updates onOzone Predictions for a Multi-Day Scenario Ozone (ppm) • Last 2 days of 5-day box model simulations with continuous emissions. Gas-Phase Atmospheric Mechanisms

23. Comparison of Different Mechanisms onOzone Predictions for a Multi-Day Scenario Ozone (ppm) • Last 2 days of 5-day box model simulations with continuous emissions. Gas-Phase Atmospheric Mechanisms

24. Approaches for Adapting Gas-Phase Mechanisms to Represent SOA Precursors Parameterized Methods • Lumped SOA species added as products of primary VOC reactions. Yields and partitioning adjusted to fit chamber data. • SOA precursor yields assumed to be independent of reaction conditions. Gas-phase mechanistic information is not used. • Used in most current regional models that represent SOA. Explicit Chemistry Methods • SOA partitioning is estimated for all product species predicted by a near‑explicit (MCM) or generated (GECKO-A) mechanism. • Explicit mechanisms are highly uncertain for many VOCs. • Validity of partitioning estimates are highly uncertain. • Predictive capability not yet evaluated against experimental data. • Used for research purposes but impractical for regional models. Gas-Phase Atmospheric Mechanisms

25. Approaches for Adapting Gas-Phase Mechanisms to Represent SOA Formation (Continued) Lumped Process Methods • Lumped SOA precursor species added as products of reactions where needed to represent the major SOA forming processes. • Processes, yields, and partitioning used are derived based on considerations of chemistry and fits to chamber data. • Needs well characterized chamber data for varied conditions • Allows models to predict how SOA precursor formation depends on chemical conditions while using lumped mechanisms. • This approach was used to develop the SAPRC-11 aromatic SOA mechanism, but further development has not been funded. In any case, representation of reactions in the particle phase is separate from the gas-phase mechanism. Gas-Phase Atmospheric Mechanisms

26. General Recommendations forGas-Phase Mechanism Development • Start by developing and evaluating the most chemically detailed mechanism possible given the available state of knowledge. • These can be used to derive more scientifically valid condensed mechanisms, traceable to detailed chemistry. • Mechanism generation is the “wave of the future”. • Only practical way to develop near-explicit mechanisms. • Provides a framework to organize theory and data. • Research needed to improve estimation methods. • Development of lumped process methods to adapt gas-phase mechanisms to predict SOA precursors needs to be supported. • Allows use of both mechanistic knowledge and chamber data for more reliable SOA modeling in the near term • Increase international collaboration to minimize duplication and make most of limited funding. Gas-Phase Atmospheric Mechanisms

27. Thoughts on the Futureof Mechanism Development • But this may take many decades. We may achieve clean air by simply reducing total emissions before this happens Optimistic about eventually understanding basic science of atmospheric chemistry Not as optimistic about making significant improvements in predictive models in a useful time frame • Funding for basic research does not seem to be driven by current practical predictive modeling needs. • Funding inadequate for needed data evaluation and compilation activities • Funding inadequate for mechanism development. Mechanism developers cannot find full-time support in the U.S. Policy makers do not seem to be convinced that mechanism development is sufficiently important for adequate funding Gas-Phase Atmospheric Mechanisms

28. Acknowledgements • Roger Atkinson (now retired from UCR): Helpful discussions on atmospheric chemistry over many years • David Cocker, his graduate students, and research staff at UCR: Assistance and collaborations on on UCR chamber experiments • Gookyoung Heo (now at NIER in S. Korea): Assistance in recent SAPRC mechanism updates through September, 2014 • U.S. EPA: Funding for mechanism multiple mechanism development efforts through the 1980’s. • California Air Resources Board: Major funding source for ongoing SAPRC mechanism development. • Various Private Sector Groups: Funding experiments and mechanism development for selected compounds of interest. • UCR CE-CERT: Support for my participation in this conference. Gas-Phase Atmospheric Mechanisms

29. Thank you Gas-Phase Atmospheric Mechanisms

30. History of the SAPRC Mechanisms Gas-Phase Atmospheric Mechanisms

31. Lumped ProductDetailed SAPRC Mechanisms • VOCs explicit but for most organic products lumped • 31 model species used to represent organic products • Ozone predictions extensively evaluated against chamber data • >2500 experiments, 11 chambers, ~120 compounds • ([O3]‑[NO]) generally simulated within  30% • Used to calculate MIR and other VOC reactivity scales • Mechanisms for >600 VOCs from mechanism generation • Mechanisms for ~130 others estimated using other means • Reactivities for 407 others based on those of other VOCs • Used in conjunction with a lumped VOC mechanism to represent the base ROGs in the ambient mixture • Used to derive the lumped VOC versions of SAPRC-07 Gas-Phase Atmospheric Mechanisms

32. SAPRC-07 Lumped VOC Mechanisms • Most VOCs and organic reaction products lumped • 10 Lumped VOC species: ALK1-5, OLE1-2, ARO1-2, TERP • Currently two versions (with different explicit VOCs): • Standard version: methane, ethene, benzene, isoprene, acetylene explicit (total of 15 VOC species) • “Toxics” version: these + 11 other selected VOCs explicit • Same inorganic and organic product mechanisms as detailed • Comparable in size and lumping as RADM and RACM. • Developed for use in airshed models • Used to represent base case when calculating reactivity scales • Derived from explicit VOC, lumped product mechanisms, with lumped VOC reactions based on the compounds they represent • Weighting factors derived from base ROG mixture used in reactivity scales. (But ambient mixture needs updating) Gas-Phase Atmospheric Mechanisms

33. CSAPRC-07 Condensed Mechanisms • Similar to but more condensed than lumped SAPRC-07 • Similar in size to Carbon Bond mechanisms (e.g., CB05) • Developed for give almost same O3 in airshed calculations as standard lumped SAPRC-07 with fewer model species. • Number of organic product model species reduced from 30 to 13, VOCs species reduced from 15 to 10. • Derived from standard lumped SAPRC-07 • Test calculations used to examine incrementally increasing lumping effects on O3, NOx, H2O2, OH and total PANs. • Static, dynamic, multi-day ambient simulations • Static simulations with individual types of VOCs • Most extensive condensations that did not significantly affect O3 results were adopted for CSAPRC07 Gas-Phase Atmospheric Mechanisms

34. Average Model Biases in Simulations of Aromatic – NOx Experiments Large improvement in fits for phenols Gas-Phase Atmospheric Mechanisms

35. Aromatic SOA Processes Used OH Aromatic Hydrocarbons Phenols Catechols NO3 OH OH O2 OH NO3 OH OH NO3 OH O2 OH-aromatic adducts Volatile Products Peroxy Radicals Volatile Products NO O2 Abstraction products (all volatile) HO2 NO OH-aromatic-O2 adducts Condensable Phenolic ROOH’s Other Phenolic Condensables Uni. Uni. OH, O2, NO Other Condensables OH + volatile products O2 Condensable ROOH’s + Volatile products Bicyclic peroxy radicals a-Dicarbonyls and other photoreactive products (all volatile) HO2 NO O2 + Volatile ROOH’s W. P. L. Carter 1/7/2020 Chamber and Model Study of SOA Formation Gas-Phase Atmospheric Mechanisms 35