CITES 2005, March 20-23, 2005, Novosibirsk, Russia.

1. Atmospheric Chemistry:Overview and Future ChallengesAllan GrossDanish Meteorological Institute,Lyngbyvej 100, 2100 Copenhagen Ø, Denmark.&University of Copenhagen, Scientific Computing Chemistry Group,Universitetsparken 5, 2100 Copenhagen Ø, Denmark. CITES 2005, March 20-23, 2005, Novosibirsk, Russia.

2. Background • There is a critical need for improving the available mechanistic data in Atmospheric Chemical Transport Models (ACTM), examples: • the chemistry of higher molecular weight organic compounds (e.g. aromatic and biogenic compounds), • radical reactions (e.g. peroxy – peroxy radical reactions), • photo-oxidation processes (quantum yields and absorption cross sections), • heterogeneous processes. • Furthermore, due to experimental difficulties most rates are measured best near 298 K, i.e. temperature dependence of many reactions is not well characterised (see NIST, IUPAC and NASA).

3. With a description of the new European project GEMS as starting point, the following aspects will be outlined: • an overview of atmospheric chemistry (boundary layer and free-troposphere), • show important areas where future studies are needed, e.g.: • aromatic chemistry, • alkene chemistry. • a comparison of some of the most frequently used lumped atmospheric chemistry mechanisms will be given (EMEP, RADM2, RACM). • Examples of atmospheric environments where these lumped mechanism need to be improved: • biogenic environment, • marine environment. Contents

4. Objectives of GEMS (EU-project, 2005-09) Develop and implement at ECMWF a new validated, comprehensive and operational global data assimilation/forecasting system for atmospheric composition and dynamics. Some components of the system: combines “all available” remotely sensed and in-situ data to achieve global tropospheric and stratospheric monitoring of the composition and dynamics of the atmosphere from global to regional scale covering the tropospheric and stratosphere: Satellite data, and near-real time measurements. global data assimilation. Point 1 will deliver current and operational forecasted 3-dim. global distributions. These distributions will be used for regional air quality modelling.

5. Data input (Assimilation, Satellite, Real-time) GEMS Global System Global Reactive Gasses Global Greenhouse Gasses Global Reac-tive Gasses (UV-forecast) oxidants green house gasses oxidants optical properties GEMS Global System Coordination System Integration Regional Air Quality Global Aerosols Regional Air Quality (RAQ modelling) boundary conditions Products, User Service Schematic illustration of the GEMS strategy to build an integrated operational system for monitoring and forecasting the atmospheric chemistry environment: Greenhouse gasses, global reactive gasses, global aerosols and regional air quality.

6. Current and forecasted 3-dim. global distributions of atmospheric key compounds (horizontal resolution 50 km): greenhouse gases (CO2, CH4, N2O and SF6), reactive gases (O3, NO3, SO2, HCHO and gradually expanded to more species), aerosols (initially a 10-parameter representation, later expanded to app. 30 parameters). The global assimilation/forecast system will provide initial and boundary conditions for operational regional air-quality and ‘chemical weather’ forecast systems across Europe: provide a methodology for assessing the impact of global climate changes on regional air quality. provide improved operational real-time air-quality forecasts. Operational deliverables

7. CLRTAP: UN Convertion on Long-Range Trans-boundary Air Polluton

8. GEMS Regional Air-Quality Monotoring and Forecastning Partners

9. Models Within RAQ Sub-Project * E : run at ECMWF ; P : run at partner institute

10. WORF-CHEM: RADM2 CMAQ: CB-IV, RADM2, RACM, ”SAPRC99” CAMX: CB-IV with improved isoprene chemistry, SAPRC99 Chemical Schemes in USA-models

11. RAQ Interfaces and Communication between ECMWF and Partner Institutes

12. The GEMS project will develop state-of-the-art variational estimates of many trace gases and aerosols, the sources/sinks, and inter-continental transports. Later on operational analyses will be designed to meet policy makers' key requirements to the Kyoto protocol, the Montreal protocol, and the UN Convention on Long-Range Trans-boundary Air Pollution. GEMS, Summary

13. Gas-Phase Chemistry Need to be Solved in Regional Air Quality Models Formation of: ozone, nitrogen oxides, peroxyacetyl nitrate (PAN), hydrogen peroxide, atmospheric acids ..... Need to understand chemical reactions of: nitrogen oxides, VOC .....

14. Chemistry of the free-troposphere: nitrogen oxides and its connection with, carbon monoxide, and simplest alkane – methane. Polluted environment we have high NOX, and VOC chemistry shall also be included.

15. Reaction Cycle of HOX and NOx, high VOCs Reaction Cycle of HOX and NOx, only VOC – methane CH4 Hydrocarbons HCHO , RCHO H2O2, CO H2O2 products hν CH3O2 RO2 RONO2 RO+NO2 O3 HO HO2 hν +H2O CH3OOH ROOH O3 RO3NO2 RO2NO2 HNO3 RNO3 H2O2 Hydrocarbons HCHO NO NO2 hν NO3 Nighttime chem. O(3P) CO Hydrocarbons O3

16. C2H6 CH3C8H17 Oxidation Steps of Hydrocarbons C9H20 C10H22 C3H8 C5H12 NO3 O2 HO2 CH3C9H19 C11H24 HO ROOH HO CH3C5H11 CH4 R’─R hν C4H8 CH3CCl C3H6 C12H26 C2H4 NO3+O2 CH3CH3CH4H8 NO2 NO3 CH3CO2CH3 C4H10 hν+O2 C5H10 HO O2 O2 HO NO NO3 CH3C4H7 RO3 C2H5CO2CH3 CH3C4H7 RO3 C4H9CHO RH R· RO2 RO2 R’CHO RO· CH3C6H13 C4H6 HO2 HO2 H2O NO2 NO2 CH3COC4H9 HNO3 O3 CH3C6H4C2H5 CH3C6H5 C7H16 C6H6 CH3Cl NO HO R’CHO C2H5OH CH3COCH3 R’─R R’O2 (CH3)3C6H3 CH3OH R(ONO2) HCHO (CH3)2CHCHO C2H2 O O CH3COC2H5 RO + R’O+ O2 O C2Cl4 R(-H)O+R’OH+O2 C2H5CHO Green: only alkene path Red: also other end products but these react further to the given end product CH3CHO C6H14 ROOR’+O2 C3H7C6H5 C3H7CHO ROOH+R’O2 C2H5C6H5 C6H5CHO CH7CH3CO2

17. Gaps in Atmospheric Chemistry, High Priorities • Inorganic chemistry is relatively well known • Problems: • alkenes • monocyclic aromatic hydrocarbons • polycyclic aromatics hydrocarbons (PAH)

18. The Chemistry of Alkenes Reasonable Established. Rate coefficients for HO-alkene reactions of most of the alkenes which have been studies appears to be reasonable accurate. Gaps, Highest Priorities the data base for RO2+ R’O2, RO2 + HO2, RO2 +NO2 ,RO2 + NO reactions and their products are very limited and complex. E.g. system with only 10 RO2 (no NOX) results in approximately 165 reactions. ozonolysis of alkenes are important in urban polluted area. Example: O3 + H2C CH2 → →HCHO + H2COO * O H2COO 37% CO+H2O 38% CO2+H2 13% O O primary ozonide Criegee biradical The rate and product yields of the stable Criegee biradical with NO, NO2 and H2O have only been studied for the simplest carbonhydrids. Higher order carbonhyrids should be investigated

19. Many of the unsaturated dicarbonyl products appear to be very photochemically active. Absorption cross sections only determined from highly uncertain gas-phase measurements. Examples of compounds it is important to determine the spectra of O O O O O O O O trans-butenedial 4-oxo-2-pentanal 3-hexene-2,5-dione 4-hexadienedials (Atmospheric oxidation products from aromatics)

20. The Chemistry of Aromatics Still Highly Uncertain Gaps is related both to the rate constant the of aromatic chemistry and the yields of the formed products

21. Rate coefficients for HO-reactions with monocyclic aromatics only 23 aromatics have been studied: only studied by one lab. p-cymene tetralin α-methyl-styrene β-methyl-styrene β-β-dimethyl-styrene studied by more than one lab. but with over all uncertainties greater than 30% iso-propyl-benzene o-m-p-ethyl-toluene tert-butyl-benzene indan indene • rate constants for only 20 of the many aromatics products of the oxidation of aromatics have been determined, 14 of these are single studies.

22. Rate coefficients for HO-reactions with polycyclic aromatics (PAHs) only 16 aromatics have been studied: only studied by one lab. 1-:2-methyl-naphthalene 2, 3-dimethyl-naphthalene acenaphthalene NO2 NO2 NO2 flouranthene 1-:2-nitronaphthalene 2-methyl-1-nitron-aphthalene

23. HO +PAH studied by more than one lab., rate constant uncertainties for seven PAHs biphenyl (30%) fluorene (fac. of 1.5) acenaphthene (fac. of 2) O O phenanthrene (fac. of 2) dibenzo-p-dioxin(fac. of 1.5) dibenzofuran (30%) anthracene: one of the most abundant and important PAH in the atmosphere Rate highly uncertain: range (18 to 289) × 10-12 cm3 molecule-1 anthracene

24. HO +PAH Rate coefficients for PAHs with vapor pressures greater than app. 10-5 Torr (298 K) should be determined since their reaction with HO may be an improtant removal process, three examples are: 3-methyl-phenanthrene pyrene benzo[a]flouorene

25. NO3 + aromatics appear unimportant in the atmosphere Exceptions: a group attached to the atomatic ring have a double bound (ex. indene, styrene), have an –OH group attached to the aromatic ring (ex. phenols, cresols). Only studies: NO3 + & & OH OH OH NO2 phenol o-:m-:p-cresol m-nitro-phenol

26. O3 + aromatics: have gaps but these reactions are not highly important in atmospheric chemistry. O(3P) + aromatics: unimportant in urban atmosphere. Atmospheric chemistry of organic compounds sorbed on particles (heterogeneous reactions) and its reactions in aerosols even more uncertain. Important. PAHs oxidation sorbed on particles. Important. PAHs + HO more studies are needed.

27. Non-aromatic products from the oxidation of aromatic compounds – additional kinetic and mechanics studies of the rates are needed: Especially the HO initiated reactions, Product studies of HO + aromatics from chamber experiments shows carbon mass losses from 30% to 50%, i.e. quite possible that some yet unidentified reactions pathways. That means the overall atmospheric oxidation mechanism of aromatics is still rather uncertain. Highest priority, a study the products from the oxidation of most important aromatics: toluene, xylenes, and trimethyl-substituted benzenes.

28. Application of Chemistry in • Atmospheric –Chemical Transport Models • Problems: • A “Complete Mechanism” would require tens of thousands of chemical species and reactions. • The reaction mechanisms and rates are not known for most of these. • The ordinary differential equation for chemical mechanisms is very stiff, i.e. numerical standard methods are not applicable. • Way of solving it: • Using lumped chemical mechanism. • Make special ad hoc adjustments to the rate equation to remove stiffness in the lumped mechanism → use a fast solver.

29. Correlation of the rates for NO3 with HO □ (line c): addition reactions Δ (lines a & b): abstraction reacs Correlation of the rates for NO3 with O(3P)

30. Correlation of Peroxy ─ Peroxy Radical Reactions Function fit depend on number of carbons Function fit depend on the rates from the and the alkyl-alkoxy substitutionreactants peroxy-self-reaction rates

31. Lumped Atmospheric Chemical Mechanisms

32. Photolysis NOX Ethene Propene tert-2-butene Chamber Experiment EC-237 RACM and RADM2 are tested against 21 Chamber Experiments included: 9 organic species. Used chamber: Statewide Air Pollution Research Center. Key species tested in the chamber: NO2, NO and O3. • n-butene • 2, 3-dimethylbutene • toulene • m-xylene

33. RACM better than RADM2 Ref. Stockwell et al., JGR, 1997

34. RACM better than RADM2 Ref. Stockwell et al., JGR, 1997

35. Problems With These Chamber Experiments • 50% or more of the total HO comes from the chamber walls (depend on the chamber). • Chamber walls can serve as sources or sinks for O3, NOX, aldehydes and ketones. • Photolysis maybe uncertain. • Chamber experiments are conducted at much higher species concentrations than in the atmosphere (i.e. have a lot of radical reactions which do not occur in the real atmosphere). • If e.g. EUPHORE chamber data were used • these problems would be smaller.

36. O3 ─ i s o p l e t s local noon Ref. Gross and Stockwell, JAC, 2004

37. O3 and HO Scatter plots Without Emissions 3 days sim. Local Noon O3 HO O3 HO Δ: urban □: rural ×: neither urban nor rural Ref. Gross and Stockwell, JAC, 2004

38. RO2 HO2 HO2 and RO2 Scatter plots Without Emissions 3 days sim. Local Noon HO2 RO2 Δ: urban □: rural ×: neither urban nor rural Ref. Gross and Stockwell, JAC, 2004

39. Mechanism Comparison, Summary • Compared to each other the mechanisms showed clear trends: • O3: EMEP > RACM > RADM2 • HO and HO2: RACM > EMEP and RACM > RADM2 • RO2: EMEP > RACM and RADM2 > RACM • The mechanism comparison showed little differences between the three mechanisms, equally good. • However, all these mechanisms are based on the same guessed rates and reactions, i.e. the same amount of uncertainty. • However, few of the simulated scenarios gave very large simulated differences between the mechanisms. This showed that only one “typical” scenario (which often has been considered to be sufficient) is not enough in order to make a proper mechanism comparison.

40. Biogenic Chemistry • Several hundreds different BVOC have been identified. Most well known are ethene, isoprene and the monoterpenes. • Isoprene is the major single emitted BVOC. • The BVOC emission depend highly on vegetation type. • BVOC emissions also contain oxygen-containing organics Estimated global Annual BVOC Emission (Tg/year)

41. ethylene isoprene 2-methyl-3-buten-2-ol many tissues methanol monoterpenes chloroplasts cell walls resin ducts or glands 100s of VOC flowers formaldehyde formic acid cell membranes acetaldehyde acetic acid ethanol C6-acetaldehydes C6-alcohols leaves, stems, roots acetone (Fall, 1999)

42. Some Biogenic Emitted Hydrocarbons isoprene α-pinene β-pinene limonene α-terpinene γ-terpinene camphene terpinolene α-phellandrene β-phellandrene myrcene ocimene Δ3-carene p-cymene

43. Some Oxygen-Containing Organics Biogenic Sources O O O formaldehyde acetaldehyde acetone butanone n-hexanal O OH O O O 3-methyl-5-hepten-2-one 3-hexenal 2-hexenal thujone methanol O OH OH OH OH ethanol n-hexanol 3-hexenol camphor linalool OH OH O O HO O O 2-methyl-3-buten-2-ol formic acid acetic acid 3-henenyl-acetate 1, 8-cineol

44. EUPHORE Chamber Experiment and Simulation without BVOCs (called base mix) Ref. Ruppert, 1999

45. EUPHORE Chamber Experiment and Simulation: base mix + 90 ppbV α-pinene Ref. Ruppert, 1999

46. EUPHORE Chamber Experiment and Simulation: base mix + isoprene Sim. with RACM Sim. with modified RACM ozone toluene ethene isoprene NO2 NO Ref. Ruppert, 1999

47. Biogenic Study, Summary The BVOC emission inventory are calculated from land-use data. The BVOCs emissions from plants are usually only given for isoprene and monoterpenes. However in Kesselmeier and Staudt (Atm. Env., 33, 23, 1999) are BVOCs from other compounds than isoprene and monoterpene presented. How shall the split of the emissions of monoterpenes into specific species (α-pinene, β-pinene, limonene etc.) be performed? This is not clear. BVOC emission inventories have uncertainties of factors ≈ 2.5-9. How good are the land-use data bases to describe the current BVOC? How good are seasonal changes of vegetation described? How good are human changes of vegetation described? The understanding of biogenic chemistry is very incomplete. Today only one lumped mch. treat other biogenic emitted species than isoprene. RACM also treat API: α-pinene and other cyclic terpenes with more than one double bound, LIM: d-limonene and other cyclic diene-terpenes. Commonly used lumped mechanisms (CBM-IV, RADM2, EMEP and RACM) do not describe the chemistry of isoprene very good.

48. DMS (DiMethyl Sulphide) ChemistryIdentified Atmospheric Sulphur Compounds HS CH3SO2OH CS2 CH3S(O)OOH COS CH3SCH2OOH SO2 CH3S(O)2OOH H2SO4 [SULF] CH3OS(O)2OH CH3SCH3 [DMS] CH3OS(O)2OCH2 CH3S(O)CH3 [DMSO] CH3S(O)2CH3OOH CH3S(O)2CH3 [DMSO2] HOCH2S(O)2OH CH3SSCH3 [DMDS] HOCH2S(O)2CH2OH CH3SH CH3SO2ONO CH3SOH [MSEA] CH3SO2ONO2 CH3S(O)OH [MSIA] CH3S(O)2OH [MSA] It is not an easy task to make a DMS gas-phase mechanism?

49. The ELCID gas-phase mch. DMS mch. for Atm Modelling A gas-phase DMS mch. was developed during the EU-project period. This DMS mch. included 30 sulphur species and 72 reactions (49 guessed & 23 experimental rates). Based on clean MBL scenarios the DMS ELCID mch. was reduced to 21 sulphur species and 34 reactions (22 guessed & 12 experimental rates). The ELCID mch. was further reduced by lumping to 15 sulphur species and 20 reactions. This mechanism was used for 3D modelling in the ELCID project.

50. The Atmospheric Box-model In the box the following processes are solved for species i (which can be either a liquid or gas phases species): dCi/dt = + chemical production – chemical loss + emission – dry deposition – wet deposition + entrainment from the free troposphereto the boundary layer + aerosol model + CCN model + cloud model Ref. Gross and Baklanov, IJEP, 2004, 22, 52