ACECHEM Atmospheric Composition Explorer for Chemistry - Climate Interactions

1. ACECHEMAtmospheric Composition Explorer for Chemistry - Climate Interactions

2. Contents 1. Scientific Rationale 2. Observational Requirements 3. Evaluation of Techniques 4. Mission Elements 5. Data Assimilation and Utilisation 6. Performance Assessment 7. Summary of Scientific Case 8. System Concept

3. Scientific Rationale Scientific Imperative • Human activities are changing atmospheric composition. • Composition changes in the UTLS couple strongly with climate. • The processes involved are complex and not well understood. • Important constituents are highly variable in the UT, requiring that vertical and horizontal structure be captured on fine scales. • There is a need for high-fidelity, global observations of Upper Tropospheric / Lower Stratospheric (UTLS) composition. • The Atmospheric Composition Explorer for CHEMistry - climate interactions (ACECHEM) mission will meet this need and will greatly improve our understanding of these issues.

4. Scientific Rationale Changing Composition & Impacts • Important changes have been detected in trace gas concentrations and related variables during recent decades: • Increases in long-lived greenhouse gases (eg CO2, CH4, N2O). • Height-dependent changes in O3, H2O and T. • Decrease in stratospheric O3 and concomitant increase in UVB. • Changes to tropospheric O3 vary from region to region. • Stratospheric H2O increase • Stratospheric cooling • Natural phenomena such as the solar cycle and volcanoes (El Chichon, ‘82; Pinatubo, ’91) also perturb composition.

5. Scientific Rationale Trends in O3 and H2O profiles 1970-96 O3 trend from European sondes at 50oN

6. Scientific Rationale The Four Mission Objectives Objective 1: Role of the UTLS for Radiative Forcing and Feedback Objective 2: Role of Stratosphere-Troposphere Exchange in Atmospheric Composition and Climate Objective 3: Interaction of Stratospheric Chemistry and Climate Objective 4: Impact of Pollution on the Upper Troposphere

7. Scientific Rationale Objectives 1 & 2 and specific questions Objective 1: Role of the UTLS for Radiative Forcing and Feedback • What are the spatial and temporal variabilities of greenhouse gases in the UTLS region and their radiative forcing or feedback? • What are the chemical and dynamical processes responsible for the variability of ozone and its precursors in the UTLS region? • What are the climatic and chemical effects associated with aerosols and cirrus in the UTLS region? Objective 2 : Role of Stratosphere-Troposphere Exchange in Atmospheric Composition and Climate • How are the stratosphere and the troposphere coupled dynamically and how do the processes involved differ with latitude? • What is the role of STE for the budgets and distributions of O3, H2O and the greenhouse gases which are destroyed in the stratosphere? • What is the spatial and temporal variability of tropopause height and temperature and how does this variability impact on STE?

8. Scientific RationaleObjectives 3 & 4 and specific questions Objective 3: Stratospheric Change and its Interactions with Stratospheric Chemistry and Climate • How will the ozone layer evolve in a cooler stratosphere where Cl begins to decrease and H2O continues to increase? • What are the mechanisms underlying future ozone changes? • How large is chemical ozone loss in the presence of substantial variations due to transport?. Objective 4: Impact of Pollution on the Upper Troposphere • What is the influence of pollutant export from industrial regions on the composition of the upper troposphere? • How does the atmosphere cleanse itself of greenhouse gases and aerosol precursors and how will its oxidizing capacity evolve in future? • What is the contribution of forest fires and other biomass burning events to the composition of the upper troposphere, particularly in tropical regions?

9. Scientific Rationale Radiative Forcing & Feedback • Tropospheric O3 has contributed ~15-20% (0.35 Wm-2) to radiative forcing since pre-industrial times. • Stratospheric O3 depletion has contributed –0.15 Wm-2 to radiative forcing between 1979 and 1997. • H2O contributes most to natural greenhouse effect and to variance in clear-sky outgoing longwave radiation. • H2O feedback amplifies radiative forcing from other gases in GCM predictions and depends on vertical profile of H2O change. • Sensitivity high at tropopause because temperatures are coldest • Vertical profiles of O3 and H2O in the UT and LS are crucial to their radiative forcing and feedback, respectively.

10. Scientific Rationale Sensitivity of Ts to O3 & H2O O3 H2O

11. Scientific Rationale Stratosphere-Troposphere Exchange • STE occurs on a range of scales and is highly variable in space and time, as is tropopause height. • Tropics: upward transport of tropospheric air • Extra-tropics: downward transport of stratospheric air • Global budgets of eg O3 in UT and H2O in LS depend on STE. • Changing atmospheric composition affects the thermal and dynamical structure of the UTLS, and hence STE. • Feedbacks on composition and climate via STE are possible. • Observations of trace gases and temperature needed at high vertical and horizontal resolution to quantify STE at global scale.

12. Scientific Rationale Mean Meridional Circulation

13. Scientific Rationale Stratosph. Chemistry - Climate Interaction • Stratosphere expected to continue to cool during coming decades • Further increase in stratospheric H2O would cause additional cooling • Change in thermal structure will also modify circulation patterns: • distribution of O3 and its radiative heating/cooling will change • feedback on temperature. • Change to stratospheric O3 will also affect penetration of solar UV into the troposphere. • Stratospheric O3 is a medium for chemistry-climate interaction • Global observations needed at end of the decade to study processes of interaction between O3 and climate in a cooler stratosphere.

14. Scientific Rationale Arctic winter/spring chemical O3 loss HALOE O3 Predicted O3

15. Scientific Rationale O3 loss in a colder stratosphere

16. Scientific Rationale Pollution in the Upper Troposphere • Chemical composition of UT affected by: • Convective transport of pollutants up from boundary layer • Local sources (O3 from CO, CH4 & NMHCs in presence of NOx; NOx from lightning) • NOx reservoir in UT is HNO3 (PAN). • Many gases emitted from biosphere and human activities (eg CH4 and HCFCs) are removed by OH. • [OH] controlled by H2O, O3 (locally and in stratosphere), CH4 and CO (CH3COCH3 & HCHO). • Tropospheric OH is a medium for chemistry - climate interaction. • Global, height-resolved measurements of key trace gases needed in the UT to: • distinguish local chemistry from pollutant transport • constrain the OH distribution in models.

17. Scientific Rationale Model Distributions of CO in UT Pollution from N.America Plume from Biomass Burning in Brazil CO (ppbv) at 200hPa in September

18. Observational Requirements • Mission objectives span tropical - polar latitudes • Measurements with fully global coverage required. • Mission must cover: two quasi-biennial oscillations; cold & warm Arctic winters; El Niño/La Niña. • Mission duration of 5 years or longer • Satellite observations essential for global perspective on UTLS. • For each objective: height-resolved observations required of a suite of trace gases and other geophysical variables • For each variable: height-range, precision, vertical resolution, horizontal & temporal sampling must be considered in parallel. • High-fidelity measurements required of a number of variables

19. Observational Requirements Quantitative Requirements Left / right = target / threshold

20. Evaluation of Techniques Limb-Sounding and Cirrus Impact • Passive techniques capable of measuring full suite of target gases over 5-years. • Vertical resolutionandsensitivityrequirements in UT and LS stringent but attainable bylimb-sounding. • Tropospheric limb-sounding possible only in mm-wave, mid-IR and near-IR “windows”. • Cloud climatology for UT limb-viewing at 1mm from SAGE-II: opaque: <10% in mid & high latitudes; ~15% in tropics sub-visual: ~20% in mid & high latitudes; ~40% in tropics • Cirrus extinction comparable to 1mm in mid-IR, but orders of magnitude smaller in mm-wave. • Limb-mm affected by only a fraction of opaque clouds and not affected by sub-visual clouds (or by aerosol or PSCs).

21. Evaluation of Techniques SAGE-II Cloud Climatology

22. Evaluation of Techniques Cirrus Extinction vs Wavelength

23. Mission ElementsDedicated Platform for UT & LS Mission to sound the UT & LS will comprise three limb-sounders on a dedicated platform: • MASTER - Spectrometer with three mm-wave bands to target: • H2O, O3 and CO in the UT • H2O, O3, CO, HNO3 and N2O in the LS and two sub-mm bands to target: • ClO, BrO and HCl in the LS • AMIPAS - Spectrometer with three mid-IR bands to target: • T, H2O, O3, HNO3and organic speciesin the UT • T, H2O, O3, HNO3, CH4, N2O, NO2, N2O5, ClONO2 & (H)CFCs in LS • LCI - Imager with mid-IR and near-IR channels for: • cirrus/PSC and aerosol detection in the UT & LS at higher spatial resolution than the spectrometers.

24. Mission ElementsFormation Flight with MetOp for LT Observations of the lower troposphere (LT) are also required: • Nadir-sounders can see to the surface in atmospheric windows. • Suite of nadir-sounders to be deployed on MetOp. GOME-2:O3 profiles; NO2, H2CO and SO2 columns in polluted air IASI:Tropospheric T and H2O profiles; CO and CH4 columns AVHRR-3:Images of cloud, column aerosol & surface properties • Combining with co-located, limb observations of UT will allow LT to be discriminated. • ACECHEM platform will fly in formation to synchronise with MetOp’s nadir observations.

25. Data Assimilation & Utilisation • Data assimilation provides an objective means to: • interpolate, integrate diverse observations and assess data quality • derive the distribution of species which interact chemically with observed species • It will play a key role for ACECHEM through integration of: • mm-wave and mid-IR limb-sounders • limb-sounders with MetOp nadir-sounders, and with other satellites • satellites with airborne and ground-based sensors • observations with dynamical information at finer scales from models • Unmeasured trace gases (eg OH) will be derived through their interaction with measured species by chemical data assimilation • The large research community will use data in diverse ways: • Process studies (the four objectives) • Constraints for global and regional chemical transport models • Trace gas climatologies (>5 yrs) for evaluation of models • Improve GCM dynamics and radiation, and hence their predictions

26. Data Assimilation & UsageAssimilation of GOME O3 columns Ozone “Mini-Hole” 30th Nov’99

27. Performance AssessmentRetrieval Simulations • The ability to fulfil research objectives can be gauged by comparing simulated retrievals with quantitative requirements. • ACECHEM sensors (as per System Concept) have been simulated extensively with realistic errors. • Simulations for MetOp and missions preceding ACECHEM performed on common basis.

28. Performance Assessment Simulations & Requirements: O3

29. Performance Assessment Simulations & Requirements: H2O

30. Performance Assessment Simulations & Requirements: CO

31. Performance Assessment Simulations & Requirements: HNO3

32. Performance Assessment Simulations & Requirements: N2O

33. Performance Assessment Simulations & Requirements: ClO

34. Performance Assessment MASTER 2-D H2O simulations

35. Performance Assessment MASTER 2-D H2O simulations

36. Performance Assessment MASTER 2-D H2O simulations

37. Performance Assessment H2O & T retrieval in NWP frame

38. Summary of Science Case • There is a compelling science case for the: Atmospheric Composition Explorer for Chemistry - Climate Interactions • ACECHEM will: • Meet the stringent, quantitative requirements for the four mission objectives • Sound UT composition with unprecedented fidelity • Add value to MetOp in data assimilation for NWP • Underpin environmental policy and contribute to GMES

39. Summary of Scientific Case Lower Stratosphere Innovation • ACECHEM will uniquely allow the processes which control stratospheric O3 at the global scale to be examined in 2010: • O3 and other key variables will be observed with higher fidelity and/or improved geographical coverage than preceding missions. • This innovation will greatly improve our understanding of stratospheric composition - climate interaction.

40. Summary of Science Case Upper Troposphere Innovation • ACECHEM is the first space mission optimised specifically to sound the composition of the upper troposphere: • O3 and H2O will be observed in the UT with unprecedented vertical resolution. • Unique, global perspective on their roles in climate and on the dynamical processes which govern their distributions, notably STE. • HNO3, CO and organic specieswill be observed in the UT simultaneously with O3 and H2O, with unprecedented resolution • Differentiate UT chemistry from upward transport of pollutants and downward transport of O3 and NOy, and constrain OH in models. • These innovations will greatly improve our understanding of the interactions between UT composition and climate.