1. Aerosol-Cloud Ocean Biology Mission (ACOB)
M. Schoeberl NASA/GSFC
C. McClain NASA/GSFC
Contributions from D. Diner, L. Remer, J. V. Martins, P. Hildebrand, J. Welton, B. Blair, M. McGill, G. Jackson, M. Mischenko, D. Starr, P. Colarco, and a bunch of other people.

2. What is ACOB? ACOB is a multi-user mission with two science goals
Quantifying Aerosol-cloud interaction
Determining Ocean Carbon Cycling and other biological processes
Why two goals?
Next generation ocean color measurements require precise estimation of the aerosol contribution to the backscatter radiation
Precise aerosol measurements are of interest to the aerosol cloud community
There are common science problems between the two communities
Aeolian fertilization of the ocean
Aerosol formation by DMS

3. ACOB will addresses the aerosol science drivers for the next decade
Climate forcing and hydrological cycle: Understanding the global significance and physical processes underlying aerosol-cloud interactions to reduce major climate uncertainty (2 W m-2 globally) associated with aerosol “indirect effects”
Human health and biological activity: Associating changes in boundary layer air quality with aerosol sources and particle types, and quantifying aerosol impacts on human and ecosystem health

4. Previous groundwork toward development of community consensus on a future aerosol mission strategy
Progressive Aerosol Retrieval and Assimilation Global Observing Network (PARAGON) initiative
Objective: To outline an integrated system for determining aerosol climate and environmental impacts
October 2004
NASA-wide aerosol strategy workshop, Williamsburg, VA, August 2005
Objective: To identify NASA’s specific contributions to PARAGON
NCAR Workshop on Air Quality Remote Sensing from Space, Boulder, CO, February 2006
Objective: To examine what observational characteristics are required for the successful use of satellite remote sensing to measure environmentally significant pollutant trace gases and aerosols.

5. Aerosol measurement recommendations
Recommendation for advanced satellite imagers and lidars to reduce indeterminacies in current aerosol microphysical property retrievals and adoption of a systems approach to the development of new satellite missions (PARAGON publications)
Emphasis upon aerosol-cloud interactions in relationship to climate change and the hydrologic cycle, and the relative impacts of anthropogenic and natural aerosols on climate and air quality (Williamsburg and GSFC workshop)
“Understanding of the composition and size characteristics of atmospheric aerosols by means of multi-angle, spectropolarimetric, and stereoscopic- imaging techniques in conjunction with active (high spectral resolution lidar) measurements.” (NCAR workshop)
More recently, GSFC Workshop Nov 2006, emphasized the role of aerosols in precipitation
Critical advances are needed in the areas of: aerosol and cloud vertical profiling, horizontal and vertical spatial resolution, global coverage, identification of precipitation processes, revisit time, and fusion of measurements to reduce uncertainties and indeterminacies

6. Evolution of aerosol/cloud research
The current decade will demonstrate improvements in our ability to observe aerosols and their affects from space
Terra Aqua: Significant improvements in quantifying direct radiative impacts; statistical inferences regarding aerosol effects on cloud properties; major improvements in determining near-surface air quality over land (MODIS, MISR)
A-Train - Aqua, Aura, CALIPSO, CloudSat, Glory
OMI: Best yet measurements of aerosols over bright surfaces ~ 20 km resolution
CALIPSO: Measurements of aerosol backscatter very close to clouds - no swath
Glory: Major advances in aerosol characterization but with sparse coverage and resolution too coarse for observing cloud boundaries or intra-urban pollution - no swath
CloudSat: Impact of aerosols on cloud formation not aligned with CALIPSO - no swath
What is missing from already-manifested missions in the 2015 time frame?
NPOESS: No vertical profiling information; no multi-angle or polarimetric imaging for reducing aerosol uncertainties to climate-quality requirements
EarthCARE: Single-wavelength lidar limits aerosol microphysical characterization; single-frequency W band radar has limited sensitivity to precipitation; lacks comprehensive passive aerosol measurement
No future missions have clear linkage to the hydrological cycle - especially impact on precipitation

7. ACOB is the NAS ACE Mission
“Science Objectives: The science goal of ACE is to reduce the uncertainty in climate forcing through two distinct processes described above. The first goal is to better constrain aerosol-cloud interaction. This goal is achieved by simultaneous measurement of aerosol and cloud properties by radar, lidar, polarimeter, and a multi-wavelength imager.
Mission and Payload: … LEO, sun-synchronous early-afternoon orbit. The orbit altitude of km. The notional mission consists of four instruments:
A multi-beam cross-track dual wavelength lidar for measurement of cloud and aerosol heights and layer thickness;
A cross-track scanning cloud radar with channels at 94 GHz and possibly 34 GHz for cloud droplet size, glaciation height, and cloud height;
A highly accurate multiangle - multiwavelength polarimeter to measure cloud and aerosol properties (This instrument, would have a cross-track and along-track swath with ~1 km pixel size.)
A multi-band cross-track visible/UV spectrometer with ~1 km pixel size, including Aqua MODIS, NPP VIIRS, and Aura OMI aerosol retrieval bands and additional bands for ocean color and dissolved organic matter.”

8. ACOB Measurement Strategy
Particle Ranges
In order to understand the interaction between pollution, clouds and precipitation we need measurements that are sensitive to the particle distribution, cloud height and particle composition. Following the measurement suite pioneered by the A-Train, a combination of active and remote multi-wavelength sensors is needed.

9. Candidate Sensor System
Passive sensors
Multiangle imaging spectropolarimeter (UV-SWIR): Global column-averaged aerosol amount, size distribution, absorption, particle shape, refractive index; some height sensitivity
High frequency µ- wave radiometer (800 GHz - W band): Cloud ice water content
Low frequency µ- wave radiometer (W - Ku band) : Cloud precipitation
Optical spectrometer (ORCA): Measurements of biomass growth rates, organic and non- organic suspended matter assessments, aerosol absorption and size sensitivity
Active sensors
Next generation aerosol lidar: Vertical profiles of aerosol abundances and microphysical properties with across-swath capability and/or direct extinction-backscatter separability
Particle Ranges
Cloud profiling radar: Vertical profiles of droplet effective radius and vertical profile of water phase, cloud base and top height, precipitation rates

10. ACOB Candidate Payload
Instrument
Purpose
Sources
Ocean Color Radiometer
Ocean biosphere measurements, aerosols
ORCA (GSFC)
Polarimeter
Aerosol properties, removal of aerosol effects for ocean biosphere
APS + Polder A (GSFC, CNES)
PACS (GSFC)
MSPI (JPL)
Multi-beam lidar*
Aerosol heights, properties
MBL (GSFC)
HSR lidar (nadir only)*
Aerosol heights, properties, microphysics
LaRC
Cloud Radar
Cloud properties
JPL, GSFC
Cloud Radiometer (HF)
Cloud IWC, ice, particles
SIRICE (joint with JPL)
Cloud Radiometer (LF)
Precipitation
GMI (Ball)
*It is unlikely we can fly both of these
HQ has asked GSFC and LaRC leads to discuss hybrid option

11. Multi-beam Lidar
Uses wider swath cross-track observations to improve aerosol and cloud parameterization in mesoscale and global transport models by providing multi-grid vertical profile data. Provides increased swath coverage for formation flight missions relying on combined lidar and imager observations (e.g. ocean color).
Nadir vs. Cross-track Lidar Example:
Forest fires in Quebec generate thick smoke plumes transported to NE United States
Cross-track lidar example:
500 km Sun Synch Orbit
7 Fixed Lidar beams
0°, ±5°, ±10°, ±15° angles
Nadir-only lidar does not provide enough spatial coverage for most aerosol plumes
Improved spatial coverage through complicated aerosol plumes
MODIS AOD
MODIS AOD
Wider swath profiling over difficult ocean color regions
nadir
Coherent aerosol time and space scales:
Average: ~5 hrs, ~100 km
Plumes: ~1 hrs, ~30 km
Cross-track spacing on the order of aerosol plume scales & model grid sizes
Cross-track
Total Swath

12. Three concepts MSPI JPL POLDER-A +EOSP PACS
Polarimeters
Three concepts
MSPI JPL
POLDER-A +EOSP
PACS

13. APS and POLDER-A Combination
The POLDER-A is a multi-channel multi-angle imaging photopolarimeter which will provide
detailed and accurate aerosol and cloud retrievals with a 2-day global coverage;
Channels 443, 490, 670, , 1650, 2130 nm
The APS is a high-precision multi-channel multi-angle photopolarimeter which will provide
continuation of the Glory APS climate record;
in-flight calibration of POLDER-A polarimetry and photometry;
improved and updated look-up tables for the POLDER-A retrievals.
Channels 412, 443, 555, 672, 865, 910, 1378, 1610, 2250 nm
The idea behind the combination is that APS would make measurements along the track and those would be extended across the track by POLDER-A
APS
Polder A
APS angular scanning

14. MSPI - Advanced MISR Instrument
Multiple cameras with extended spectral range, polarimetry, and wider swath
Synergistic use of multiple techniques reduces retrieval indeterminacies
multiangle: particle size, shape, retrievals over bright regions (deserts, cities)
multispectral: particle size (visible and SWIR), absorption and height (near-UV)
nominal bands: 380, 412, 446, 558, 650, 865, 1375, 1610, 2130 nm
polarimetric: size-resolved refractive index and size distribution width
nominal bands: 650, 1610 nm
Intensity only
2% polarimetry
0.5% polarimetry
NPOESS reqmt
0.5% polarimetric uncertainty is a challenging requirement for a wide field-of-view imager

15. PACS - Passive Aerosol Cloud Suite
TIR
Thermal
Cloud Scanner
Cloud-Aerosol Polarimeter
TIR
VIS/NIR
UV-VIS
NIR
Thermal Imager
ls: 8550, 11030,12020nm
X-track Swath: 90dg (single imager)
2 Angles: Nadir and Fwd 15dg apart
Spatial resolution 1.2km at nadir
Rainbow
Angles
Multi-Angle Views along track
Specs for coarse resolution component:
ls: (360?), 380, 410, 440, 550, 660, 870, 910, 1230, 1380, 1550, 1640, 2100nm
Polarization: selected channels X all channels
Along track MultiAngle views: 9-20 angles all wavelengths angles rainbow l (660nm)
Wide Swath: along and cross track

16. Cloud-Aerosol Polarimeter Detailed/High Resolution
PACS - Passive Aerosol Cloud Suite
Cloud-Aerosol Polarimeter
TIR
Thermal
Cloud Scanner
TIR
VIS/NIR
UV-VIS
NIR
Pointing
System
Detailed/High Resolution
Cloud Microphysics
VIS-NIR: 660, 870, 940, 1230, 1380, 1550,1640, 2100
TIR: 8550, 11030,12020nm
Nadir Resolution: VIS=110m, TIR=340m (less for larger array)
Pointing Capability +/- 60dg
X-track FOV options: 20dg
Must be small size/mass for pointing
Specs for high
resolution component

17. OCEAN Color Radiometer (ORCA)
MODIS/OMI
Type: Passive radiometer
Fore-optic: Rotating telescope
Aft-optic: Grating and filter-based spectrometer
Cross-track swath: ±60°
Approx. dimension: 1 m3
Measurement range: 317–1375 nm
Measurement specifics: 2 nm bandwidth ozone channel centered at 317 nm; 4–5 nm spectral resolution 345 nm – 800 nm (w/ 700 – 800 nm included for terrestrial applications); four 30 to 50 nm wide bands between 865 – 1375 nm; CCD arrays in 3 focal planes
Ground resolution at nadir: 1.1 km
SNR requirements (based on 20 nm integrated bandwidths for 345 to 800 nm & nm nm: >1000 for 345 – 400 nm; >1500 for 400 – 720 nm; >750 for 720 – 900 nm; > 400 for 1000 – 1400 nm
Global coverage: 2 days
MODIS
OMI

18. High Frequency µ-wave Radiometer
Submillimeter/Millimeter (SM4) Radiometer
Conical Scanning Imager with 1600 km swath
10-km spatial resolution => 0.36 pencil beam
6 Receivers > 12 Channels
Vertical + Dual Polarization at 643 GHz
{183V, 325V, 448V, 643 V&H, and 874V GHz}
Three-point calibration (hot, cold, space cold)
Heritage: MLS, CoSSIR, HERSHEL, MIRO
Earth

19. Cloud Radar
Products:
Cloud top height
Microphysical profile information
Particle phase/Glaciation height
IWC and CWC
Precipitation detection
What we would like:
Swath as well as dual frequencies (W and Ka)
Even a narrow swath will be hard due to narrow back scattering phase function
Lower frequencies mean larger antenna
More sensitivity to precipitation
Sensitivity to low clouds (aerosols probably have more effect on them)
(-30dBz)
It is unlikely that the cloud radar can point more than 10º off nadir
New Strategy: as with GPM and TRMM use a low frequency radiometer to increase the precipitation measurement swath

20. Low Frequency µ-wave Radiometer (GMI)
GMI Key Products
Rain rates from ~0.3 to 110 mm/hr
Increased sensitivity to light rain over land and falling snow
CM1 would be a GPM daughter satellite
GMI Key Parameters
Mass (with margin):~150 kg
Power:~125 W
Data Rate:~30 kbps
Antenna Diameter:~1.2 m
Channel Set:
10.65 GHz, H & V Pol
18.7 GHz, H & V Pol
23.8 GHz, V Pol
36.5 GHz, H & V Pol
89.0 GHz, H & V Pol
166 GHz, H & V Pol,
183±3 GHz, V (or H) Pol
183±8 GHz, V (or H)
(166 and 183 GHz to have same resolution as 89 GHz)
Same as HF radiometer
Ball Aerospace and Technology
Corporation (BATC) is developing GMI

21. ACOB: Two Spacecraft Observing Geometry
Orbit: 650 km SS
ORCA
Multi-angle
multi-wavelength
polarimeter
Radiometers
HF (Orange)
LF (Purple)
Cloud Radar
Multi-beam Lidar
ORCA (120º)
Polarimeter & Radiometers (90º)
Lidar (30º)
90º
Radar (20º)
20º
30º

22. Next Steps Community driven STM and white paper
IMDC studies of payload
Cost estimates
cheaper than the space station
more near term than the human settlement of Mars
HQ buy in

23. Synergies between aerosol and ocean ecosystem/biomass measurements
Ocean measurement requirement
Novel use of near-UV wavelengths to separate non-living organic material from phytoplankton
Biomass assessment in coastal and turbid waters
Suspended matter concentrations
Aerosol payload benefit
Accurate characterization of aerosol properties is essential because optical depths are high in this spectral region; passive and active combination provides sensitivity to aerosol absorption and height
Multiangle observations at shortwave-IR wavelengths permit atmospheric correction over bright waters. Observations within and outside of glint pattern constrain surface wind speed, aerosol optical depth, and particle size distribution
Independent assessment using lidar observations
Ocean color payload benefit
Simultaneous measurement of ozone concentration
UV spectrometery to 345 nm provides associated trace gas sensitivity and potential simplification of aerosol radiometer design
Aerosol measurement requirement
Stratospheric ozone correction
Aerosol absorption, height, and chemical environment

24. ACOB and Climate
ACOB will link the whole spectrum of particles from aerosols-clouds-precipitation to untangle the climate/aerosol impacts
ACOB will provide simultaneous measurements of these key parameters within the same footprint.
ACOB will quantify the ocean carbon cycling and the biological pump component