aerosol composition and radiative properties · 7. aerosol number concentration 8. cloud...
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Aerosol Composition and Radiative PropertiesUrs Baltensperger
Laboratory of Atmospheric ChemistryPaul Scherrer Institut, 5232 Villigen PSI, Switzerland
WMO-BIPM Workshop Geneva, 30 March – 1 April 2010
Aerosols particles: Solid or liquid particles suspended in the atmosphere
Examples:
Pollen: 10 - 100 m
Diesel soot: ca. 0.1 m
Ammonium sulfate: ca. 0.1 m
Sea salt: 0.2 - 10 m Mineral dust: 0.2 - 10 m
PM10 = Particles with aerodynamic diameter <10m
Aerosols affect our health and have an impact on climate
Source: www.ecocouncil.dk http://saga.pmel.noaa.gov/aceasia/
Direct and indirect aerosol effect on climate
Direct effect:Scattering and absorption of incoming sunlight by aerosol particles
Indirect effect:The number concentration of cloud condensation nuclei (CCN) influences the cloud droplet size and thereby changes the cloud albedo and lifetime
Indirect aerosol effect
Large dropletsWeak reflection
Small droplets Strong reflection
Indirect effectNumber of CCN influences the dropletnumber and size(Twomey-Effect) and thereby the cloudalbedo and lifetime.
Some of the major issues of aerosol particles• Aerosol particles have a very wide variety of sources• Many emission inventories have large uncertainties• A large fraction of aerosol mass is secondary (i.e., they are formed in the
atmosphere from gaseous precursors)• Many emission inventories of precursors have large uncertainties as well
(e.g. VOC‘s)• The aerosol yield from gaseous precursors has uncertainties of up to a
factor of 10• Impact is not only defined by mass, but also by size distribution and
morphology of particles• Short residence time (~1 week) results in high spatial variability, which
asks for many stations• Many of the components are semivolatile, resulting in measurement
issues (temperature dependence) • Aerosols cannot be packed into a bottle and sent around for certification
Uncertainty in the radiative forcing of black carbon (BC)
()
For comparison: CO2 forcing is 1.6 W m-2 Chung and Seinfeld (JGR 2002)
IPCC value forforcing by BC
The uncertainty in the aerosol forcingis a major reason for our limited
understanding of the total radiative forcing
IPCC (2007)
WMO GAW SAG:Scientific Advisory Group For Aerosols
Continuous Measurement 1. Multiwavelength optical depth 2. Mass in two size fractions 3. Major chemical components in two size fractions 4. Light absorption coefficient 5. Light scattering coefficient at various wavelengths 6. Hemispheric backscattering coefficient at various
wavelengths 7. Aerosol number concentration 8. Cloud condensation nuclei at 0.5% supersaturation
Intermittent Measurement 1. Aerosol size distribution 2. Detailed size fractionated chemical composition 3. Dependence on relative humidity 4. CCN spectra (various supersaturations) 5. Vertical distribution of aerosol properties
GAW Report # 153. WMO/GAW Aerosol Measurement procedures guidelines and recommendations (September 2003)
List of comprehensive aerosol measurements with a subset of core variables (identified in bold) that are recommended by the GAW Scientific Advisory Group on Aerosols for long-term measurements in the global network.ftp://ftp.wmo.int/Documents/PublicWeb/arep/gaw/gaw153.pdf
In an ideal world
• There is an agreed method• Instruments of different institutions have successfully
been intercompared• There is a hierarchy of standards
In an ideal world
• There is an agreed method• Instruments of different institutions have successfully
been intercompared• There is a hierarchy of standards
The ideal world does not exist for any of the aerosolvariables
Closest to the ideal world: Aerosol optical depth(i.e., aerosol extinction coefficient integrated over full column)
Aeronet
Different instruments,Different calibrationprocedures, but:AOD usually showsexcellent agreementbetween proper instrumentsCan also be measuredfrom satellites
Plus many othernational networks(Russia, ……)
A WMO/GAW Experts Workshop“A Global Surface-Based Network for Long Term Observations of
Column Aerosol Optical Properties”March 2004, WORCC Davos, GAW Report # 162
ftp://ftp.wmo.int/Documents/PublicWeb/arep/gaw/gaw162.pdf
There is still a lack of coordination between networks, but there is an agreement for further harmonization under the lead of the SAG
Vertical profiles
In addition:- MPLNET, a global network
- Satellites
Different instruments, different procedures, much less agreementbetween data than for AOD
Distribution of stations
ALINE, Latin America
AD-Net, East Asia
CIS-LINET, Commonwealth of Independent States
EARLINET, Europe
NDACC, Global Stratosphere
REALM, Eastern North America
MPLNET, Global, Micropulse Lidar
Plan for the implementation of the GAW Aerosol Lidar Observation Network
GALION, WMO/GAW Report No. 178http://www.wmo.int/pages/prog/arep/gaw/documents/gaw178-galion-27-Oct.pdf
PM10, PM2.5 and/or PM1 and chemistryLargely separated networks, Within individual networks: regular quality control, but mostly no formally established intercomparison between networks
EUSAAR
The worst of all possible situations:Organic and elemental carbon
Schmid et al., 2001: Factor of 10 difference between individual methods;Today: Factor of ~2
Until recently two competingprocedures in the US, from whichthe IMPROVE procedure survived
Today, intercomparison betweenIMPROVE, ENV-CANADA and EUSAAR (Europe): still large discrepancies.
We do not even have decent reference samples: reference material NIST 8785 was shown to have drifts and inhomogeneities in their samples.
A possible way out: use advanced optical methods (e.g., SP2) to determineblack carbon, rather than chemical determination of EC.
More promising:Physical and optical properties
‘Advantage‘: no established network when SAG started in 1997 (except NOAA) Adoption of guidelines by SAG (GAW Report # 153, 2003) by most stationsFurther development involved all major players, under the lead of the Aerosol SAG (ftp://ftp.wmo.int/Documents/PublicWeb/arep/gaw/gaw153.pdf)
EUSAAR: a Blueprint for harmonizationExample: Stations reporting physical or optical properties inEurope:
in 2002 in 2008
Stations are using the same protocol,Instruments are regularly intercompared,Data are stored in agreed format in World Data Center for Aerosols,Some recommendations were transferred to the German UBA, the German DWD, and to the British National Physical Laboratory, NPL
Long-term trends are becoming possibleExample: Jungfraujoch, Switzerland, 3580 m asl
10 years of data are necessary
June - August: no significant trend ofbs, babs, and CN
September - December: significant positive trend of 2 - 4% per year (for bs, babs, and CN)
Collaud-Coen et al. JGR 2007
Capacity building is an important issue
• On-site audits (mainly done by World Calibration Center for Aerosol Physics)
• Instrument intercomparisons• Traveling standards (size distribution, PFR for AOD
measurement)• Training courses• Twinning activities
Towards an integrated aerosol approach
An important issue for this integrationin situ variables are measured ‘dry‘ (<40%RH),
for comparison e.g. with satellite products these need to beconverted to ambient conditions
Example: Enhancement of scattering coefficient with increasing RH
Summary
• Aerosol observations have made great progress in the last decade, in method development, standardization, harmonization between networks, capacity building, and data availability (at new World data center for aerosols, http://ebas.nilu.no/
• Challenges remain, in all above aspects• We need to work together towards an Integrated observing
system building on satellite, ground-based remote sensing and in situ data
Where could BIPM contribute?
• Formulating SOPs• Supporting intercomparisons• Providing suitable reference maetrials• Develop standards for aerosol number concentration• Develop standards for sampling inlets and sample
conditioning• Transferring knowledge to instrument manufacturers etc.• ….• A representative of BIPM in the Aerosol SAG could be useful
Thank you for your attention