Aerosol Composition and Radiative Properties Urs Baltensperger Laboratory of Atmospheric Chemistry Paul Scherrer Institut, 5232 Villigen PSI, Switzerland WMO-BIPM Workshop Geneva, 30 March 1 April 2010
Examples: Aerosols particles: Solid or liquid particles suspended in the atmosphere PM10 = Particles with aerodynamic diameter <10 m Diesel soot: ca. 0.1 m Ammonium sulfate: ca. 0.1 m Pollen: 10-100 m Sea salt: 0.2-10 m Mineral dust: 0.2-10 m
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 Indirect effect Large droplets Weak reflection Small droplets Strong reflection Number of CCN influences the droplet number and size (Twomey-Effect) and thereby the cloud albedo 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) ( ) IPCC value for forcing by BC For comparison: CO 2 forcing is 1.6 W m -2 Chung and Seinfeld (JGR 2002)
The uncertainty in the aerosol forcing is a major reason for our limited understanding of the total radiative forcing IPCC (2007)
WMO GAW SAG: Scientific Advisory Group For Aerosols 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 GAW Report # 153. WMO/GAW Aerosol Measurement procedures guidelines and recommendations (September 2003) 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
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 aerosol variables
Closest to the ideal world: Aerosol optical depth (i.e., aerosol extinction coefficient integrated over full column) Plus many other national networks (Russia, ) Aeronet Different instruments, Different calibration procedures, but: AOD usually shows excellent agreement between proper instruments Can also be measured from satellites
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 agreement between 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. 178 http://www.wmo.int/pages/prog/arep/gaw/documents/gaw178-galion-27-oct.pdf
PM10, PM2.5 and/or PM1 and chemistry Largely 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 competing procedures in the US, from which the IMPROVE procedure survived Today, intercomparison between IMPROVE, 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 determine black 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 stations Further 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 harmonization Example: Stations reporting physical or optical properties in Europe: 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 possible Example: Jungfraujoch, Switzerland, 3580 m asl 10 years of data are necessary June - August: no significant trend of b s, b abs, and CN September - December: significant positive trend of 2-4% per year (for b s, b abs, 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 integration in situ variables are measured dry (<40%RH), for comparison e.g. with satellite products these need to be converted 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