Analysis of SPUV aerosol data for the May 2003

BBC2 campaign

Piet StammesWouter Knap

KNMI

The SPUV sunphotometerhas 6 spectral channels: 368 nm501 nm675 nm780 nm871 nm 940 nm (H2O)

These channels are 2-15 nm wide.

Data description

• SPUV data available for all days of May 2003, except 22 May.

• Data are 1-minute values of aerosol optical thickness at 368, 501, 675, 780 and 871 nm.

• AOT spectral dependence is characterized by the Angstrom relation:

AOT () -

Data processing

1. Absolute calibration uses the De Bilt time-series (June 1997- March 2003) and the Jungfraujoch calibration campaign (Aug.-Nov. 2003).

2. Correction for Rayleigh scattering and ozone absorption uses actual data of surface pressure and ozone column.

3. Cloud detection is improved: repeated triplet test.

4. Only days with >2 % cloud-free points for the 501 nm channel are considered.

Percentage cloud-free points

Morning values Afternoon values

NB: - Not many cloud-free days during the flights between 8 and 21 May- Clear period end of the month- More cloud-free points in the morning than in the afternoon.

AOT at 501 nm

Morning averages ± Afternoon averages ±

NB: - lower AOT values until 15-20 May, higher values at end of month- PM data lower than AM data in first half of the month.

Angstrom parameter

Morning averages ± Afternoon averages ±

NB: - lower values until 15-20 May, higher values at end of month- high values of Angstrom parameter at end of the month.- low AOT, large particles in first half of month- high AOT, small particles at end of month

Examples of daily variation in aerosol on three days:

4 May, 8 May and 31 May 2003

Diurnal variation of aerosol on 4 May 2003

AOT at 501 nm Angstrom parameter

Diurnal variation of aerosol on 8 May 2003

AOT at 501 nm Angstrom parameter

Diurnal variation of aerosol on 31 May 2003

AOT at 501 nm Angstrom parameter

Number of channels to fit AOT () to -

varies over the day; this changes .

Conclusions

• Aerosol optical thickness can vary strongly over the day, probably due to advection and turbulence.

• The BBC2 days with aircraft data are not well suited for aerosol studies.

• Future work:- Intercomparison with CIMEL aerosol data- Combine with the broadband irradiances at the ground to determine the direct aerosol effect.