Ronald H. Brown ship Aerosol in-situ scattering and absorption during NEAQS 2004

 

AUTHORS:

 

Berko Sierau* (bsierau@atmos.washington.edu  ph: 206-543-6674)

David S. Covert (dcovert@u.washington.edu  ph: 206-685 7461)

Dept. of Atmospheric Science

University of Washington

Box 351640

Seattle, WA  98195 USA

fax: 206-543-0308

* contact for questions about the data

 

Patricia Quinn (patricia.k.quinn@noaa.gov)

NOAA Pacific Marine Environmental Laboratory

7600 Sand Point Way NE

Seattle, WA, USA

 

 

INSTRUMENT DESCRIPTION:

 

A suite of instruments was used to measure light scattering and absorption.  Two TSI, Inc. integrating nephelometers (Model 3563) measured integrated total scatter  and hemispheric backscatter at 450, 550, and 700nm wavelengths (Anderson et al, 1996; Anderson and Ogren, 1998).  One nephelometer ("SupermicronScattering/Neph10") always measured aerosols of aerodynamic diameter D<10micron; the second nephelometer("SubmicronScattering/Neph1") measured only aerosol of aerodynamic diameter D<1micron. Both nephs measured at approximately 60% RH. The 10 and 1micron cut-off was accomplished by using standard berner impactors. One Radiance Research Particle Soot Absorption Photometer ('SubmicronAbsorption/PSAP1') was used to measure light absorption by aerosols at 467, 530, and 660nm (Bond et al., 1999; Virkkula et al.,2005) under'dry' (<25% RH) conditions.

 

Additionally, a 'dry' system measured total scatter ('OECneph', extinction ('OEC'), and absorption ('PSAP2') at RH<25%. The system includes a TSI, Inc. integrating nephelometer measuring scattering only at 550nm, a three-wavelength (467, 530, 660nm) optical extinction cell (Virkkula, 2005), and a three-wavelength Radiance Research Particle Soot Absorption Photometer (again 467, 530, and 660nm). A Condensation Particle Counter (TSI, Inc. 3010) measured total particle counts, as sampled from the nephelometers, for quality-control only. 

 

 

3.0 DATA COLLECTION AND PROCESSING

 

Data of the 'wet' system were collected at between 1 and 5 sec resolution, depending on the instrument (1 sec for PSAP, 5 sec for Neph). However, while reported at 1-5 Hz, light absorption and scattering measurements represent 60-second averages which is also the time resolution the data will be provided. Data from each instrument are corrected and adjusted as described below, allowing for derivation of extensive parameters (light scattering and absorption) and intensive parameters (single scatter albedo, Angstrom exponent). For the 60-second data files light scattering values are smoothed over a window 40-seconds wide before calculating the Angstrom exponents.  Similarly, light absorption is instrument-internally smoothed over a window 10-seconds wide. The smoothed neph data are used *only* to calculate the intensive parameters; in all cases, the reported extensive parameter light scattering is un-smoothed. The Angstrom exponent is smoothed over a 40-seconds window before averaging.  Data of the 'dry' system were also collected between 1 and 5 sec resolution but are reported at a 1080 sec resolution. This is due to the determination of the light extinction which is based on a 1080 sec average time.

 

For all parameters, the bad value code is "NaN" (-999 in the .acf files). If no data was acquired by the data acquisition system, time lacks exist in the data files.  Intensive parameters are set to NaN when the extensive properties used in their calculation fell below the measurement noise threshold (v2 noise thresholds are based on former campaigns). Both extensive and intensive properties are set to NaN during certain events, such as during filter changes, instrument calibration, obvious instrument failure etc. Negative values of absorption might occur during periods of absorption signals near or in the

range of the instrument noise, and are partly shifted into the negative range due to  scattering correction.

 

STP values to be adjusted to are p_STP=1013.2 hPa, T_STP=273.2 K.

 

 

DERIVATI ON OF MEAN VALUES:

 

EXTENSIVE PARAMETERS

 

Data from the TSI integrating nephelometers Neph10 and Neph1, and OECneph are processed as  follows:

      1) Span gas (air and CO2) calibrations were made before the field

campaign. However, due to problems of the Neph1 calibration shutter during

the cruise several additional calibrations had to be made during the cruise

to determine TSI nephelometer gain and offset calibration coefficients. This

calibration correction was applied for the Neph1 scattering data in the data

processing code.

      2) The TSI nephelometers measure integrated light scattering into 7-170

degrees.  To derive total scatter (0-180degrees) and hemispheric backscatter

(90-180degrees) angular truncation correction factors were applied as

recommended by Anderson and Ogren (1998).

      3) Total and hemispheric backscatter were adjusted to STP.

     

 

Data from the Radiance Research Particle Soot Absorption Photometers, PSAPs 1 and 2,

are processed as follows:

      1) Reported values of light absorption are corrected for spot size, flow  rate, artifact response to scattering, and error in the manufacturer's  calibration, all given by Bond et al. (1999). Except the spot size, all corrections were made after data collection, i.e. they are not integrated into the PSAP firmware. However, the PSAP's were flow-calibrated prior to the campaign, and a flow correction was applied based on routine flow checks during the cruise.

      2) Light absorption is adjusted to STP

 

     

DATA FORMAT

 

All varibles are in STP.

 

REFERENCES

 

Anderson, T.L., D.S. Covert, S.F. Marshall, M. L. Laucks, R.J. Charlson, A.P. Waggoner, J.A. Ogren, R. Caldow, R. Holm, F. Quant, G. Sem, A. Wiedensohler, N.A. Ahlquist, and T.S. Bates, "Performance characteristics of a high-sensitivity, three-wavelength, total scatter/backscatter nephelometer", J. Atmos. Oceanic Technol., 13, 967-986, 1996.

 

Anderson, T.L., and J.A. Ogren, "Determining aerosol radiatve properties using the TSI 3563 integrating nephelometer", Aerosol Sci. Technol., 29, 57-69, 1998.

 

Bond, T.C., T.L. Anderson, and D. Campbell, "Calibration and intercomparison of filter-based measurements of visible light absorption by aerosols", Aerosol Sci. and Tech., 30, 582-600, 1999.

 

A. Virkkula, N. C. Ahquist, D. S. Covert, P. J. Sheridan, W. P. Arnott, J. A Ogren,  "A three-wavelength optical extinction cell for measuring aerosol light extinction and its application to determining absorption coefficient", Aero. Sci. and Tech., 39, 52-67, 2005

 

A. Virkkula, N. C. Ahquist, D. S. Covert, W. P. Arnott, P. J. Sheridan, P. K. Quinn, D. J. Coffman,"Modification, calibration and a field test of an instrument for measuring light absorption by particles", Aero. Sci. and Tech., 39, 68-83, 2005