PSAP corrections. Observations Based Research. OBR Technical Note No th August Kate Turnbull. Amendment to MRF Technical Note No.

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1 Observations Based Research corrections OBR Technical Note No th August 2010 Kate Turnbull _OBR_TN80_v1_0-1 rown copyright 2008 Amendment to MRF Technical Note No. 31

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3 ontents 1. Introduction The requirement for corrections Flow rate correction Spot size correction Summary of corrections omparing the new and old corrections References...6 rown copyright

4 1. Introduction A Particle Soot Absorption Photometer () designed by Radiance Research is flown as part of the core aerosol cability on the FAAM research aircraft. The instrument measures the aerosol absorption coefficient at a single wavelength (λ=567nm). By combining the data with scattering data from the nephelometer, the single scattering albedo can be derived. This is an important optical property of atmospheric aerosols used in radiative transfer calculations. MRF Technical note 31 describes correction factors based on work by Bond et al. (1999) (hereafter B1999) that should be plied to the uncorrected data contained in the FAAM core NetDF. The publication of a comment by Ogren (2010) clarifying the original work in B1999 caused the correction algorithms to be revisited and errors to be uncovered. This note details corrections that should be plied to all data gathered by FAAM, both past and future. In the hope of minimising confusion, the only references to corrections derived in Tech Note 31 are included in section 4 which examines how this updated correction compares with earlier work. 2. The requirement for corrections operation is based on the measurement of a change in light transmission of a 567nm LED through a quartz-fibre filter as particles are deposited on it. The theoretical absorption coefficient at 567nm wavelength, 567 is given by: A I = ln (1) V I where A is the area of the spot caused by the aerosol on the filter, V is the volume of air drawn through the filter during a given time period and I 0 and I are the average filter transmittances at the start and end of this time period, respectively. The reported absorption coefficient,, is related to 567 by an empirical calibration plied internally by the instrument to account for the magnification of absorption by the filter medium and non-linearities in the response of the unit as the filter becomes loaded. From equation (1), Bond et al. (1999) deduce that errors in data arise from 1) Inaccurate assumptions about spot size, hence A. 2) Inaccurate assumptions about flow rate, hence V. 3) Multiple opportunities for the particle to absorb light. 4) Reduction in transmission owing to particulate light scattering. rown copyright

5 Data can be usted,,for these effects using = (2) where is a flow-rate correction and corrects the spot-size assumption. B1999 assumes that the usted instrumental response is a linear function of both the absorption coefficient,, and the scattering coefficient, sp, such that K1 sp + K 2 = (3) For an ideal measurement, scattering particles have no effect and there is 100% efficiency in sampling absorption; K 1 = 0 and K 2 = Flow rate correction B1999 found that the sample flow rate measured internally by the can be in error by as much as 20% and varies between instruments. The flow rate should be measured directly using e.g. an electronic bubble flow meter to derive the flow correction factor, Q = (4) QTRUE where Q is the air flow rate measured internally by and Q TRUE is measured directly. Essentially, this is a re-calibration of the internal flow meter. Haywood and Osborne (2000) reported a systemic under-estimation of the actual flow resulting in = 0.84 ± The calibration was repeated by FAAM staff (Jamie Trembath) in October 2006 giving = It is recommended that should be measured annually and that data-users ensure they use the most recent calibration. 2.2 Spot size correction The manufacturer s calibration of assumes a spot area (A ) of 17.83mm 2, equivalent to a circular spot diameter of 4.765mm. This is the diameter of the hole in the filter holder, which is smaller than the exposed filter area owing to the use of O-rings in the filter holder to provide an airtight seal. A is used internally by the instrument in its derivation of. B1999 observed some variation in spot size among instruments and recommend that the actual spot area (A TRUE ) for each instrument should be measured and used to correct the spot area of the manufacturer s reference instrument (A REF ). rown copyright

6 A TRUE = (5) AREF However, Ogren (2010) notes that the spot size correction should in fact compare the actual spot area A TRUE for the instrument with the spot area assumed by in its internal calculations, A, rather than comparing it to the reference instrument. Thus, the spot size correction factor should really be and = A A TRUE where the asterisk denotes the modified definition. (6) = (7) The calibrations in B1999 use the definition of in equation 4 to relate the usted absorption coefficient to the actual absorption coefficient (at 550nm) and derive K 1 and K 2. Thus, in order to be able to use the corrected definition in equation 5 while using the calibration constants derived in B1999, must be related to the usted absorption coefficient from the original per using equation 5 of Ogren s work. A = A REF A = A TRUE This introduces an additional factor of since A REF = 20.43mm 2 and A = 17.83mm 2 (B1999). For the operated by FAAM, the diameter of the spot was found to be 5.19mm (Haywood and Osborne, 2000), equivalent to an area A TRUE = 21.16mm 2. This results in = 1.186, in contrast to = using the B1999 technique. It is recommended that A TRUE should be measured annually and that data-users ensure they use the most recent calibration. (8) 3. Summary of corrections By reference to equations 3 and 8, the correction that should be plied to the FAAM is given in equation K1 sp = (9) K 2 rown copyright

7 where K 1 and K 2 are defined by extensive calibration tests in B1999 and values plicable to the FAAM can be found in table (Oct 09) K ± 0.02 K ± Table 1: onstants as plicable to FAAM at time of writing Ogren (2010) observes that, by virtue of the reference absorption being evaluated at 550nm in their tests, these values of K 1 and K 2 implicitly include a wavelength ustment and result in an estimate of absorption at 550nm, 550. Since TSI 3563 scattering measurements used in the tests were NOT corrected for angular nonidealities, the corrections described by Anderson and Ogren (1998) must not be plied to 550nm nephelometer data when correcting data. 4. omparing the new and old corrections The calculation for provided in Tech Note 31 is = (10) Haywood and Osborne (2000) recognised that the true spot size should be compared with the spot size assumed internally by rather than the manufacturer s reference instrument. However, the inverse of the fraction was used and the need for the additional factor of required to use the B1999 calibration coefficients was over-looked. Using the values from table 1 in equations 8 and 10 reveals that this amounts to a 19% underestimation in. If the reader has been using the Tech Note 31 value of 0.84 for, a 25% under-estimation in would result. There are further differences between the value of as calculated using Tech Note 31 shown in equation 11, rown copyright

8 567 = K 2 K and the calculation described in this document that data users should use (equation 9). In converting scattering data to 567nm before using it to correct, an error is introduced which is compounded if the scattering data is also corrected for angular non-idealities first. Another source of error arises if is usted to 550nm for use in e.g. the derivation of the single scattering albedo, since the output from the sp corrections in B1999 is already the absorption coefficient at 550nm. For flights during the EM25 project in June 2009, particle absorption corrected using Tech Note 31 would be systematically under-estimated by proximately 30% if equation 11 were used rather than the updated correction described in this document (equation 9). (11) 5. Acknowledgements Thanks to Ellie Highwood and Gavin McMeeking for their help in sorting this out! 6. References Anderson, T.L. and Ogren, J.A., 1998: Determining aerosol radiative properties using the TSI 3563 integrating nephelometer. Aerosol Sci. and Technol., 29, Bond, T.., Anderson, T.L. and ampbell, D., 1999: alibration and inter-comparison of filter-based measurements of visible light absorption by aerosols. Aerosol Sci. and Technol., 30, Haywood, J.M. and Osborne, S.R., 2000: orrections to be plied to the and nephelometer for determination of the absorption coefficient, scattering coefficient and single scattering albedo. MRF Technical Note No. 31. Ogren, J.A., 2010: omment on alibration and Intercomparison of filter-based measurements of visible light absorption by aerosols. Aerosol Sci. and Technol., 44, rown copyright

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