Correction for a measurement artifact of the Multi-Angle Absorption Photometer (MAAP) at high black carbon mass concentration levels

Atmos. Meas. Tech., 6, 81–90, 2013 www.atmos-meas-tech.net/6/81/2013/ doi:10.5194/amt-6-81-2013 © Author(s) 2013. CC Attribution 3.0 License. Atmospheric Measurement Techniques Correction for a measurement artifact of the Multi-Angle Absorption Photometer (MAAP) at high black carbon mass concentration levels A.-P. Hyvärinen1 , V. Vakkari2 , L. Laakso1,3 , R. K. Hooda1,4 , V. P. Sharma4 , T. S. Panwar4,5 , J. P. Beukes3 , P. G. van Zyl3 , M. Josipovic3 , R. M. Garland6,7 , M. O. Andreae6 , U. Pöschl6 , and A. Petzold8 1 Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland 2 Department of Physics, University of Helsinki, P.O. BOX 64, 00014 Helsinki, Finland 3 School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa 4 The Energy and Resources Institute (TERI), Darbari Seth Block, IHC Complex, Lodhi Road, 110 003 New Delhi, India 5 WWF India, Lodhi Road, 110 003 New Delhi, India 6 Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany 7 Natural Resources and the Environment, The Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa 8 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research IEK-8: Troposphere, 52425 Jülich, Germany Correspondence to: A.-P. Hyvärinen () Received: 10 August 2012 – Published in Atmos. Meas. Tech. Discuss.: 12 September 2012 Revised: 13 December 2012 – Accepted: 14 December 2012 – Published: 11 January 2013 Abstract. The Multi-Angle Absorption Photometer (MAAP) is a widely-used instrument for aerosol black carbon (BC) measurements. In this paper, we show correction methods for an artifact found to affect the instrument accuracy in environments characterized by high black carbon concentrations. The artifact occurs after a filter spot change – as BC mass is accumulated on a fresh filter spot, the attenuation of the light (raw signal) is weaker than anticipated. This causes a sudden decrease, followed by a gradual increase in measured BC concentration. The artifact is present in the data when the BC concentration exceeds ∼ 3 µg m−3 at the typical MAAP flow rate of 16.7 L min−1 or 1 m3 h−1 . The artifact is caused by erroneous dark counts in the photodetector measuring the transmitted light, in combination with an instrument internal averaging procedure of the photodetector raw signals. It was found that, in addition to the erroneous temporal response of the data, concentrations higher than 9 µg m−3 (at the flow rate of 16.7 L min−1 ) are underestimated by the MAAP. The underestimation increases with increasing BC accumulation rate. At a flow rate of 16.7 L min−1 and concentration of about 24 µg m−3 (BC accumulation rate ∼ 0.4 µg min−1 ), the underestimation is about 30 %. There are two ways of overcoming the MAAP artifact. One method is by logging the raw signal of the 165◦ photomultiplier measuring the reflected light from the filter spot. As this signal is not affected by the artifact, it can be converted to approximately correct absorption and BC values. However, as the typical print formats of the MAAP do not give the reflected signal as an output, a semi-empirical correction method was developed based on laboratory experiments to correct for the results in the post-processing phase. The correction function was applied to three MAAP datasets from Gual Pahari (India), Beijing (China), and Welgegund (South Africa). In Beijing, the results could also be compared against a photoacoustic spectrometer (PAS). The correction improved the quality of all three MAAP datasets substantially, even though the individual instruments operated at different flow rates and in different environments. 1 Introduction A widely used method for measuring atmospheric black carbon (BC) mass concentration involves the determination of absorption of an aerosol sample collected on an appropriate filter matrix. The most common instruments utilized Published by Copernicus Publications on behalf of the European Geosciences Union. 82 A.-P. Hyvärinen et al.: Correction for a measurement artifact of the MAAP today for this purpose are the filter-tape-based Aethalometer (Hansen et al., 1984), Multi-Angle Absorption Photometer (MAAP) (Petzold et al., 2002; Petzold and Schönlinner, 2004), and the single-filter-based Particle Soot Absorption Photometer (PSAP) (e.g., Bond et al., 1999). Since BC by definition cannot be unambiguously measured with these instruments, it is customary to refer to the measured carbonaceous light absorbing aerosol constituent as equivalent BC (BCe) or light-absorbing carbon (LAC). For the sake of simplicity, we use the term BC throughout. For a detailed discussion of the nomenclature used for black carbon or light-absorbing carbon components of the atmospheric aerosol, see, e.g., Bond and Bergstrom (2006) and Andreae and Gelencsér (2006). It is well known that filter-based BC measurements suffer from several artifacts. These include the filter loading effect that causes a decrease in the measured BC concentration with increasing filter load, and the sample matrix effect that causes scattering aerosols on the filter to increase the measured BC concentration. These artifacts can be corrected to some extent by using different numerical methods (e.g., ond et al., 1999; Weingartner et al., 2003; Arnott et al., 2005; Virkkula et al., 2007; Collaud Coen et al., 2010). All of the correction schemes have their advantages and disadvantages under field conditions. Thus far, the MAAP has been deemed as the most reliable filter-based instrument for measurement of BC, since the instrument design and software take the typical filter-related artifact effects into account. We have conducted aerosol field measurements in Gual Pahari (India) from December 2007 to January 2010, including BC measurements with the MAAP (Hyvärinen et al., 2010). During this campaign, we observed that at high BC concentrations the MAAP is not free of measurement artifacts. The observed artifact is different from those seen with other filter-based BC instruments, and to our knowledge has not been reported before in the literature. Here, we quantify this artifact with the assistance of laboratory measurements utilizing two MAAPs operating at different flow rates. The focus of this paper is to raise awareness of the MAAP artifact within the aerosol community, and to demonstrate how the artifact can be circumvented by logging the reflected photodetector signal. In addition, we present a method for correcting the results from the typical instrument print formats in the post-processing phase. The correction is applied to three MAAP datasets: Gual Pahari (India) (Hyvärinen et al., 2010); Beijing (Garland et al., 2009) (China), and Welgegund (South Africa) (Beukes et al., 2012; www.welgegund.org). In Beijing, the results could be compared against a photoacoustic spectrometer (PAS; Garland et al., 2009). Fig. 1. Schematic of the MAAP. The transmitted light is measured with (...truncated)