Improving Performance of an Aethalometer for Measuring Aerosol Absorption

Fossil fuel and biomass combustion releases aerosols into Earth’s atmosphere which strongly absorb incoming solar radiation, contributing significantly to positive radiative forcing (global warming). Given the uncertainty on the extent of this warming effect by aerosols (Alexander et al., 2013), it is paramount that aerosol absorption be measured accurately for translation to reliable climate models. The Aethalometer measures the attenuation of light by aerosol particles deposited on a filter and is the most popular instrument for real-time monitoring of aerosol absorption. These instruments, however, suffer from a reduction in sensitivity as loading on the filter increases, a phenomenon called the filter-loading effect (FLE) (Gundel et al., 1984). In 2015, Drinovec et al. introduced DualSpot® technology in the seven-wavelength AE33 Aethalometer (Aerosol Magee Scientific), providing a way to correct for the FLE by generating a compensation factor (k) in real-time. While the AE33 is currently the most widely deployed aethalometer, and generally regarded as the most reliable absorption instrument on the market, we observe that it performs poorly compared to a reference instrument. In the present study, we demonstrate how the basis of DualSpot® technology can be used to generate an internal consistency metric to diagnose issues in AE33 operation, wherein we identify three sources of error: (1) the weighting method used to smooth values of k, (2) the use of cumulative as opposed to instantaneous sample volume, and (3) unnecessary correction for discrepancies in the sample flow. Finally, we employ an alternative approach for calculating a compensation parameter using DualSpot® which significantly outperforms the default AE33 method, providing far more reliable measures of aerosol absorption, and hence, reducing the uncertainty of the role of aerosols in climate change. 

References 

Alexander, L.; Allen, S.; Bindoff, N.; Breon, F.; Church, J.; Cubasch, U.; Emori, S.; Forster, P.; Friedlingstein, P.; Gillett, N.; Gregory, J.; Hartmann, D.; Jansen, E.; Kirtman, B.; Knutti, R.; Kanikicharla, K.; Lemke, P.; Marotzke, J.; Masson-Delmotte, V.; Xie, S, Climate change 2013: The physical science basis, in contribution of Working Group I (WGI) to the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC), 2013.

Gundel, L.A., Dod, R.L., Rosen, H., and Novakov, T. (1984). The relationship between optical attenuation and black carbon concentration for ambient and source particles. Sci. Total Environ. 36:197–202. doi:10.1016/0048-9697(84)90266-3. 

Drinovec, L., Močnik, G., Zotter, P., Prévôt, A.S.H., Ruckstuhl, C., Coz, E., Rupakheti, M., Sciare, J., Müller, T., Wiedensohler, A., and Hansen, A.D.A. (2015). The “dual-spot” Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation. Atmos. Meas. Tech. 8 (5):1965–1979. doi:10.5194/amt-8-1965-2015.