Spectral light absorption by ambient aerosols influenced by biomass burning in the Amazon Basin – I. Comparison and field calibration of absorption measurement techniques

Abstract

Abstract. Spectral aerosol light absorption is an important parameter for the assessment of the radiation budget of the atmosphere. Although on-line measurement techniques for aerosol light absorption, such as the Aethalometer and the Particle Soot Absorption Photometer (PSAP), have been available for two decades, they are limited in accuracy and spectral resolution because of the need to deposit the aerosol on a filter substrate before measurement. Recently, a 7-wavelength (λ) Aethalometer became commercially available, which covers the visible (VIS) to near-infrared (NIR) spectral range (λ=450–950 nm), and laboratory calibration studies improved the degree of confidence in these measurement techniques. However, the applicability of the laboratory calibration factors to ambient conditions has not been investigated thoroughly yet. As part of the LBA-SMOCC (Large scale Biosphere atmosphere experiment in Amazonia – SMOke aerosols, Clouds, rainfall and Climate) campaign from September to November 2002 in the Amazon basin we performed an extensive field calibration of a 1-λ PSAP and a 7-λ Aethalometer utilizing a photoacoustic spectrometer (PAS, 532 nm) as reference device. Especially during the dry period of the campaign, the aerosol population was dominated by pyrogenic emissions. The most pronounced artifact of integrating-plate type attenuation techniques is due to multiple scattering effects within the filter matrix. For the PSAP, we essentially confirmed the laboratory calibration factor by Bond (1999). On the other hand, for the Aethalometer we found a multiple scattering enhancement of 5.23 (or 4.55, if corrected for aerosol scattering), which is significantly larger than the factors previously reported (~2). While the exact reason for this discrepancy is unknown, the available data from the present and previous studies suggest aerosol mixing (internal versus external) as a likely cause. While it is well-known that RH may (moderately) affect aerosol absorption, we found no dependence of either PSAP or Aethalometer on relative humidity (RH) for 30%<RH<55% and 40%<RH<80%, respectively. However, a substantial decrease in PSAP sensitivity was observed for low RH (20%<RH<30%). In addition, while the PSAP demonstrated no sensitivity to gaseous adsorption, the Aethalometer response was clearly positively correlated with the gradient in pollution level. Hence, although very similar in measurement principle, the PSAP and Aethalometer require markedly different correction factors, which is probably due to the different filter media used. Although on-site calibration of the PSAP and Aethalometer is suggested for best data quality, we recommend a set of PSAP and Aethalometer correction factors for ambient sampling based on the data from the present and previous studies. For this study, the estimated accuracies of the absorption coefficients determined by the PAS, PSAP and Aethalometer were 10, 15 and 20%, respectively.

Comparison of elemental and black carbon measurements during normal and heavy haze periods: implications for research

“…Two methods can be used to obtain b ap . For the first method, a nephelometer can be used to monitor aerosol scattering (b scat ) simultaneously with an aethalometer to monitor ATN; b ap can then be calculated using a correction procedure (Arnott et al 2005;Coen et al 2010;Schmid et al 2006;Weingartner et al 2003). In the second approach, b ap can be monitored directly using non-filter-based instruments.…”

Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols

“…When absorption is stronger at shorter wavelengths, i.e., the particles appear brownish or yellow,Å will be significantly greater than unity. Values of 2.0 and more were found in aerosol collected near a lignite combustion plant (Bond et al, 1999b), in biomass smoke (Kirchstetter et al, 2004;Schmid et al, 2005;Schnaiter et al, 2005b), environmental tobacco smoke (Lawless et al, 2004), rural aerosol (Lindberg et al, 1993), and in particles produced by oxygen-deficient combustion of propane (Schnaiter et al, 2006). The highest values ofÅ are found for the yellowish to brownish products of pure smoldering combustion, e.g., tobacco smoke (3.5, Lawless et al, 2004) or water-soluble HULIS from biomass burning (6-7, Hoffer et al, 2005).…”

“…The absorption cross section of C brown increases very sharply with decreasing wavelength, with a slope of λ −2 to λ −6 , whereas C soot (e.g., from diesel vehicles) shows a λ −1 dependence (Kirchstetter et al, 2004;Hoffer et al, 2005;Schmid et al, 2005) (see also Sect. 3).…”

A critical review of filter transmittance measurements for aerosol light absorption, and de novo calibration for a decade of monitoring on PTFE membranes

“…Hansen et al (1984) developed a continuous integrating plate monitor using optically thick quartz fiber filters, and instrument vendors have since produced a variety of user-ready monitors derived from this and other integrating plate designs. A great deal of work has been done to characterize and reconcile the responses of such instruments to realistic aerosols, both in the lab (Gerber and Hindman 1982;Clarke et al 1987;Hitzenberger 1993;Horvath 1993;Bond et al 1999;Weingartner et al 2003;Sheridan et al 2005;Muller et al 2011) and through comparisons in the field (Liousse et al 1993;Reid et al 1998;Schmid et al 2006;Chow et al 2009;Collaud Coen et al 2010). These studies have identified sources of measurement bias that can be loosely grouped into three categories requiring different forms of correction: filter effects, loading dependence, and particle scattering interference.…”