Determination and analysis of in situ spectral aerosol optical properties by a multi-instrumental approach

Abstract

Abstract. Continuous in situ measurements of aerosol optical properties were conducted from 29 June to 29 July 2012 in Granada (Spain) with a seven-wavelength Aethalometer, a Multi-Angle Absorption Photometer, and a three-wavelength integrating nephelometer. The aim of this work is to describe a methodology to obtain the absorption coefficients (babs) for the different Aethalometer wavelengths. In this way, data have been compensated using algorithms which best estimate the compensation factors needed. Two empirical factors are used to infer the absorption coefficients from the Aethalometer measurements: C – the parameter describing the enhancement of absorption by particles in the filter matrix due to multiple scattering of light in the filter matrix – and f, the parameter compensating for non-linear loading effects in the filter matrix. Spectral dependence of f found in this study is not very strong. Values for the campaign lie in the range from 1.15 at 370 nm to 1.11 at 950 nm. Wavelength dependence in C proves to be more important, and also more difficult to calculate. The values obtained span from 3.42 at 370 nm to 4.59 at 950 nm. Furthermore, the temporal evolution of the Ångström exponent of absorption (αabs) and the single-scattering albedo (ω0) is presented. On average αabs is around 1.1 ± 0.3, and ω0 is 0.78 ± 0.08 and 0.74 ± 0.09 at 370 and 950 nm, respectively. These are typical values for sites with a predominance of absorbing particles, and the urban measurement site in this study is such. The babs average values are of 16 ± 10 Mm−1 (at 370 nm) and 5 ± 3 Mm−1 (at 950 nm), respectively. Finally, differences between workdays and Sundays have been further analysed, obtaining higher babs and lower ω0 during the workdays than on Sundays as a consequence of the diesel traffic influence.

The "dual-spot" Aethalometer: an improved measurement of aerosol black carbon with real-time loading compensation

“…For the TFE-coated glass filter we obtained C ≈ 1.57 using the 2-week Klagenfurt campaign data, assuming C = 2.14 for quartz filter (Weingartner et al, 2003). There is some uncertainty with this value since values of C for quartz filter as obtained by comparisons with other techniques differ (Weingartner et al, 2003;Collaud Coen et al, 2010;Müller et al, 2011a, Segura et al, 2014.…”

“…The factor 100 is there for convenience only (Gundel et al, 1984). The loading effect was analyzed using a plot of the BC dependence on attenuation (Park et al, 2010;Segura et al, 2014). This approach is based on the idea that within a long enough measurement campaign the probability to measure a certain BC value in an ATN bin is constant and independent of the ATN:…”

“…The enhancement parameter C has no statistically significant spectral dependence (Weingartner et al, 2003;Segura et al, 2014), with its value determined by comparing different methods and instruments in laboratories and in ambient observatories (Weingartner et al, 2003;Collaud Coen et al, 2010;Müller et al, 2011a;Segura et al, 2014), resulting in a range of values. The value of C may depend on the particle properties, as shown for the particle soot absorption photometer (PSAP) (Nakayama et al, 2010).…”

“…Providing that the concentration does not change in the time, during which the filter tape is being advanced (typically 20 min), these discontinuities in the BC concentration at the time of the tape advance display the "goodness" of the compensation. A simpler method, making no assumptions, follows the method described in Park et al (2010) and is developed into a criterion for the test of the compensation in Segura et al (2014), examining the nonlinearity by plotting the dependence of the measured BC on the accumulated surface loading on the filter as reported by the measured optical attenuation.…”

Spatial and temporal variability of carbonaceous aerosols: Assessing the impact of biomass burning in the urban environment

“…Atmospheric aerosol optical properties have been widely investigated in the city of Granada at surface level (e.g., Lyamani et al, 2008;Lyamani et al, 2010;Titos et al, 2012;Segura et al, 2014), in the vertical column with height resolution (e.g., GuerreroRascado et al, 2008;Alados-Arboledas et al, 2011;Bravo-Aranda et al, 2015 and references therein) and column-integrated (e.g., Lyamani et al, 2005;Valenzuela et al, 2012). The meteorological conditions together with the topography of the city sited in a valley surrounded by mountains of high elevation (about 3500 m a.s.l.)…”