Absorption  Angstr€ om exponents of aerosols and light absorbing carbon (LAC) obtained from in situ data in Covilh~ a, central Portugal S. Mogo, * abc V. E. Cachorro, a A. de Frutos a and A. Rodrigues b Received 2nd May 2012, Accepted 18th October 2012 DOI: 10.1039/c2em30345k A field campaign was conducted from October 2009 to July 2010 at Covilh~ a, a small town located in the region of Beira Interior (Portugal) in the interior of the Iberian Peninsula. The ambient light- absorption coefficient, s a (522 nm), obtained from a Particle Soot Absorption Photometer (PSAP), presented a daily mean value of 12.1 Mm 1 (StD ¼ 7.3 Mm 1 ). The wavelength dependence of aerosol light absorption is investigated through the  Angstr€ om parameter, a a . The a a values for the pair of wavelengths 470–660 nm ranged from 0.86 to 1.47 during the period of measurements. The PSAP data were used to infer the mass of light absorbing carbon (LAC) and the daily mean varied from 0.1 to 6.8 mgm 3 . A detailed study of special events with different aerosol characteristics is carried out and, to support data interpretation, air masses trajectory analysis is performed. 1 Introduction The climatic effect of aerosols depends strongly on their optical properties. In fact, recent research shows that the understanding of aerosol effects on climate requires information not only on the aerosol amount, but also on aerosol characteristics such as size, composition and optical properties, especially including absorption, which is the least known. Further, knowledge of the connection between the chemical composition and the measured optical properties of the aerosols is largely absent. Reduction in the intensity of a direct solar beam during its propagation through the atmosphere is determined by absorp- tion and scattering processes. These two different mechanisms contribute to extinction of light, a term that means the loss of light in the atmosphere from a directly transmitted beam. The absorption, measured by the light absorption coefficient, s a , is primarily attributed to sub-micron carbon particles at solar radiation wavelengths. Light absorbing carbon (LAC) is an anthropogenic component of atmospheric aerosols produced by incomplete combustion of fossil fuel and biomass burning. On the other hand, scattering mainly arises from sulfate and nitrate aerosols from anthropogenic sources and is measured by the scattering coefficient, s s . It has also been suggested that the organic species, biogenic scattering aerosols and several other particles play important roles in aerosol light scattering. 1 The extinction coefficient, s e , is obtained from s e ¼ s s + s a . The wavelength dependence of the aerosol extinction coefficients is traditionally studied using a parameter named  Angstr€ om exponent, a. This parameter was introduced by  Angstr€ om 2 and subsequently used to represent the turbidity of the atmosphere and to model the effect of aerosols on solar radiation. 3 It is defined as the negative slope of the loga- rithm of extinction coefficient as a function of wavelength and is given by: s e ¼ Kl ae . (1) a Grupo de  Optica Atmosf erica, Univ. Valladolid, Valladolid, Spain. E-mail: sipmogo@gmail.com b Universidade da Beira Interior, Covilh~ a, Portugal c Instituto Dom Luis, Portugal Environmental impact Automated instruments such as particle soot absorption photometers (PSAPs), aethalometers and nephelometers have become usual in practice, allowing for the continuous obtainment of spectrally resolved optical properties of aerosols. The spectral variation of these properties is a valuable tool to infer the particles chemical composition and size. We present a methodology to study the spectral dependence of the absorption data that can be used to infer the composition of the particles when no chemical data are available. However, the data obtained from a PSAP need to be corrected with data obtained from a nephelometer and not always an absorption and a scattering instrument are simultaneously available. In this work we also present a method for obtaining scattering corrected absorption coefficients of aerosols. 3174 | J. Environ. Monit., 2012, 14, 3174–3181 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Journal of Environmental Monitoring Cite this: J. Environ. Monit., 2012, 14, 3174 www.rsc.org/jem PAPER