Characterization of atmospheric aerosol particles using in-situ techniquesoptical, chemical and hygroscopic properties

Abstract

Abstract This PhD dissertation focuses on the characterization of atmospheric aerosol particles by means of ground-based in-situ techniques. To this end, a complete analysis of aerosol chemical and optical properties, including aerosol hygroscopicity, has been done. A complete chemical analysis has been performed for the period 2006-2010 with special focus on the seasonality of fine and coarse fractions. A significant decreasing trend in PM10 levels has been observed related with a decrease in most of its constituents, specially marked in mineral matter levels and non-mineral carbon. In addition, the main aerosol sources contributing to the fine and coarse aerosol mass concentration were identified and apportioned by means of the Positive Matrix Factorization technique. This analysis resulted in the identification of five sources in the coarse fraction and four in the fine fraction. In spite of being relatively uncommon, the use of fine and coarse PM in the PMF analysis separately was found to be very useful to discriminate additional sources. Chemical composition of aerosol particles contributes, among other factors, to the aerosol optical properties observed. In order to find the relationship between aerosol optical properties and the chemical composition, the total scattering and absorption coefficients and speciated PM10 and PM1 data were combined for a period of one year to calculate the mass scattering and absorption efficiencies. Different existing methodologies (measurement method and Multiple Linear Regression method) were applied to determine the mass efficiencies of fine and coarse aerosol particles and of the different chemical constituents. This contributes to a better knowledge of the scattering properties of the different species since many previous works used only sulphate or bulk PM to account for the total scattering coefficient. Fine particles were found to extinct light more efficiently than coarse particles. Among the different aerosol constituents, SO42-nm exhibited the largest mass scattering efficiency and dust aerosols presented the lowest mass scattering efficiency. On the other hand, the absorption process was found to be mainly dominated by carbonaceous particles. The ability of aerosol particles to take up water also affects the aerosol optical properties observed. Aerosol particles can take up water depending on their size, chemical composition and ambient relative humidity, RH. They become larger in size than their dry counterparts, and hence, scatter more light. This change in the scattering coefficient results also in changes in the radiative forcing estimations. Since the aerosol scattering coefficient is typically measured at dry conditions, knowledge of the scattering enhancement due to water uptake is of great importance in order to convert dry measurements into more relevant ambient data. For measuring the scattering enhancement due to water uptake, a humidification system was developed and built in the frame of this thesis for an integrating nephelometer. After successfully testing the humidifier in the laboratory, two measurement campaigns were conducted in Granada during winter and spring seasons. The results obtained during these measurement campaigns are presented in this thesis. The scattering enhancement factor, f(RH=85%), was found to undergo a clear diurnal pattern. The two f(RH=85%) minima were connected to the relative increase of the non-hygroscopic fraction (such as black carbon and road dust) due to traffic emissions during the traffic rush hours. The chemical composition was found to be very important in determining the aerosol hygroscopic properties. The f(RH=85%) decreased for increasing mass fraction of particulate organic matter. Finally, the effect of RH on the radiative forcing estimations was accounted for. The RH dependency of the scattering coefficient was also investigated at a different location, Cape Cod (Massachusetts, USA). The f(RH=85%) was found to be higher than in Granada mainly because of the influence of marine aerosols. The aerosol deliquescence was studied by investigating the differences in the f(RH) versus RH curve for RH above and below 65%. A clear relationship between the single scattering albedo, ¿0, and the scattering Ångström exponent, SAE, with aerosol hygroscopicity was observed. In this sense, we propose an exponential equation that successfully estimates aerosol hygroscopicity as a function of ¿0 at Cape Cod. Referencias destacadas: Amato F, Pandolfi M, Escrig A, Querol X, Alastuey A, Pey J, et al. Quantifying road dust resuspension in urban environment by multilinear engine: a comparison with PMF2. Atmos. Environ., 43 (17), 2770-2780, 2009. Anderson TL, and Ogren JA. 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