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In situ measurements of aerosol optical properties and number size distributions in a coastal region of Norway during the summer of 2008

by S. Mogo, V. E. Cachorro, J. F. Lopez, E. Montilla, B. Torres, E. Rodríguez, Y. Bennouna, A. M. De Frutos
Atmospheric Chemistry and Physics ()
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In situ measurements of aerosol optical properties and particle size\ndistributions were made in the summer of 2008 at the ALOMAR station\nfacility (69 degrees 16 ` N, 16 degrees 00 ` E), located in a rural site\nin the north of the island of Andoya (VesterAyenlen archipelago),\napproximately 300 km north of the Arctic Circle. The extended\nthree-month campaign was part of the POLARCAT Project (Polar Study using\nAircraft, Remote Sensing, Surface Measurements and Models, of Climate,\nChemistry, Aerosols, and Transport) of the International Polar Year\n(IPY-2007-2008). Our goal was to characterize the aerosols of this\nsub-Arctic area, which are frequently transported to the Arctic region.\nData from 13 June to 26 August 2008 were available and the statistical\ndata for all instruments were calculated based on the hourly averages.\nThe overall data coverage was approximately 72%. The hourly mean values\nof the light-scattering coefficient, Sigma(s), and the light-absorption\ncoefficient, Sigma(a), at 550 nm were 5.41 Mm(-1) (StD = 3.55 Mm(-1))\nand 0.40 Mm(-1) (StD = 0.27 Mm(-1)), respectively. The\nscattering/absorption Angstrom exponents, alpha(s,a), were used in a\ndetailed analysis of the variations of the spectral shape of Sigma(s,a).\nWhile alpha(s) indicates the presence of two particle sizes\ncorresponding to two types of aerosols, alpha(a) indicates only one type\nof absorbing aerosol particle. alpha(a) values greater than 1 were not\nobserved. The single-scattering albedo, omega(0), ranged from 0.62 to\n0.99 (mean = 0.91, StD = 0.05), and the relationships between this\nparameter and the absorption/scattering coefficients and the Angstrom\nexponents are presented. Any absorption value may lead to the lowest\nvalues of omega(0), whereas only the lowest scattering values were\nobserved in the lowest range of omega(0). For a given absorption value,\nlower omega(0) were observed for smaller alpha(s). The submicrometer,\nmicrometer and total concentrations of the particles presented hourly\nmean values of 1277 cm(-3) (StD = 1563 cm(-3)), 1 cm(-3) (StD = 1\ncm(-3)) and 2463 cm(-3) (StD = 4251 cm(-3)), respectively, and the modal\ncorrelations were also investigated. The optical and microphysical\nparameters, as well as their relationship with each other, are reported.\nSigma(s) correlated strongly with the number concentration of\naccumulation mode particles and more strongly with the micrometer\nfraction of particles, but weak correlations were observed for the\nAitken and nucleation modes. The origins and pathways of the air masses\nwere examined, and based on sector classification, a relationship\nbetween the air mass origin, the optical parameters and the size\ndistributions was established.\nThe low values of the optical and microphysical parameters indicate that\nthe predominant regional aerosol is mostly clean and the shape of the\nsize distribution is characterized by bimodal median size distributions.\nHowever, the relationships between the air mass origins and the\nparameters studied allow us to describe two characteristic situations:\nthe one of the northern and western air masses, which were predominantly\ncomposed of marine aerosols and presented the lowest optical and\nmicrophysical values observed, indicating predominantly non-absorbent\nand coarser particles; and the one of the eastern and southern air\nmasses, in which continental aerosols were predominant and exhibited\nhigher values for all parameters, indicating the presence of smaller\nabsorbent particles. The north-northeastern air masses presented the\nstrongest Aitken mode, indicating more recently formed particles, and\nthe southeastern air masses presented the strongest accumulation mode\n(however, the southeastern air masses were the least common, accounting\nfor only 3% of occurrences).

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