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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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Volume 16, issue 4
Atmos. Chem. Phys., 16, 1863-1876, 2016
https://doi.org/10.5194/acp-16-1863-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 1863-1876, 2016
https://doi.org/10.5194/acp-16-1863-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Feb 2016

Research article | 18 Feb 2016

Four-year long-path monitoring of ambient aerosol extinction at a central European urban site: dependence on relative humidity

A. Skupin, A. Ansmann, R. Engelmann, P. Seifert, and T. Müller A. Skupin et al.
  • Leibniz Institute for Tropospheric Research, Permoserstraße 15, 04318 Leipzig, Germany

Abstract. The ambient aerosol particle extinction coefficient is measured with the Spectral Aerosol Extinction Monitoring System (SÆMS) along a 2.84km horizontal path at 30–50m height above ground in the urban environment of Leipzig (51.3°N, 12.4°E), Germany, since 2009. The dependence of the particle extinction coefficient (wavelength range from 300 to 1000nm) on relative humidity up to almost 100% was investigated. The main results are presented. For the wavelength of 550nm, the mean extinction enhancement factor was found to be 1.75 ± 0.4 for an increase of relative humidity from 40 to 80%. The respective 4-year mean extinction enhancement factor is 2.8 ± 0.6 for a relative-humidity increase from 40 to 95%. A parameterization of the dependency of the urban particle extinction coefficient on relative humidity is presented. A mean hygroscopic exponent of 0.46 for the 2009–2012 period was determined. Based on a backward trajectory cluster analysis, the dependence of several aerosol optical properties for eight air flow regimes was investigated. Large differences were not found, indicating that local pollution sources widely control the aerosol conditions over the urban site. The comparison of the SÆMS extinction coefficient statistics with respective statistics from ambient AERONET sun photometer observations yields good agreement. Also, time series of the particle extinction coefficient computed from in situ-measured dry particle size distributions and humidity-corrected SÆMS extinction values (for 40% relative humidity) were found in good overall consistency, which verifies the applicability of the developed humidity parameterization scheme. The analysis of the spectral dependence of particle extinction (Ångström exponent) revealed an increase of the 390–881nm Ångström exponent from, on average, 0.3 (at 30% relative humidity) to 1.3 (at 95% relative humidity) for the 4-year period.

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