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The ambient aerosol particle extinction coefficient is measuredwith the Spectral Aerosol Extinction Monitoring System (SÆMS) along a2.84 km horizontal path at 30–50 m height above ground inthe 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 1000 nm) on relative humidity up toalmost 100 % was investigated. The main results are presented. For thewavelength of 550 nm, the mean extinction enhancement factor wasfound to be 1.75 ± 0.4 for an increase of relative humidity from 40 to80 %. The respective 4-year mean extinction enhancement factor is2.8 ± 0.6 for a relative-humidity increase from 40 to 95 %. Aparameterization of the dependency of the urban particle extinctioncoefficient on relative humidity is presented. A mean hygroscopic exponent of0.46 for the 2009–2012 period was determined. Based on a backward trajectorycluster analysis, the dependence of several aerosol optical properties foreight air flow regimes was investigated. Large differences were not found,indicating that local pollution sources widely control the aerosol conditionsover the urban site. The comparison of the SÆMS extinction coefficientstatistics with respective statistics from ambient AERONET sun photometerobservations yields good agreement. Also, time series of the particleextinction coefficient computed from in situ-measured dry particle sizedistributions and humidity-corrected SÆMS extinction values (for 40 %relative humidity) were found in good overall consistency, which verifies theapplicability of the developed humidity parameterization scheme. The analysisof the spectral dependence of particle extinction (Ångström exponent)revealed an increase of the 390–881 nm Ångström exponent from, onaverage, 0.3 (at 30 % relative humidity) to 1.3 (at 95 % relativehumidity) for the 4-year period

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