The optical properties of atmospheric aerosol particles were measured close to ground level using different methods at Lindenberg/Falkenberg (Germany) during the Lindenberg Aerosol Characterization Experiment (LACE 98), 13 July 1998 to 14 August 1998 <Ansmann et al., 2002>. The experimental setup consisted of (a) an aerosol photometer, which measured a complete set of aerosol optical properties, such as extinction, scattering, and absorption coefficients, single scattering albedo, apparent complex refractive index, asymmetry parameter of the phase function of scattering, and apparent volume soot content, (b) an integrating plate, which measured the absorption coefficient, and (c) a nephelometer and (d) a particle soot absorption photometer (PSAP) for measurements of scattering and absorption coefficients. All these measurements were performed under dry conditions. A telephotometer and a horizontal lidar were used to determine the aerosol extinction coefficient under ambient conditions. This paper presents a closure study of the different measurement methods. An empirical correction function for the PSAP is introduced and compared with a semiempirical correction function previously introduced by Bond et al. <1999>. A detailed model of humidity effects on the extinction coefficient is presented using coated sphere calculations based on measured input values of the size distribution, the chemical composition, the growth factor at 90% relative humidity, the water-soluble fraction of particulate materials, the temperature, and the relative humidity. Model calculations allowed an intercomparison of measured values of particles in the dry state and at ambient relative humidity and showed good agreement.