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In the field, aerosol in-situ measurements are often performed under dryconditions (relative humidity RH<30–40%). Since ambient aerosolparticles experience hygroscopic growth at enhanced RH, theirmicrophysical and optical properties – especially the aerosol lightscattering – are also strongly dependent on RH. The knowledge of this RH effect isof crucial importance for climate forcing calculations or for the comparisonof remote sensing with in-situ measurements. Here, we will present resultsfrom a four-month campaign which took place in summer 2009 in Cabauw, TheNetherlands. The aerosol scattering coefficient σsp(λ)was measured dry and at various, predefined RH conditions between 20 and95% with a humidified nephelometer. The scattering enhancement factorf(RH,λ) is the key parameter to describe the effect of RH onσsp(λ) and is defined as σsp(RH,λ)measured at a certain RH divided by the dry σsp(dry,λ).The measurement of f(RH,λ) together with the dry absorptionmeasurement (assumed not to change with RH) allows the determination of theactual extinction coefficient σep(RH,λ) at ambient RH.In addition, a wide range of other aerosol properties were measured inparallel. The measurements were used to characterize the effects of RH on theaerosol optical properties. A closure study showed the consistency of theaerosol in-situ measurements. Due to the large variability of air mass origin(and thus aerosol composition) a simple parameterization of f(RH,λ)could not be established. If f(RH,λ) needs to be predicted, thechemical composition and size distribution need to be known. Measurements offour MAX-DOAS (multi-axis differential optical absorption spectroscopy)instruments were used to retrieve vertical profiles of σep(λ). The values of the lowest layer were compared to the in-situvalues after conversion of the latter ones to ambient RH. The comparison showed agood correlation of R2 = 0.62–0.78, but the extinction coefficients from MAX-DOAS werea factor of 1.5–3.4 larger than the in-situ values. Best agreement isachieved for a few cases characterized by low aerosol optical depths and lowplanetary boundary layer heights. Differences were shown to be dependent on theapplied MAX-DOAS retrieval algorithm. The comparison of the in-situ extinction data to aRaman LIDAR (light detection and ranging) showed a good correlation andhigher values measured by the LIDAR (R2 = 0.82−0.85, slope of 1.69–1.76) if the Ramanretrieved profile was used to extrapolate the directly measured extinctioncoefficient to the ground. The comparison improved if only nighttimemeasurements were used in the comparison (R2 = 0.96, slope of 1.12)

Comparison of ambient aerosol extinction coefficients obtained from in-situ, MAX-DOAS and LIDAR measurements at Cabauw