Comparison of ambient aerosol extinction coefficients obtained from in-situ, MAX-DOAS and LIDAR measurements at a continental site (Cabauw)

In the field, aerosol in-situ measurements are often performed under dry conditions (relative humidity RH<30-40%). Since ambient aerosol particles experience hygroscopic growth at enhanced RH, their microphysical and optical properties - especially the aerosol light scattering - are strongly dependent on RH. The knowledge of this RH effect is of eminent importance for climate forcing calculations or for the comparison of remote sensing with in-situ measurements. We will present results from a four-month campaign which took place in summer 2009 in Cabauw (Netherlands) and was part of the Cabauw Intercomparison Campaign of Nitrogen Dioxide measuring Instruments (CINDI). During this campaign different remote sensing and in-situ instruments were used to derive atmospheric gas species and aerosol properties. The aerosol scattering coefficient σsp(λ) was measured dry and at various, predefined RH conditions between 20 and 95% with a recently developed humidified nephelometer (WetNeph). The scattering enhancement factor f(RH,λ) is the key parameter to describe the effect of RH on σsp(λ) and is defined as σsp(λ,RH) measured at a certain RH divided by the dry σsp(λ,RH=dry). The measurement of f(RH) together with the dry absorption measurement allows the determination of the actual extinction coefficient σep(λ) at ambient RH. In addition, a wide range of further aerosol properties were measured in parallel. The measurements were used to characterize the effects of RH on the aerosol optical properties. A closure study showed the consistency of the aerosol in-situ measurements. Due to the large variability of airmass origin and aerosol composition a simple categorization could not be established. If f(RH) needs to be predicted, the chemical composition needs to be known. Four MAX-DOAS (multi-axis differential optical absorption spectroscopy) instruments retrieved vertical profiles of σep(λ). The aerosol extinction corresponding to the lowest profile layer can be directly compared to the in-situ values, which were re-calculated to ambient RH. The comparison showed a good correlation of R2=0.59-0.78, but a factor of 1.4-2.76 higher extinction coefficients compared to the in-situ measured values. Best agreement is achieved for a few cases characterized by low aerosol optical depths and low planetary boundary layer heights. Differences showed to be dependent on the applied MAX-DOAS retrieval algorithm. The comparison of the in-situ data to a Raman lidar (light detection and ranging) showed much better agreement if the Raman retrieved profile was used to extrapolate the directly measured extinction coefficient to ground.