Vertical profiling of aerosol hygroscopic properties in the planetary boundary layer during the PEGASOS campaigns
Authors: Rosati B. , Gysel M. , Rubach F. , Mentel T. F. , Goger B. , Poulain L. , Schlag P. , Miettinen P. , Pajunoja A. , Virtanen A. , Bialek J. , Klein Baltink H. , Henzing J. S. , Größ J. , Gobbi G. P. , Wiedensohler A. , Kiendler-Scharr A. , O'Dowd C. , Decesari S. , Facchini M. C. , Weingartner E. , Baltensperger U. 
Published: April 2015

Abstract. Airborne measurements of the aerosol hygroscopic and optical properties as well as chemical composition were performed in the Netherlands and northern Italy on board of a Zeppelin NT airship during the PEGASOS field campaigns in 2012. The vertical changes in aerosol properties during the development of the mixing layer were studied. Hygroscopic growth factors (GF) at 95% relative humidity were determined using the white-light humidified optical particles spectrometer (WHOPS) for dry diameters of 300 and 500 nm particles. These measurements were supplemented by an aerosol mass spectrometer (AMS) and an aethalometer providing information on the aerosol chemical composition. Several vertical profiles between 100 and 700 m a.g. were flown just after sunrise close to the San Pietro Capofiume ground station in the Po Valley, Italy. During the early morning hours the lowest layer (newly developing mixing layer) contained a high nitrate fraction (20%) which was coupled with enhanced hygroscopic growth. In the layer above (residual layer) small nitrate fractions of ~ 2% were measured as well as low GFs. After full mixing of the layers, typically around noon and with increased temperature, the nitrate fraction decreased to 2% at all altitudes and led to similar hygroscopicity values as found in the residual layer. These distinct vertical and temporal changes underline the importance of airborne campaigns to study aerosol properties during the development of the mixed layer. The aerosol was externally mixed with 22 and 67% of the 500 nm particles in the range GF < 1.1 and GF > 1.5, respectively. Contributors to the non-hygroscopic mode in the observed size range are most likely mineral dust and biological material. Mean hygroscopicity parameters (κ) were 0.34, 0.19 and 0.18 for particles in the newly forming mixing layer, residual layer and fully mixed layer, respectively. These results agree well with those from chemical analysis which found values of κ = 0.27, 0.21 and 0.19 for the three layers. The highest κ values in the new mixed layer and lower values in the fully developed mixed layer were additionally confirmed by ground measurements. The aerosol sampled in the Netherlands did not show any altitude dependent characteristics because only the fully mixed layer or the entrainment zone between mixed and the residual layer were probed. The airborne hygroscopicity measurements agreed well with ground based composition measurements. However, the fraction of the hygroscopic particles (GF > 1.5) was enhanced compared to the results from Italy amounting to 82%, while 12% showed low hygroscopicity (GF < 1.1). The mean κ value measured by the WHOPS was 0.28 and therefore considerably higher than the value measured in the fully mixed layer in Italy. The effective index of refraction reached values of 1.43 and 1.42 for the 500 nm particles in Italy and the Netherlands, respectively. This coincides well with literature data for airmasses with predominant organic contribution as was the case during our flights.