Vertical profiling of aerosol hygroscopic properties in the planetary boundary layer during the PEGASOS campaigns

Vertical profiles of the aerosol particles hygroscopic properties, their mixing state as well as chemical composition were measured above northern Italy and the Netherlands. An aerosol mass spectrometer (AMS; for chemical composition) and a white-light humidified optical particle spectrometer (WHOPS; for hygroscopic growth) were deployed on a Zeppelin NT airship within the PEGASOS project. This allowed one to investigate the development of the different layers within the planetary boundary layer (PBL), providing a unique in situ data set for airborne aerosol particles properties in the first kilometre of the atmosphere. Profiles measured during the morning hours on 20 June 2012 in the Po Valley, Italy, showed an increased nitrate fraction at similar to aEuro-100aEuro-m above ground level (a.g.l.) coupled with enhanced hygroscopic growth compared to similar to aEuro-700aEuro-maEuro-a. g. l. This result was derived from both measurements of the aerosol composition and direct measurements of the hygroscopicity, yielding hygroscopicity parameters (kappa) of 0.34aEuro- +/- 0.12 and 0.19aEuro- +/- 0.07 for 500aEuro-nm particles, at similar to aEuro-100 and similar to aEuro-700aEuro-maEuro-a. g. l., respectively. The difference is attributed to the structure of the PBL at this time of day which featured several independent sub-layers with different types of aerosols. Later in the day the vertical structures disappeared due to the mixing of the layers and similar aerosol particle properties were found at all probed altitudes (mean kappa a parts per thousand 0.18aEuro- +/- 0.07). The aerosol properties observed at the lowest flight level (100aEuro-maEuro-a. g. l.) were consistent with parallel measurements at a ground site, both in the morning and afternoon. Overall, the aerosol particles were found to be externally mixed, with a prevailing hygroscopic fraction. The flights near Cabauw in the Netherlands in the fully mixed PBL did not feature altitude-dependent characteristics. Particles were also externally mixed and had an even larger hygroscopic fraction compared to the results in Italy. The mean kappa from direct measurements was 0.28aEuro-+/- 0.10, thus considerably higher than kappa values measured in Italy in the fully mixed PBL.