
Atmos. Chem. Phys., 16, 7295–7315, 2016 www.atmos-chem-phys.net/16/7295/2016/ doi:10.5194/acp-16-7295-2016 © Author(s) 2016. CC Attribution 3.0 License. Vertical profiling of aerosol hygroscopic properties in the planetary boundary layer during the PEGASOS campaigns Bernadette Rosati1,a, Martin Gysel1, Florian Rubach2,3,b, Thomas F. Mentel2, Brigitta Goger1,c, Laurent Poulain3, Patrick Schlag2, Pasi Miettinen4, Aki Pajunoja4, Annele Virtanen4, Henk Klein Baltink5, J. S. Bas Henzing6, Johannes Größ3, Gian Paolo Gobbi7, Alfred Wiedensohler3, Astrid Kiendler-Scharr2, Stefano Decesari8, Maria Cristina Facchini8, Ernest Weingartner1,d, and Urs Baltensperger1 1Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232 Villigen, Switzerland 2Institute for Energy and Climate Research (IEK-8), Forschungszentrum Jülich, 52428 Jülich, Germany 3Leibniz Institute for Tropospheric Research, 04318 Leipzig, Germany 4Department of Applied Physics, University of Eastern Finland, 1627 Kuopio, Finland 5Royal Netherlands Meteorological Institute (KNMI), 3730 De Bilt, the Netherlands 6Netherlands Organization for Applied Scientific Research (TNO), 80015 Utrecht, the Netherlands 7Institute of Atmospheric Sciences and Climate (ISAC-CNR), National Research Council, 00133 Rome, Italy 8Institute of Atmospheric Sciences and Climate (ISAC-CNR), National Research Council, 40129 Bologna, Italy anow at: Institute for Aerosol Physics and Environmental Physics, University of Vienna, 1090 Vienna, Austria bnow at: Max Planck Institute for Chemistry, 55128 Mainz, Germany cnow at: Institute of Meteorology and Geophysics, University of Innsbruck, 6020 Innsbruck, Austria dnow at: Institute for Aerosol and Sensor Technology, University of Applied Science Northwestern Switzerland, 5210 Windisch, Switzerland Correspondence to: Martin Gysel ([email protected]) Received: 26 January 2015 – Published in Atmos. Chem. Phys. Discuss.: 31 March 2015 Revised: 13 May 2016 – Accepted: 23 May 2016 – Published: 10 June 2016 Abstract. Vertical profiles of the aerosol particles hygro- 0:19 ± 0:07 for 500 nm particles, at ∼ 100 and ∼ 700 ma:g:l:, scopic properties, their mixing state as well as chemical respectively. The difference is attributed to the structure of composition were measured above northern Italy and the the PBL at this time of day which featured several inde- Netherlands. An aerosol mass spectrometer (AMS; for chem- pendent sub-layers with different types of aerosols. Later ical composition) and a white-light humidified optical par- in the day the vertical structures disappeared due to the ticle spectrometer (WHOPS; for hygroscopic growth) were mixing of the layers and similar aerosol particle properties deployed on a Zeppelin NT airship within the PEGASOS were found at all probed altitudes (mean κ ≈ 0:18 ± 0:07). project. This allowed one to investigate the development The aerosol properties observed at the lowest flight level of the different layers within the planetary boundary layer (100 ma:g:l:) were consistent with parallel measurements at (PBL), providing a unique in situ data set for airborne aerosol a ground site, both in the morning and afternoon. Overall, particles properties in the first kilometre of the atmosphere. the aerosol particles were found to be externally mixed, with Profiles measured during the morning hours on 20 June 2012 a prevailing hygroscopic fraction. The flights near Cabauw in the Po Valley, Italy, showed an increased nitrate frac- in the Netherlands in the fully mixed PBL did not feature tion at ∼ 100 m above ground level (a.g.l.) coupled with en- altitude-dependent characteristics. Particles were also exter- hanced hygroscopic growth compared to ∼ 700 ma:g:l. This nally mixed and had an even larger hygroscopic fraction result was derived from both measurements of the aerosol compared to the results in Italy. The mean κ from direct mea- composition and direct measurements of the hygroscopicity, surements was 0:28 ± 0:10, thus considerably higher than κ yielding hygroscopicity parameters (κ) of 0:34 ± 0:12 and values measured in Italy in the fully mixed PBL. Published by Copernicus Publications on behalf of the European Geosciences Union. 7296 B. Rosati et al.: Vertical profiling of aerosol hygroscopic properties 1 Introduction is the hygroscopicity tandem differential mobility analyzer (HTDMA; see e.g. Liu et al., 1978, or McMurry and Stolzen- Aerosol particles directly have an impact on climate by ab- burg, 1989, for details). It explores particle growth in the sub- sorbing or scattering the solar radiation. The optical proper- saturated RH range and was employed successfully in several ties depend on the particles’ size as well as their chemical ground-based campaigns (compare the review by Swietlicki composition and both can be altered at elevated relative hu- et al., 2008). This method investigates GFs at high precision midities (RH) if the particles are hygroscopic (Boucher et al., but needs several minutes for a full measurement cycle which 2013). makes it rather unsuited for airborne measurements. Besides, Most particles are emitted or formed in the planetary its detection range is limited to a maximal dry mobility diam- boundary layer (PBL), the lowermost layer of the tropo- eter of approximately 250 nm. HTDMA and chemical com- sphere. The PBL is subject to changes depending on the position hygroscopicity closures were performed at various strength of the incident solar radiation (compare Stull, 1988): sites including urban (see e.g. Kamilli et al., 2014) and rural under clear sky conditions heating of the Earth’s surface by regions (see e.g. Wu et al., 2013), as well as elevated moun- solar radiation induces convective turbulence and therefore tain sites (e.g. Kammermann et al., 2010). a well-mixed PBL builds up after midday ranging up to an To our knowledge, only few campaigns report airborne hy- altitude of approximately 2 km. During night when the sur- groscopicity results due to the lack of suitable instruments face cools down, several sub-layers are present in the PBL for this kind of measurements. The first instrument built for where the uppermost part is defined as the residual layer this special task is the aerosol hydration spectrometer (AHS; (RL). The RL contains a mixture of emissions and back- Hegg et al., 2007). The set-up comprises two optical instru- ground aerosol from the day before and is decoupled from ments, to measure the properties of the dry and humidified the surface. Close to the ground a stable nocturnal layer (NL) aerosol particles. However, quantifying hygroscopic growth develops where local and/or regional emissions accumulate. with the AHS is difficult as the measurement is performed Once the sun rises a new mixing layer (ML) is formed which for poly-disperse rather than size-selected aerosol samples. is separated from the other layers through a temperature in- The differential aerosol sizing and hygroscopicity spectrom- version. Throughout the day this ML evolves until it reaches eter probe (DASH-SP; Sorooshian et al., 2008) is using a up to the free troposphere and extends across the whole PBL. combination of differential mobility analysis (DMA) and op- The dynamics of the sub-layers in the PBL and its effect on tical particle spectrometry (OPS) and has successfully been the properties of the aerosol particles in these layers are still applied for airborne GF measurements at sub-saturated RH. poorly understood. This instrument is limited to small sizes in the range of Airborne studies were previously performed to investi- 150–225 nm dry diameters. In the aircraft campaigns an at- gate the aerosol chemical composition as a function of al- tempt was made to reconcile simultaneously measured chem- titude utilizing an aerosol mass spectrometer and show- ical composition and hygroscopic growth using an AMS and ing that chemical composition varies with height (Morgan DASH-SP. Herein, Hersey et al.(2009, 2013) focused their et al., 2009, 2010a,b; Pratt and Prather, 2010). Accordingly, studies on the free troposphere in the marine atmosphere off aerosol hygroscopicity, which depends on chemical compo- the coast of California. sition and can also be inferred from it, is also expected to be Most measurement techniques to study hygroscopicity se- height dependent. Morgan et al.(2010a) conducted flights lect particles smaller than ∼ 300 nm, which implies that over north-western Europe, focusing on changes between species that are more abundant at larger sizes (e.g. sea salt, 0 and 10 km a.g.l. In general aircraft campaigns commonly mineral dust) cannot be easily investigated. This may induce stretch over large areas and altitudes, often above the PBL, a bias in estimates of the humidity effects on hygroscopic therefore providing only limited information on the PBL it- growth and light scattering efficiency of particles in the upper self. On the other hand, extensive data sets studying the vari- accumulation mode size range. Indeed, Zieger et al.(2011) ability of aerosol composition and hygroscopic properties are presented a comparison between HTDMA measurements for available from ground-based studies. However, surface mea- dry diameters of 165 nm and calculated GFs using size dis- surements are not always representative for aerosol proper- tributions, scattering enhancement factors (based on poly- ties at elevated altitudes. disperse aerosol particles in the PM1 range) and Mie theory. One way to explore the hygroscopic properties of aerosols The comparison revealed that indeed in the presence of sea
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