Sub-3 nm particle size and composition dependent response of a nano-CPC battery

Open Access Atmospheric Measurement Techniques Atmos. Meas. Tech., 7, 689–700, 2014 www.atmos-meas-tech.net/7/689/2014/ doi:10.5194/amt-7-689-2014 © Author(s) 2014. CC Attribution 3.0 License. Sub-3 nm particle size and composition dependent response of a nano-CPC battery J. Kangasluoma1 , C. Kuang2 , D. Wimmer1 , M. P. Rissanen1 , K. Lehtipalo1,3 , M. Ehn1 , D. R. Worsnop1,4 , J. Wang2 , M. Kulmala1 , and T. Petäjä1 1 Department of Physics, P.O. Box 64, 00014, University of Helsinki, Helsinki, Finland Sciences Division, Brookhaven National Laboratory, Upton, New York, 11789, USA 3 Airmodus Ltd., Finland, Gustaf Hällströmin katu 2 A, 00560 Helsinki, Finland 4 Aerodyne Research Inc., Billerica, MA, USA 2 Atmospheric Correspondence to: C. Kuang () Received: 1 August 2013 – Published in Atmos. Meas. Tech. Discuss.: 8 October 2013 Revised: 17 January 2014 – Accepted: 30 January 2014 – Published: 4 March 2014 Abstract. In this study we built a nano-CPC (condensation particle counter) battery, consisting of four ultrafine CPCs optimized for the detection of sub-3 nm particles. Two of the CPCs use diethylene glycol as a working fluid: a laminar type diethlylene glycol CPC and a mixing type Airmodus A09 particle size magnifier. The other two CPCs are a laminar type TSI 3025A and a TSI 3786 with butanol and water as the working fluids, respectively. The nanoCPC battery was calibrated with seven different test aerosols: tetraheptyl ammonium bromide, ammonium sulfate, sodium chloride, tungsten oxide, sucrose, candle flame products and limonene ozonolysis products. The results show that ammonium sulfate and sodium chloride have a higher activation efficiency with the water-based 3786 than with the butanolbased 3025A, whereas the other aerosols were activated better with butanol than with water as the working fluid. It is worthwhile to mention that sub-2 nm limonene ozonolysis products were detected very poorly with all of the CPCs, butanol being the best fluid to activate the oxidation products. To explore how the detection efficiency is affected if the aerosol is an internal mixture of two different chemical substances, we made the first attempt to control the mixing state of sub-3 nm laboratory generated aerosol. We show that we generated an internally mixed aerosol of ammonium sulfate nucleated onto tungsten oxide seed particles, and observed that the activation efficiency of the internally mixed clusters was a function of the internal mixture composition. 1 Introduction Atmospheric aerosols play a significant role in impacting the global atmospheric energy balance by directly scattering the sunlight and indirectly participating in cloud formation and affecting the properties of clouds (IPCC, 2007). According to Merikanto et al. (2010), about 75 % of surface level aerosol particles are formed in the atmosphere via nucleation and condensation of pre-existing vapours, but the processes and vapours responsible for new particle formation are not well known. Recent field (Riipinen et al., 2007; Kuang et al., 2008) and laboratory (Sipila et al., 2010; Kirkby et al., 2011) studies show that while sulfuric acid plays a major role in atmospheric nucleation, compounds that stabilize sulfuric acid in the initial cluster formation and compounds, other than sulfuric acid, that contribute to particle growth above 1.5 nm are still mostly unknown (Smith et al., 2010; Riipinen et al., 2012; Kuang et al., 2012a). The chemical composition of particles from 4 to 10 nm can be measured indirectly with a tandem differential mobility analyser (HTDMA) (Sakurai et al., 2005; Ehn et al., 2007; Keskinen et al., 2013), which measures the hygroscopic and volatile fraction of the sampled aerosols. However, due to large sampling losses associated with low charging, transport and counting efficiencies of the smallest particles, the lowest measurable size for the HTDMA is about 4 nm, with most field measurements restricted to sizes above 10 nm. Alternatively, the chemical composition of the nucleation mode particles can be measured by means of mass spectrometry. Published by Copernicus Publications on behalf of the European Geosciences Union. 690 The thermal desorption chemical ionization mass spectrometer (TD-CIMS) (Voisin et al., 2003) is able to measure the chemical composition of freshly nucleated particles as small as 8 nm. For example, Smith et al. (2010) found that nanoparticle composition was dominated by the presence of aminium salts. A different sampling approach is employed in the nanoaerosol mass spectrometer (NAMS) (Bzdek et al., 2011), where the sample particle is broken into elements, and thereon the elemental distribution is measured. Using this method Bzdek et al. were able to observe the elemental composition of 20 nm particles. With an atmospheric pressure inlet time of flight mass spectrometer (APi-TOF) (Junninen et al., 2010) it is possible to characterize the precise chemical composition of atmospheric ions up to about 1.5 nm in mobility diameter, which corresponds to about 1000 Th, and by using different ionization methods also information from neutral clusters can be obtained (Zhao et al., 2010; Jokinen et al., 2012; Berndt et al., 2012). These two methods leave a gap between 1.5 and 8 nm, where the measurement of chemical composition (both direct and indirect) has been difficult so far. Also, while measurement of the total aerosol size distribution down to around 2.5 nm is a routine measurement by DMPS (differential mobility particle sizer) and NAIS (neutral air ion spectrometer) (based on the detection limit for the DMPS using a 3025A as a detector and on the upper size limit of corona-generated ions as seen in Manninen et al. (2011), Aalto et al.(2001) and Manninen et al.(2009)), below that size there is only a very limited literature available (Lehtipalo et al., 2009, 2010; Jiang et al., 2011b; Kuang et al., 2012a; Kulmala et al., 2013) due to lack of instruments capable of measuring sub-2 nm neutral clusters. In earlier studies Kulmala et al. (2007) have inferred aerosol hygroscopicity with a CPC (condensation particle counter)-battery (CPCb) using four CPCs: an ultrafine water and butanol CPC (d50 = 3 nm for silver, where d50 is the size corresponding to 50 % detection efficiency) and a conventional water and butanol CPC (d50 = 11 nm). In the laboratory, the CPCs were tuned to have equal cut-off diameters for both working fluids for non-hygroscopic silver test aerosol. Using the CPCb with those operating conditions, they found that salts (hygroscopic test aerosol) are activated better with water than butanol. In the following work Riipinen et al. (2009) were able to infer the hygroscopicities of atmospheric aerosols in Hyytiälä for two different cutoff diameters. The results showed for both CPC pairs clear WCPC / CPC ratios over 1 during a nucleation event, indicating more hygroscopic compounds participating in formation and initial growth especially at the smaller siz (...truncated)