A chemical analyzer for charged ultrafine particles

and Physics cess Atmospheric Measurement Techniques Open Access Biogeosciences Open Access Atmos. Meas. Tech., 6, 2339–2348, 2013 www.atmos-meas-tech.net/6/2339/2013/ doi:10.5194/amt-6-2339-2013 © Author(s) 2013. CC Attribution 3.0 License. Climate 1 Junior Professorship in Atmospheric Chemistry, University of Bayreuth, 95440 Bayreuth, Germany of the Past S. G. Gonser1,2 and A. Held1,2 2 Bayreuth Center of Ecology and Environmental Research, University of Bayreuth, 95440 Bayreuth, Germany Received: 28 March 2013 – Published in Atmos. Meas. Tech. Discuss.: 18 April 2013 Revised: 15 July 2013 – Accepted: 31 July 2013 – Published: 11 September 2013 1 Introduction Geoscientific Instrumentation Methods andfor the Earth’s Aerosol particles are of significant relevance radiation balance (ForsterData et al., Systems 2007) and human health Open Access (Delfino et al., 2005; Stieb et al., 2002). It is well known that part of the aerosol forms in the atmosphere through nucleation of gas-phase precursor molecules. The formation of Geoscientific these so-called secondary aerosol particles can be observed in the planetaryModel boundaryDevelopment layer at a multitude of locations around the world both in natural backgrounds as well as in polluted industrial areas (e.g. Kulmala et al., 2004). Typically, particle nucleation events begin in the morning hours Hydrology and and the nucleated particles grow for several hours to a few Earth System days by coagulation and condensation, reaching diameters in the order of 100 nm. Typical aerosol growth Sciencesand nucleation rates are in the range of 1–20 nm h−1 and 0.01–10 cm−3 s−1 , respectively (Kulmala et al., 2004). Once the initial nuclei are formed the particles will grow due to condensation of certain trace gases. The main contributors to the growth of Science particles are thought toOcean be sulfuric acid, oxidation products of volatile organic compounds (VOCs), ammonia salts such as ammonium sulfate and aminium salts, as has been shown by several studies using direct as well as indirect methods (e.g. Barsanti et al, 2009; Held et al., 2004; Laaksonen et al., 2008; Riipinen et al., 2009; Smith et al., 2010). Even Solid though there is a general idea about Earth the mechanisms governing the growth of freshly nucleated particles a lot of processes remain unrevealed. This is due to the challenging task of performing online chemical analysis of sub-30 nm particles. The small mass of these ultrafine particles is a limiting factor. For example, a particle of 10 nm diameter has a mass The Cryosphere of a few attograms (10−18 g). The preferred method to perform such measurements is the mass spectrometric approach, Open Access Open Access Open Access Open Access Abstract. New particle formation is a frequent phenomenon in the atmosphere and of major significance for the Earth’s climate and human health. To date the mechanisms leading to the nucleation of particles as well as to aerosol growth are not completely understood. A lack of appropriate measurement equipment for online analysis of the chemical composition of freshly nucleated particles is one major limitation. We have developed a Chemical Analyzer for Charged Ultrafine Particles (CAChUP) capable of analyzing particles with diameters below 30 nm. A bulk of size-separated particles is collected electrostatically on a metal filament, resistively desorbed and subsequently analyzed for its molecular composition in a time of flight mass spectrometer. We report on technical details as well as characterization experiments performed with the CAChUP. Our instrument was tested in the laboratory for its detection performance as well as for its collection and desorption capabilities. The manual application of defined masses of camphene (C10 H16 ) to the desorption filament resulted in a detection limit between 0.5 and 5 ng, and showed a linear response of the mass spectrometer. Flow tube experiments of 25 nm diameter secondary organic aerosol from ozonolysis of alpha-pinene also showed a linear relation between collection time and the mass spectrometer’s signal intensity. The resulting mass spectra from the collection experiments are in good agreement with published work on particles generated by the ozonolysis of alpha-pinene. A sensitivity study shows that the current setup of CAChUP is ready for laboratory measurements and for the observation of new particle formation events in the field. Earth System Dynamics Open Access Correspondence to: S. G. Gonser () Open Access A chemical analyzer for charged ultrafine particles Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. M 2340 S. G. Gonser and A. Held: A chemical analyzer for charged ultrafine particles as the achieved detection limits are low enough to analyze chemical components of ultrafine particles. To date there are two instruments available which are capable of performing online measurements of the chemical composition of sub-30 nm aerosol particles in the field: (I) NAMS – the Nano Aerosol Mass Spectrometer (Wang et al., 2006) and (II) TDCIMS – the Thermal Desorption Chemical Ionization Mass Spectrometer (Voisin et al., 2003). NAMS is capable of measuring the quantitative atomic composition of individual particles down to diameters of 7 nm (Wang et al., 2006). Its principle of operation is the laser-induced ablation of single particles captured in an ion trap, and the subsequent elemental analysis with a time of flight mass spectrometer (TOF-MS). Due to the high energetic laser pulse the particles break up into ionized atomic elements. Thus, molecular information about the sampled aerosol is not available. The TDCIMS, in contrast, analyzes the molecular composition of a particle bulk of one size range, down to diameters of 6 nm (Smith et al., 2004). The particles are charged in a unipolar charger, size selected in a differential mobility analyzer (DMA), collected on a filament and subsequently evaporated. The resulting gas is chemically ionized and analyzed for its molecular composition in a triple-quadrupole mass spectrometer, an ion trap mass spectrometer (Held et al., 2009), or a high-resolution time of flight mass spectrometer (Winkler et al., 2012). Both instruments are custom-built. Considering the scarcity of available instrumentation to measure the molecular composition of sub-30 nm particles, we present the design and first measurement results of an additional aerosol mass spectrometer for this size range. In principle, our instrument is similar to the TDCIMS, with a few major differences: firstly, for charging the particles we optionally use a non-radioactive source, thus facilitating the transport of the instrument to field sites in consideration of the transport restrictions regarding radioactive material. Secondly, we use a compact TOF-MS, minimizing the overall size of our instrument while accepting a lower mass resolution. Thirdly, we use electron ioniza (...truncated)