An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions

Transcription

1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions Mikael Ehn, Heikki Junninen, Siegfried Schobesberger, Hanna E. Manninen, Alessandro Franchin, Mikko Sipilä, Tuukka Petäjä, VeliMatti Kerminen, Hannes Tammet, Aadu Mirme, Sander Mirme, Urmas Hõrrak, Markku Kulmala & Douglas R. Worsnop To cite this article: Mikael Ehn, Heikki Junninen, Siegfried Schobesberger, Hanna E. Manninen, Alessandro Franchin, Mikko Sipilä, Tuukka Petäjä, VeliMatti Kerminen, Hannes Tammet, Aadu Mirme, Sander Mirme, Urmas Hõrrak, Markku Kulmala & Douglas R. Worsnop (211) An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions, Aerosol Science and Technology, 45:4, To link to this article: Published online: 18 Jan 211. Submit your article to this journal Article views: 753 Citing articles: 34 View citing articles Full Terms & Conditions of access and use can be found at

2 Aerosol Science and Technology, 45: , 211 Copyright C American Association for Aerosol Research ISSN: print / online DOI: 1.18/ An Instrumental Comparison of Mobility and Mass Measurements of Atmospheric Small Ions Mikael Ehn, 1 Heikki Junninen, 1 Siegfried Schobesberger, 1 Hanna E. Manninen, 1 Alessandro Franchin, 1 Mikko Sipilä, 1,2 Tuukka Petäjä, 1 VeliMatti Kerminen, 1,3 Hannes Tammet, 4 Aadu Mirme, 4 Sander Mirme, 4 Urmas Hõrrak, 4 Markku Kulmala, 1 and Douglas R. Worsnop 1,3,5 1 Department of Physics, University of Helsinki, Finland 2 Helsinki Institute of Physics, University of Helsinki, Helsinki, Finland 3 Finnish Meteorological Institute, Research and Development, Helsinki, Finland 4 Institute of Physics, University of Tartu, Tartu, Estonia 5 Aerodyne Research Inc, Billerica, Massachusetts, USA Ambient, naturally charged small ions (<2 Da) were measured in Hyytiälä, Finland, with a mass spectrometer (atmospheric pressure interface timeofflight, APiTOF) and two mobility spectrometers (air ion spectrometer, AIS, and balanced scanning mobility analyzer, BSMA). To compare these different instrument types, a mass/mobility conversion and instrumental transfer functions are required to convert highresolution mass spectra measured by the APiTOF into lowresolution mobility spectra measured by the AIS and BSMA. A modified version of the StokesMillikan equation was used to convert between mass and mobility. Comparison of APiTOF and BSMA results showed good agreement, especially for sizes above 2 Da (Pearson s R =.7.9). Below this size, agreement was fair, and broadening BSMA transfer functions improved the correlation. To achieve equally good agreement between APi TOF and AIS, AIS results needed to be shifted by mobility channels. The most likely cause was incorrect sizing in the AIS. In summary, the mass and mobility spectrometers complement each other, with the APiTOF giving superior chemical information, limited to relatively small ions (<2.5 nm diameter), whereas the mobility spectrometers are better suited for quantitative concentration measurements up to 4 nm. The BSMA and AIS were used to infer a transmission function for the APiTOF, making it possible to give quantitative estimates of the concentrations of detected chemical ions. Received 1 July 21; accepted 17 October 21. This work has been supported by the European Commission 6th Framework program project EUCAARI (contract no ), Advanced Grant EUFP7ATMNUCLE (project no ), and by the Academy of Finland Center of Excellence program (project no ). S. S. and A. F. were supported by the European Community under the FP7 Marie Curie Initial Training Network CLOUDITN (PITNGA ). Address correspondence to Mikael Ehn, Department of Physics, University of Helsinki, PO Box 64, Helsinki FI14, Finland. mikael.ehn@helsinki.fi INTRODUCTION Nucleation is the most important source of new particles in the global atmosphere (Kulmala et al. 24; Spracklen et al. 21). Although our understanding on the atmospheric nucleation process has progressed during the recent years, significant questions remain (Kerminen et al. 21). This is due to the lack of experimental information on the smallest atmospheric aerosol particles and clusters, including number concentration, size distribution, charging state and, most importantly, chemical composition. Resolving initial stages of new particle formation requires instruments capable of measuring the size range 1 2 nm, where the gastoparticle conversion occurs (Kulmala et al. 27a). Concentration of particles down to 3 nm in electrical mobility equivalent diameter has been available for about two decades (Stolzenburg and McMurry 1991; McMurry 2; for an overview on different diameter definitions, see DeCarlo et al. 24). Multiple research groups have reported instrumental detection limits down to 2 nm or below (Saros et al. 1996; Gamero Castano and de la Mora 2; Kulmala et al. 25; Mordas et al. 28; Sipilä et al. 28; Sipilä et al. 29; Vanhanen et al. 21). Sizeresolved measurements on sub3 nm particles have been obtained from pulse height CPCs (Sipilä et al. 28), as well as from various ion mobility spectrometers. The air ion spectrometer (AIS, Mirme et al. 27) and balanced scanning mobility analyzer (BSMA, Tammet 26) are able to measure the size distribution of air ions down to below 1 nm in mobility diameter. Additionally, neutral cluster and air ion spectrometers (NAIS) have reported size distributions of neutral particles down to about 2 nm (Kulmala et al. 27a; Manninen et al. 29; Mirme et al. 21). It should be noted that at sizes approaching 1 nm, particles/clusters are likely not spherical. Thus the concept of diameter is no longer welldefined and is used

3 MOBILITY AND MASS MEASUREMENTS OF ATMOSPHERIC SMALL IONS 523 here as a rough measure of size in order to compare to larger particles. Physical characterization of atmospheric nanoparticles does not provide sufficient information to fully explain atmospheric new particle formation; i.e., knowledge of chemical composition is a prerequisite to elaborate this research question. A limiting factor in such chemical analysis is typically the low absolute mass concentration of freshly nucleated particles. Indirectly, the chemical nature of particles has been inferred down to 1 nm (or even below) by measuring particle volatility (Wehner et al. 25; Ehn et al. 27b), hygroscopicity (Sakurai et al. 25; Ehn et al. 27a), and ethanol affinity (Vaattovaara et al. 25). CPC batteries have also provided indirect composition information down to 3 nm (Kulmala et al. 27b; Riipinen et al. 29). However, direct observation of aerosol chemistry below 1 nm is rare (Smith et al. 28, 29). In the sub3 nm diameter size range, only mass spectrometric approaches provide direct information on the composition of charged clusters and molecules (Eisele and Tanner 199; Hanson and Eisele 22; Eichkorn et al. 22; Eisele et al. 26) and in some cases also on neutral clusters and molecules (Zhao et al. 21). The latest development in this regard is the atmospheric pressure interface timeofflight mass spectrometer (APiTOF; Junninen et al. 21; Ehn et al. 21), which is able to measure the mass spectrum of naturally charged ions in the range of about 4 2 Da, corresponding to particle mobility diameters from below 1 nm up to nm. With multiple instruments now available to measure atmospheric small ions below 2 nm, the aim of this study was to compare measurements of mass and mobility spectrometers under ambient sampling conditions. In the present investigation, we compared these different instruments under field conditions and evaluated their overall performance. The deployed instruments were the high mass resolution APiTOF and two lowresolution, high sensitivity mobility spectrometers, BSMA and AIS. Challenging issues are: (1) these instruments are difficult to calibrate (Asmi et al. 29; Junninen et al. 21), (2) they measure different metrics of particles size, i.e., mass and mobility, whose exact relation is not welldetermined below a few nm (e.g., de la Mora et al. 25; Ku and de la Mora 29), and (3) they have very different resolution and accuracy in measuring concentration and size of air ions. 2. EXPERIMENTAL METHODS An APiTOF and three mobility spectrometers were deployed in Hyytiälä, southern Finland. Two different types of mobility spectrometers were used: an AIS and two BSMAs. More detailed descriptions of the instruments and the measurement site are given in sections 2.1 and 2.2, respectively. For the units in this work, masses are expressed in dalton (Da), and for acquired mass spectra the unit of mass/charge (m/q) is thomson (Th), corresponding to Da/e, where e is the number of elementary charges on the ion. In other words, an ion with one excess electron or proton will have e = 1. When discussing particle sizes, mobility diameter, d p, is used, except where clearly noted otherwise. The conversion between mass, mobility, and size is discussed in detail in section Instruments APiTOF The APiTOF measures the mass/charge ratio of ions of either positive or negative polarity. The atmospheric pressure interface (APi) guides ions from the critical orifice inlet to the timeofflight region (TOF) while sampled gas is differentially pumped in three stages. Mass/charge of the ions is determined in the TOF region, where the pressure is 1 6 mbar. The resulting mass spectra have a mass accuracy of 2 ppm or less (i.e., at 1 Th the error is <.2 Th), and a resolution of 3 Th/Th (i.e. at 1 Th the peak width at half maximum is roughly.33 Th). For a more detailed description of the instrument, see Junninen et al. (21). Due to imperfect ion sampling in the APi, the ion transmission of the instrument is of order.5% for m/q 1 6 Th. At low m/q there is a fairly abrupt cutoff where transmission drops to zero. Depending on tuning, this cutoff can be below 2 Th. At the high mass range, the transmission drops more slowly, starting at 6 Th, decreasing to zero at 15 2 Th. These are typical values, that ultimately depend on instrument tuning. For instance, in Ehn et al. (21), the low m/q cutoff was 8 Th, leaving some important small ions undetected. More discussion on transmission can be found in Junninen et al. (21), and section 4 of this study. It should be noted, that at the low pressures within the APi, both fragmentation (via ionmolecule collisions) and evaporation of weakly bound species (e.g., water molecules) can occur BSMA The BSMA measures ions in a mobility range of cm 2 /Vs, which corresponds to a mobility diameter range of nm (Tammet 26). In this study, two separate BSMAs were utilized. The instrument consists of two plane type differential mobility analyzers (DMA; Knutson and Whitby 1975) in order to measure both positive and negative ion polarities. The mobility distribution of the ions is scanned by varying the voltage over both DMAs using a single discharging capacitor. As opposed to standard DMAs, where the ions of a specific size continue through a slit in one of the electrodes, in the BSMA the slit has been replaced by a detector plate. The detector plates of both DMAs are connected to a single electrometer bridge circuit, which compensates for induced currents that are produced in each DMA as the voltage changes. However, with the common electrometer, only one polarity can be measured at a time. An electrostatic filter is applied alternatively in front of each DMA while measuring the other polarity, and in addition, background measurements are conducted with both filters applied. Sample flow is 4 l s 1, with a sampletosheath air flow

4 524 M. EHN ET AL. ratio of 1:3. For a full description of the instrument, see Tammet (26) AIS The AIS consists of two cylindrical DMAs with outer cylinders (collecting electrodes) lined with 21 electrometers, which measure current produced by impacting ions. DMA voltages are not scanned as in the BSMA, rather the central electrodes have sections of different diameters and different fixed voltages. In this way, the AIS is able to measure a broad mobility range of cm 2 /Vs, corresponding to mobility diameters of nm. Inlet flow is 6 l min 1, with a sampletosheath air flow ratio of 1:2. For a full description of the instrument, see Mirme et al. (27) Measurements Measurements were performed at the SMEAR II station in Hyytiälä, Finland (Hari and Kulmala 25) during an intensive campaign in June 21. Although all instruments measured at the station, the exact sampling positions differed slightly. The APiTOF and one BSMA (BSMA3) were located in a container at the edge of the forest, with several tens of meters of open area from south to west. The AIS was located in the aerosol cottage together with most SMEAR II aerosol instrumentation, under the forest canopy about 5 m northwest of the container. The second BSMA (BSMA1) sampled in the REA cottage, roughly 12 m north of the container, also within the forest. 3. CONVERTING BETWEEN MASS, MOBILITY, AND DIAMETER Sizeselected measurements of aerosol particles are typically made by measuring the electrical mobility of the particles with some type of DMA, with ion detection either via electrometers (as in the BSMA and AIS) or with a CPC. For convenience, size distributions are typically reported as a function of diameter converted from directly measured mobility, assuming particles are singly charged and spherical. For liquid droplets, this assumption is valid, and even in situations where the particles are nonspherical, the error is often small, unless the particles are fractallike agglomerates. The conversion between mobility and diameter is typically based on the StokesMillikan law (e.g., Friedlander 1977). As proposed by Tammet (1995), the finite size of gas molecules needs to be taken into account. This is typically done by setting the particle (mobility) diameter d p = d m +d g, where d m = 3 6 m / πρ is the mass diameter of the particle, and d g is a free parameter corresponding to the diameter of a gas molecule. m and ρ are the mass and density of the particles, respectively. The resulting StokesMillikan equation is 1 Q Z = 1 + m g / m 3πµ 1 + Kn( e 1.1/Kn ) d m + d g, [1] where Z is the electrical mobility, m g the mass of a carrier gas molecule, Q the charge of the particle (typically the elementary charge), µ the viscosity of the carrier gas, and Kn the Knudsen number defined as 2λ/(d m + d g ), with λ the mean free path of the carrier gas. In this work we used µ = kg/ms and λ = 68 nm, corresponding to typical spring time conditions in Hyytiälä. The first factor on the righthand side was not originally included in the StokesMillikan formula, but is needed to account for the finite mass of the particle (Tammet 1995; Ku and de la Mora 29). The drawback of including this factor is that it requires knowledge of the mass of the particle, and can therefore not be used when converting between mobility and diameter. For more discussion on diameter/mobility conversion, see Mäkelä et al. (1996b). As this work focuses on conversions between mass (using m p and ρ p from the APiTOF to calculate d m ) and mobility, the factor is always included unless stated otherwise. Omitting it gives a deviation of <5% above 25 Da, 1% at 12 Da, and 15% at 75 Da, when assuming m g = 28 Da (the integer mass of N 2 ). Based on mobility measurements of ions with known mass and bulk density, Ku and de la Mora (29) chose d g =.3 nm in Equation (1), resulting in a simplified conversion between mass diameter and mobility diameter: d p = d m +.3 nm. However, with the inclusion of the (1 + m g /m p ) 1/2 factor in Equation (1), this does not hold rigorously for the smallest ions. Figure 1 gives an overview of the relationship between mass, mobility and diameter. The mobility diameter (right axis) is calculated from mobility (left axis) using Equation (1) excluding the factor (1 + m g /m p ) 1/2. This factor is omitted to allow a direct conversion from mobility to diameter without knowledge of the particle mass. The colored dots are the different compounds used by Ku and de la Mora (29) for their mobility measurements (at room temperature and atmospheric pressure). Each color corresponds to a specific compound, and the different points of the same color are clusters containing n anions and n + 1 cations of the used salts. With these accurate mobility measurements, n, and thus the mass, was resolved for each cluster. Cluster density was assumed to be that of the bulk. Dashed lines correspond to Equation (1), assuming different densities, and the solid lines to the model by Tammet (1995). Brown crosses are mass/mobility data reported by Kilpatrick (1971), which has been used as a reference for massmobility conversions (Mäkelä et al. 1996a; Tammet 1995). Overall, the agreement between the points measured by Ku and de la Mora (29) and the corresponding theoretical values are very good down to a few hundred Da. Keeping in mind all effects ignored by the StokesMillikan equation, such as carrier gas polarization (Tammet 1995; Larriba et al. 21), Equation (1) works surprisingly well realizing, for example, that the use of bulk densities at the small cluster or molecular level adds uncertainty. For comparison of ambient ion distributions, where much of the sample is not identified, it is not possible to take all factors into account. Thus, the StokesMillikan equation will be used over the whole size range, enabling extension of this

5 MOBILITY AND MASS MEASUREMENTS OF ATMOSPHERIC SMALL IONS 525 analysis to larger ion sizes in future. A sensitivity study was performed (section 5.3) to compare to the Tammet (1995) model, which, by incorporating most known molecular size effects, produces a transition between the polarization limit in the free molecule limit and the StokesMillikan equation at large ion sizes. 4. METHODS USED FOR INSTRUMENT COMPARISON For this study, we used only APiTOF integer mass resolution data, i.e., ignored high resolution analysis. All APiTOF data were analyzed using the Matlabbased software package toftools (Junninen et al. 21). The BSMA and AIS report ion concentrations in evenly spaced logarithmic mobility bins. However, the limits of the BSMA mobility bins coincide with midpoints (geometric mean mobilities) of the AIS bins. We will henceforth number the mobility bins starting from the smallest particle sizes, i.e., the particles with the highest mobilities. The first AIS bin center, and consequently the upper limit of the first BSMA bin, is at 3.16 cm 2 /Vs. The following bins are found by multiplying by (3/4) N, resulting in 8 bins/decade. The horizontal blue lines in Figure 1 represent BSMA bin limits and AIS bin centers. Keeping in mind that the upper limit for the APiTOF here is between 1 and 2 Da, we use only the first 6 7 bins of the mobility spectrometers for APiTOF comparisons. The simplest way to compare mass and mobility is to convert the APiTOF mass spectra to mobility using Equation (1), and then sum all APiTOF masses in corresponding mobility spectrometer bins. For density, we used bulk density values for the major identified ions, mainly sulfuric acid and nitric acid and their multimers, and some specific organic acids (e.g., malonic, malic, tartaric) in the negative ion spectrum. For the remainder of the spectrum, one common value (1.66 g cm 3 ) was estimated based on the results by Ehn et al. (21), who characterized the larger unidentified peaks with average O:C:H ratios of around 1:1:1 based on observed mass defect. Reference compounds, such as citric acid (C 6 H 8 O 7 ) and aconitic acid (C 6 H 6 O 6 ), have densities of 1.67 and 1.66 g cm 3, respectively. An example comparison of measurement data in Hyytiälä, a 4 h average on the evening of June 1, 21, is plotted in Figure 2. In the APiTOF mass spectrum of negative ions (panel A), the largest peaks are labeled with the dominant ion at each peak. The peaks at 3 4 Da are believed to be organic molecules with a C:H:O ratio of roughly 1:1:1, based on measured highresolution masses (Ehn et al. 21). The dashdotted vertical blue lines in panel B (corresponding to the horizontal blue lines in Figure 1) delineate BSMA channel limits, converted to mass assuming a density of 1.66 g cm 3. The solid brown line in panel C displays BSMA3 data (again with ρ = 1.66 g cm 3 ) for the 4 h period, and the dashed orange line shows the sum of APiTOF ions in each BSMA bin. While there is fair agreement, the local minimum in the second bin of the APiTOF data is not present in the BSMA data. The reason is that the low resolution of the BSMA leads to leakage between adjacent bins. The BSMA transfer function (TF) accounts for this blurring of signals among neighboring bins. Data reported by the BSMA acquisition software is not inverted with the theoretical instrument function (Tammet 26). This theoretical diffusionless TF is plotted for channel 3 as a dashed line in panel B of Figure 2. In this work we broadened this TF by a factor of 1.5 for each channel, since, as pointed out by Tammet (1995), the theoretical TF width is a lower limit. The widened TF for channel 3 is also plotted in panel B (red dashed line). Note, since these wide transfer functions overlap with adjacent channels, the sum of the transfer functions was scaled to 1 at each mass in order to conserve total ion counts. Therefore, the widened TF does not reach unity. Converting APiTOF data to BSMA bins with the widened TF removes the minimum in channel 2 (green dashed line in panel C). In panel C in Figure 2, plotting BSMA data on the left axis and APiTOF data (dashed lines) ion the right, APiTOF concentrations are roughly 2 times lower than the BSMA, consistent with APiTOF transmission of.5%. Ratios of APi TOF/BSMA concentrations in each bin are plotted in panel D of Figure 2, using both conversion methods (simple summing, and applying a TF). For most of the mass range, the ratio is.4, but, as expected, is lower at larger sizes due to decreasing APiTOF transmission. Panel D also shows an estimated APi TOF transmission function based on these ratios. The blue solid line in panel C shows the resulting APiTOF data on the BSMA grid (now also plotted on the left axis). It should be noted that this transmission function, derived by making the two instruments match, includes inlet losses in the APiTOF, which are not separable from ion transmission inside the mass spectrometer and its interface. For comparison with the AIS, APiTOF data were converted in a similar way as described above. The higher sampletosheath flow rate lowers the resolution in the AIS relative to the BSMA. Since no theoretical TF for the AIS has been published, we used the BSMA TF widened by a factor 2 instead of 1.5, reflecting the relative sampletosheath flow rates. For more discussion on AIS transfer functions, see Asmi et al. (29) who found the transfer functions of several different AIS instruments to be fairly broad and nonsymmetrical. Additionally, as AIS total flow rate is lower than that of the BSMA and sampling lines are longer and more turbulent, both the ion losses and uncertainties are expected to be higher. 5. INSTRUMENTAL COMPARISON OF TIME SERIES 5.1. APiTOF versus BSMA Figure 3 shows a channelbychannel comparison of time series from the APiTOF and both BSMA negative ion signals during 6 days in June 21. APiTOF data were converted to the BSMA grid using a default density of 1.66 g cm 3 for all ions except major identified ions, and with the theoretical BSMA transfer function widened by a factor of 1.5, as explained in

6 526 M. EHN ET AL. 3 BSMA channel limits, AIS channel mid points Mobility [cm 2 /Vs] Kilpatrick, 1971 Tammet (1995) 1.5 g/cm 3.8 g/cm 3 StokesMillikan d g =.3 nm 2. g/cm g/cm g/cm 3.8 g/cm Mobility diameter [nm] Ku & de la Mora, 29: EMIBeti, 1.57 g/cm 3 (Dodecyl) 4 NBr,.879 g/cm 3 DMPIMe, 1.52 g/cm 3 Cyclodextrin, g/cm 3 EMIIm (112mM), g/cm 3 Bradykinin, 1.98 g/cm 3 EMIIm (1mM), g/cm 3 (Hexyl) 4 NBr,.911 g/cm 3 TBAPOS, g/cm Mass [Da] FIG. 1. Comparison between mass, mobility, and diameter based on the StokesMillikan law with variable densities. The circles depict different salts and ionic liquids measured by Ku and de la Mora (29), and the crosses by Kilpatrick (1971). Horizontal lines depict channels in the ion mobility spectrometers used in this study. Subtracting.3 nm from the right axis gives a fair estimate for the mass diameter. APiTOF conc. [cm 3 ] Signal fraction into channel 3 Bin conc. (solid) (dn/dm) [cm 3 Da 1 ] Transmission x NO 3 C 3 H 3 O 4 HSO 4 A B C D HNO 3 NO 3 C 3 H 4 O 4 NO 3 C 5 H 6 O 6 NO 3 NO 3 containing ions HSO 4 containing ions Others BSMA channel limits BSMA (l) Summed TF applied Applied transmission BSMA channel 3 TF Theoretical Widened by factor 1.5 APiTOF on BSMA grid: Summed (r) TF applied (r) TF and transmission applied (l) 5x (dn/dm) [cm 3 Da 1 ] Bin conc. (dashed) 2 4 Mass [Da] FIG. 2. Overview of the conversion process from APiTOF mass spectra to mobility bins. Panel A (top) shows a 4 h average negative mass spectrum measured by the APiTOF. Panel B depicts the theoretical BSMA transfer function (black) and the transfer function widened by a factor of 1.5 (pink). The vertical blue dashdotted lines correspond to the BSMA channel limits, similar to the horizontal lines in Figure 1. The brown line in panel C shows the BSMA data for the 4 h period, and the other lines show the APiTOF data calculated onto the BSMA grid using different methods. The solid lines in panel C correspond to the left axis, and the dashed to the right. Panel D shows the ratios of the APiTOF bins to the BSMA bins, together with the final estimated transmission function for the APiTOF.

7 MOBILITY AND MASS MEASUREMENTS OF ATMOSPHERIC SMALL IONS 527 section 4. Both the BSMA and APiTOF data were averaged to 15 min, as an optimal tradeoff of smoothing signaltonoise and blurring atmospheric variability. The background in each subplot shows the average chemical composition of ions in each bin as measured by the APiTOF (as in mass spectra in Figure 2a; see Ehn et al. 21). The nominal range for each channel, both in mobility and mass scale, is presented on the left axis labels. These values correspond to the reported channel limits, though, as in Figure 2, each bin contains ion signal from adjacent bins. BSMA3 and BSMA1 were located adjacently inside the forest, until noon 1 June, when BSMA3 was moved to the APiTOF container to check observations at the two locations. The top 4 panels (channels/bins 3 6, as defined previously with numbering starting at the smallest ion bin) show a good correlation among the three instruments. During some periods, such as midnight between June 3 and 4, the APiTOF seems to agree better with BSMA3 than BSMA1, indicating some small scale variability within the station area. The bottom two channels do not correlate as well as the larger ones, but this is not surprising, as sampling of small ions is more difficult due to increased diffusional losses. For example, the upper limit of channel 1 corresponds to a mass of only 4 times the mass of an air molecule. The highest concentrations of ions were measured during night, when minimum boundary layer heights maximized surface radon concentration (emitted from the ground) and resulting surface ion concentration. Total ion concentration appears to be the dominant cause of variability in channels 3 6, leading to relatively high correlations between the channels. This in turn makes results less sensitive to both the chosen TF width and chemical density, and we can thus not definitively validate these choices based on this data. A simple sensitivity analysis (described in section 5.3) shows that decreasing the density made the correlations worse than increasing the density. As an example, the correlation coefficient for channel 3 between the APiTOF and BSMA1 became negative when using a default density of 1. g cm 3. The largest change typically happens in channel 3 when changing the densities. The strongest new particle formation event during this period occurred on June 4, 21 (particle data not shown). During this period, APITOF results show a high fraction of HSO 4 in the ions in the bottom three bins, typical of strong nucleation events in Hyytiälä (Ehn et al. 21). The strong increase in APiTOF signal in bin 2 is not seen by either BSMA. Similar discrepancies have been observed during other periods (not shown). As neither BSMA shows any increase, the cause cannot be attributed to local variability, nor to the fact that the APiTOF inlet is in direct sunlight, whereas other instruments measure below the canopy where photochemical activity is lower. The explanation could be instrumental. The APiTOF samples ions through stages where iongas collisions occur, with ion detection at 1 6 mbar pressure. Thus, both ion cluster fragmentation and evaporation can perturb observed ion mass spectra. However, as the APITOF and mobility instruments agree well during other periods, this alone cannot explain this deviation during nucleation events. The agreement of the BSMAs might point toward flawed methodology for BSMA/APiTOF comparison. For this purpose, Figure 4 compares the lowest three channels with variable broadening factors for the transfer functions (the red and black and red lines are as in Figure 3). The changes are not large, especially in the third channel (top panel). The Pearson s correlation coefficients, R, comparing APiTOF and BSMA for each bin, calculated with the different TF widths, are listed in Figure 4. The largest difference is seen for channel 1, where increasing TF from 1.5 to 2. increased R from.37 to.5, suggesting that increased diffusional broadening is observable at the smallest BSMA ion sizes. During June 4 where the BSMA/APiTOF discrepancy was noted, broadening the TF does decrease the signal in channel 2, improving agreement with the BSMA. The correlation might be further improved by using nonsymmetric transfer functions, but as the potential error sources at these small sizes are so numerous, this issue will not be analyzed further. Increased diffusion in the smallest channels, together with possible evaporation of water or other semivolatile species, is believed to be the most likely explanation for the observed discrepancy. The lowresolution ion distribution does not change although the APiTOF detects a clear change in ion composition. Figure 5 displays the BSMA/APiTOF comparison as a scatter plot, with each color representing data in one channel, the channels again numbered starting from the smallest ion sizes. The underestimation in channel 2 (purple) is again visible, but overall this figure shows how well the instruments agree. While the transmission function was chosen to optimize agreement for each comparable channel, the simultaneous variability measured by both instruments for each of the channels is the basis for the good correlation. Channel 4 shows the largest variation, going from around 5 cm 3 to above 3 cm 3, and the agreement is very good, staying within some tens of percent throughout the measurement period. For bin 6, the spread is larger along the xaxis, consistent with the fact that the BSMA detects ions outside the APiTOF measurement range. Overall, the good agreement between the BSMA and APi TOF suggests that no considerable fragmentation/evaporation occurs inside the APiTOF. Since no water clustering is observed in the APiTOF, all water molecules evaporate. However, there would need to be large amounts of water clustered with all ions for water evaporation to shift the results enough to be detected by the ion mobility spectrometers. Due to the extremely high flow rate of the BSMA, ions are sampled very close to ambient temperature and humidity, in contrast to the other two instruments used in this study, where the sample has time to reach room temperature before detection. During the reported measurements, ambient temperature varied in the range 5 15 C. Data presented here are for negative ion detection. Many daytime photooxidation products have low proton affinities (e.g., sulfuric and malonic acid), producing large diurnal variability in

8 528 M. EHN ET AL. Mobility ranges [cm 2 / Vs] & mass ranges (density = 1.66 g/cm 3 ) [Da] Concentration [cm 3 ] APiTOF BSMA3 moved next to APiTOF BSMA1 BSMA3 HSO 4 containing fraction NO3 containing fraction Other : : : TIme : : FIG. 3. Channelbychannel comparison of negative ions measured by the APiTOF and the BSMA during 6 days in Hyytiälä. The background of each subplot is colored according to the amount of sulfate, nitrate, and other compounds in each bin as measured by the APiTOF. Other contains mainly organic peaks, and all unidentified peaks. Before the afternoon of June 1, BSMA3 was measuring next to BSMA1, and after that it was moved next to the APiTOF. Concentration [cm 3 ] Da Da APiTOF, with TF width BSMA3 1.5x 2.x 2.5x R =.71 R =.73 R =.73 R =.61 R =.61 R = Da 2 1 R =.37 R =.5 R =.47 : : : Time : : FIG. 4. Channelbychannel comparison of negative ions measured by the APiTOF and the BSMA. The APiTOF data was calculated using three different transfer function widths.

9 MOBILITY AND MASS MEASUREMENTS OF ATMOSPHERIC SMALL IONS 529 the mass spectra. For positive ions spectral variability is much smaller (Ehn et al. 21), and consequently the variation in each BSMA/AIS bin is mainly due to fluctuations in the total ion concentration. The same analysis was performed for positive ions, and the agreement was as good as, or better than, for the negative ions. But as the correlation between all channels was relatively high, i.e., the time series of all channels were similar, good agreement could be found with a large range of densities and TF widths. However, it should be noted that based on the ions identified by Ehn et al. (21), the bulk densities of compounds in the positive ions were mostly below 1. g cm 3, i.e., clearly lower than the typical negative ions APiTOF versus AIS As an additional test, the APiTOF was also compared against the AIS, and the result is plotted in Figure 6. The background is again colored according to the composition measured by the APiTOF, and 15 min averages were used. Unfortunately, the AIS was not working during the period used for the BSMA comparison, and therefore the comparison period starts 1 days later. The agreement between the APiTOF and the AIS is as good as the comparison to the BSMAs for all the size bins. For this comparison the applied transfer function width was 2 times the width of the theoretical BSMA transfer function but, more importantly, the APiTOF data were shifted towards larger particle sizes to achieve the good correlation shown in Figure 6. The reason was most likely problems in the AIS, as will be discussed later. The shift was accomplished by decreasing the density by a factor 2, i.e., the default density was lowered to.83 g cm 3 and densities for all identified compounds were divided by 2. Using the expected densities as in the BSMA comparison, there was little correlation between the two instruments in channel 3 (as shown in section 5.3), corresponding to a mass range of Da after the density shift. This change in density corresponds to a shift of roughly bin widths in the AIS, as can be seen from Figure 1. A factor 2 in density corresponds to a 25% change in particle diameter, and a 4% change in ion mobility. Note also that Figure 6 contains data for channels 2 7 instead of 1 6, since the lower limit for channel 2 (2.7 cm 2 /Vs) with a density of.83 g cm 3 already corresponds to Da using the modified StokesMillikan equation. The equation is not expected to be accurate below 1 Da, but there was little signal in channel 1 of the AIS, as would be expected if no ions had high enough mobilities to be collected on the first electrometer. The need for the density modification is of course not due to the ions changing density, but most likely due to incorrect sampling in the AIS, either through incorrect flows or unwanted turbulence inside the instrument. As the sample temperature inside the AIS is slightly above room temperature, more evaporation would be expected to take place in the AIS than in the BSMA, and therefore this can be ruled out as a reason for the discrepancy, since the ions were observed to be larger in the AIS than in the BSMA or APiTOF. Incorrect application of the transfer functions may also affect the results. If using the correct densities, but strongly skewing the TF towards smaller sizes, a similar result as shown in Figure 6 can be obtained. However, Asmi et al. (29) found the TF to be skewed towards larger sizes, and therefore this is unlikely the only factor to explain the discrepancy. Varying the TF shape for each channel individually would be warranted, as both the shape and electric field in the DMAs in the AIS change along the columns (Mirme et al. 27), but this is outside the scope of this study Sensitivity to Density and MassMobility Conversion The good agreement found between the APiTOF and both mobility spectrometers shows that also the methods used to compare the instruments were adequate. The sensitivity to transfer function width was discussed in section 5.1. Here we will briefly discuss the impact of density and the use of the modified StokesMillikan equation for mass/mobility conversion. The default density (1.66 g cm 3 ) for the APiTOF comparison with the BSMA was chosen based on previous measurements by Ehn et al. (21), and most identified peaks were given their specific bulk densities. For the AIS comparison, the APiTOF data was converted to mobility space using the default density of.83 g cm 3, and all identified ion densities were also halved, as described. There have been known issues with turbulence in the AIS inlet, and this turbulence may directly perturb the sizing in the DMA, but at least produces (sizedependent) ion losses. These large losses then require large corrections which can further perturb the distributions. In Figure 7 we have plotted the correlation coefficient R (solid lines) between the APiTOF and both mobility spectrometers for each channel (bins 1 6 for the BSMA, bins 2 7 for the AIS), as a function of density. The thicker black lines correspond to the average of the six plotted bins. A signalweighted average would result in a higher R, as most of the signal is found in the larger channels with the best correlations. The steady increase in bin 1 in the BSMA comparison is mostly due to the fact that there is very little signal from the APiTOF attributed to that bin at low densities. For this sensitivity study, the entire mass spectra, including identified ions, were given the same default density. The selected densities, marked by vertical dotted lines, are clearly close to the optimal values, although shifts of a 2% in either direction would not have changed the picture significantly. However, applying the density of 1.66 g cm 3 also for the AIS comparison would have resulted in worse agreement. The dashed lines in Figure 7 depict R as a function of density using the model by Tammet (1995) that approaches the StokesMillikan equation at larger sizes and the polarization limit at smaller sizes. With the densities used in this study, the difference between the two formulations is small, although the modified StokesMillikan equation seems to better correlate the instrument signals for the BSMA comparison. This surprising result, that the relatively simple StokesMillikan equation can be used down to such small sizes without large errors was

10 53 M. EHN ET AL. APiTOF conc. [cm 3 ] BSMA channels: Bin 6, Da Bin 5, Da Bin 4, Da Bin 3, Da Bin 2, Da Bin 1, 3416 Da 1: BSMA conc. [cm 3 ] 3 4 FIG. 5. Scatter plot of negative ions measured by the APiTOF and BSMA3 for the same 6day period as plotted in Figure 3. Mobility ranges [cm 2 / Vs] & mass ranges (density =.83 g/cm 3 ) [Da] Concentration [cm 3 ] APiTOF AIS HSO 4 containing fraction NO3 containing fraction Other TIme FIG. 6. Channelbychannel comparison of the APiTOF and the AIS during 6 days in Hyytiälä. The background of each subplot is colored according to the amount of sulfate, nitrate, and other compounds in each bin as measured by the APiTOF. Other contains mainly organic peaks, and all unidentified peaks. BSMA3 vs APiTOF AIS vs APiTOF.8.8 Correlation coefficient R.6.4 Correlation coefficient R StokesMillikan Tammet Bin 7 Bin 7 Bin 6 Bin 6 Bin 5 Bin 5 Bin 4 Bin 4 Bin 3 Bin 3 Bin 2 Bin 2 Bin 1 Bin 1 Average Average Density [g/cm 3 ] Density [g/cm 3 ] FIG. 7. Correlation coefficients as a function of applied density for APiTOF vs. BSMA3 (left) and AIS (right). Each spectrometer bin is plotted with a separate color, and the average of the 6 plotted bins in each figure is plotted in black. It should be noted, again, that the bins in the AIS and BSMA do not correspond to the same mobility ranges. The vertical dotted lines show the densities used for the previously plotted comparisons. The solid lines refer to the modified StokesMillikan equation used throughout this study, and the dashed lines to the model by Tammet (1995), which incorporates several effects important in the free molecule regime. The difference between the two formulations are small at the densities used for comparison. Outside this range, StokesMillikan seems to do a better job in the BSMA comparison, whereas the Tammet (1995) model does better in the AIS comparison.

11 MOBILITY AND MASS MEASUREMENTS OF ATMOSPHERIC SMALL IONS 531 attributed by Larriba et al. (21) to different effects canceling each other out when nearing the free molecule regime. CONCLUSIONS A mass spectrometer (APiTOF) and two types of mobility spectrometers (BSMA and AIS) were compared in ambient small ion measurements at a boreal forest site where typical concentrations range from 1 to 1 ions cm 3. The instruments were found to agree well, especially in the mass range 2 15 Da with correlations coefficients close to.9. Ion densities were adjusted down by about a factor of 2 in order to obtain similar agreement in the APiTOF AIS comparison. The underlying reason for this discrepancy was most likely incorrect sizing in the AIS. Fair agreement was also seen below 2 Da for the instruments, but as the ions approach the sizes of gas molecules in the air, diffusion in the mobility measurements and/or potential fragmentation/evaporation in the APiTOF may become significant. It was shown that broadening the mobility spectrometer transfer functions, used to convert the APiTOF data into mobility bins, improved the agreement, particularly at the smallest sizes. An additional potential error source is the mobility/mass conversion itself, with uncertainties increasing rapidly as the ion mass decreases. However, the good overall agreement presented in this work suggests that the updated StokesMillikan conversion (Equation 1) is adequate for a study such as this, down to masses below a few hundred Da, even though it does not account for all effects at sizes around 1 nm. The mass spectrum measured by the APiTOF resolves over 1 different ions, whereas the mobility spectrometers produce 6 7 data points over this range. Moreover, the high correlation of the time series of adjacent channels in the mobility spectrometers practically reduces the number of independent data points in the comparison. Nevertheless, the APiTOF mobility spectrometer comparisons (especially with the BSMA) confirmed that assumed density and transmission function shape produce good agreement, indicating that evaporation/fragmentation inside the APiTOF does not dominate the detected ion distributions. This is an important validation for the APiTOF, showing that it can be deployed for ambient measurements without considerable perturbation of the ion distributions. The APiTOF provides detailed chemical composition data, but accurate m/q dependent ion transmission is needed for quantitative concentration measurements. Currently, calibrations are not convenient in the field and, additionally, common mobility standards are mostly limited to positive ions, making the negative polarity more difficult to calibrate. The AIS/BSMA on the other hand are optimized for quantitative ion counting and thus give accurate ion concentrations, even down to small ions, albeit with low mobility resolution. Concurrent measurements with both instruments give quantitative estimates of the concentration for each detected ion, as has been applied by Ehn et al. (21). For the mobility spectrometers, good agreement with the mass spectrometer validates the data at small ion sizes. Together with previous validation of the instruments at sizes above 5 nm (e.g., Vana et al. 26), it can be stated that the instruments are capable of measuring the whole size range that is important for new particle formation: from molecules and clusters where the nucleation is believed to take place, up to 4 nm (for the AIS) nearing the climatically relevant sizes where the particles can act as cloud condensation nuclei. In summary, the APiTOF and mobility spectrometers are complementary instruments. The APiTOF provides detailed chemical information to help interpret the more commonly deployed AIS. As the APiTOF operation and tuning are improved, characterization of both types of measurement will also improve. 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