Aerosol Chemistry Resolved by Mass Spectrometry: Linking Field Measurements of Cloud Condensation Nuclei Activity to Organic Aerosol Composition

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1 Aerosol Chemistry Resolved by Mass Spectrometry: Linking Field Measurements of Cloud Condensation Nuclei Activity to Organic Aerosol Composition Item Type Article Authors Vogel, Alexander; Schneider, Johannes; Mueller-Tautges, Christina; Phillips, Gavin J.; Poehlker, Mira L.; Rose, Diana; Zuth, Christoph; Makkonen, Ulla; Hakola, Hannele; Crowley, John N.; Andreae, Meinrat O.; Poeschl, Ulrich; Hoffmann, Thorsten Citation Vogel, A., et. al. (2016). Aerosol Chemistry Resolved by Mass Spectrometry: Linking Field Measurements of Cloud Condensation Nuclei Activity to Organic Aerosol Composition. Environmental Science & Technology, 50(20), DOI: /acs.est.6b01675 DOI /acs.est.6b01675 Publisher American Chemical Society Journal Environmental Science & Technology Download date 08/09/ :01:49 Item License Link to Item

2 1 2 3 Aerosol Chemistry Resolved by Mass Spectrometry Linking Field Measurements of CCN Activity to Organic Aerosol Composition Alexander L. Vogel 1,, *, Johannes Schneider 2, Christina Müller-Tautges 1, Gavin J. Phillips 3,, Mira L. Pöhlker 4, Diana Rose 4,, Christoph Zuth 1, Ulla Makkonen 5, Hannele Hakola 5, John N. Crowley 3, Ulrich Pöschl 4 and Thorsten Hoffmann 1, * Institute for Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg-University Mainz, Mainz, Germany 9 2 Max Planck Institute for Chemistry, Particle Chemistry Department, Mainz, Germany 10 3 Max Planck Institute for Chemistry, Air Chemistry Department, Mainz, Germany 11 4 Max Planck Institute for Chemistry, Multiphase Chemistry Department, Mainz, Germany 12 5 Finnish Meteorological Institute, FI-00560, Helsinki, Finland now at: Laboratory for Environmental Chemistry & Laboratory for Atmospheric Chemistry, Paul Scherrer Institute, Villigen 5232, Switzerland 15 now at: Department of Natural Sciences, University of Chester, Chester CH2 4NU, UK now at: Institute for Atmospheric and Environmental Sciences, Goethe University Frankfurt am Main, Frankfurt, Germany 1

3 18 19 KEYWORDS: Secondary Organic Aerosol, Aerosol Mass Spectrometry, Chemical Ionization Mass Spectrometry, Cloud Condensation Nuclei, Hygroscopicity ABSTRACT: Aerosol hygroscopic properties were linked to its chemical composition by using complementary online mass spectrometric techniques in a comprehensive chemical characterization study at a rural mountaintop station in central Germany in August In particular, an atmospheric pressure chemical ionization mass spectrometer (( )APCI-MS) provided measurements of organic acids, organosulfates and nitrooxy-organosulfates in the particle phase at one minute time resolution. Offline analysis of filter samples enabled us to determine the molecular composition of signals appearing in the online ( )APCI-MS spectra. An aerosol mass spectrometer (C-ToF-AMS) provided quantitative measurements of total submicron organics, nitrate, sulfate and ammonium. Inorganic sulfate measurements were achieved by semi-online ion chromatography and were compared to the AMS total sulfate mass. We found that up to 40% of the total sulfate mass fraction can be covalently bonded to organic molecules. This finding is supported by both on- and off-line soft ionization techniques, which confirmed the presence of several organosulfates and nitrooxy-organosulfates in the particle phase. The chemical composition analysis was compared to hygroscopicity measurements derived from a cloud condensation nuclei counter. We observed that the hygroscopicity parameter (κ) that is derived from organic mass fractions determined by AMS measurements may overestimate the observed κ up to 0.2, if high a fraction of sulfate is bonded to organic molecules and little photochemical aging is exhibited. 2

4 INTRODUCTION One of the major uncertainties in global climate models is the role of aerosol particles and its effect on clouds, complicated by their high spatial and temporal variation throughout the atmosphere. 1 The ability of aerosol particles to indirectly influence the radiative budget of the planet by acting as cloud condensation nuclei (CCN) requires insight into their sources, properties and transformation processes. 2,3 It has been shown that throughout the northern hemisphere, the organic fraction of submicron particles accounts for 20-90% 4 of total mass and that most of the particulate organic compounds are of secondary nature. 5 The highly diverse composition of the aerosol s organic fraction 6 might mask important chemical transformation processes and associated changes of the physico-chemical properties of submicron particles. 7 The application of chemical ionization mass spectrometric techniques, which can resolve the chemical composition of the organic aerosol (OA) fraction at a high measurement frequency, 8 12 enables a better understanding of chemical transformation processes of ambient aerosol particles. Chemical transformation such as oxidative aging or condensation reactions can alter the aerosols chemical composition significantly on short timescales. 13 Heterogeneous aging can occur due to reactive uptake of gaseous oxidants or gas-phase epoxides that can react with sulfuric acid in the condensed phase forming organosulfates Other categories of aging are condensational aging due to repartitioning following gas phase oxidation of semi-volatiles and aging due to condensed phase chemistry such as oligomerization reactions However, the atmospheric implications, such as the overall impact of chemical transformation processes on the CCN ability of submicron particles, remain highly uncertain. It has been demonstrated that the organic mass fraction measured by aerosol mass spectrometry (AMS) 28,29 can be used to approximate the effective aerosol particle 3

5 hygroscopicity Field measurements have also demonstrated that particles show a reduced hygroscopicity and CCN efficiency during new particle formation 33,34 and also during summer time along with fresh organic emissions, 35 both indicating that oxidative aging of organic precursors has an important effect on particle hygroscopicity. Unfortunately, AMS measurements barely account for the chemical complexity of the organic fraction, such as the differentiation between inorganic sulfate and sulfate-fragments emerging from organosulfates. 36,37 Thus, due to the occurrence of organosulfates (OS), measurements are biased regarding the ratio of the inorganic and organic fraction. The measurement of the O/C ratio, which is used to parameterize the aging history and thus the hygroscopicity of the organic fraction, is also biased, and can be systematically underestimated if high amounts of OS or organonitrates are present. 37 Evidence for the ubiquitous abundance of OS and nitrooxyorganosulfates (NOS) has been provided in a variety of studies using different offline measurement techniques. 18,41 47 Quantification of the contribution of organosulfates to the total organic matter is a current challenge. Average contributions vary between 5 to 10%, 48 and upper estimations amount up to 30% or 50%. 44,49 In this paper we show results of online APCI-MS measurements of organic acids, organosulfates and nitrooxy-organosulfates in ambient aerosols. Semi-online ion chromatography (IC) was applied for measuring the pure inorganic sulfate. Periods with high levels of organosulfates were determined by comparison between ion chromatography and AMS measurements. Determination of the elemental composition of organic species was achieved by the analysis of co-located filter samples using ultrahigh performance liquid chromatography coupled to electrospray ionization ultrahigh resolution mass spectrometry (UHPLC/( )ESI- UHRMS). Simultaneously, the effective particle hygroscopicity was measured using a CCN 4

6 84 85 counter in order to assess the influence of particle age and organosulfate fraction on the cloud formation ability of submicron aerosol particles EXPERIMENTAL For a detailed description of the applied online and offline mass spectrometric techniques, as well as for a description of the field site and the mobile laboratory, refer to the companion paper of this work. 50 Briefly, the measurements were conducted at the Taunus Observatory on the Mt Kleiner Feldberg in central Germany in August 2012 as part of the Ice Nuclei Research Unit Taunus Observatory (INUIT-TO) campaign. Two online mass spectrometers were operating in-situ to resolve the chemical composition of submicron aerosols: (1) a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS, Aerodyne Research Inc.) 51 and (2) an atmospheric pressure chemical ionization ion trap mass spectrometer (APCI-MS, Model: LCQ, Thermo Finnigan). 9 Aerosol particles from ~20 nm to 2.5 µm were enriched by using an online aerosol concentrator. 52 Gas-phase molecules were removed from the sampled air by a charcoal denuder. The aerosol particle stream (1 SLPM) was then guided through a ceramic heater tube (350 C setpoint, ~200 C actual temperature, estimated by CFD calculations) in which particles were vaporized without being deposited on a surface. Reagent ions were produced by corona discharge ( 3.3 kv) and the ion mode was set to detect negatively charged ions. A minor flow of nitrogen sheath gas (0.1 SLPM) was added to the ion-source to improve the sensitivity. Mass spectra from 80 to 500 amu were averaged and saved at one minute frequency. Thermal decomposition of molecules in the particle phase is a potential source of negative and positive artefacts. Therefore we also conducted offline analysis of PM 2.5 filter samples using an ultrahigh performance liquid chromatography system (Thermo Scientific, Dionex UltiMate 3000) equipped with a Hypersil gold column (C18, 50 x 2.0 mm, 1.9 µm particle size, Thermo 5

7 Scientific) coupled to a (heated) electrospray ion source on an ultrahigh resolution mass spectrometer (UHPLC/( )ESI-UHRMS, Thermo Scientific, Q-Exactive). The filters (PTFEcoated quartz fibre filters, Pallflex T60A20, Pall Life Science, USA) were sampled between 6 and 30 hours throughout the campaign. For analysis they were cut into small pieces and extracted using methanol (HPLC grade) and placing the vial on a vortex shaker for 30 minutes. Methanol extracts from each filter (1 x 1.5mL, 2 x 1mL) were evaporated to dryness and the residue redissolved in a 10% methanolic solution. 10 µl of the sample were injected and separated by a gradient method with two eluents A and B (A: ultrapure water with 2% acetonitrile and 0.04% formic acid; B: acetonitrile with 2% ultrapure water). The gradient method is described in detail in the companion paper. Exemplary chromatograms of authentic standards are shown in Figure S5. A non-target analysis of the high resolution data (R=70.000) was conducted to screen the chromatograms for all signals above a certain threshold value. An elemental composition was assigned to the resulting peak list of the exact masses by XCalibur 2.2 (Thermo Scientific) using the following constraints: # 12 C: 1 to 45, # 1 H: 1 to 60, # 14 N: 0 to 10, # 16 O: 2 to 45, # 31 P: 0 to 2, # 32 S: 0 to 4 and # 35 Cl: 0 to 2 and a mass tolerance of less than ±2 ppm (further details are given in the companion paper of this work). Black carbon was measured using a multi angle absorption photometer (MAAP, Thermo Electron Corp., USA) Monitor for Aerosols and Gases in Ambient Air (MARGA) The MARGA system 53 is a commercialized version of the GRAEGOR analyser developed by ECN, Netherlands, 54,55 and produced by Metrohm Applikon BV, Netherlands. The instrument was deployed inside a ventilated metal cabinet on the roof of the observatory. The air sample was drawn at 1m 3 h 1 through a Teflon coated PM10 cutoff cyclone and through a short (<25 cm length) tube of high density polyethylene (HDPE) into a wet rotating denuder (WRD). The 6

8 samples from both the WRD and the steam-jet aerosol collector were analyzed online every hour using both cation and anion ion chromatography (IC). In some cases the analytical ion may result from more than one ambient species and care must be taken when interpreting MARGA signals, 56 e.g. N2O5 and HNO3 can give rise to NO3 - in the gas-phase as measured during INUIT- TO. The detection system was continuously calibrated by the use of an internal standard of LiBr. In addition, before starting the ambient measurements standard solutions were injected to check the retention times and the analytical system. Our inlet system including cyclone was cleaned after approximately 2 weeks of operation Cloud condensation nuclei counter The number concentration of CCN was measured using a continuous-flow streamwise thermal gradient CCN counter (CCNC), commercially available from Droplet Measurement Technologies, Inc. (DMT, CCN-100, 57 ). The instrument was coupled to a Differential Mobility Analyzer (DMA, TSI 3071, selecting quasi-monodisperse aerosol) and to a condensation particle counter (CPC, TSI 3762, detecting the number concentration of aerosol particles (CN)) to measure size-resolved CCN efficiency spectra, i.e., the size-resolved fraction of activated particles at a certain water vapor supersaturation. 58,59 The effective water vapor supersaturation (S) in the CCNC was alternately set to 0.11, 0.2, 0.3, 0.5 and 0.7%. For this work, only the dataset of the lowest supersaturation has been used. The CCNC was calibrated regularly during the campaign with ammonium sulfate aerosol particles applying the method described in Rose et al. 59 (using Köhler model AP3). The CCN efficiency spectra (CCN/CN) were corrected for multiple charges, the DMA transfer function, and different counting efficiencies of the CCNC and the CPC. 58,60 From the diameter at which the CCN efficiency spectrum at a certain S reaches half its maximum value (activation diameter Da) the hygroscopicity parameter κ was calculated 7

9 using Eq. 6 of Petters & Kreidenweis. 61 The κ parameter can be used to describe the influence of chemical composition on the CCN activity of aerosol particles. Its values range from zero for completely insoluble materials like pure soot or mineral dust to 1.28 for sodium chloride aerosol. Typical values for organic aerosol and ammonium sulfate are κ ~ 0.1 and ~ 0.6, respectively (Petters & Kreidenweis 61 and references therein). On average, mixed aerosol of continental background tends to cluster around κ ~ 0.3 (e.g. Pöschl et al.) RESULTS AND DISCUSSION Online mass spectrometry of ambient (nitrooxy)-organosulfates Here we provide evidence that organosulfates (CHOS) and nitrooxy-organosulfates (CHNOS) can temporarily be substantial constituents of ambient aerosols in a rural region of central Germany. In particular, during the second week of the INUIT-TO 2012 campaign, we observed periods when CHNOS containing signals were strongly enhanced compared to signals that contain only carbon, hydrogen and oxygen (CHO). In the upper panel of Figure 1, the online ( )APCI-MS time series of m/z 294 is shown, as this unit-mass represents the most abundant nitrooxy-organosulfate C10H17NO7S. However, the unit-mass resolution of the APCI-MS is not capable to resolve the nitrooxy-organosulfate from isobaric interferences. We estimated the fraction of C10H17NO7S contributing to the total intensity at m/z 294. Therefore we used the linear fit in the correlation plot between APCI-MS m/z 294 and AMS OA when the contribution of nitrate and sulfate were both below 2 µg/m 3 (Figure S1 (F)). Thus, only periods with a high organic fraction are considered for the linear fit. The fraction of m/z 294 above the linear fit can be interpreted as signal that originates from the nitrooxy-organosulfate C10H17NO7S. This fraction appears as the blue part of the time series at m/z 294, which peaks during the night of 22 August. However, the assumption that nitrooxy-organosulfates and OA are independent variables 8

10 is rather unlikely. Thus, we account with the described procedure only for the contribution of C10H17NO7S to m/z 294 on top of the average aerosol composition and therefore representing a lower-limit estimation of the contribution of C10H17NO7S to the total signal at m/z 294. Although we observe a good correlation between sulfate determined by AMS and MARGA (see Figure S1 A+B), we see a systematic bias resulting in higher AMS sulfate at elevated sulfate concentrations. We interpret this bias as follows: as organosulfates almost quantitatively fragment in the AMS, inorganic sulfate can hardly be distinguished from the sulfate groups of organosulfates in the AMS spectrum. The MARGA technique instead separates inorganic sulfate from organosulfates on the IC column and represents the signal of the inorganic sulfate only. Thus, the fraction of organic bonded sulfate was determined as 186 1,,. (1) The uncertainty of MARGA sulfate concentration can be determined within a relative error of 20 %, 53 and the AMS sulfate concentration within 30 %. By error propagation, the relative error of is thus 36 %. For the tentative identifcation of all (nitrooxy)-organosulfates in the ( )APCI-MS spectrum, a period with low concentrations was compared to a period with enhanced concentrations of (nitrooxy)-organosulfates (low and high OS, Figure 1). The average spectra of both periods were normalized to the base peak (m/z 203) and depicted on top of each other (middle panel, Figure 1). Beside the enhanced inorganic ions m/z 97 (sulfate) and 125 (HNO3*NO3 ), many signals above m/z 200 emerge from the mass spectrum that are enhanced during the high OS period. In the lower panel of Figure 1, the difference between the two normalized spectra is depicted. The colored symbols represent masses at which elemental formulas were determined by the offline LC/ESI-MS analysis. The signals are classified into their elemental composition 9

11 group. We found that during the high OS period the average difference of the signals associated with the three groups CHOS, CHNO and CHNOS are significantly different from zero, while the average of the signals of the CHO group is not different from zero. Furthermore, the detection of nitrogen and sulfur containing organics by the offline analysis indicates that the majority of enhanced signals in the online APCI-MS spectrum are not ion source artefacts of organics clustering with nitrate or sulfate. Still, future laboratory work must focus on the investigation of potential artefact generation and matrix effects of both APCI and ESI. Although it is known that ESI compared to APCI shows better sensitivity towards very polar molecules, 63 the offline LC/ESI-MS measurements do not reveal any compounds that were not detected by the online APCI-MS unit-mass spectra. The non-target-analysis of the UHPLC/( )ESI-UHRMS measurements resulted in a total number of 407 components above the signal threshold value. A molecular formula with a mass deviation of less than 2 ppm could be assigned to 261 compounds. The comprehensive list of the 261 different compounds (156 different molecular formulas) is given in the supplementary material (Table S1). All signals could be classified into CHO, CHNO, CHOS or CHNOS subgroups. The numerically largest group is the CHO-subgroup accounting for 35% of all molecular formulas, followed by the CHNOS subgroup with 26%, followed by the CHOSsubgroup with 21%. The CHNO subgroup is the numerically smallest group accounting for 18% of all molecular formulas. Nitrooxy-organosulfates that have been tentatively identified by UHPLC/( )ESI-UHRMS at m/z 294 are five isomers of C10H17NO7S which are separated from other isobaric interferences (Figure 2). All five C10H17NO7S-isomers show a strong retention on the reversed phase column compared to organic acids (see Table S1 a-d), which indicates that C10-nitrooxy-organosulfates 10

12 are less hydrophilic than organic acids. Further chromatograms and mass spectra of authentic standards are shown in Figure S5 and chromatograms of tentatively identified nitrooxyorganosulfates are shown in Figure S6. Possible fragmentation mass traces of the C10H17NO7Sisomers are shown in Figure S7. Calculation of the intensity weighted average molecular weight of organosulfates enables to derive the organosulfate fraction of the total OA. The mean sulfate concentration determined by AMS and MARGA during the high OS event are 4.9 µg/m 3 ± 30% and 3.2 µg/m 3 ± 20%, respectively. Thus, the mass of sulfate which is bonded to organics is 1.7 µg/m 3 ± 36%. Using 161 g/mol as the average molecular weight of the organic moiety of organosulfates results in 2.8 µg/m 3 ± 36% organosulfate mass (see SI). The total OA mass during the high OS event is 6.1 µg/m 3, and thus up to approximately 46% of the OA signal can be attributed to (nitrooxy)- organosulfates (Figure 3), which is in accordance with the upper-limit described by Lukacs et al.. 49 These numbers are associated with a large relative error due to the subtraction of two large values from each other. Furthermore, the intensity-weighted average molecular weight can be biased towards the molecular weight of compounds which have a high ionization efficiency. However, it illustrates that (nitrooxy)-organosulfates during the high OS event on 22 August 2012 are potentially large contributors to the total aerosol mass. Other field studies have reported organically bonded sulfate fractions between 5 and 14 %, 48,49 that NOS are usually present on % of all particles between 50 and 100 nm, 65 and can show peak values in spring, 45 summer 48 and fall

13 Composition group classification The information obtained on the elemental composition, compared to the temporal variation (between 20 August and 27 August) measured by ( )APCI-MS, was used to investigate the correlation between the CHO, CHOS and CHNOS subgroups (Figure S4). For the comparison of subgroups we have chosen representative m/z, namely for the CHO group we choose the highly abundant nominal mass m/z 187 and m/z 203, for the CHOS group m/z 209 and m/z 279 and for the CHNOS group m/z 294 and m/z 296. A high temporal correlation between different compounds suggests similar sources and formation pathways. By comparing different m/z signals of the same group (e.g. m/z 209 vs. m/z 279), a high degree of correlation can be observed (R ). From previous studies, it is known that the organic acid MBTCA, C8H12O6 (m/z 203) and C8H12O5 (m/z 187) are both of biogenic origin, 67,68 resulting in a high degree of correlation (R 2 =0.99). During the high OS period, organosulfates (CHOS group) show an increased signal compared to organic acids (CHO group), and nitrooxy-organosulfates (CHNOS group) deviate even more when compared with the CHO group. To classify the signals of the whole ( )APCI-MS spectrum into composition groups, we used the above mentioned period for a detailed correlation analysis. The squared correlation coefficient matrix for all m/z ratios (between m/z 80 and m/z 500) was calculated (Figure S3) (negative R-values were set to zero). In Figure 4 each correlation coefficient of the nominal masses in the range between m/z 150 to m/z 400 is plotted against m/z 203 (upper panel, representing the CHO group) and against m/z 294 (lower panel, representing the CHNOS group). The temporal correlation analysis also shows the four different molecular composition groups. It can be seen in the upper panel of Figure 4, that the majority of compounds which belong to the CHO group are highly correlated with m/z 203 (R ). In contrast, all CHO compounds show 12

14 a lower temporal correlation to the CHNOS-containing compound at m/z 294 (R ). In the lower panel of Figure 4, several different compounds exhibit a high degree of correlation with m/z 294. Those compounds can mainly be attributed to the CHNOS group. Furthermore, it can be seen that some CHNO-compounds show a higher degree of correlation than organosulfates from the CHOS group (e.g. m/z 264, 278, 336, 348). This observation suggests that under the prevailing conditions the CHOS compounds either further react to form CHNOS-compounds or have a different formation mechanism. So far, the formation pathways of terpene derived nitrooxy-organosulfates are not completely elucidated. The study by Minerath et al. showed that the rate of OS formation by alcohol-sulfuric acid esterification is kinetically insignificant under atmospherically relevant timescales. 69 Instead it was found that the reactive uptake of epoxides can explain the formation of organosulfates. 70,71 Despite this finding, the formation of the most abundant nitrooxy-organosulfate C10H17NO7S still cannot be explained by the epoxide pathway, as it is discussed by Lin et al., 64 and it is likely that NO3 night time chemistry is involved in the formation process. A consequence of the high correlation of other nitrooxy-organosulfates to m/z 294 might be that the NO3 night time chemistry is also an important formation pathway for those compounds, which in principal incorporate sufficient oxygen atoms necessary to be explained by the epoxide pathway, but in fact, can also be formed via NO3 radical chemistry Impact of photochemical aging and organic bonded sulfate on particle hygroscopicity The photochemical age of the aerosol was monitored by the evolution of the ratio of 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) / Pinic acid (m/z 203 / m/z 185) measured by the APCI-MS. High values of this ratio indicate that aerosol from biogenic emissions has undergone gas-phase oxidation by OH radicals. 50,72 The fraction of organically bonded sulfate in 13

15 the particle phase was determined according to Equation (1). The physicochemical properties of the submicron aerosol were characterized by a CCN counter, and the influence of the chemical composition on the effective hygroscopicity (κ) of the CCN active particles was investigated. We show that photochemical aging increases particle hygroscopicity while particles that contain high amounts of organic bonded sulfate tend to exhibit decreased κ-values compared to particles in which sulfate appears as purely inorganic. In contrast to the work by Gunthe et al. and Dusek et al., 30,34 our analysis focusses only on the κ parameter derived from CCN measurements performed at the lowest supersaturation (S=0.11%), which is most closely representing the aerosol bulk composition. The composition of smaller diameters (κ derived from higher supersaturations) was not considered because the utilized instruments, which provide a more detailed picture of the organic fraction (APCI-MS and MARGA), are not capable of measuring size-resolved chemical composition. At the supersaturation of S=0.11%, the critical particle diameter for CCN activation, which is used to derive κ, was observed to be in the size range of 120 to 220 nm (predominantly in the range 130 to 180 nm), whereas for larger supersaturations the activation diameters were much smaller. Depending on Da (at S=0.11%), the size-resolved chemical composition by the C-ToF-AMS was hourly averaged for the size bins between nm (for Da<150 nm) and between nm (for Da>150nm) (Figure 5A). An average density of 1.5 g/cm 3 was assumed in order to calculate the electrical mobility into the vacuum aerodynamic mobility. The size-resolved ammonium concentrations were calculated from the sulfate and nitrate concentrations under the assumption that the aerosol was fully neutralized. Figure 5B shows the observed hygroscopicity parameter κa as derived from the CCNC measurements at S = 0.11% as well as a predicted hygroscopicity parameter, κp. This value was 14

16 calculated according to the parameterization as described in Gunthe et al. 30 using the sizeresolved mass fractions of sulfate and organics from AMS measurements and the BC mass fractions from MAAP measurements. The observed and predicted values agree very well between 13 and 19 August despite a short period on 14 August where the predicted κ-value underestimates the measured κ-value. To investigate the cause of the observed deviation between κa and κp, we show the relative particle aging state by the ratio of the ( )APCI-MS signals m/z 203 / m/z 185 (Fig. 5C) and the fraction of organic bonded sulfate (fobso4) from the difference between MARGA and AMS sulfate measurements (Figure 5D). On 18 August, fobso4 increased to 30%, and coincidently κp appears larger than κa. During this period AMS measurements also indicated peak nitrate concentrations of 3 µg/m 3, which might suggest that the chemistry of the formation of organonitrates is also linked to the formation of nitrooxy-organosulfates. The largest offset between measured and predicted κ can be observed from 22 August on, which was accompanied by continuously high fobso4 but also by a low aging state of the organic aerosol. As shown above, the highest fraction of (nitrooxy)-organosulfates was measured by the ( )APCI-MS during the night of 22 August. Thus, if the highly hygroscopic sulfate and nitrate is bonded to organic molecules, the resulting total hygroscopicity appears to be lower from what would be derived from AMS measurements. However, the nitrooxy-organosulfate fraction measured by APCI-MS did not always appear to be high when a high fobso4 was observed (e.g. night of 23 August). Reasons for that might be caused by the underestimation of the contribution of nitrooxy-organosulfates to the unit-mass signals, especially at low mass concentrations. In Figure 6, the measured κa at S=0.11% is plotted against the size-resolved organic mass fraction. The ( )APCI-MS aging proxy (Figure 5C) and the fraction of sulfate that is bonded to 15

17 organics (Figure 5D) are now depicted by color and size-coded data points, respectively. It can clearly be seen that the measured κ agrees with the AMS dependent parameterization if the fraction of organic bonded sulfate is low and the aerosol is moderately to highly aged (represented by small orange/red points). However, the parameterization is not applicable with decreasing aerosol age (blue and green points) and an increasing fraction of organic bonded sulfate (large points). The observed overestimation of κ by the AMS-parameterization can, under certain conditions, be above 0.2 and thus larger than the uncertainty of the measurements. Most of the measurements that show a smaller κ compared to the predicted κ, suggest they are affected by both effects that can shift κ to lower values: they are sparsely aged and contain a high fraction of organically bonded sulfate (see also Figure S8). This means that both effects can have a pronounced influence on the aerosol hygroscopicity. However, to investigate the overall impact of aging and organosulfate formation on aerosol hygroscopicity, future chamber studies and field campaigns at different locations are needed. Overall, our observations suggest that the occurrence of ambient levels of (nitrooxy)- organosulfate can affect particle hygroscopicity. We found that under certain atmospheric conditions, the generally accepted linear relationship of κ versus forg is not applicable. Considering other hygroscopicity closure studies, a systematic overprediction of κ derived from AMS measurements has also been described: i.e. Wu et al. 32 explained the observed gap by possible evaporation artefacts of NH4NO3 in the HTDMA system, however, the observed bias can also be explained by substantial amounts of nitrooxy-organosulfates in the particle phase. These observations imply that chemical reactions between anthropogenic inorganic emissions and biogenic organic emissions can suppress the hydrophilic contribution of purely inorganic sulfate and nitrate when sulfate and nitrate is bonded to organic molecules. Recent work has 16

18 shown that organosulfates can also form from anthropogenic precursor VOCs, which might have important implications for the physico-chemical properties of urban aerosol particles. 73 Since the chemical composition of large particles does not affect the CCN-activity of a particle, it should be a future analytical aim to achieve chemical analysis of particles of sizes for which the chemical composition can become the crucial factor in determining whether a particle can grow at a certain supersaturation into a cloud droplet or not. 17

19 Figure 1. Time series of ( )APCI-MS mass trace m/z 294 being either attributed to C10H17NO7S or to other isobaric interference (upper panel). Periods of low and high contribution of (nitrooxy)-organosulfates are indicated by green and blue rectangles, respectively. The mass spectra in the middle panel are normalized to the base peak (in both spectra m/z 203). The lowermost panel shows the difference between the two normalized spectra. The tentatively identified molecular formulas by the UHPLC/( )ESI-UHRMS analysis are depicted as symbols on top of the APCI-MS difference spectrum according to the elemental composition group (CHO, CHOS, CHNO or CHNOS). Air mass histories during the period are provided as an animation in the supplementary material. 18

20 Figure 2. The extracted ion chromatogram (EIC) of the accurate mass at m/z resolves that five different isomers occur on this exact mass. The UHPLC/ESI-UHRMS analysis separates four compounds on the nominal mass at m/z 294 which cannot be resolved by the online APCI-MS. The mass spectrum is the average over the entire chromatogram. 19

21 Figure 3. Estimated fraction of (nitrooxy)-organosulfates to total sulfate and organic aerosol mass during the high OS event on 22 August The exposed slices illustrate the fraction of organic bonded (o. b.) sulfate (light red) and the fraction of the organic moiety of nitrooxyorganosulfates (NOS) and organosulfates (OS) (light green). The numbers in the slices represent aerosol mass in µg/m 3. 20

22 Figure 4. Correlation analysis (20 to 28 August 2012) between the APCI-MS signals m/z 150 to m/z 400 and APCI-MS m/z 203 (primarily C8H12O6, upper panel) and APCI-MS m/z 294 (primarily C10H17NO7S, lower panel). The tentatively identified molecular formulas by the UHPLC/( )ESI-UHRMS analysis (Table S1) are depicted as color coded symbols according to the elemental composition group (CHO, CHOS, CHNO, or CHNOS). 21

23 Figure 5. Time series of (A) aerosol mass fractions determined with C-ToF-AMS within the size bins of nm or nm; (B) observed hygroscopicity parameter (κa) at S = 0.11% determined from CCNC measurements and predicted values, κp, from AMS mass fractions using the parameterization of Gunthe et al. 30 ; (C) the biogenic aerosol aging proxy m/z 203 / m/z 185 at the temporal resolution of the CCN counter measurements; and (D) the fraction of organic bonded sulfate determined from the difference between MARGA and AMS sulfate measurements. 22

24 Figure 6. Effective hygroscopicity parameter κa of CCN active particles at S=0.11% versus sizeresolved AMS organic fraction. The size of the data points represents the fraction of organic bonded sulfate and the color the relative extent of the aerosol age (blue: fresh aerosol, red: aged aerosol). The orange and blue line represent the parameterization of κ derived from forg. The shaded area represents the uncertainty of the forg-derived parameterization. 23

25 404 ASSOCIATED CONTENT 405 Supporting Information This material is available free of charge via the Internet at It contains a detailed description of AMS and MARGA comparison for sulfate measurements, the description of the average molecular weight calculation, figures which are mentioned in the main text, an animation of the backtrajectories, and a table containing the results of the non-target analysis AUTHOR INFORMATION Corresponding Authors *Alexander L. Vogel; Paul Scherrer Institut, 5232 Villigen, Switzerland; ; alexander.vogel@psi.ch or *Thorsten Hoffmann; Duesbergweg 10-14, Johannes Gutenberg-Universität, Mainz; ; t.hoffmann@uni-mainz.de Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interest. 421 ACKNOWLEDGMENT This work was supported by the Max Planck Graduate Center together with the Johannes Gutenberg-Universität Mainz (MPGC). The INUIT-TO 2012 field campaign was initiated by the DFG funded research unit INUIT (FOR 1525). We thank H. Bingemer and University of 24

26 Frankfurt technicial staff for field site support; F. Drewnick for providing the Mola, and T. Böttger and S.-L. von der Weiden-Reinmüller for support during the setup of the aerosol inlets and the Mola. 25

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