Molecular constraints on particle growth. during new particle formation

Transcription

1 PUBLICATIONS Geophysical Research Letters RESEARCH LETTER Key Points: Identity and contribution of carbonaceous matter to growth are better defined Carbonaceous matter is highly oxidized with composition consistent with extremely low volatility organic compounds Organic nitrogen is a substantial component of the carbonaceous matter Supporting Information: Readme Table S1 Table S2 Table S3 Table S4 Text S1 Correspondence to: M. V. Johnston, mvj@udel.edu Citation: Bzdek, B. R., M. J. Lawler, A. J. Horan, M. R. Pennington, J. W. DePalma, J. Zhao, J. N. Smith, and M. V. Johnston (2014), Molecular constraints on particle growth during new particle formation, Geophys. Res. Lett., 41, , doi: / 2014GL Received 7 APR 2014 Accepted 22 JUN 2014 Accepted article online 26 JUN 2014 Published online 27 AUG 2014 Molecular constraints on particle growth during new particle formation Bryan R. Bzdek 1, Michael J. Lawler 2,3, Andrew J. Horan 1, M. Ross Pennington 1, Joseph W. DePalma 1, Jun Zhao 4, James N. Smith 2,3, and Murray V. Johnston 1 1 Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, USA, 2 Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA, 3 Applied Physics Department, University of Eastern Finland, Kuopio, Finland, 4 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, USA Abstract Atmospheric new particle formation (NPF) produces large numbers of nanoparticles which can ultimately impact climate. A firm understanding of the identity and contribution of the inorganic and carbonaceous species to nanoparticle growth is required to assess the climatic importance of NPF. Here, we combine elemental and molecular nanoparticle composition measurements to better define the composition and contribution of carbonaceous matter to nanoparticle growth in a rural/coastal environment. We show that carbonaceous matter can account for more than half of the mass growth of nanoparticles and its composition is consistent with that expected for extremely low volatility organic compounds. An important novel finding is that the carbonaceous matter must contain a substantial amount of nitrogen, whose molecular identity is not fully understood. The results advance our quantitative understanding of the composition and contribution of carbonaceous matter to nanoparticle growth, which is essential to more accurately predict the climatic impacts of NPF. 1. Introduction Atmospheric nanoparticles having one or more dimension <100 nm typically constitute the largest portion of ambient aerosol loading by number [Seinfeld and Pandis, 2006]. A major source of atmospheric nanoparticles is new particle formation (NPF), a process whereby clusters nucleate from gas phase precursors on the order of a few nanometers and then grow rapidly to climatically relevant sizes [Kulmala et al., 2004; Zhang et al., 2012]. A substantial fraction of cloud condensation nuclei (CCN) are thought to arise from NPF [Kuang et al., 2009; Merikanto et al., 2009]. In order to better predict the frequency, growth rates, and climatic impacts of NPF, knowledge of the chemical mechanisms by which nucleated nanoparticles grow is required [Bzdek and Johnston, 2010]. Identification of the specific chemical compounds contributing to growth and a quantitative description of their contribution to nanoparticle growth enable elucidation of the relative importance of different pathways to NPF. Sulfuric acid is a key component of new particle growth, and its role is well understood. The contribution of sulfuric acid to growing nanoparticles can be quantitatively explained by diffusion-limited condensation [Bzdek et al., 2012, 2013a, 2013b; Kuang et al., 2010; Nieminen et al., 2010; Pennington et al., 2013; Smith et al., 2008; Weber et al., 1996, 1997]. However, sulfuric acid condensation alone is inadequate to describe measured nanoparticle growth during NPF [Stolzenburg et al., 2005; Weber et al., 1997; Wehner et al., 2005]. Indeed, in many environments, sulfuric acid can account for less than half of the total growth of the particles [Bzdek et al., 2012, 2013b; Kuang et al., 2010, 2012; Pennington et al., 2013; Smith et al., 2008]. Species associated with neutralization of sulfuric acid (e.g., ammonia and amines) are also fairly well understood with respect to the types of molecular species that may contribute to neutralization and the mechanisms by which they can contribute to growth [Almeida et al., 2013; Bzdek et al., 2010, 2011a, 2013a; Chen et al., 2012; Kirkby et al., 2011; Qiu et al., 2011; Smith et al., 2010; Wang et al., 2010]. The largest uncertainty with respect to understanding particle growth is associated with carbonaceous matter, which in some locations can account for most of the nanoparticle mass growth [Bzdek et al., 2012, 2013b; Pennington et al., 2012]. Indirect and direct chemical composition measurements indicate a substantial contribution of organic matter to nanoparticle growth [Ehn et al., 2007; Kulmala et al., 2013; Modini et al., 2009; Riipinen et al., 2009; Ristovski et al., 2010; Smith et al., 2008, 2010; Vaattovaara et al., 2006]. Models describing nanoparticle growth generally need to invoke low volatility organic compounds to BZDEK ET AL American Geophysical Union. All Rights Reserved. 6045

2 describe measured growth rates [Donahue et al., 2011; Pierce et al., 2011; Riipinen et al., 2012, 2011]. Recently, highly oxidized organic matter has been proposed to substantially contribute to the nucleation and growth of new particles [Ehn et al., 2014; Schobesberger et al., 2013; Zhao et al., 2013]. The ability to accurately describe the role of carbonaceous matter in nanoparticle growth is one of the greatest remaining uncertainties for determining the impact of NPF on climate [Carslaw et al., 2013]. In order to refine models, more complete and precise knowledge of the composition and quantitative contribution of the carbonaceous component of growth is needed. In this work, we combine elemental and molecular mass spectrometric approaches to impose molecular constraints on the identities and quantitative contributions of species contributing to nanoparticle growth during NPF in a rural/coastal environment. Although quantitative elemental composition alone provides substantial information about the evolution of particle composition during NPF, assumptions as to the identities of the molecular species involved are required. Collocated molecular composition measurements permit more precise apportionment of the elemental composition to inorganic species and thereby better define the elemental and molecular constraints on the carbonaceous matter. Relative to previous studies in this location [Bzdek et al., 2011b, 2013b], the key advance provided by this work is the collocated molecular composition measurement, which allows substantially improved quantitative assessment of the identity and contribution of various molecular species to nanoparticle growth. These field measurements are supported by thermodynamic modeling of aerosol composition based on measured gas phase concentrations, which helps to validate the molecular constraints. The combination of multiple measurement strategies and thermodynamic modeling allows a more precise definition of the chemical identity and quantitative contribution of carbonaceous matter to nanoparticle growth. Such knowledge is essential to the refinement of models to more accurately predict the climatic impact of NPF. 2. Methods Field measurements were conducted at the Hugh R. Sharp Campus of the University of Delaware in Lewes, Delaware, USA ( N, W) from 23 July to 31 August 2012 [Bzdek et al., 2013b]. The site is described in detail in the supporting information, Text S1. Nanoparticle composition was measured by two complementary methods. The first is the Nano Aerosol Mass Spectrometer (NAMS), which gives quantitative elemental composition of individual nanoparticles in the nm size range [Pennington and Johnston, 2012; Wang and Johnston, 2006; Wang et al., 2006]. For this campaign, NAMS was set to analyze the composition of 18 ± 3 nm mobility diameter particles. The second method is the Thermal Decomposition Chemical Ionization Mass Spectrometer (TDCIMS) [Smith et al., 2004; Voisin et al., 2003]. TDCIMS was set to analyze the composition of 30 nm mobility diameter particles. Note that multiple charging of aerosol will impact the actual size range analyzed by TDCIMS. Details of both mass spectrometers are provided in the supporting information, Text S1. The discussion below focuses primarily on the NPF event on 21 August 2012, a day where both instruments were in full operation. Analysis of this particular day is instructive because the measured chemical composition exhibits key features that are typical of NPF in Lewes and other locations. After a detailed discussion of the event on 21 August, we then generalize to the rest of the events studied in Lewes. In addition to nanoparticle chemical composition measurements, gas phase species and particle size distributions were also measured so that thermodynamic modeling of the particle phase could be performed. Ambient gas phase ammonia and amine concentrations were measured using the Ambient pressure Proton transfer Mass Spectrometer (AmPMS) [Freshour et al., 2014; Hanson et al., 2011]. Gas phase sulfuric acid was measured using the Cluster-Chemical Ionization Mass Spectrometer (Cluster CIMS) [Zhao et al., 2010, 2011]. Nitric acid was not directly measured. In its place, NO 2 measurements were taken using a chemiluminescence analyzer (model 42c, Thermo Environmental Instruments, Franklin, Massachusetts, USA). The underlying assumption was that these measured NO 2 levels represent upper limits for the HNO 3 concentrations because the measurement represents the sum of NO 2 and HNO 3 since both are converted to NO by the molybdenum catalyst. Particle size distributions were measured using a Scanning Mobility Particle Sizer (SMPS, electrostatic classifier model 3080, condensation particle counter model 3788, TSI, Inc., St. Paul, Minnesota, USA). Aerosol thermodynamic calculations were performed BZDEK ET AL American Geophysical Union. All Rights Reserved. 6046

3 Figure 1. (a) Scanning Mobility Particle Sizer (SMPS)-measured aerosol size distribution, (b) Nano Aerosol Mass Spectrometer (NAMS)-measured S and N elemental mole fractions, (c) Thermal Decomposition Chemical Ionization Mass Spectrometer (TDCIMS)-measured sulfate ion intensity fraction, and d) TDCIMS-measured cation-forming nitrogen ion intensity fraction for the new particle formation (NPF) event on 21 August The horizontal lines in Figure 1a indicate the NAMS- and TDCIMS-measured sizes. Note that the dotted line for TDCIMS does not consider the impact of multiply charged aerosol on analysis. NAMS data were subject to six-point smoothing. with the Extended Aerosol Inorganics Model (E-AIM) [Clegg et al., 1998, 2013; Ge et al., 2011; Wexler and Clegg, 2002]. 3. Results and Discussion Figure 1a presents the evolution of the SMPS-measured particle size distribution on 21 August 2012, a day when NPF was observed and both NAMS and TDCIMS were in full operation. Particles appear around 10 nm diameter at about 11:00 and grow over the course of the day to larger sizes. The dotted lines indicate the nominal mobility diameter at which NAMS and TDCIMS measured nanoparticle chemical composition. Note that for size selection with TDCIMS, multiple charging of the aerosol can skew the measured distribution to larger sizes if the aerosol is dominated by particles >>30 nm diameter, whereas during periods where the aerosol size distribution dominates at 30 nm diameter aerosol (i.e., during NPF) multiple charging does not BZDEK ET AL American Geophysical Union. All Rights Reserved. 6047

4 impact the measurement. In the case of NAMS, multiple charging of the aerosol does not significantly impact the size range of analysis because particles are selected on the basis of a mass-to-charge ratio. Based on Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) back trajectories for this day [Draxler and Rolph, 2013], the air mass transporting the NPF event came from the northwest, although substantial air recirculation occurred around the field site. One characteristic feature of NPF in Lewes is that on NPF days air masses generally arrive at the site from the northwest [Bzdek et al., 2011b, 2013b]. Figure 1b shows NAMS-measured sulfur and nitrogen elemental mole fractions as the event progresses and illustrates three key observations about NPF. First, particle phase S increases during the NPF event, and this increase can be quantitatively ascribed to sulfuric acid condensation onto the growing particles [Bzdek et al., 2012]. The initial increase in S mole fraction is coincident with an increase in gas phase sulfuric acid, which occurs before the mode diameter passes through the NAMS-measured size range. These characteristics of the S mole fraction increase have been observed in every NAMS study of NPF [Bzdek et al., 2011b, 2012, 2013b; Pennington et al., 2013]. Second, particle phase N is correlated with the change in particle phase S. This, too, is a general observation of NPF and has been observed in every NAMS study. In Figure 1b, N is scaled by a factor of 2 larger than S. If N and S overlap, they are exactly at a 2:1 ratio. At the beginning of the event, N/S 2, suggesting that sulfate is completely neutralized. Third, later on in the event, N increases more substantially than S, resulting in N/S > 2. This ratio indicates that some nitrogen exists in excess of that required to neutralize sulfate (termed Excess N ). The presence of Excess N appears to be a general characteristic of NPF in Lewes [Bzdek et al., 2011b, 2013b] but not in other locations where NPF has been studied by NAMS [Pennington et al., 2013]. Figures 1c and 1d show the variation in TDCIMS-measured sulfate ion intensity (Figure 1c, from negative mode mass spectra) and ammonia and amine ion intensity (Figure 1d, from positive mode mass spectra) during the event day. The reported values are the intensity of a given ion relative to the total ion current at that polarity, and the width of the bar represents the time period of analysis. Both sulfate and ammonia ion intensities increase during the event period. These observations from TDCIMS confirm the previous interpretation of the NAMS elemental signal changes. Specifically, the changes in NAMS-measured S can be attributed to sulfate, and the concurrent NAMS-measured increase in N can be attributed to ammonium. Note that both ammonia and amines are detected with similar sensitivity on a molar basis by TDCIMS, so the relative signal intensities give an approximate measure of the mole ratio of the two species. The observed larger ammonium ion signal suggests that ammonia is the dominant cation-forming nitrogen compound in the aerosol. However, as will be discussed in more detail later, the observed ammonium signal is too large to be only associated with neutralization of sulfate. Figure 2 explores in more detail the nitrogen-containing compounds measured by TDCIMS and how they relate to the NAMS-measured Excess N (=N-2S). In Figure 2a, Excess N is plotted as a function of time of day. Excess N gives a quantitative description of the amount of N that cannot be associated with sulfate neutralization. If Excess N < 0, then all measured N could be apportioned to cation neutralization of sulfate (as either ammonia or amine). If Excess N = 0, then the aerosol is described as neutralized ammonium sulfate (N/S = 2). If Excess N > 0, then there exists additional N not associated with sulfate. A general feature of NPF in Lewes is that Excess N increases at the same time as the nanoparticle mode grows through the measured size range [Bzdek et al., 2013b]. Figure 2b shows how the TDCIMS-measured ammonia intensity fraction evolves over the course of the event day. When Excess N measured by NAMS reaches its peak value (corresponding to when the mode moves through the NAMS size range), the ammonia intensity fraction reaches its peak value (accounting for over 70% of the total positive ion signal from TDCIMS). The slight time lag of the TDCIMS data relative to the NAMS data most likely arises from the slightly different particle sizes measured by each method. Figure 2c shows the evolution of the TDCIMS-measured amine intensity fraction on this day. Before the event begins (i.e., before 11:00), the amine accounts for several percent of the total positive ion signal and represents 3 20% of the cation-forming nitrogen signal (sum of ammonia and amine signals). At the very start of the event but before the mode of new particles grows into the measured size range (i.e., at 12:30), both the ammonia and amine signals increase, but the amine signal increases more substantially relative to ammonia (going from 3% of the cation-forming nitrogen signal at 8:30 to more than 8% of the signal at 12:30). This observation suggests that amines might be important to the early stages of particle formation and growth. However, as BZDEK ET AL American Geophysical Union. All Rights Reserved. 6048

5 Figure 2. (a) NAMS-measured Excess N mole fraction, (b) TDCIMS-measured ammonia ion intensity fraction, (c) TDCIMS-measured amine ion intensity fraction, and (d) TDCIMS-measured inorganic nitrate ion intensity fraction for the NPF event on 21 August NAMS data were subject to six-point smoothing. the event moves through the analyzed size range and Excess N reaches its maximum value (from 14:00 to 16:00), the amine signal is close to zero and in fact reaches its minimum value for the day. This observation suggests that amines are not responsible for the NAMS-measured increase in Excess N during NPF. Figure 2d shows TDCIMS-measured inorganic nitrate intensity fraction. Nitrate appears to be a more significant component of the aerosol during the nighttime and early morning. Indeed, the nitrate intensity fraction is very low during the period where Excess N is very high. An important note is that TDCIMS is generally 100 times more sensitive in the negative mode to detection of nitrate than to detection of sulfate (determined by comparing ammonium nitrate and ammonium sulfate samples). Therefore, a large TDCIMS-measured nitrate signal does not necessarily correlate to a large particulate nitrate fraction. In fact, during the period when Excess N is highest, the nitrate/sulfate ratio is about Based on the relative sensitivities of the two species determined from standard aerosol measurements, such a nitrate/sulfate signal BZDEK ET AL American Geophysical Union. All Rights Reserved. 6049

6 Table 1. Results of Extended Aerosol Inorganics Model (E-AIM) Modeling for Nanoparticle Events in Lewes based on Measured Gas Phase Concentrations a Date Solid Phase: Predicted N/S Molar Ratio Liquid Phase: Predicted N/S Molar Ratio Liquid Phase: Amine Mole Fraction of Total Cationic N Liquid Phase: Inorganic Nitrate Mole Fraction of Anions 10 August No solid phase predicted % 0.6% 12 August 2 (solid AS) No aerosol predicted No aerosol predicted No aerosol predicted 13 August 2 (solid AS) % 0.4% 21 August 2 (solid AS) % 3% a AS = (NH4 ) 2 SO 4. Liquid phase results were obtained by disabling solid phase formation. intensity ratio would correspond to a nitrate/sulfate mole ratio of <1% during NPF. As a result, inorganic nitrate can be discounted as a substantial contributor to Excess N. Although these observations argue against inorganic nitrate as a substantial component of the nanoparticle composition, organonitrates cannot be completely excluded. Laboratory-based TDCIMS measurements of standards composed of a C 4 dihydroxy dinitrate and ammonium nitrate suggest that the TDCIMS-measured NO 2 ion may correspond to particulate inorganic nitrate, whereas the TDCIMS-measured NO 3 ion may be associated with particulate organonitrate. Although little signal attributable to organonitrates (NO 3 ) was measured, the sensitivity of TDCIMS to different organonitrate compounds, including the role of particle-phase acidity, has not been extensively explored. Experimental measurements of nanoparticle composition by NAMS and TDCIMS can be compared to predictions by E-AIM modeling of aerosol composition from measured gas phase concentrations of sulfuric acid, ammonia, dimethylamine, and NO 2 (measured by chemiluminescence analyzer and serves as an upper limit for HNO 3 ). Note that an underlying assumption is that measured NO 2 levels represent upper limits for the HNO 3 concentrations because the measurement represents the sum of NO 2 and HNO 3. Table 1 shows the modeled E-AIM results. (Average gas phase concentrations used as model inputs are provided in Table S1 of the supporting information.) For the conditions during the event on 21 August, E-AIM modeling predicts formation of solid ammonium sulfate without any amine or inorganic nitrate in the particle phase. If modeling is performed by disabling solid formation (i.e., requiring the aerosol to be exclusively in the liquid phase), the predicted N/S molar ratio is slightly larger than 2, amine is expected to account for only 27% of the mole fraction of cations, and inorganic nitrate is expected to account for only 3% of the mole fraction of anions. The TDCIMS measurements are broadly consistent with E-AIM modeling in that the amine and inorganic nitrate signals fall between what would be expected for the limiting cases of a solid-only phase vs. a liquid-only phase. The small amounts of amine and inorganic nitrate detected by TDCIMS suggest a combination of the two phases. Importantly, neither amine nor inorganic nitrate accounts for a significant fraction of the particle composition during NPF. Based on the above, only the ammonia signal from TDCIMS measurements correlates with the increase in Excess N measured by NAMS. This observation suggests two possibilities for the molecular identity of the Excess N. Both possibilities are consistent with the TDCIMS-measured ammonium to sulfate signal intensity ratio during NPF, which is too large to arise from (NH 4 ) 2 SO 4 alone. During NPF in Lewes, the TDCIMSmeasured ammonium to sulfate signal intensity ratio is ~5 15, whereas the ratio is only ~1.8 for an ammonium sulfate standard. The first possibility is that Excess N is composed of ammonium salts of organic acids. This possibility seems unreasonable, since these salts are probably too volatile to remain in the aerosol phase and if there are ammonium salts, aminium salts would also be expected [Barsanti et al., 2009; Yli-Juuti et al., 2013]. A second possibility is that a portion of the ammonia signal arises from organic compounds that decompose to give ammonia upon thermal desorption and subsequent ionization. One candidate for such a process would be imines, which upon thermal desorption and chemical ionization (with H 3 O + as the reagent ion) could potentially undergo hydrolysis to release ammonia [McMurry, 2004; Moldoveanu, 2009]. Imines could arise from reaction of carbonyl-containing compounds with gas phase ammonia and have been measured frequently in aerosol [O Brien et al., 2013a, 2013b]. Although the specific molecular identity of this organic nitrogen is not fully determined, the broader conclusion is that the carbonaceous matter for this particular event contains a substantial amount of nitrogen. Figure 3 maps the NAMS-measured elemental mole fractions onto molecular species for the event on 21 August as well as the other four event days during the Lewes campaign that were studied by NAMS. Molecular mass fractions are determined by apportioning the NAMS-measured elemental mole fractions to BZDEK ET AL American Geophysical Union. All Rights Reserved. 6050

7 Figure 3. Quantitative molecular mass fractions for all five events measured by NAMS during the campaign. Molecular mass fractions were determined by apportioning the quantitative elemental composition (determined by NAMS) to molecular species based on TDCIMS-measured nanoparticle molecular composition. molecular species based on the measured TDCIMS molecular composition along with support from E-AIM modeling (see Table 1). First, sulfur and the accompanying oxygen (=4S) are apportioned to sulfate. Then, Si and the accompanying oxygen (=2Si) are apportioned as silicon dioxide. Note that Si accounts for only a small portion of the total elemental signal (<1%) and virtually any reasonable assumption on the molecular identity of the silicon would have little impact on the apportioned mass fraction. Third, ammonium is apportioned to sulfate until the sulfate is fully neutralized. Based on the previously discussed TDCIMS and E- AIM results, neither amines nor inorganic nitrate contributes significantly to the nanoparticle mass. Therefore, the rest of the elemental signal is considered carbonaceous matter, with corresponding elemental ratios given in the boxes in Figure 3. For the event on 21 August, the carbonaceous matter accounts for nearly 50% of the mass growth and has an average elemental composition of CN 0.2 O 0.8. Note NAMS measurements do not allow for quantitative measurement of H elemental mole fraction. Therefore, H is not included in the reported elemental composition of the carbonaceous matter. The systematic error that arises from excluding H in the mass fraction calculation is within the measurement uncertainty of NAMS. If we generalize the observations for the event on 21 August to all events studied by NAMS during the Lewes campaign, the results imply that carbonaceous matter is responsible for more than 50% of the nanoparticle growth on the other event days and that nitrogen is an important component of the carbonaceous matter. The NAMS-measured elemental ratios in Lewes show that N/C ranges between 0.1 and 0.2 and O/C between 0.6 and 0.9. If every organic molecule contains one nitrogen atom, these formulas imply only 5 10 C atoms per N atom. Note that the updated O/C ratios in Figure 3 for 12 and 13 August differ from those reported previously [Bzdek et al., 2013b]. In our previous work, ammonium nitrate was used as a surrogate for Excess N. The updated values reported here are based on a combination of elemental and molecular composition measurements, which permit more precise refinement of the elemental apportionment and ultimately allow for a better definition of the composition and contribution of carbonaceous matter to nanoparticle growth. Table 1 provides E-AIM modeling results for all events where gas phase concentrations were measured. Even if liquid-like particles are assumed, including the small amount of inorganic nitrate and amine indicated in the table does not change the results presented in Figure 3 beyond the precision of the NAMS measurements (~10%). The measured elemental ratios for carbonaceous matter are consistent with the concept of extremely low volatility organic compounds (ELVOC) contributing to NPF [Ehn et al., 2014; Schobesberger et al., 2013; Zhao et al., 2013], as the O/C ratios measured here are generally between 0.6 and 1.0 depending on the particular event day BZDEK ET AL American Geophysical Union. All Rights Reserved. 6051

8 and the assumptions underlying the molecular apportionment. However, previous reports of ELVOC do not explain the large amount of organic nitrogen in the Lewes nanoparticles. This observation of a substantial amount of organic nitrogen contributing to nanoparticle growth highlights the fact that the identities of the carbonaceous species contributing to nanoparticle growth during NPF are still not fully resolved. 4. Conclusions In this work, quantitative elemental composition measurements by NAMS are combined with molecular composition measurements by TDCIMS to better define the carbonaceous component contributing to the growth of 20 nm nanoparticles during NPF. This is accomplished by a more precise partitioning of the elemental composition to the inorganic components contributing to growth. Although ammonium (and some aminium) sulfate is important to the growth of new particles, the major fraction of particulate mass is carbonaceous. In Lewes, this carbonaceous matter is consistent with the O/C ratio of ELVOC. Unlike previously reported ELVOC, the carbonaceous matter in Lewes contains a substantial amount of organic nitrogen, which has not previously been observed during NPF. More work is required to confirm the molecular identities of these organic nitrogen compounds. The Lewes observations are in contrast to early springtime studies in a remote boreal forest, where the carbonaceous matter does not contain a substantial nitrogen component [Pennington et al., 2013]. The combination of elemental and molecular measurements permits determination of the key molecular components contributing to nanoparticle growth, the quantitative contribution of each molecular component to growth, and more precise determination of the elemental ratios necessary to describe carbonaceous matter. This study is broadly relevant to aerosol modelers, as it better defines the contribution of carbonaceous matter to nanoparticle growth, a key uncertainty in understanding the climatic impacts of NPF. Future work should focus on products and formation mechanisms of highly oxidized carbonaceous matter containing nitrogen. Models describing sulfuric acid incorporation into growing nanoparticles are robust. However, similarly robust models to describe incorporation of organic nitrogen into growing nanoparticles are lacking. Acknowledgments Data supporting the figures are provided in Tables S2-S4. MVJ acknowledges support from the US National Science Foundation (NSF) (grant AGS ). BRB acknowledges a STAR Graduate Fellowship (FP ) awarded by the US Environmental Protection Agency (EPA). The views expressed in this publication are solely those of the authors, and the US EPA does not endorse any products or commercial services mentioned in this publication. JNS acknowledges funding from the Finnish Academy (grant ) and US Department of Energy (grant DE-SC ). The National Center for Atmospheric Research is sponsored by NSF. NO 2 measurements were performed by the Delaware Department of Natural Resources and Environmental Control. The authors acknowledge Coty N. Jen for assistance in acquisition of the gas phase sulfuric acid data, David R. Hanson for provision of the gas phase ammonia and amine data, Peter H. McMurry for constructive comments in the preparation of this manuscript, and George W. Luther, III, for providing access to the Lewes site and facilitating the measurement campaign. The Editor thanks two anonymous reviewers for assistance in evaluating this manuscript. References Almeida, J., et al. (2013), Molecular understanding of sulphuric acid-amine particle nucleation in the atmosphere, Nature, 502(7471), , doi: /nature Barsanti, K. C., P. H. McMurry, and J. N. Smith (2009), The potential contribution of organic salts to new particle growth, Atmos. Chem. Phys., 9(9), , doi: /acp Bzdek, B. R., and M. V. Johnston (2010), New particle formation and growth in the troposphere, Anal. Chem., 82(19), , doi: /ac100856j. Bzdek, B. R., D. P. Ridge, and M. V. Johnston (2010), Size-dependent reactions of ammonium bisulfate clusters with dimethylamine, J. Phys. Chem. A, 114(43), 11,638 11,644, doi: /jp106363m. Bzdek, B. R., D. P. Ridge, and M. V. Johnston (2011a), Amine reactivity with charged sulfuric acid clusters, Atmos. Chem. Phys., 11(16), , doi: /acp Bzdek, B. R., C. A. Zordan, G. W. Luther, and M. V. Johnston (2011b), Nanoparticle chemical composition during new particle formation, Aerosol Sci. Technol., 45(8), , doi: / Bzdek, B. R., C. A. Zordan, M. R. Pennington, G. W. Luther, and M. V. Johnston (2012), Quantitative assessment of the sulfuric acid contribution to new particle growth, Environ. Sci. Technol., 46(8), , doi: /es204556c. Bzdek, B. R., J. W. DePalma, D. P. Ridge, J. Laskin, and M. V. Johnston (2013a), Fragmentation energetics of clusters relevant to atmospheric new particle formation, J. Am. Chem. Soc., 135(8), , doi: /ja Bzdek, B. R., A. J. Horan, M. R. Pennington, J. W. DePalma, J. Zhao, C. N. Jen, D. Hanson, J. N. Smith, P. H. McMurry, and M. V. Johnston (2013b), Quantitative and time-resolved nanoparticle composition measurements during new particle formation, Faraday Discuss., 165(1), 25 43, doi: /c3fd00039g. Carslaw, K. S., L. A. Lee, C. L. Reddington, G. W. Mann, and K. J. Pringle (2013), The magnitude and sources of uncertainty in global aerosol, Faraday Discuss., 165(1), , doi: /c3fd00043e. Chen, M., et al. (2012), Acid base chemical reaction model for nucleation rates in the polluted atmospheric boundary layer, Proc. Natl. Acad. Sci. U.S.A., 109(46), 18,713 18,718, doi: /pnas Clegg, S. L., P. Brimblecombe, and A. S. Wexler (1998), Thermodynamic model of the system H + -NH 4 + -SO4 2 -NO3 -H2 O at tropospheric temperatures, J. Phys. Chem. A, 102(12), , doi: /jp973042r. Clegg, S. L., C. Qiu, and R. Y. Zhang (2013), The deliquescence behaviour, solubilities, and densities of aqueous solutions of five methyl- and ethyl-aminium sulphate salts, Atmos. Environ., 73(1), , doi: /j.atmosenv Donahue, N. M., E. R. Trump, J. R. Pierce, and I. Riipinen (2011), Theoretical constraints on pure vapor-pressure driven condensation of organics to ultrafine particles, Geophys. Res. Lett., 38, L16801, doi: /2011gl Draxler, R. R., and G. D. Rolph (2013), HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website, NOAA Air Resources Laboratory, Silver Spring, Md. [Available at Ehn, M., T. Petaja, W. Birmili, H. Junninen, P. Aalto, and M. Kulmala (2007), Non-volatile residuals of newly formed atmospheric particles in the boreal forest, Atmos. Chem. Phys., 7(3), , doi: /acp Ehn, M., et al. (2014), A large source of low-volatility secondary organic aerosol, Nature, 506(7489), , doi: /nature BZDEK ET AL American Geophysical Union. All Rights Reserved. 6052

9 Freshour, N. A., K. K. Carlson, Y. A. Melka, S. Hinz, B. Panta, and D. R. Hanson (2014), Quantifying amine permeation sources with acid neutralization: calibrations and amines measured in coastal and continental atmospheres, Atmos. Meas. Tech. Discuss., 7(4), , doi: /amtd Ge, X. L., A. S. Wexler, and S. L. Clegg (2011), Atmospheric amines - Part II. Thermodynamic properties and gas/particle partitioning, Atmos. Environ., 45(3), , doi: /j.atmosenv Hanson, D. R., P. H. McMurry, J. Jiang, D. Tanner, and L. G. Huey (2011), Ambient pressure proton transfer mass spectrometry: Detection of amines and ammonia, Environ. Sci. Technol., 45(20), , doi: /es201819a. Kirkby, J., et al. (2011), Role of sulphuric acid, ammonia and galactic cosmic rays in atmospheric aerosol nucleation, Nature, 476(7361), , doi: /nature Kuang, C., P. H. McMurry, and A. V. McCormick (2009), Determination of cloud condensation nuclei production from measured new particle formation events, Geophys. Res. Lett., 36, L09822, doi: /2009gl Kuang, C., I. Riipinen, S. L. Sihto, M. Kulmala, A. V. McCormick, and P. H. McMurry (2010), An improved criterion for new particle formation in diverse atmospheric environments, Atmos. Chem. Phys., 10(17), , doi: /acp Kuang, C., M. Chen, J. Zhao, J. Smith, P. H. McMurry, and J. Wang (2012), Size and time-resolved growth rate measurements of 1 to 5 nm freshly formed atmospheric nuclei, Atmos. Chem. Phys., 12(7), , doi: /acp Kulmala, M., H. Vehkamaki, T. Petaja, M. Dal Maso, A. Lauri, V. M. Kerminen, W. Birmili, and P. H. McMurry (2004), Formation and growth rates of ultrafine atmospheric particles: A review of observations, J. Aerosol Sci., 35(2), , doi: /j.jaerosci Kulmala, M., et al. (2013), Direct observations of atmospheric aerosol nucleation, Science, 339(6122), , doi: /science McMurry, J. (2004), Organic Chemistry, 697 pp., Thomson, Belmont, Calif. Merikanto, J., D. V. Spracklen, G. W. Mann, S. J. Pickering, and K. S. Carslaw (2009), Impact of nucleation on global CCN, Atmos. Chem. Phys., 9(21), , doi: /acp Modini, R. L., Z. D. Ristovski, G. R. Johnson, C. He, N. Surawski, L. Morawska, T. Suni, and M. Kulmala (2009), New particle formation and growth at a remote, sub-tropical coastal location, Atmos. Chem. Phys., 9(19), , doi: /acp Moldoveanu, S. C. (2009), Pyrolysis of amines and imines, in Pyrolysis of Organic Molecules With Applications to Health and Environmental Issues, pp , Elsevier, Amsterdam, Netherlands. Nieminen, T., K. E. J. Lehtinen, and M. Kulmala (2010), Sub-10 nm particle growth by vapor condensation Effects of vapor molecule size and particle thermal speed, Atmos. Chem. Phys., 10(20), , doi: /acp O Brien,R.E.,A.Laskin,J.Laskin,S.Liu,R.Weber,L.M.Russell,andA.H. Goldstein (2013a), Molecular characterization of organic aerosol using nanospray desorption/electrospray ionization mass spectrometry: CalNex 2010 field study, Atmos. Environ., 68(1), , doi: /j.atmosenv O Brien, R. E., T. B. Nguyen, A. Laskin, J. Laskin, P. L. Hayes, S. Liu, J. L. Jimenez, L. M. Russell, S. A. Nizkorodov, and A. H. Goldstein (2013b), Probing molecular associations of field-collected and laboratory-generated SOA with nano-desi high-resolution mass spectrometry, J. Geophys. Res. Atmos., 118, , doi: /jgrd Pennington, M. R., and M. V. Johnston (2012), Trapping charged nanoparticles in the nano aerosol mass spectrometer (NAMS), Int. J. Mass Spectrom., 311(1), 64 71, doi: /j.ijms Pennington, M. R., J. P. Klems, B. R. Bzdek, and M. V. Johnston (2012), Nanoparticle chemical composition and diurnal dependence at the CalNex Los Angeles Ground Site, J. Geophys. Res., 117, D00V10, doi: /2011jd Pennington, M. R., B. R. Bzdek, J. W. DePalma, J. N. Smith, A.-M. Kortelainen, L. Hildebrandt Ruiz, T. Petaja, M. Kulmala, D. R. Worsnop, and M. V. Johnston (2013), Identification and quantification of particle growth channels during new particle formation, Atmos. Chem. Phys., 13(20), 10,215 10,225, doi: /acp Pierce,J.R.,I.Riipinen,M.Kulmala,M.Ehn,T.Petaja,H.Junninen,D.R.Worsnop,andN.M.Donahue(2011),Quantification of the volatility of secondary organic compounds in ultrafine particles during nucleation events, Atmos. Chem. Phys., 11(17), , doi: /acp Qiu, C., L. Wang, V. Lal, A. F. Khalizov, and R. Y. Zhang (2011), Heterogeneous reactions of alkylamines with ammonium sulfate and ammonium bisulfate, Environ. Sci. Technol., 45(11), , doi: /es Riipinen, I., H. E. Manninen, T. Yli-Juuti, M. Boy, M. Sipila, M. Ehn, H. Junninen, T. Petaja, and M. Kulmala (2009), Applying the Condensation Particle Counter Battery (CPCB) to study the water-affinity of freshly-formed 2 9 nm particles in boreal forest, Atmos. Chem. Phys., 9(10), , doi: /acp Riipinen, I., et al. (2011), Organic condensation: A vital link connecting aerosol formation to cloud condensation nuclei (CCN) concentrations, Atmos. Chem. Phys., 11(8), , doi: /acp Riipinen, I., T. Yli-Juuti, J. R. Pierce, T. Petaja, D. R. Worsnop, M. Kulmala, and N. M. Donahue (2012), The contribution of organics to atmospheric nanoparticle growth, Nat. Geosci., 5(7), , doi: /ngeo1499. Ristovski, Z. D., T. Suni, M. Kulmala, M. Boy, N. K. Meyer, J. Duplissy, A. Turnipseed, L. Morawska, and U. Baltensperger (2010), The role of sulphates and organic vapours in growth of newly formed particles in a eucalypt forest, Atmos. Chem. Phys., 10(6), , doi: /acp Schobesberger, S., et al. (2013), Molecular understanding of atmospheric particle formation from sulfuric acid and large oxidized organic molecules, Proc. Natl. Acad. Sci. U.S.A., 110(43), 17,223 17,228, doi: /pnas Seinfeld, J. H., and S. N. Pandis (2006), Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 2nd ed., Wiley, Hoboken, N. J. Smith,J.N.,K.F.Moore,P.H.McMurry,andF.L.Eisele(2004),Atmosphericmeasurementsofsub-20nmdiameterparticle chemical composition by thermal desorption chemical ionization mass spectrometry, Aerosol Sci. Technol., 38(2), , doi: / Smith, J. N., M. J. Dunn, T. M. VanReken, K. Iida, M. R. Stolzenburg, P. H. McMurry, and L. G. Huey (2008), Chemical composition of atmospheric nanoparticles formed from nucleation in Tecamac, Mexico: Evidence for an important role for organic species in nanoparticle growth, Geophys. Res. Lett., 35, L04808, doi: /2007gl Smith, J. N., K. C. Barsanti, H. R. Friedli, M. Ehn, M. Kulmala, D. R. Collins, J. H. Scheckman, B. J. Williams, and P. H. McMurry (2010), Observations of aminium salts in atmospheric nanoparticles and possible climatic implications, Proc. Natl. Acad. Sci. U.S.A., 107(15), , doi: /pnas Stolzenburg, M. R., P. H. McMurry, H. Sakurai, J. N. Smith, R. L. Mauldin, F. L. Eisele, and C. F. Clement (2005), Growth rates of freshly nucleated atmospheric particles in Atlanta, J. Geophys. Res., 110, D22S05, doi: /2005jd Vaattovaara, P., P. E. Huttunen, Y. J. Yoon, J. Joutsensaari, K. E. J. Lehtinen, C. D. O Dowd, and A. Laaksonen (2006), The composition of nucleation and Aitken modes particles during coastal nucleation events: Evidence for marine secondary organic contribution, Atmos. Chem. Phys., 6(12), , doi: /acp BZDEK ET AL American Geophysical Union. All Rights Reserved. 6053

10 Voisin, D., J. N. Smith, H. Sakurai, P. H. McMurry, and F. L. Eisele (2003), Thermal desorption chemical ionization mass spectrometer for ultrafine particle chemical composition, Aerosol Sci. Technol., 37(6), , doi: / Wang, L., V. Lal, A. F. Khalizov, and R. Zhang (2010), Heterogeneous chemistry of alkylamines with sulfuric acid: Implications for atmospheric formation of alkylaminium sulfates, Environ. Sci. Technol., 44(7), , doi: /es Wang, S. Y., and M. V. Johnston (2006), Airborne nanoparticle characterization with a digital ion trap-reflectron time of flight mass spectrometer, Int. J. Mass Spectrom., 258(1 3), 50 57, doi: /j.ijms Wang, S. Y., C. A. Zordan, and M. V. Johnston (2006), Chemical characterization of individual, airborne sub-10-nm particles and molecules, Anal. Chem., 78(6), , doi: /ac052243l. Weber, R. J., J. J. Marti, P. H. McMurry, F. L. Eisele, D. J. Tanner, and A. Jefferson (1996), Measured atmospheric new particle formation rates: Implications for nucleation mechanisms, Chem. Eng. Commun., 151(1), 53 64, doi: / Weber, R. J., J. J. Marti, P. H. McMurry, F. L. Eisele, D. J. Tanner, and A. Jefferson (1997), Measurements of new particle formation and ultrafine particle growth rates at a clean continental site, J. Geophys. Res., 102(D4), , doi: /4396jd Wehner, B., T. Petaja, M. Boy, C. Engler, W. Birmili, T. Tuch, A. Wiedensohler, and M. Kulmala (2005), The contribution of sulfuric acid and nonvolatile compounds on the growth of freshly formed atmospheric aerosols, Geophys. Res. Lett., 32, L17810, doi: /2005gl Wexler, A. S., and S. L. Clegg (2002), Atmospheric aerosol models for systems including the ions H +,NH 4 +,Na +,SO4 2,NO3,Cl,Br, and H 2 O, J. Geophys. Res., 107(D14), 4207, doi: /2001jd Yli-Juuti, T., K. Barsanti, L. Hildebrandt Ruiz, A. J. Kieloaho, U. Makkonen, T. Petäjä, T. Ruuskanen, M. Kulmala, and I. Riipinen (2013), Model for acid base chemistry in nanoparticle growth (MABNAG), Atmos. Chem. Phys., 13(24), 12,507 12,524, doi: /acp Zhang, R., A. Khalizov, L. Wang, M. Hu, and W. Xu (2012), Nucleation and growth of nanoparticles in the atmosphere, Chem. Rev., 112(3), , doi: /cr Zhao, J., F. L. Eisele, M. Titcombe, C. Kuang, and P. H. McMurry (2010), Chemical ionization mass spectrometric measurements of atmospheric neutral clusters using the cluster-cims, J. Geophys. Res., 115, D08205, doi: /2009jd Zhao, J., J. N. Smith, F. L. Eisele, M. Chen, C. Kuang, and P. H. McMurry (2011), Observation of neutral sulfuric acid-amine containing clusters in laboratory and ambient measurements, Atmos. Chem. Phys., 11(21), 10,823 10,836, doi: /acp Zhao, J., J. Ortega, M. Chen, P. H. McMurry, and J. N. Smith (2013), Dependence of particle nucleation and growth on high-molecular-weight gas-phase products during ozonolysis of alpha-pinene, Atmos. Chem. Phys., 13(15), , doi: /acp BZDEK ET AL American Geophysical Union. All Rights Reserved. 6054