Atmos. Chem. Phys., 12, 2809–2822, 2012 www.atmos-chem-phys.net/12/2809/2012/ Atmospheric doi:10.5194/acp-12-2809-2012 Chemistry © Author(s) 2012. CC Attribution 3.0 License. and Physics + + Reactions of H (pyridine)m(H2O)n and H (NH3)1(pyridine)m(H2O)n with NH3: experiments and kinetic modelling M. J. Ryding1, A.˚ M. Jonsson2, A. S. Zatula3, P. U. Andersson1, and E. Uggerud3 1Department of Chemistry, Atmospheric Science, University of Gothenburg, 412 96 Goteborg,¨ Sweden 2IVL Swedish Environmental Research Institute Ltd., P.O. Box 5302, 400 14 Goteborg,¨ Sweden 3Mass Spectrometry Laboratory and Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway Correspondence to: P. U. Andersson ([email protected]) Received: 23 June 2011 – Published in Atmos. Chem. Phys. Discuss.: 1 September 2011 Revised: 5 February 2012 – Accepted: 19 February 2012 – Published: 16 March 2012 Abstract. Reactions between pyridine containing water itively and negatively charged cluster ions may occur. For + + cluster ions, H (pyridine)1(H2O)n,H (pyridine)2(H2O)n small clusters, this is believed to be associated with exten- + and H (NH3)1(pyridine)1(H2O)n (n up to 15) with NH3 sive fragmentation; for large clusters coalescence may occur. have been studied experimentally using a quadrupole time- These larger neutral clusters formed by the recombination of-flight mass spectrometer. The product ions in the reac- are suggested to be large enough to continue to grow sponta- + tion between H (pyridine)m(H2O)n (m = 1 to 2) and NH3 neously into new aerosol particles by condensation (Yu and have been determined for the first time. It is found Turco 2000; Yu 2003). that the reaction mainly leads to cluster ions of the form + Several air ion mobility measurements have identified H (NH3)1(pyridine)m(H2O)n−x, with x = 1 or 2 depending cluster ions in the troposphere. However, the chemical na- on the initial size of the reacting cluster ion. For a given ture of these cluster ions is often difficult to identify (Horrak number of water molecules (from 5 to 15) in the cluster ion, et al., 2000; Vana et al., 2008). By contrast, there have been rate coefficients are found to be slightly lower than those for measurements and identification of molecule ions in the tro- protonated pure water clusters reacting with ammonia. The posphere during the last two decades. A large fraction of the rate coefficients obtained from this study are used in a ki- molecule ions observed in these studies has likely originated netic cluster ion model under tropospheric conditions. The from cluster ions that fragment before mass analysis (Eisele, disagreement between ambient ground level measurements 1983, 1986, 1988; Eisele and McDaniel, 1986; Eisele and and previous models are discussed in relation to the results Tanner, 1990; Schulte and Arnold, 1990). The first ground from our model and future experimental directions are sug- based measurement of atmospheric ion composition was per- gested. formed by Perkins and Eisele in 1983 (Eisele, 1983; Perkins and Eisele, 1984). In the measurements, several unidentified positive ions were observed (Perkins and Eisele, 1984). Im- 1 Introduction proved measurements conducted a few years later revealed the unidentified ions that had a mass-to-charge ratio of 80, + Atmospheric ions are initially formed by solar radiation, 94 and 108 to be protonated pyridine (C5H5NH ), pro- galactic cosmic rays and radioactive decay. The ions are tonated picoline (methyl-pyridine) and protonated lutidine found in the entire atmosphere, although the formation mech- (dimethyl-pyridine), respectively (Eisele, 1986, 1988). Sev- anisms vary with altitude, region and time of day. The main eral other ions have been identified in the troposphere in + + products of the ionisation of air are O2 ,N2 and free elec- addition to these, although pyridinium and its derivatives trons (Wayne, 2000). Attachment of neutral polar molecules are often found to dominate the mass spectrum. For exam- to the ions leads to charged molecular clusters. Subsequent to ple, Schulte and Arnold (1990) identified pyridinium as the cluster formation and growth, ion-ion recombination of pos- dominating ion in air-plane based measurements in the free Published by Copernicus Publications on behalf of the European Geosciences Union. 2810 M. J. Ryding et al.: Experiments and kinetic modelling troposphere over Europe. Recently, Junninen et al. (2010) for the formation of these pyridinated cluster ions from + measured day-time air ions at an urban site (the SMEAR III H (H2O)n clusters. The first reaction pathway starts with station in Helsinki), using an Atmospheric Pressure Inter- addition of NH3 to a protonated water cluster. The formed face Time-of-Flight instrument. They identified protonated cluster can thereafter react with a pyridine type molecule X: poly(alkyl) pyridines as one of the main positive compound + + types. Ehn et al. (2010) measured day and night-time air H (H2O)n +pNH3 → H (NH3)p(H2O)n (R1a) ions at a remote site (the SMEAR II station in Hyytial¨ a)¨ us- + + ing the same instrument. They observed pyridine ions and H (NH3)p(H2O)n +X → H (X)1(NH3)p−x(H2O)n−y alkyl substituted pyridine ions in both the day and night-time +xNH +yH O (R1b) ion spectra, with approximately a factor two higher concen- 3 2 tration during night-time. In the second reaction pathway, a pyridine molecule reacts Sources of atmospheric pyridine and pyridine derivatives with a protonated water cluster. The pyridine is thereafter are supposed to be biomass burning, automobile exhaust, ejected when ammonia attaches to the cluster in a second coal tars and tobacco smoke (Clemo 1973; Saintjalm and step: Moreetesta, 1980; Beig, 2008). The main atmospheric sink is considered to be reaction with OH radicals (Eisele, + + H (H2O)n +X → H (X)1(H2O)n−x +xH2O (R2a) 1986; Atkinson et al., 1987; Eisele, 1988; Yeung and El- + + rod, 2003). Yeung and Elrod (2003) calculated atmospheric H (X)1(H2O)n +pNH3 → H (NH3)p(H2O)n +X (R2b) lifetimes based on experimentally determined reaction rate coefficients for pyridine and for various substituted pyridine The second step can thereafter be followed by Reac- compounds to be 44 days and around 1 to 10 days, respec- tion (R1b) above to form a cluster containing both ammo- tively. Other suggested atmospheric sinks of significance nia and pyridine. The driving force behind these reactions are reaction with HNO3 in polluted environments (Atkin- – forming cluster ions containing ammonia and pyridine son et al., 1987) and reaction with atomic chlorine (Zhao derivatives – appears connected to the high basicities of am- et al., 2007). Due to the localised and sometimes irregular monia and the pyridine derivatives. Note that loss of water nature of the sources – as well as the relatively short at- is likely to occur also in the first step of the first mechanism mospheric lifetimes – the concentration of pyridine is ex- (Reaction R1a) and in the second step of the second mecha- pected to be highly variable with time and location (Beig nism (Reaction R2b); however, this was not included in the and Brasseur 2000; Yeung and Elrod, 2003). Few mea- notation by Beig and Brasseur. surements of pyridine concentrations in the atmosphere ex- The rate coefficients for the first three reactions have been ist. Among these, Tanner and Eisele (1991) measured a determined by Viggiano et al. for the case X = pyridine (Vig- concentration of about 2.5 ppt ± 50 % (6.2 × 107 cm−3) of giano et al., 1988a, b). The rate coefficients were found to be molecular pyridine at Mauna Loa Observatory, Hawaii. In approximately equal to the collision rate constant. The rate the measurements by Junninen et al. (2010) about 1 cm−3 of Reaction (R2b) is unknown; Beig and Brasseur assumed of protonated pyridine was observed at the urban SMEAR 10−11 cm−3 s−1 as an upper limit for the rate coefficient for III station in Helsinki. However, in contrast to most previ- all pyridine derivatives in their study, this is two orders of ous measurements they found up to six times higher con- magnitude lower than the rate coefficient for Reaction (R1b) centrations for ionic alkyl substituted pyridine compounds at 298 K. + + H C5H5N(CH2)n, 1 ≤ n ≤ 6, including picoline and luti- The pyridinated cluster ions, H (X)1(NH3)p(H2O)n, dine. The reason for this is unknown but interesting and the which may be the dominating positive cluster ions in the findings show that there is a need to better understand the at- atmosphere, as suggested by Beig and Brasseur, could po- mospheric chemistry of these compounds. Ehn et al. (2010) tentially be an important source for new aerosol formation. reported average concentrations of pyridinium and alkyl sub- However, these cluster ions have to date not been measured stituted pyridine ions from the SMEAR II station in Hyytial¨ a¨ in the atmosphere. This discrepancy has motivated us to per- during 4 days in early May 2009: pyridinium 36.4 cm−3, pi- form well controlled experiments to investigate the formation colinium 57.3 cm−3, lutidinium 33.5 cm−3. Also in this case, mechanisms of these clusters. The reactions of two types of alkyl substituted pyridine ions were higher in concentration cluster ions with NH3 in a cluster beam experiment are stud- + than the pyridine ion. ied; the clusters being H (pyridine)m(H2O)n (m = 1 to 2, + A kinetic cluster ion model by Beig and Brasseur (2000) n ≤ 15) and H (NH3)1(pyridine)1(H2O)n (also n ≤ 15). The indicate that pyridine-containing clusters may be the dom- results from the experiments are input to improve the present inating positive ions in the lower free troposphere (from kinetic model by Beig and Brasseur for atmospheric positive 1 to 6 km above ground).