10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Impact of Ammonia on Atmospheric (H2SO4)n(HSO4-) Ions: Thermochemistry and Implications for New Particle Formation
ALEXEY NADYKTO, Jason Herb, Kirill Nazarenko, Fangqun Yu, Moscow State University of Technology; SUNY at Albany
Abstract Number: 1348 Working Group: Aerosol Chemistry
Abstract Ion-Mediated Nucleation (IMN), which involves, in addition to H2SO4 and H2O, common airborne ions and other stabilizing species, is an essential source of secondary particles [1-3]. Typically, airborne ions have great advantage over neutrals with regards to uptake of highly dipolar (μ(H2SO4) = 3.02 D; μ(H2O)=1.85 D; μ(NH3)=1.44 D) monomers of atmospheric nucleation precursors and multicomponent neutral clusters. However, bonding of NH3, the most abundant atmospheric base, to common atmospheric ion HSO4- exhibits non-classical behavior. In particular, charged (HSO4-)(NH3) complex appears to be much less stable thermodynamically than its neutral analog ammonium bisulfate (H2SO4)(NH3), exhibiting a perfect example of inability of the Classical Nucleation Theory (CNT) to adequately describe nucleation thermochemistry . Nonetheless, larger stable (H2SO4)n(HSO4-)(NH3)m clusters have been observed in laboratory experiments under atmospherically relevant conditions. However, both the molecular nature of the stabilizing effect of NH3 on (H2SO4)n(HSO4-)(NH3)m formation mechanism remain puzzling.
The main goals of this paper is to gain new insights of the stabilizing effect of NH3 on (H2SO4)n(HSO4-) clusters and to elucidate the (H2SO4)n(HSO4-)(NH3)m formation mechanism. In order to achieve the goals, we have carried a DFT study at PW91PW91/6-311++G(3df,3pd) level, have examined structural trends, have obtained and analyzed new thermochemical data. We have also investigated dependencies of cluster formation energies on n and m, explored cluster distributions under atmospherically relevant conditions and discussed implications of new data for interpreting nucleation experiments and observations of NPF and constraining nucleation models.
The present study leads us to conclude that
(1) Ammonia affects both average S-S and N-S distances in (H2SO4)n(HSO4-)(NH3)m. While NH3 effect is size- and composition- dependent, at n>2, addition of any number of NH3 molecules to pure negatively charged (H2SO4)n(HSO4-) leads to the reduction in the average S-S distances that in turn indicates the enhanced overall stability of larger (H2SO4)n(HSO4-)(NH3)m clusters.
(2) The strength of the interaction of (H2SO4)n(HSO4-)(NH3)m clusters with H2SO4 , the key atmospheric nucleation precursor, increases as the molar fraction of NH3 is growing. This indicates that the presence of NH3 enhances the stability of nucleating (H2SO4)n(HSO4-)(NH3)m ions.
(3) Although small negative ions are known to be unstable under typical atmospheric conditions, the analysis of the thermochemical data obtained in the present study confirms that NH3 can get into and stabilize larger (n>2) negatively charged ions. This seems to explain the presence of ammonia in large stable (H2SO4)n(HSO4-) clusters and a pronounced effect of NH3 on stability of large negatively charged H2SO4 cluster ions observed in laboratory experiments despite the extreme instability of the ammonium bisulfate ion.
(4) The comparison of distributions of (H2SO4)n(HSO4-)(NH3)m clusters shows that at n=3 and n=4 most abundant ions under typical atmospheric conditions contain, in agreement with observations, at least one and two NH3 molecules, respectively.
References [1] F.Yu, R.P. Turco, Ultrafine aerosol formation via ion‐mediated nucleation, Geophys.Res. Lett. 27 (6) (2000) 883-886. [2] F.Yu, R.P. Turco, From molecular clusters to nanoparticles: Role of ambient ionization in tropospheric aerosol formation, J. Geophys. Res. 106 (D5) (2001) 4797-4814. [3] F.Yu, G.Luo, X.Liu, R.C.Easter, X.Ma, S.J. Ghan, Indirect radiative forcing by ion-mediated nucleation of aerosol, Atmos. Chem. Phys. 12 (2012) 11451-11463.