Abstract
Abstract. The distribution of charged species produced by electrospray of an ammonium sulfate solution in both positive and negative polarities is examined using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Positively-charged ammonium bisulfate cluster composition differs significantly from negatively-charged cluster composition. For positively-charged clusters all sulfuric acid is neutralized to bisulfate, whereas for negatively-charged clusters the degree of sulfuric acid neutralization is cluster size-dependent. With increasing cluster size (and, therefore, a decreasing role of charge), both positively- and negatively-charged cluster compositions converge toward ammonium bisulfate. The reactivity of negatively-charged sulfuric acid-ammonia clusters with dimethylamine and ammonia is also investigated by FTICR-MS. Two series of negatively-charged clusters are investigated: [(HSO4)(H2SO4)x]− and [(NH4)x(HSO4)x+1(H2SO4)3]−. Dimethylamine substitution for ammonia in [(NH4) x(HSO4) x+1(H2SO4)3]− clusters is nearly collision-limited, and subsequent addition of dimethylamine to neutralize H2SO4 to bisulfate is within one order of magnitude of the substitution rate. Dimethylamine addition to [(HSO4) (H2SO4) x]− clusters is either not observed or very slow. The results of this study indicate that amine chemistry will be evident and important only in large ambient negative ions (>m/z 400), whereas amine chemistry may be evident in small ambient positive ions. Addition of ammonia to unneutralized clusters occurs at a rate that is ~2–3 orders of magnitude slower than incorporation of dimethylamine either by substitution or addition. Therefore, in locations where amine levels are within a few orders of magnitude of ammonia levels, amine chemistry may compete favorably with ammonia chemistry.
Highlights
The process of forming small clusters from gas-phase molecules and their subsequent growth to a size where they may serve as cloud condensation nuclei (CCN) and thereby affect global climate is poorly understood
One reason for this uncertainty is that particle formation occurs in the 1 nm diameter size range, whereas aerosols are activated to serve as CCN around 100 nm diameter
Negatively-charged dimethylammonium bisulfate clusters were unstable upon mass selection; no kinetic data were obtained for exposure of these clusters to dimethylamine or ammonia gas
Summary
The process of forming small clusters from gas-phase molecules and their subsequent growth to a size where they may serve as cloud condensation nuclei (CCN) and thereby affect global climate is poorly understood One reason for this uncertainty is that particle formation occurs in the 1 nm diameter size range, whereas aerosols are activated to serve as CCN around 100 nm diameter. Growth from the nucleating cluster to the CCN size range requires a 106fold increase in particle mass This process may be a significant contributor to global CCN levels (Merikanto et al, 2009). A better understanding of aerosol climate effects requires improved capability in prediction of ambient CCN levels under varying conditions (Kuang et al, 2010; Spracklen et al, 2008) Such predictive capability necessitates a mechanistic understanding of new particle formation. The results of this work will facilitate interpretation of the mass spectra of ambient molecular clusters, as well as improve our understanding of the chemistry and composition of ca. 1–2 nm diameter particles
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