DFT Study of the Formation of Atmospheric Aerosol Precursors from the Interaction between Sulfuric Acid and Benzenedicarboxylic Acid Molecules.

Dicarboxylic acids are ubiquitous products of the photooxidation of volatile organic compounds which are believed to play a significant role in the formation of secondary organic aerosols in the atmosphere. In this paper, we report high-level quantum investigations of the clustering properties of sulfuric acid and benzenedicarboxylic acid molecules. Up to four molecules have been considered in the calculations, and the behavior of the three isomers of the organic diacid species have been compared. The most stable geometries have been characterized together with the corresponding thermodynamic data. From an atmospheric point of view, the results of the DFT calculations show that the organic diacid molecules may significantly enhance the nucleation of small atmospheric clusters, at least from an energetic point of view. In this respect, the phthalic acid isomer seems more efficient than the two other isomers of the benzenedicarboxylic acid, in particular because the internal distance between the two carboxyl groups in the organic diacids appears to play an important role in the stabilization of the H-bond network inside the corresponding heterocluster formed with sulfuric acid molecules.

A comprehensive protocol for chemical analysis of flame combustion emissions by secondary ion mass spectrometry.

RATIONALE: Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide detailed information on the surface chemical composition of soot. An analytical protocol is proposed and tested on a laboratory flame, and the results are compared with our previous measurements provided by two-step laser mass spectrometry (L2MS). METHODS: This work details: (1) the development of a dedicated apparatus to sample combustion products from atmospheric flames and deposit them on substrates suitable for TOF-SIMS analysis; (2) the choice of the deposition substrate and the material of the sampling line, and their effect on the mass spectra; (3) a method to separate the contributions of soot and condensable gas based on impact deposition; and finally (4) post-acquisition data processing. RESULTS: Compounds produced during flame combustion are detected on the surface of different deposition substrates and attributed a molecular formula based on mass defect analysis. Silicon and titanium wafers perform similarly, while the surface roughness of glass microfiber filters results in a reduced mass resolution. The mass spectra obtained from the analysis of different locations of the deposits obtained by impaction show characteristic patterns that are attributed to soot/condensable gas. CONCLUSIONS: A working method for the analysis of soot samples and the extraction of useful data from mass spectra is proposed. This protocol should help to avoid common experimental issues like sample contamination, while optimizing the setup performance by maximizing the achievable mass resolution.