ABSTRACT
Reactions of dinitrogen pentoxide (N2O5) greatly affect the concentrations of NO3, ozone, OH radicals, methane, and more. In this work, we employ ab initio molecular dynamics and other tools of computational chemistry to explore reactions of N2O5 with anions hydrated by 12 water molecules to shed light on this important class of reactions. The ions investigated are Cl-, SO42-, ClO4-, and RCOO- (R = H, CH3, C2H5). The following main results are obtained: (i) all the reactions take place by an SN2-type mechanism, with a transition state that involves a contact ion pair (NO2+NO3-) that interacts strongly with water molecules. (ii) Reactions of a solvent-separated nitronium ion (NO2+) are not observed in any of the cases. (iii) An explanation is provided for the suppression of ClNO2 formation from N2O5 reacting with salty water when sulfate or acetate ions are present, as found in recent experiments. (iv) Formation of novel intermediate species, such as (SO4NO2-) and RCOONO2, in these reactions is predicted. The results suggest atomistic-level mechanisms for the reactions studied and may be useful for the development of improved modeling of reaction kinetics in aerosol particles.
ABSTRACT
Gas-liquid scattering and product-yield experiments are used to investigate reactions of N2O5 with glycerol containing Br- and surfactant ions. N2O5 oxidizes Br- to Br2 for every solution tested: 2.7 M NaBr, 0.03 M tetrahexylammonium bromide (THABr), 0.03 M THABr + 0.5 M NaBr, 0.03 M THABr + 0.5 M NaCl, 0.03 M THABr + 0.01 M sodium dodecyl sulfate (SDS), and 0.01 M cetyltrimethylammonium bromide (CTABr). N2O5 also reacts with glycerol itself to produce mono- and dinitroglycerin. Surface tension measurements indicate that 0.03 M THABr and 2.7 M NaBr have similar interfacial Br- concentrations, though their bulk Br- concentrations differ by 90-fold. We find that twice as much Br2 is produced in the presence of THA+, implying that the conversion of Br- to Br2 is initiated at the interface, perhaps mediated by the charged, hydrophobic pocket within the surface THA+ cation. The addition of 0.5 M NaBr, 0.5 M NaCl, or 0.01 M SDS to 0.03 M THABr lowers the Br2 production rate by 23%, 63%, and 67% of the THABr value, respectively. When CTA+ is substituted for THA+, Br2 production drops to 12% of the THABr value. The generation of Br2 under such different conditions implies that trace amounts of surface-active alkylammonium ions can catalyze interfacial N2O5 reactions, even when salts and other surfactants are present.
ABSTRACT
Aromatic organosulfates are identified and quantified in fine particulate matter (PM2.5) from Lahore, Pakistan, Godavari, Nepal, and Pasadena, California. To support detection and quantification, authentic standards of phenyl sulfate, benzyl sulfate, 3-and 4-methylphenyl sulfate and 2-, 3-, and 4-methylbenzyl sulfate were synthesized. Authentic standards and aerosol samples were analyzed by ultra-performance liquid chromatography (UPLC) coupled to negative electrospray ionization (ESI) quadrupole time-of-flight (ToF) mass spectrometry. Benzyl sulfate was present in all three locations at concentrations ranging from 4 - 90 pg m-3. Phenyl sulfate, methylphenyl sulfates and methylbenzyl sulfates were observed intermittently with abundances of 4 pg m-3, 2-31 pg m-3, 109 pg m-3, respectively. Characteristic fragment ions of aromatic organosulfates include the sulfite radical (â¢SO3-, m/z 80) and the sulfate radical (â¢SO4-,m/z 96). Instrumental response factors of phenyl and benzyl sulfates varied by a factor of 4.3, indicating that structurally-similar organosulfates may have significantly different instrumental responses and highlighting the need to develop authentic standards for absolute quantitation organosulfates. In an effort to better understand the sources of aromatic organosulfates to the atmosphere, chamber experiments with the precursor toluene were conducted under conditions that form biogenic organosulfates. Aromatic organosulfates were not detected in the chamber samples, suggesting that they form through different pathways, have different precursors (e.g. naphthalene or methylnaphthalene), or are emitted from primary sources.
