Seasonality of the Water-Soluble Inorganic Ion Composition and Water Uptake Behavior of Urban Grime.

Impervious surfaces, especially in urban environments, are coated with a film composed of a complex mixture of substances, referred to as urban grime. Despite its ubiquity, the factors that dictate urban grime composition are still not well understood. Here, we present the first study of the seasonal variation in composition of water-soluble inorganic ions present in urban grime, performed by analyzing samples collected in Toronto for 4-week intervals over the course of a year. A clear seasonality in the composition is evident, with NaCl dominating in the winter months and Ca2+ and NO3- dominant in the summer. We compare the grime composition to the water-soluble ion composition of PM2.5 and PM10 in order to infer chemistry occurring within the grime and find evidence that chemistry occurring within the urban grime matrix could provide a source of ClNO2 and NH3 to the urban atmosphere. The uptake of water by urban grime also shows a clear seasonality, which may be driven by the changing proportions of nitrate salts and/or oxidized organic compounds over the year.

Chemistry of Urban Grime: Inorganic Ion Composition of Grime vs Particles in Leipzig, Germany.

Deposition of atmospheric constituents--either gas phase or particulate--onto urban impervious surfaces gives rise to a thin "urban grime" film. The area exposed by these impervious surfaces in a typical urban environment is comparable to, or greater than, that of particles present in the urban boundary layer; however, it is largely overlooked as a site for heterogeneous reactions. Here we present the results of a field campaign to determine and compare the chemical composition of urban grime and of particles collected simultaneously during the autumn of 2014 at an urban site in central Leipzig, Germany. We see dramatically reduced ammonium and nitrate levels in the film as compared to particles, suggesting a significant loss of ammonium nitrate, thus enhancing the mobility of these species in the environment. Nitrate levels are 10% lower for films exposed to sunlight compared to those that were shielded from direct sun, indicating a possible mechanism for recycling nitrate anion to reactive nitrogen species. Finally, chloride levels in the film suggest that urban grime could represent an unrecognized source of continental chloride available for ClNO2 production even in times of low particulate chloride. Such source and recycling processes could prove to be important to local and regional air quality.

Energetics of intermolecular hydrogen bonds in a hydrophobic protein cavity.

This work explores the energetics of intermolecular H-bonds inside a hydrophobic protein cavity. Kinetic measurements were performed on the gaseous deprotonated ions (at the -7 charge state) of complexes of bovine ß-lactoglobulin (Lg) and three monohydroxylated analogs of palmitic acid (PA): 3-hydroxypalmitic acid (3-OHPA), 7-hydroxypalmitic acid (7-OHPA), and 16-hydroxypalmitic acid (16-OHPA). From the increase in the activation energy for the dissociation of the (Lg + X-OHPA)7⁻ ions, compared with that of the (Lg + PA)7⁻ ion, it is concluded that the ­OH groups of the X-OHPA ligands participate in strong (5-11 kcal mol⁻¹) intermolecular H-bonds in the hydrophobic cavity of Lg. The results of molecular dynamics (MD) simulations suggest that the ­OH groups of 3-OHPA and 16-OHPA act as H-bond donors and interact with backbone carbonyl oxygens, whereas the ­OH group of 7-OHPA acts as both H-bond donor and acceptor with nearby side chains. The capacity for intermolecular H-bonds within the Lg cavity, as suggested by the gas-phase measurements, does not necessarily lead to enhanced binding in aqueous solution. The association constant (Ka) measured for 7-OHPA [(2.3 ± 0.2) × 105 M⁻¹] is similar to the value for the PA [(3.8 ± 0.1) × 105 M⁻¹]; Ka for 3-OHPA [(1.1 ± 0.3) × 106 M⁻¹] is approximately three-times larger, whereas Ka for 16-OHPA [(2.3 ± 0.2) × 104 M⁻¹] is an order of magnitude smaller. Taken together, the results of this study suggest that the energetic penalty to desolvating the ligand ­OH groups, which is necessary for complex formation, is similar in magnitude to the energetic contribution of the intermolecular H-bonds.

Fluorine bonding enhances the energetics of protein-lipid binding in the gas phase.

This paper reports on the first experimental study of the energies of noncovalent fluorine bonding in a protein-ligand complex in the absence of solvent. Arrhenius parameters were measured for the dissociation of gaseous deprotonated ions of complexes of bovine ß-lactoglobulin (Lg), a model lipid-binding protein, and four fluorinated analogs of stearic acid (SA), which contained (X =) 13, 15, 17, or 21 fluorine atoms. In all cases, the activation energies (E(a)) measured for the loss of neutral XF-SA from the (Lg + XF-SA)7⁻ ions are larger than for SA. From the kinetic data, the average contribution of each > CF2 group to E(a) was found to be ~1.1 kcal mol⁻¹, which is larger than the ~0.8 kcal mol⁻¹ value reported for > CH2 groups. Based on these results, it is proposed that fluorocarbon­protein interactions are inherently stronger (enthalpically) than the corresponding hydrocarbon interactions.

Photochemical renoxification of nitric acid on real urban grime.

The fate of NO(x) (=NO + NO(2)) is important to understand because NO(x) is a significant player in air quality determination through its role in O(3) formation. Here we show that renoxification of the urban atmosphere may occur through the photolysis of HNO(3) deposited onto urban grime. The photolysis occurs 4 orders of magnitude faster than in water with J values at noon on July 1 in Toronto of 1.2 × 10(-3) s(-1) for nitrate on urban grime and 1.0 × 10(-7) s(-1) for aqueous nitrate. Photolysis of nitrate present on urban grime probably follows the same mechanism as aqueous nitrate photolysis, involving the formation of NO(2), OH, and possibly HONO. Thus NO(x) may be rapidly returned to the atmosphere rather than being ultimately removed from the atmosphere through film wash off.