The nanostructures of a series of conjugated oligo(p-phenylene-ethynylene)s (OPE) adsorbed on a surface were thoroughly studied using scanning tunneling microscopy (STM). These oligomers have different backbone lengths and side chains. As a result, various nanostructures displaying periodic linear patterns at a single molecule level were obtained. Based on careful measurements on the STM images in combination with density functional theory (DFT) calculations, it could be found that the vertical and parallel distances between neighboring oligomers were responsible for the specific arrangement of the backbone and side chains. The results showed that these molecular designs strongly affect their self-assembled structure, which is important to clarify the structure-property relationship in the nanoscience field.
Substantial biogenic secondary organic aerosol (BSOA) formation was investigated in a coniferous forest mountain region in Whistler, British Columbia. A largely biogenic aerosol growth episode was observed, providing a unique opportunity to investigate BSOA formation chemistry in a forested environment with limited influence from anthropogenic emissions. Positive matrix factorization of aerosol mass spectrometry (AMS) measurement identified two types of BSOA (BSOA-1 and BSOA-2), which were primarily generated by gas-phase oxidation of monoterpenes and perhaps sesquiterpenes. The temporal variations of BSOA-1 and BSOA-2 can be explained by gas-particle partitioning in response to ambient temperature and the relative importance of different oxidation mechanisms between day and night. While BSOA-1 arises from gas-phase ozonolysis and nitrate radical chemistry at night, BSOA-2 is likely less volatile than BSOA-1 and consists of products formed via gas-phase oxidation by OH radical and ozone during the day. Organic nitrates produced through nitrate radical chemistry can account for 22-33aEuro-% of BSOA-1 mass at night. The mass spectra of BSOA-1 and BSOA-2 have higher values of the mass fraction of m/zaEuro-91 (f(91)) compared to the background organic aerosol. Using f(91) to evaluate BSOA formation pathways in this unpolluted, forested region, heterogeneous oxidation of BSOA-1 is a minor production pathway of BSOA-2.
Vertical column densities (VCDs) of SO2 retrieved by a Pandora spectral sun photometer at Fort McKay, Alberta, Canada, from 2013 to 2015 were analysed. The Fort McKay site is located in the Canadian oil sands region, approximately 20 km north of two major SO2 sources (upgraders), with total emission of about 45 kt yr(-1). Elevated SO2 VCD values were frequently recorded by the instrument, with the highest values of about 9 Dobson Units (DU; DU = 2.69 x 10(16) molecules cm(-2)). Comparisons with co-located in situ measurements demonstrated that there was a very good correlation between VCDs and surface concentrations in some cases, while in other cases, elevated VCDs did not correspond to high surface concentrations, suggesting the plume was above the ground. Elevated VCDs and surface concentrations were observed when the wind direction was from south to southeast, i.e. from the direction of the two local SO2 sources. The precision of the SO2 measurements, estimated from parallel measurements by two Pandora instruments at Toronto, is 0.17 DU. The total uncertainty of Pandora SO2 VCD, estimated using measurements when the wind direction was away from the sources, is less than 0.26DU (1 sigma). Comparisons with integrated SO2 profiles from concurrent aircraft measurements support these estimates.
The self-assembly behaviors of two series of monodispersed oligomers consisting of perylenediimide (PDI) linked by ethynylene and butadiynylene spacers are investigated in solutions. In spite of the very similar chemical structures, the two sets of oligomers manifest completely different optical properties upon self-aggregation, implying differed aggregate structures. While the oligomers containing butadiynylene spacers form H-aggregates, those featuring ethynylene linkers display J-aggregation characteristics. Thermodynamic analysis revealed that the self-association constants of both series of oligomers increase with the number of PDI units in the backbones. Oligomers containing the same number of PDI units but different spacers display nearly identical enthalpy changes. According to the molecular exciton theory, the observed H-and J-aggregates are suggested to comprise similar packing motifs with slightly varied slipping angles, giving rise to greatly disparate optical properties.