Chemical mechanisms for the production of secondary organic material (SOM) are developed in focused laboratory studies but widely used in the complex modeling context of the atmosphere. Given this extrapolation, a stringent testing of the mechanisms is important. In addition to particle mass yield as a typical standard for model-measurement comparison, particle composition expressed as O:C and H:C elemental ratios can serve as a higher dimensional constraint. A paradigm for doing so is developed herein for SOM production from a C(5)-C(10)-C(15) terpene sequence, namely isoprene, a-pinene, and beta-caryopyhllene. The model MCM-SIMPOL is introduced based on the Master Chemical Mechanism (MCM v3.2) and a group contribution method for vapor pressures (SIMPOL). The O:C and H:C ratios of the SOM are measured using an Aerosol Mass Spectrometer (AMS). Detailed SOM-specific AMS calibrations for the organic contribution to the H(2)O(+) and CO(+) ions indicate that published O:C and H:C ratios for SOM are systematically too low. Overall, the measurement-model gap was small for particle mass yield but significant for particle-average elemental composition. The implication is that a key chemical pathway is missing from the chemical mechanism. The data can be explained by the particle-phase homolytic decomposition of organic hydroperoxides and subsequent alkyl-radical-promoted oligomerization.
What determines large-scale patterns of species richness remains one of the most controversial issues in ecology. Using the distribution maps of 11 405 woody species in China, we compared the effects of habitat heterogeneity, human activities and different aspects of climate, particularly environmental energy, water–energy dynamics and winter frost, and explored how biogeographic affinities (tropical versus temperate) influence richness–climate relationships. We found that the species richness of trees, shrubs, lianas and all woody plants strongly correlated with each other, and more strongly correlated with the species richness of tropical affinity than with that of temperate affinity. The mean temperature of the coldest quarter was the strongest predictor of species richness, and its explanatory power for species richness was significantly higher for tropical affinity than for temperate affinity. These results suggest that the patterns of woody species richness mainly result from the increasing intensity of frost filtering for tropical species from the equator/lowlands towards the poles/highlands, and hence support the freezing-tolerance hypothesis. A model based on these results was developed, which explained 76–85% of species richness variation in China, and reasonably predicted the species richness of woody plants in North America and the Northern Hemisphere.
Peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) were measured sequentially in situ by an online gas-phase chromatograph with electron capture detector at urban (Peking University, PKU) and suburban (Yufa, A town in the south of Beijing) sites 5 in Beijing during the photochemical season in 2006. Maximum and average values of PAN were 11.22 ppbv and 1.95 ppbv at PKU during 15 to 27 August, and maximum and average values of PPN were 2.51 ppbv and 0.41 ppbv at Yufa during 2 to 12 September. Average mixing ratios (PAN/PPN) were 5.60 (at PKU) and 5.83 (at Yufa), which is much lower than those in other metropolitan areas. High correlation between PAN and 10 PPN reflects similar volatile organic compound origins. Thermal loss of PAN and PPN was remarkable when compared with their ambient concentrations. PAN and PPN with ozone have similar trend by day. Thermal decompositions of PAN and PPN were calculated, and results indicated that thermal losses influence their atmospheric lifetime significantly. The percentages of PAN loss at the two sites were very similar; however, 15 PPN urban loss was higher than that in suburban.
A question of how electrons can escape from one surfaces has seldom been studied and is still not properly answered. Herein, lateral electron emission from a one-atom-thick surface is thoroughly studied for the first time. We study election emission from side surface of individual electrically biased carbon nanotubes (CNTs) both experimentally and theoretically and discover a new electron emission mechanism named phonon-assisted electron emission. A kinetic model based on coupled Boltzmann equations of electrons and optical phonons is proposed, and well describes experimentally measured lateral electron emission from CNTs. It is shown that the electrons moving along biased CNT can overflow from the one-atom-thick surface due to the absorption of hot forward-scattering optical phonons. A low working voltage, high emission density, and side emission character make phonon-assisted electron emission pin:nadir promising in electron source applications.