Ozone concentrations in the Po Valley of northern Italy often exceed international regulations. As both a source of radicals and an intermediate in the oxidation of most volatile organic compounds (VOCs), formaldehyde (HCHO) is a useful tracer for the oxidative processing of hydrocarbons that leads to ozone production. We investigate the sources of HCHO in the Po Valley using vertical profile measurements acquired from the airship Zeppelin NT over an agricultural region during the PEGASOS 2012 campaign. Using a 1-D model, the total VOC oxidation rate is examined and discussed in the context of formaldehyde and ozone production in the early morning. While model and measurement discrepancies in OH reactivity are small (on average 3.4 ± 13%), HCHO concentrations are underestimated by as much as 1.5 ppb (45%) in the convective mixed layer. A similar underestimate in HCHO was seen in the 2002–2003 FORMAT Po Valley measurements, though the additional source of HCHO was not identified. Oxidation of unmeasured VOC precursors cannot explain the missing HCHO source, as measured OH reactivity is explained by measured VOCs and their calculated oxidation products. We conclude that local direct emissions from agricultural land are the most likely source of missing HCHO. Model calculations demonstrate that radicals from degradation of this non-photochemical HCHO source increase model ozone production rates by as much as 0.6 ppb h−1 (12%) before noon.
Particle shape plays a crucial role in determining packing characteristics. Real particles in nature usually have rounded corners. In this work, we systematically investigate the rounded corner effect on the dense packings of spherotetrahedral particles. The evolution of dense packing structure as the particle shape continuously deforms from a regular tetrahedron to a sphere is investigated, starting both from the regular tetrahedron and the sphere packings. The dimer crystal and the quasicrystal approximant are used as initial configurations, as well as the two densest sphere packing structures. We characterize the evolution of spherotetrahedron packings from the ideal tetrahedron (s = 0) to the sphere (s = 1) via a single roundness parameter s. The evolution can be partitioned into seven regions according to the shape variation of the packing unit cell. Interestingly, a peak of the packing density Φ is first observed at s ≈ 0.16 in the Φ-s curves where the tetrahedra have small rounded corners. The maximum density of the deformed quasicrystal approximant family (Φ ≈ 0.8763) is slightly larger than that of the deformed dimer crystal family (Φ ≈ 0.8704), and both of them exceed the densest known packing of ideal tetrahedra (Φ ≈ 0.8563).
Most pollutants in the Earth's atmosphere are removed by oxidation with highly reactive hydroxyl radicals. Field measurements have revealed much higher concentrations of hydroxyl radicals than expected in regions with high loads of the biogenic volatile organic compound isoprene(1-8). Different isoprene degradation mechanisms have been proposed to explain the high levels of hydroxyl radicals observed(5,9-11). Whether one or more of these mechanisms actually operates in the natural environment, and the potential impact on climate and air quality, has remained uncertain(12-14). Here, we present a complete set of measurements of hydroxyl and peroxy radicals collected during isoprene-oxidation experiments carried out in an atmospheric simulation chamber, under controlled atmospheric conditions. We detected significantly higher concentrations of hydroxyl radicals than expected based on model calculations, providing direct evidence for a strong hydroxyl radical enhancement due to the additional recycling of radicals in the presence of isoprene. Specifically, our findings are consistent with the unimolecular reactions of isoprene-derived peroxy radicals postulated by quantum chemical calculations(9-11). Our experiments suggest that more than half of the hydroxyl radicals consumed in isoprene-rich regions, such as forests, are recycled by these unimolecular reactions with isoprene. Although such recycling is not sufficient to explain the high concentrations of hydroxyl radicals observed in the field, we conclude that it contributes significantly to the oxidizing capacity of the atmosphere in isoprene-rich regions.
Lake eutrophication is associated with excessive nutrient enrichment and unobserved internal nutrient cycling. In spite of advances in understanding the role of nitrogen (N) and phosphorus (P) cycling in eutrophication, the relative importance of N and P limitation and release from sediment is still an open question. The complicated interaction between N and P cycling and external driving factors leads to dynamics in N or P limitation patterns and internal release that change over time. We developed a nutrient-driven model of phytoplankton dynamics including the critical nutrient cycling processes. It was fitted using Bayesian inference to explore the roles of N and P inputs from external sources, net sediment release, and internal dynamics in Lake Yilong, a shallow eutrophic lake in China. The model provided a good fit to observations, with time-varying parameters required to fit time-dependent variations in the sediment release process. The results demonstrated that, in Lake Yilong, the pattern of nutrient limitation showed a transformation from P limitation to N and P co-limitation after an observed regime shift occurred in 2008. After the shift in 2008, sediment release had an increasing influence on N and P supply, which could make eutrophication remediation more difficult. For Lake Yilong, it would not be possible to reverse eutrophication solely with watershed nutrient load reduction so in-lake manipulation of physical chemical conditions to inhibit the sediments release should also be considered. (C) 2015 Elsevier B.V. All rights reserved.
Fe(III) hydroxide nanoparticles are an essential carrier for aqueous heavy metals. Particularly, iron hydroxide precipitation on mineral surfaces can immobilize aqueous heavy metals. Here, we used grazing-incidence small-angle X-ray scattering (GISAXS) to quantify nucleation and growth of iron hydroxide on quartz in 0.1 mM Fe(NO3)3 solution in the presence of Na+, Cu2+, Pb2+, or Cr3+ at pH = 3.7 ± 0.1. In 30 min, the average radii of gyration (Rg) of particles on quartz grew from around 2 to 6 nm in the presence of Na+ and Cu2+. Interestingly, the particle sizes remained 3.3 ± 0.3 nm in the presence of Pb2+, and few particles formed in the presence of Cr3+. Quartz crystal microbalance dissipation (QCM-D) measurements showed that only Cr3+ adsorbed onto quartz, while Cu2+ and Pb2+ did not. Cr3+ adsorption changed the surface charge of quartz from negative to positive, thus inhibiting the precipitation of positively charged iron hydroxide on quartz. Masses and compositions of the precipitates were also quantified. This study provided new insights on interactions among quartz, iron hydroxide, and metal ions. Such information is helpful not only for environmental remediation but also for the doping design of iron oxide catalysts.
There are two sides to every story of visual saliency modeling in the frequency domain. On the one hand, image saliency can be effectively estimated by applying simple operations to the frequency spectrum. On the other hand, it is still unclear which part of the frequency spectrum contributes the most to popping-out targets and suppressing distractors. Toward this end, this paper tentatively explores the secret of image saliency in the frequency domain. From the results obtained in several qualitative and quantitative experiments, we find that the secret of visual saliency may mainly hide in the phases of intermediate frequencies. To explain this finding, we reinterpret the concept of discrete Fourier transform from the perspective of template-based contrast computation and thus develop several principles for designing the saliency detector in the frequency domain. Following these principles, we propose a novel approach to design the saliency detector under the assistance of prior knowledge obtained through both unsupervised and supervised learning processes. Experimental results on a public image benchmark show that the learned saliency detector outperforms 18 state-of-the-art approaches in predicting human fixations.