Effects Of Photochemical Oxidants Research Articles

Effects of photochemical oxidation on the mixing state and light absorption of black carbon in the urban atmosphere of China

The relationship between the refractory black carbon (rBC) aerosol mixing state and the atmospheric oxidation capacity was investigated to assess the possible influence of oxidants on the particles’ light absorption effects at two large cities in China. The number fraction of thickly-coated rBC particles (FrBC) was positively correlated with a measure of the oxidant concentrations (OX = O3 + NO2), indicating an enhancement of coated rBC particles under more oxidizing conditions. The slope of a linear regression of FrBC versus OX was 0.58% ppb−1 for Beijing and 0.84% ppb−1 for Xi’an, and these relationships provide some insights into the evolution of rBC mixing state in relation to atmospheric oxidation processes. The mass absorption cross-section of rBC (MACrBC) increased with OX during the daytime at Xi’an, at a rate of 0.26 m2 g−1 ppb−1, suggesting that more oxidizing conditions lead to internal mixing that enhances the light-absorbing capacity of rBC particles. Understanding the dependence of the increasing rates of FrBC and MACrBC as a function of OX may lead to improvements of climate models that deal with the warming effects, but more studies in different cities and seasons are needed to gauge the broader implications of these findings.

Meteorological and Chemical Behavior of Gaseous Sulfur Compounds in and around an Urban Valley

The effects of photochemical oxidation of reduced sulfur compounds (RSCs: H2S, CH3SH, DMS, and DMDS) on SO2 production were evaluated at high (HV) and low ventilation (LV) conditions, based on a CALPUFF dispersion model coupled with photo-chemical oxidation mechanisms for four RSCs. The RSC emission concentrations used in the modeling were measured in and around an urban valley during a field campaign held in October 2008. SO2 production with LV (up to 156 ppb at 0900 LST) was found to be significantly higher than that with HV (up to 30 ppb). SO2 produced by photochemical oxidation of RSCs with LV (78% of total SO2 concentrations) was much higher than that with HV (27%), while the predominant RSC species were similar: DMDS (≥ 60% of the total contributions) with HV and LV when compared to three other RSCs (＜ 20%). The difference in SO2 concentration between HV and LV might be caused by the combined effects of photochemical oxidation of RSCs and ventilation condition.

The effects of photochemical oxidation and chain conformation on green emission of poly(9,9-dioctylfluorene)

The effects of chain conformation on the photo-oxidation and green emission of poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) has been investigated at both single-molecule and ensemble levels. Single-molecule studies reveal the conformation of PFO chains to be more globular when cast from THF than from toluene solutions. Intensity transients of the single molecules show that the elongated molecules cast from toluene have more fluctuations due to limited energy transfer along the polymer chains. The more globular chains show monotonic decays with isotropic polarization. Emission spectra of the single molecules show that photochemical oxidation leads to a reduction in the emission of the molecule with no change in the emission spectra. No green emission is ever detected for single molecules, indicating that formation of emissive ketone defects is a rare event. Ensemble studies of chains show that molecules cast from THF develop some green emission upon photodegradation, while those cast from toluene do not. The increase in green emission in the globular molecules suggests that interchain contacts improve energy transfer for emissive ketone defects in the PFO cast from THF.

Evaluation of NOx and O3 Concentrations in the Atmosphere of Bursa, Turkey

This study summarizes seasonal variations of NOx and O3 concentration levels. Samples were taken between May 2001 and April 2002 in a monitoring site located in the city center of Bursa, Turkey. Owing to the emission rates and meteorological conditions, air pollutant concentrations varied. The annual average NOx and O3 concentrations were found to be 78.96 ± 70.39 μ g/m3 and 55.10 ± 32.53 μg/m3, respectively. In general, the maximum NOx concentrations (115.35 ± 38.83) occurred during the winter while minimum values (44.86 ± 17.10) were observed during the summer. This fluctuation probably caused by the residential heating, mixing height and photochemical activity in the region. On the other hand, ozone concentrations were higher in the summer by the effect of photochemical oxidation. This study may help in forensic investigations dealing with O3 sources in the atmosphere.

Setting international targets for controlling atmospheric emissions of pollutants — now and in the future

Current discussions on controlling emissions of atmospheric nitrogen pollutants in Europe are adopting a multi-pollutant, multieffect strategy where acidification, eutrophication and the effects of photochemical oxidants are all taken into account. Parallel activities by both the Convention on Long Range Transboundary Air Pollution (ratified by 42 governments of the 55 countries in the UNECE area) and the EU are making use of the critical loads approach and integrated assessment models to arrive at cost optimized scenarios for environmental protection. Critical loads and critical levels are used to define the thresholds for effects; as these are usually difficult to attain they are being used to set.intermediate target objectives. A number of different methods for setting such targets, the so-called “gap closures” are being investigated. While the basis for estimating critical loads and levels has been extensively researched, collection of the necessary data and the development of methods continue to evolve. Current discussions on international agreements for emission reductions are based upon the best available information, but this has limitations which are often difficult to build into the decision making process. It is to be expected that any currently derived results will be an interim and very approximate solution for the problems resulting from pollutant emissions. Further steps will require better information on impacts and probably alternative approaches to the critical loads concept as it currently stands.

Control of Fermi level pinning and recombination processes at GaAs surfaces by chemical and photochemical treatments

The effects of photochemical oxidation in water, Na2 S deposition and exposure to NH3 and HCl gas ambients on the Fermi level pinning, and the surface recombination process at GaAs surfaces were studied by measuring the band edge photoluminescence (PL) intensity, x-ray photoelectron spectroscopy spectra, and surface current transport(SCT). Computer simulation of the surface recombination process was also made on the basis of the disorder-induced gap state model. Marked increase of PL intensity was observed after both photochemical oxidation and deposition of Na2 S as previously reported. However, SCT measurements on n-type materials detected increase of surface band bending in the dark. Exposure to NH3 resulted in a slight reduction in the band bending with little change in the PL intensity. Exposure to HCl, on the other hand, resulted in marked reduction of the band bending with marked increase of the PL intensity. The computer simulation shows that the contradictory behavior of the PL intensity and band bending, as observed after photochemical oxidation and Na2 S deposition, is explicable by a shift of the Fermi level pinning position towards the valence band edge due to a fixed negative interface charge which reduces the effective recombination velocity. On the other hand, exposure to HCl reduces the surface-state density and weakens the pinning remarkably.

The effects of photochemical oxidants on the yield of snap beans

Effects of photochemical oxidants (primarily ozone) in the ambient air of Eastern United States on the yields of selected snap bean ( Phaseolus vulgaris L.) cultivars grown from the seedling stage to maturity in “open-top” field chambers were studied. The chambers were supplied with either carbon-filtered or nonfiltered air. The research was conducted at Beltsville, Maryland, near Washington, D.C. during 1972–1976 and at Raleigh, North Carolina, in 1972. Cultivars Bush Blue Lake (BBL) 290, BBL 274 and Gallatin 50 represented processing type beans and Astro, fresh market beans. Two crops, an early and a late crop, were grown each year. At Beltsville from 1972 to 1974, BBL 290, the most sensitive cultivar to oxidants, averaged 14% less bean yield (range 5–27%) in nonfiltered air than in filtered air. Gallatin 50 and Astro, two oxidant tolerant cultivars, yielded about the same in the two environments. The results were similar in North Carolina. In 1975 yields were greatly reduced due to heavy rains. Consequently, no measurable yield effects attributable to oxidants could be determined. In the late crop in 1976, however, BBL 274 yielded 22% less in the nonfiltered air. The other three cultivars did not exhibit statistically significant oxidant induced yield losses. Yields in ambient air field plots without chambers were essentially equivalent to those in the nonfiltered chambers indicating that chamber effects per se were minimal. The microclimates of the chambers are described in an Appendix.

Effect of photochemical oxidation on polypropylene phosphorescence

AbstractPhosphorescence from polyolefins was studied from the aspects of excitation and emission wavelength and lifetime. Effects of photochemical oxidation on polypropylene phosphorescence are discussed in contrast to the effects of thermal oxidation.