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Nitrous acid HONO is an important component of the nitrogen cycle, being widespread in the environment. Either in its protonated form HONO or HNO 2 08318 as nitrite ions NO lfgrand – it can be found not only in the gas phase, on aerosol particles, in clouds and in dew droplets but also in soil, seawater and sediments Foster et al. It plays a key role in the oxidizing capacity of the atmosphere, as it is an important precursor of the OH radical, which initiates most atmospheric oxidations.

OH radicals react with pollutants in the atmosphere to form mostly less toxic compounds e. In this way HONO has an indirect effect on the radiative budget and climate. However, field measurements 0381 remote and rural locations as well as urban legran polluted regions found elgrand daytime HONO concentrations than model predictions, suggesting a large unknown source Kleffmann et al. Heterogeneous reactions on aerosols have been proposed as an explanation for the missing source.

In a recent study, Weber et al. Many studies have shown decreasing HONO mixing ratios with altitude in the lowest few hundred meters of the troposphere, due to respective short atmospheric lifetime compared to vertical transport time Wong et al. According to the modeling results of Wong et al. This correlation can be explained either legrrand the aforementioned photosensitized reactions or by temperature-dependent soil—atmosphere exchange Su et al. According to Su et al. This temperature dependence exists not only for equilibrium over soil solution but also for adsorption—desorption equilibrium over dry and humid soil surfaces Li et al.

In this study we measured HONO and lwgrand suite of other atmospherically relevant trace gases in a coastal area on the Mediterranean island of Cyprus in summer Due to low local anthropogenic impact and low NO x levels in aged air masses, but high solar radiation, this is an ideal site at which to investigate possible HONO sources and to gain a better understanding of HONO chemistry.

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HONO was measured with a commercial long-path absorption photometry instrument effective light path 1. To avoid potential interferences induced by long inlet lines and heterogeneous formation or loss of HONO on the inlet walls Kleffmann et al.

In the first stripping coil HONO is quantitatively collected. Due to the acidic stripping solution, interfering species are collected less efficiently but in both channels. This correction of chemical interferences ascertained excellent agreement with the absolute differential optical absorption spectroscopy DOAS measurements, both in a smog chamber and under urban atmospheric conditions Kleffmann et al.

A possible interference from peroxynitric acid HNO 4 has been proposed Liao et al. The stripping coils are temperature-controlled by a water-based thermostat, and the whole external sampling unit is shielded from sunlight by a small plastic housing. The reagents were all high-purity-grade chemicals, i. The four red points mark the main cities of Cyprus: Nicosia, Larnaca, Limassol and Paphos clockwise ordering.

This procedure allows the removal of potential interference species. The instrument is calibrated by measuring signals from known amounts of OH and HO 2 generated by photolysis of water vapor in humidified zero air.

Aerosol measurements were also performed during the campaign. In this study particulate nitrate and aerosol surface data were used. The mobility- and aerodynamics-based size distributions were combined based on the algorithm proposed by Khlystov et al. Meteorological parameters temperature, relative humidity RHwind speed and direction, pressure, solar radiation, precipitation were detected by the weather station Vantage Pro2 from Davis Instruments.

The measuring site was located on a military compound in Ineia, Cyprus The field site is characterized by light vegetation cover, mainly comprising small shrubs like Pistacia lentiscus, Sarcopoterium spinosum and Nerium oleander ; herbs like Inula viscosa and Foeniculum vulgare ; and few typical Mediterranean trees like Olea europaea, Pinus sp.

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No rain was observed, but the elevated field site was impacted by fog during nighttime and early morning due to adiabatic cooling of ascending marine humid air masses. Within the main local wind direction of SW Fig.

During the remaining half of the campaign air masses originated from north of Cyprus, from eastern European countries Turkey, Bulgaria, Rumania, Ukraine and Russia.

Westerly air masses have been shown to exhibit lower concentration of gaseous and aerosol pollutants than the predominant northerly air masses that typically reach the site Kleanthous et al. They spent more time over continental terrestrial surface and were likely to be additionally affected by biomass burning events detected in eastern Europe within the measurement periods FIRMS, MODIS, web fire mapper, Fig.

S1 in the Supplement. Previous back-trajectory studies in the eastern Mediterranean support this assumption Kleanthous et al. Most of the time the advected letrand mass was loaded with high humidity as legeand result of sea breeze circulation.

These two situations will be contrasted below. The yellow and blue boxes reflect the dry and the humid periods, respectively. Error bars represent standard deviation of diel mean. Besides two different air mass origins, two periods with different behavior of relative humidity were identified, as illustrated by blue and yellow boxes in Fig.

In both periods we found northerly and westerly air mass origins. The diel profiles of trace gas mixing ratios and meteorological variables of the humid period blue box are shown in Fig. Similar profiles were also observed for other trace gases, like isoprene or DMS, which are transported in upslope winds. This similarity and their dependency on relative humidity are suggestive of a common source for both reactive nitrogen species.

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Low-NO x conditions at this remote field site in photochemically aged marine air were found to be an ideal prerequisite to trace as yet undefined local HONO sources. On Cyprus, diel profiles of HONO showed peak values in the late morning and persistently high mixing ratios during daytime, as has been reported for some other remote regions Acker et al.

This is not the case for rural and urban sites, where atmospheric HONO mixing ratios are normally observed to continuously build up during nighttime, presumably due to heterogeneous reactions involving NO x and decline in the morning due to strong photodissociation e. The ratio average of 0.

However, high values were observed at remote mountain sites, with mean values of 0. During the humid period, during nighttime the estimated according Eq. During the drier period the observed HONO mixing ratios were lower than the ones calculated with a NO 2 conversion rate of 1. Here the approach for the nighttime conversion frequency by, e. The 7-day average conversion rate for the dry nights was 0. As already mentioned above, it is apparent that HONO mixing ratios under low-RH conditions during nighttime were much lower than under humid conditions, and HONO morning peaks were most pronounced compare Fig.

HONO Donaldson et al. In contrast, in a humid regime HONO mixing ratios were continuously high during nighttime and showed less pronounced morning peaks, suggesting lower nighttime deposition of NO 2 and lower HONO emissions in the morning, respectively. As morning HONO peak mixing ratios were most pronounced after dry nights on Cyprus, our observations are to some extent contradictory to earlier results that have proposed that dew formation on the ground surface may be responsible for HONO nighttime accumulation in the aqueous phase, followed by release from this reservoir after dew evaporation the next morning Zhou et al.

We cannot rule out that the latter could have contributed to nighttime accumulation of HONO during humid conditions, as we had no means to measure dew formation at the site, and high daytime HONO mixing ratios were observed under all humidity regimes. However, kinetic models of competitive adsorption of trace gases and water onto particle surfaces predict exchange behavior explicitly distinct from the liquid phase Donaldson et al.

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Likewise, HONO formation rates in surface-bound water are about 4 orders of magnitude larger than expected for the aqueous-phase reaction Pitts et al.

For the latter study, formation of dew could be ruled out as relative humidity was mostly well below saturation. On Cyprus the sea breeze, driven by the growing difference between sea and soil surface temperature, brings air to the site which interacted with the soil surface and vegetation and is loaded by respective trace gas emissions.

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This is endorsed by the simultaneous increase of DMS and isoprene, markers for transportation of marine air and emission by vegetation.

In the late afternoon, when the surface cools, down-welling air from aloft would dominate, being less influenced by ground surface processes. Error bars indicate standard deviation of diel mean. As has been previously established by many other studies Su et al. Observed daytime values were up to 30 times higher than calculated based on PSS, indicating strong additional local daytime sources of HONO.

In this study, the missing source was calculated according to Su et al. Accordingly, the respective air mass travel time over land is estimated to be about half an hour, which is somewhat longer than the daytime lifetime of HONO and might provide enough time for the equilibrium processes. Furthermore and in strong contrast to Lee et al. In the late morning around For both periods the contributions are quiet similar; just the absolute values are different. To compensate the strong loss via photolysis, a comparably strong unknown source is necessary as the heterogeneous NO 2 conversion or the gas-phase reaction of OH and NO is insignificant.

Other light-dependent reactions such as the photolysis of nitrate might additionally contribute to high daytime HONO. Based on a formula for photo-enhanced conversion of NO 2 on humic acid aerosols which was derived by Stemmler et al. Recently an enhancement of the photolysis frequency of particulate nitrate relative to gaseous or aqueous nitrate was found Ye et al.

A missing source of NO can be calculated as shown in Eq. Total [RO 2 ] was estimated to be maximum 1. The budget analysis for NO for both humidity regimes is illustrated in Fig. On Cyprus, two different atmospheric humidity regimes can be differentiated. Due to low and invariant acetonitrile levels, anthropogenic activity and local biomass burning can be excluded as an NO source at this specific site. A net NO source during humid conditions is assumed to result from biogenic NO emission from soil.

As shown in Fig. After irrigation they did not find direct emission, but when the soil had almost dried out gravimetric soil water content: In addition they observed soil-temperature-dependent emission of reactive nitrogen. Analyzing microbial surface communities from drylands, Weber et al.

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Error bars indicate standard deviation of diel average. We propose that HONO is emitted from nitrogen compounds being accumulated on mountain slope soil surfaces produced either biologically by soil microbiota or from previously deposited NO y. This forms the major daytime HONO source responsible for morning concentration peaks and consistently high daytime mixing ratios at the Cyprus field site.

While biological formation is assumed to be more relevant for humid conditions, physical NO y accumulation can be assumed to be stronger under dry conditions, as uptake coefficients for a variety of trace gases were shown to be significantly higher for dry surfaces, among them NO 2 Wang et al. The strongest HONO morning peaks observed after dry nights were accompanied by an increase in relative humidity driven by the sea breeze Fig.

OH formation yields of the reactions of alkenes with O 3 were taken from Paulson et al. The results of this study are shown in Fig.

All three production routes show a clear diel profile with higher production rates during daytime.