Abstract:Nitrogen oxides (NOx) have a central role in the chemistry of the atmosphere, especially in key processes relating to ozone, hydroxyl-radical (OH) and acid formation. High reactivity of NOx (lifetime of 0.5-2 days) precludes hemispheric-scale transport and it has been proposed that non-methane hydrocarbons present in the troposphere can transform NOx into its organic forms principally as peroxyacetyl nitrate (PAN). PAN is highly stable in the colder regions of the middle and upper troposphere and can provide a… Show more
“…These high concentrations can probably be explained by long-range transport of PAN-enriched air from lower latitudes or higher altitudes. Several airborne and ship-based "eld experiments have demonstrated that PAN mixing ratios are normally below 10 pptv in the MBL of the Southern Hemisphere (Singh et al, 1986(Singh et al, , 1998Rudolph et al, 1987;MuK ller and Rudolph, 1992;Schultz et al, 1999). In contrast, recent investigations onboard RV Polarstern showed that over the South Atlantic mean PAN concentrations of 60 pptv north of 493S can occur .…”
Because investigations of PAN at higher southern latitudes are very scarce, we measured surface PAN concentrations for the "rst time in Antarctica. During the Photochemical Experiment at Neumayer (PEAN'99) campaign mean surface PAN mixing ratios of 13$7 pptv and maximum values of 48 pptv were found. When these PAN mixing ratios were compared to the sum of NO V and inorganic nitrate they were found to be equal or higher. Low ambient air temperatures and low PAN concentrations caused a slow homogeneous PAN decomposition rate of approximately 5;10\ pptv h\ . These slow decay rates were not su$cient to "rmly establish the simultaneously observed NO V concentrations. In addition, low concentration ratios of [HNO ]/[NO V ] imply that the photochemical production of NO V within the snow pack can in#uence surface NO V mixing ratios in Antarctica. Alternate measurements of PAN mixing ratios at two di!erent heights above the snow surface were performed to derive #uxes between the lower troposphere and the underlying snow pack using calculated friction velocities. Most of the concentration di!erences were below the precision of the measurements. Therefore, only an upper limit for the PAN #ux of $1;10 molecules m\ s\ without a predominant direction can be estimated. However, PAN #uxes below this limit can still in#uence both the transfer of nitrogen compounds between atmosphere and ice, and the PAN budget in higher southern latitudes.
“…These high concentrations can probably be explained by long-range transport of PAN-enriched air from lower latitudes or higher altitudes. Several airborne and ship-based "eld experiments have demonstrated that PAN mixing ratios are normally below 10 pptv in the MBL of the Southern Hemisphere (Singh et al, 1986(Singh et al, , 1998Rudolph et al, 1987;MuK ller and Rudolph, 1992;Schultz et al, 1999). In contrast, recent investigations onboard RV Polarstern showed that over the South Atlantic mean PAN concentrations of 60 pptv north of 493S can occur .…”
Because investigations of PAN at higher southern latitudes are very scarce, we measured surface PAN concentrations for the "rst time in Antarctica. During the Photochemical Experiment at Neumayer (PEAN'99) campaign mean surface PAN mixing ratios of 13$7 pptv and maximum values of 48 pptv were found. When these PAN mixing ratios were compared to the sum of NO V and inorganic nitrate they were found to be equal or higher. Low ambient air temperatures and low PAN concentrations caused a slow homogeneous PAN decomposition rate of approximately 5;10\ pptv h\ . These slow decay rates were not su$cient to "rmly establish the simultaneously observed NO V concentrations. In addition, low concentration ratios of [HNO ]/[NO V ] imply that the photochemical production of NO V within the snow pack can in#uence surface NO V mixing ratios in Antarctica. Alternate measurements of PAN mixing ratios at two di!erent heights above the snow surface were performed to derive #uxes between the lower troposphere and the underlying snow pack using calculated friction velocities. Most of the concentration di!erences were below the precision of the measurements. Therefore, only an upper limit for the PAN #ux of $1;10 molecules m\ s\ without a predominant direction can be estimated. However, PAN #uxes below this limit can still in#uence both the transfer of nitrogen compounds between atmosphere and ice, and the PAN budget in higher southern latitudes.
“…The photocatalytic efficiency (ξ) was calculated according to equation (1). The catalyst selectivity for nitrate (S) was calculated according to equation (2).…”
Section: Photocatalytic Performancementioning
confidence: 99%
“…NOx gases are not only toxic in their own right but they contribute to the formation of other toxic atmospheric pollutants, such as ground level ozone, PAN (peroxyacytyl nitrate), etc. [1][2][3] . Consequently, EU and EPA guidelines for maximum atmospheric NO and NO 2 concentrations have been set but these are regularly exceeded in urban centers because the largest contribution to NOx gases in the urban atmosphere comes from automotive emissions [4][5][6][7] .…”
Abstract:The nitrate selectivity of TiO 2 has important consequences for its efficiency as a NOx depollution photocatalyst. Most emphasis is typically given to photocatalyst activity, a measure of the rate at which NOx concentrations are reduced, but a reduction in NOx concentration (mainly NO + NO 2 ) is not necessarily a reduction in atmospheric NO 2 concentration because the catalytic process itself generates NO 2 . With NO 2 being considerably more toxic than NO, more emphasis on nitrate selectivity, a measure of the NOx conversion to nitrate, and how to maximise it, should be given in engineering photocatalytic systems for improved urban air quality. This study, on the importance of adsorbed water in the photocatalytic oxidation of NOx, has identified important correlations which differentiate the role that water plays in the oxidation of NO and NO 2 . This observation is significant and offers insights into controlling nitrate selectivity on TiO 2 and the potential for increased effectiveness in environmental photocatalyst applications.
“…oxidants may arise in atmosphere for peroxyacetyl nitrate [34,35]. which is rather an unstable compound, we used mchloroperoxybenzoic acid, which is stable for 10 min in solution, to study oxidation of Met residues in a-l-PI.…”
myeloperoxidase/H,O,/chloride system and the related compound NH,CI. With taurine monochloramine, another myeloperoxidase-related oxidant, 1.05 mol Met(O) were generated per mol a-l-PI during inactivation. These oxidants attack preferentially one Met residue in a-l-PI, which is identical with Met 358, as concluded from the parallelism of loss of elastase inhibitory activity and oxidation of Met. A similar high specificity for Met oxidation was determined for the xanthine oxidase-derived oxidants. In contrast, the ratio found for ozone and m-chloroperoxybenzoic acid was 6.0 and 5.0, respectively, indicating oxidation of additional Met residues besides the reactive site Met in a-l-PI, i.e. unselective action of these oxidants. Further studies were performed on the efficiency of oxidants for total depletion of the elastase inhibitory capacity of a-l-PI. Ozone and m-chloroperoxybenzoic acid were IO-fold less effective and the superoxide anion/hydroxyl radicals were 3&50-fold less effective to inactivate the elastase inhibitory activity as compared to the myeloperoxidase-derived oxidants. The myeloperoxidase-related oxidants are discussed as important regulators of a-l-PI activity in vivo.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.