The heterogeneous formation of N2O over bulk condensed phases in the presence of SO2 at high humidities

Abstract

In view of the uncertainty of the origin of the secular increase of N20, we studied heterogeneous processes that contribute to formation of N20 in an environment that comes as close as possible to exhaust conditions containing NO and SOZ, among other constituents. The N20 formation was followed using electron capture gas chromatography (ECD-GC). The other reactants and intermediates (SOZ, NO, NO2 and HONO) were monitored using gas phase UV-VIS absorption spectroscopy. Experiments were conducted at 298 and 368 K as well as at dry and high humidity (approaching 100% rh) conditions. There is a significant heterogeneous rate of N20 formation at conditions that mimic an exhaust plume from combustion processes. The simultaneous presence of NO, SO2,02 in the gas phase and condensed phase water, either in the bulk liquid or adsorbed state has been confirmed to be necessary for the production of significant levels of N20. The stoichiometry of the overall reaction is: 2 NO + SO2 + Hz0 -+ N20 + H2S04. The maximum rate of N20 formation occurred at the beginning of the reaction and scales with the surface area of the condensed phase and is independent of its volume. A significant rate of N20 formation at 368 K at 100% rh was also observed in the absence of a bulk substrate. The diffusion of both gas and liquid phase reactants is not rate limiting as the reaction kinetics is dominated by the rate of N20 formation under the experimental conditions used in this work. The simultaneous presence of high humidity (90-100% rh at 368 K) and bulk condensed phase results in the maximum rate andfinal yield of N20 approaching 60% and 100% conversion after one hour in the presence of amorphous carbon and fly-ash, respectively.

Isotopomeric characterization of N2O produced, consumed, and emitted by automobiles

“…The pretreatment column was a polytetrafluoroethylene (PTFE) tube (9 mm i.d., 1 m long) packed with Mg(ClO 4 ) 2 (8–24 mesh; Wako Pure Chemical Industries, Osaka, Japan), Ascarite® (NaOH on support, 8–20 mesh; Thomas Scientific, Swedesboro, NJ, USA), soda lime (granule, Wako), and Mg(ClO 4 ) 2 (20–48 mesh, Wako) in series for removing H 2 O, CO 2 , and other acidic gases (NO 2 , SO 2 , etc.) that potentially produce N 2 O during exhaust gas storage 20, 21. The gases were allowed to flow through the sample bottle for 5 min at ca.…”

The effects of nitrate on the heterogeneous uptake of sulfur dioxide on hematite

“…It has been reported that S (IV) species such as H 2 SO 3 can be easily oxidized by HONO at low pH (0.6-3.2) and the gas-phase product is N 2 O (Martin et al, 1981). Therefore, it is reasonable to speculate that N 2 O is formed from the reduction of HONO by S (IV) species such as sorbed or surface-coordinated H 2 SO 3 , HSO − 3 , and SO 2− 3 species on the mixture sample surface before HONO is released (Pires et al, 1996(Pires et al, , 1997. This observation provides evidence for the formation of nitrous oxide from the hematite-nitrate mixtures at ambient temperature, and suggests a new potential atmospheric source of N 2 O.…”

The heterogeneous formation of N2O in the presence of acidic solutions: Experiments and modeling

“…The effect of O 2 on the kinetics of the reaction system is important but at high O 2 excess used in this study this parameter is not limiting such that we calculate a pseudo n th -order rate constant considering the oxygen concentration to be constant. The effect of water vapor is not negligible and was studied only in extreme cases, namely in a dry system with neither liquid nor water vapor present and in a wet system with approximately 100% of relative humidity (rh) at each given temperature [11]. However, these simplifications do not invalidate the results obtained in the kinetic part of the work because we expect both high O 2 excess and elevated humidity under real conditions.…”

“…In this context we assume that polar surfaces of atmospheric particulate such as liquid aerosols and droplets present an ideal substrate supporting the reactions suspected of occurring within the exhaust gas plumes of power stations [9,10]. The H 2 O-saturated surfaces of soot and fly-ash have also been studied and the obtained results are presented in a forthcoming article [11]. Previously we studied the heterogeneous oxidation of NO by O 2 on polar, that is water-saturated surfaces to yield HONO [12], which has been suggested to be the oxidizing species in the SO 2 to S(IV) conversion [13,14].…”