A low-pressure reactor was used to study the heterogeneous chemistry of NO2 interacting with three types of amorphous carbon with widely differing physicochemical properties at ambient temperature. Theuptake kinetics and the resulting products were measured using molecular beam sampled electron-impact mass spectrometry and in situ laser-induced fluorescence. The only major product resulting from the heterogeneous interaction of NO2 with all three carbon samples was NO with the oxidation product of carbon apparently remaining on the surface at ambient temperature. Both pulsed valve dosing and steady-state experiments were performed and revealed a complex reaction mechanism for both the uptake as well as product formation. The initial uptake coefficient yo for NO2 was (6.4 f 2.0) X I t 2 and proved to be identical for all three types of amorphous carbon.The initial NO2 uptake rate was mass independent and scaled with the external surface. Applying a simple chemical kinetic model to both pulsed dosing and steady state experiments resulted in an effective surface for uptake on amorphous carbon ranging between factors of 2.8 and 8.4 larger than the geometrical surface area but inversely proportional to the measured BET surface area of the three types of amorphous carbon. The chemical kinetic model included competing processes between Langmuir-type adsorption and inhibition controlling the adsorption kinetics of NO2 and revealed that the NO generation rate differed greatly between the three carbon samples examined. All samples showed saturation effects of differing degree that were partially reversible through prolonged pumping at 1 V Torr and/or heating. Virgin amorphous carbon samples did not take up H20 vapor at 20 mTorr, and no HONO and/or HNO3 was detected in simultaneous NO2/H2O exposure experiments. CO and C02 weredetected when previously dosed amorphous carbon was heated by an incandescent lamp. Upon heating a sample previously exposed to NO2, a MS signal m / e 62 originating from NO3 and/or N205 was detected.

A new low-pressure flow reactor operated as a Knudsen cell and intended for chemical kinetic studies is described. The reactor is specifically designed to study the kinetics of heterogeneous reactions. Gas-phase species are detected either by mass-spectrometric sampling or by in situ optical techniques, e.g., laser-induced fluorescence, resonantly enhanced multiphoton ionization. A feature of the reactor is its modular design, allowing full interchangeability of several sample holders at minimal effort, allowing the measurement of uptake coefficients ranging from 10−7 to 1.0. Sample supports operating at low and high temperatures have been developed which cover the stated temperature range. Several experimental examples of the utility of the reactor are detailed. The reliability and error bars of the kinetic results due to the errors and uncertainties associated with the experimental procedures are discussed, in particular for fast heterogeneous processes. It is found that even in the molecular flow regime, for fast reaction, the effects of diffusion limitations within the cell must be taken into account. This fact has been shown here from an experimental point of view. In a companion article the phenomena are studied using Monte Carlo simulation of the gas dynamics under molecular flow conditions.

Rotationally and vibrationally resolved CARS spectra of the O2(1Δg) photofragment produced by the photodissociation of O3 at 17 wavelengths between 230 and 311 nm are reported. The spectra are taken under collision-free conditions, therefore, they reveal the nascent rotational and vibrational state distributions of the O2(1Δg) photofragment. At all photolysis wavelengths studied the vibrational distribution peaks very sharply at v=0, although all energetically allowed vibrational states are observed. The rotational state distributions are narrow, and peak typically at high J. The rotational distribution shifts to lower J as the photolysis wavelength increases. These observations imply vibrationally adiabatic, rotationally impulsive energy release in the dissociation. The shape and width of the rotational distributions can be completely accounted for by the spread in the O3 thermal rotation and zero-point vibration contributions to the O2(1Δg) photofragment angular momentum. The most striking observation about the O2(1Δg) photofragment quantum state distribution is an apparent propensity for even-J states. Experiments with 18O enriched ozone indicate that this propensity is observed only for 16O16O, not for 18O16O, and by implication not for 17O16O. We show that this is the consequence of a selective depletion of only odd-J rotational states of 16O16O(1Δg) by a curve crossing to O2(3Σg), but an equal depletion of both even-J and odd-J rotational states of 18O16O and 17O16O(1Δg) by the curve crossing. The odd-J selectivity for 16O16O is a consequence of the restriction of 3Σg to only odd-J states, due to the requirement of even nuclear exchange symmetry for this homonuclear species with spin-zero nuclei. As a result of the different curve crossing behavior, the quantum yield for 3Σg is twice as great for 18O16O and 17O16O as it is for 16O16O, and this imposes a mass-independent isotopic fractionation in the photodissociation: the O2(1Δg) fragments are depleted of 17O and 18O, while the O2(3Σg) fragments are enriched in these isotopes.

The heterogeneous reaction of NO 2 with different carbon aerosol particles was investigated in situ. The NO 2 was labeled with the + -emitter 13 N (half-life 10.0 min) which allowed application of NO 2 at very low concentrations. The carbon aerosol was either produced by a spark discharge generator using graphite electrodes or by a brush generator resuspending commercial soot material. Monodisperse size cuts between 50-and 490-nm diameter were selected and mixed with the 13 NO 2 . After a defined reaction time, the different reaction products were separated by means of selective traps and detected on-line by γ-spectrometry. A sticking coefficient for chemisorption of NO 2 between 0.3 × 10 -4 and 4.0 × 10 -4 and a rate constant for the reduction of adsorbed NO 2 to NO(g) between 4.0 × 10 -4 and 9.4 × 10 -4 s -1 were determined for both aerosols. The sticking coefficient obtained in this study in situ with aerosol particles is 2 orders of magnitudes smaller than the uptake coefficient recently reported with bulk carbon material.