N2O5 Photolysis Research Articles

Kinetic Studies of Nitrate Radicals: Flash Photolysis at 193 nm

Nitrate radicals, NO3, were produced for the first time by 193 nm laser flash photolysis of N2O5 and HNO3. Detection was achieved due to NO3's strong absorption at 622.7 nm confirmed by measurements of the absorption spectrum in the range of 617–625 nm using both NO3 precursors. Time-resolved kinetic studies allowed the determination of room temperature rate coefficients for the reactions of NO3 with 2-methylbut-2-ene and NO2 of (1.28 ± 0.11) × 10−11 and (8.4 ± 1.2) × 10−13 cm3 molecule−1 s−1, respectively. The rate coefficients compare well to previous measurements with alternative techniques, suggesting that the reported method is valid and may be applied in follow-up studies. The rate coefficient for 2-methylbut-2-ene is compared to previous measurements and predictions for the alkene as well as the related alkenol. The new data are consistent with a previously suggested deactivation of the reactive site of the double bond if adjacent to an OH group. A calculated atmospheric lifetime for 2-methylbut-2-ene with respect to NO3-initiated oxidation of less than 3 min suggests predominant removal by NO3 in the atmosphere.

Constraining the N<sub>2</sub>O<sub>5</sub> UV absorption cross section from spectroscopic trace gas measurements in the tropical mid-stratosphere

Abstract. The absorption cross section of N2O5, σN2O5(λ, T), which is known from laboratory measurements with the uncertainty of a factor of 2 (Table 4-2 in (Jet Propulsion Laboratory) JPL-2011; the spread in laboratory data, however, points to an uncertainty in the range of 25 to 30%, Sander et al., 2011), was investigated by balloon-borne observations of the relevant trace gases in the tropical mid-stratosphere. The method relies on the observation of the diurnal variation of NO2 by limb scanning DOAS (differential optical absorption spectroscopy) measurements (Weidner et al., 2005; Kritten et al., 2010), supported by detailed photochemical modelling of NOy (NOx(= NO + NO2) + NO3 + 2N2O5 + ClONO2 + HO2NO2 + BrONO2 + HNO3) photochemistry and a non-linear least square fitting of the model result to the NO2 observations. Simulations are initialised with O3 measured by direct sun observations, the NOy partitioning from MIPAS-B (Michelson Interferometer for Passive Atmospheric Sounding – Balloon-borne version) observations in similar air masses at night-time, and all other relevant species from simulations of the SLIMCAT (Single Layer Isentropic Model of Chemistry And Transport) chemical transport model (CTM). Best agreement between the simulated and observed diurnal increase of NO2 is found if the σN2O5(λ, T) is scaled by a factor of 1.6 ± 0.8 in the UV-C (200–260 nm) and by a factor of 0.9 ± 0.26 in the UV-B/A (260–350 nm), compared to current recommendations. As a consequence, at 30 km altitude, the N2O5 lifetime against photolysis becomes a factor of 0.77 shorter at solar zenith angle (SZA) of 30° than using the recommended σN2O5(λ, T), and stays more or less constant at SZAs of 60°. Our scaled N2O5 photolysis frequency slightly reduces the lifetime (0.2–0.6%) of ozone in the tropical mid- and upper stratosphere, but not to an extent to be important for global ozone.

Formation of NO3 in the Photolysis of N2O5

The spectra of excited nitrate radicals (NO3*) produced from the photolysis of N2O5 at 248 and 193 nm are reported for the first time. We measured the time-resolved absorption spectra from 400 to 700 nm and separated the contributions of excited- and ground-state species. The spectra of NO3* are broad and featureless in this spectral region. An induction period was observed before the formation of thermalized NO3, consistent with the existence of NO3* that is then quenched by collisions with the buffer gas. We re-examined the kinetics of the formation of the room-temperature NO3 and determined the formation rate constants in N2 at 248 and 193 nm photolysis to be (7.1±1.0)×10−13 and (7.9±1.0)×10−13 cm3molecule−1s−1, respectively. We also measured the relative formation rate constants in O2, Ar, CO2 and SF6 buffer gases.

Note: A transient absorption spectrometer using an ultra bright laser-driven light source

An apparatus to measure transient absorption spectra for short-lived species in the gas phase was built. This was achieved by coupling a laser-driven plasma light source to a time-gated intensified-CCD spectrometer. Although the laser-driven light source features high brightness, ultra broad bandwidth and long lifetime, we found it possesses a plasma oscillation at a frequency of ~200 kHz with a peak-to-peak amplitude of ~7%. This oscillation caused significant variation of the baseline of the transient absorption spectra even after averaging. To reduce this problem, we synchronized the detector gate time with the phase of the plasma oscillation. This arrangement results in much greater stability of the spectral baseline. We have tested the performance of the whole system with the time-resolved absorption spectra of excited NO3 radicals produced by pulsed laser photolysis of N2O5.

Measurement from sun-synchronous orbit of a reaction rate controlling the diurnal NO<sub>x</sub> cycle in the stratosphere

Abstract. A reaction rate associated with the nighttime formation of an important diurnally varying species, N2O5, is determined from MIPAS-ENVISAT. During the day, photolysis of N2O5 in the stratosphere contributes to nitrogen-catalysed ozone destruction. However, at night concentrations of N2O5 increase, temporarily sequestering reactive NOx NO and NO2 in a natural cycle which regulates the majority of stratospheric ozone. In this paper, the reaction rate controlling the formation of N2O5 is determined from this instrument for the first time. The observed reaction rate is compared to the currently accepted rate determined from laboratory measurements. Good agreement is obtained between the observed and accepted experimental reaction rates within the error bars.

Photodissociation of BrONO2 and N2O5: Quantum Yields for NO3 Production at 248, 308, and 352.5 nm

Quantum yields for NO3 production in the photolysis of BrONO2 and N2O5 were measured at 248, 308, and 352.5 nm. The measured values for BrONO2 were found to be independent of pressure over the range 150−600 Torr and bath gas (N2 or O2) and are 0.28 ± 0.09, 1.01 ± 0.35, and 0.92 ± 0.43 at 248, 308, and 352.5 nm, respectively. Quantum yields of Br and BrO in the photolysis of BrONO2 were also estimated. The measured values for NO3 production in the photolysis of N2O5 were 0.64 ± 0.10, 0.96 ± 0.15, and 1.03 ± 0.15 at 248, 308, and 352.5 nm, respectively. Rate coefficients for the reactions Br + BrONO2 → Br2 + NO3 (18) and Cl + BrONO2 → ClBr + NO3 (19) were measured at 298 K to be k18 = (6.7 ± 0.7) × 10-11 cm3 molecule-1 s-1 and k19 = (1.27 ± 0.16) × 10-10 cm3 molecule-1 s-1. The NO3 product yields for reactions 18 and 19 were measured to be 0.88 ± 0.08 and 1.04 ± 0.24, respectively. The absorption cross sections for N2O5 between 208 and 398 nm are also reported. All quoted uncertainties are 2σ and include estimated systematic errors. On the basis of the measured quantum yields of NO3, the atmospheric photolysis rate of BrONO2 is discussed.

Nitrogen and chlorine species in the spring Antarctic stratosphere: Comparison of models With Airborne Antarctic Ozone Experiment observations

The Atmospheric and Environmental Research, Inc., photochemical model has been used to simulate the concentrations and time development of key trace gases in the Antarctic stratosphere before, during, and after the Airborne Antarctic Ozone Experiment (AAOE). The model includes complete gas phase photochemistry and heterogeneous reactions of ClNO3 (g) and N2O5 (g) with HCl (s) and H2O (s). Observations of long‐lived species by the AAOE instruments have been used to constrain the initial conditions in our calculations. We present results from four cases illustrating the evolution of the trace gases for a range of possible initial conditions and duration of heterogeneous activity. The amount of ClO produced by heterogeneous conversion of HCl is determined not only by the initial concentrations of NOx (NO + NO2 + NO3), N2O5, and ClNO3 during winter, but also by the rate at which NOx is resupplied by photolysis of N2O5 and HNO3, or by transport. Results from the four cases presented bracket column measurements of HCl, ClNO3, and HNO3 by the Jet Propulsion Laboratory and National Center for Atmospheric Research infrared spectrometers onboard the NASA DC‐8, and in situ measurements of ClO and NOy by instruments aboard the NASA ER‐2. Comparison of results and measurements of HCl and ClO suggests that heterogeneous chemistry was maintained throughout the month of September in 1987. We suggest field observations and kinetic data which would further constrain the photochemistry of the spring Antarctic stratosphere. The behavior of ozone is discussed in a companion paper (Ko et al., this issue).

Nitric oxide profiles measured in situ during the Globus '85 campaign

Nitric oxide profiles obtained from three flights of chemiluminescent instruments during the Globus '85 campaign held in France in the autumn of 1985 are reported. When the profile obtained in the early morning of 20 September is compared with the flight made the previous afternoon, an average morning to midday NO ratio of 0.7 for the region between 26 and 33 km is obtained. This value is in good agreement with theoretical studies involving the photolysis of N2O5 and the establishment of the NO2−NO equilibrium.

Nitrogen dioxide fluorescence from N2O5 photolysis

The products of ultraviolet photolysis of N2O5 are NO3 and a wavelength dependent mixture of NO2, NO*2, and NO+O, where NO*2 represents one or more excited electronic states of nitrogen dioxide. This NO*2 emits the visible fluorescence spectrum of nitrogen dioxide (photolysis induced fluorescence, PIF), and this spectrum was compared with monochromatically excited NO2 fluorescence spectra (laser induced fluorescence, LIF). In a series of experiments, dispersed PIF and LIF spectra were measured where reactant pressure was 200 mTorr, delay time was 30 ns, and observation time was 600 ns. According to results obtained by Sugimoto and co-workers, under these conditions the continuous spectrum, which reflects the overall internal energy of NO*2, had been little modified by collision, although there was degradation of fine structure. The continuous LIF spectra were fit to an empirical function, and the PIF spectra were shown to be well represented by a linear combination of these mono-energetic excitation spectra. The coefficients of this linear combination plus other considerations were interpreted to give the almost nascent internal energy distribution of the electronically excited nitrogen dioxide molecules produced by N2O5 photolysis. This non-Boltzmann internal energy distribution indicates that electronically excited nitrogen dioxide is produced in the 2B1 state when N2O5 is photolyzed.

N2O5 photolysis: Quantum yields for NO3 and O(3P)

The quantum yield for the production of NO3 in the N2O5 photolysis at 248 nm was measured to be unity. The concentration of NO3 produced upon photolysis was measured using 662‐nm laser absorption. The quantum yields for O(3P) production in the photolysis of N2O5 at 248 nm, 266 nm, 287 nm, and 289 nm were also measured using resonance fluorescence detection of O(3P). The quantum yield for O(3P) decreased from 0.7 at 248 nm to 0.15 at 289 nm. It is postulated that N2O5 photolysis at wavelengths less than 298 nm yields NO3 + NO + O(3P)in addition to NO3 + NO2.

Short‐term variability of nitrogen dioxide in the winter stratosphere

Measurements of limb radiance from the Solar Mesosphere Explorer (SME) satellite are used to infer the NO2 density at the 10‐ and 16‐mbar pressure levels from January 1 to March 31 of 1982. A photochemical‐dynamical model is developed using the presently accepted chemistry of the NOx family. The dynamical model produces isentropic trajectories which simulate the history of air parcels. From the trajectories the photochemical model calculates NO2 densities, which are compared to those observed by SME. Although the model generally reproduces the spatial and temporal variations rather well, some disagreement was noted for conditions of exceptionally low temperatures. Further analysis indicates that the temperature sensitivity of the N2O5 photolysis cross sections may be overestimated at low temperatures.

N2O5 photolysis products investigated by fluorescence and optoacoustic techniques

AbstractPulsed laser photolysis of N2O5 near 290 nm coupled with fluorescence detection (calibrated by NO2 photolysis) showed that the O(3P) quantum yield is ≤0.1. A pulsed laser optoacoustic technique in a flow tube (ca. 6 torr of N2) was tested by photolysis of NO2 and then applied to N2O5. Nitric oxide was added to react with NO3 free radical and the resulting increase in the optoacoustic signal confirmed the presence of NO3 free radicals. Based on the relative optoacoustic signals observed for NO2 and N2O5, the quantum yield for NO3 production is 0.8 ± 0.2.

Sunrise measurements of stratospheric nitric oxide

The chemiluminescence technique has been used to measure the growth of stratospheric nitric oxide from before sunrise until about 2 h prior to local noon at a constant altitude of 26.5 km at 33.1 °N. The instrument has sufficient time resolution to record the rapid growth of NO due to photolysis of NO2 during ultraviolet sunrise. As well, NO was observed to increase slowly throughout the remainder of the experiment. This slow growth is tentatively assigned to the release of NO2 and NO3 and subsequently NO from the photolysis of N2O5 formed during the night. A time dependent computer simulation of diurnal measurements is used to investigate the sensitivity of the computed NO growth curves to O3, total NOx, and the photolysis coefficient of N2O5.