NO2 Photolysis Research Articles

Comparison of measured and modeled stratospheric UV/Visible actinic fluxes at large solar zenith angles

Measured and modeled stratospheric filter sensitivity weighted ultraviolet/visible (UV/vis) actinic fluxes—approximating the NO2 photolysis rate coefficients (jNO2)— are compared. The measurements were performed with two calibrated 2π‐actinometers assembled on the azimuth angle‐controlled LPMA/DOAS (Laboratoire de Physique Moléculaire et Applications/Differential Optical Absorption Spectroscopy) gondola during a series of balloon flights. Since the actinometer's spectral sensitivity curve did not exactly match the actinic spectrum of NO2 and the skylight's spectrum shape changes with atmospheric height and solar illumination, only proxies (proxy‐)jNO2 rather than true jNO2 values were monitored during balloon ascents (0–30 km) for solar zenith angle (SZA) 75° < SZA < 86°, and at balloon float altitude during solar occultation (86° < SZA < 95°). The measured direct and diffuse total proxy‐jNO2 values compare excellently with radiative transfer (RT) modeling. That finding allows us to rule out uncertainties in computing UV/vis actinic fluxes as a significant factor in the still insufficient modeling of stratospheric NO2 at large SZAs.

Dynamics of the O(3P) + CS2(XΣg+) → SO(X3Σ-) + CS(X1Σ+) Reaction

The rovibrational state distributions of both the CS and the SO products following the reaction of O(3P) + CS2 have been investigated. The O(3P) atoms are generated by photolysis of NO2 using a frequency-tripled Nd:YAG laser at 355 nm. The SO(X3Σ-, v‘ ‘ = 0−6) and CS(X1Σ+, v‘ ‘ = 0−3) are observed directly using laser induced fluorescence (LIF) spectroscopy on the (B3Σ- − X3Σ-) and (A1Π − X1Σ+) transitions, respectively. The SO(X3Σ-) product is found to be highly excited with the vibrational state distribution inverted at v‘ ‘ = 1 and detectable population up to v‘ ‘ = 6, while the CS(X1Σ+) product vibrational state distribution is characterized as Boltzmann with a vibrational temperature of 1230 ± 155 K. The total vibrational excitation of both products accounts for 21% of the available energy to the products. The near nascent SO(X3Σ-, v‘ ‘ = 0−4) rotational state distributions are characterized by rotational temperatures in the range of 882−1312 K, and the near nascent CS(X3Σ+, v‘ ‘ = 0) is characterized by a temperature of 2986 ± 607 K. The total rotational energy of the products accounts for 34.8% of the available energy. Correlated ab initio calculations of the reaction pathway have been performed, resulting in accurate energies for the reactants, products, intermediates, and transition states. Optimized geometries for the intermediates and transition states have been obtained. The inverted vibrational state distribution of the SO(X3Σ-) product and the excited rotational state distribution of the CS(X1Σ+) product suggest a short-lived, nonlinear intermediate structure as the primary pathway for the reaction. The results from the ab initio calculations corroborate this model.

The origin of the anomalous or “mass‐independent” oxygen isotope fractionation in tropospheric N2O

Analysis of the complete oxygen isotopic composition (16O, 17O, 18O) of tropospheric N2O from various northern hemispheric locations reveals a mass independent anomaly with a 17O excess of Δ17O = 1.0±0.2‰ at δ18O = 20.7±0.3‰. So far, the origin of this intriguing isotope signature has remained elusive. New laboratory experiments demonstrate that the fractionation during UV photolysis of N2O, which causes 15N and 18O enrichments in the stratosphere, is strictly mass dependent (Δ17O = 0). To explain the isotope anomaly in atmospheric N2O, we propose a chemical mechanism for heavy oxygen transfer from O3 to N2O. In a first step, the NOx‐O3 photochemical interaction leads to the formation of NO2 with significant excess 17O. In a second step, the heavy oxygen anomaly is transferred to N2O via the reaction , as part of the gas phase degradation of ammonia. This small but significant N2O source is of the right magnitude to explain the tropospheric observations.

Photochemical Formation of OH Radicals in Dew Formed on the Pine Needles at Mt. Gokurakuji

Photochemical formation rates and sources of the hydroxyl (OH) radical were determined in dew water formed on the surface of Japanese red pine (Pinus densiflora) needles of declining (NO2 polluted area) and healthy pine stands at Mt. Gokurakuji located west of Hiroshima city in western Japan. The measured OH radical photoformation rates in dew water (n=10), which were normalized to the rate at midday on May 1 at 34°N, ranged from 0.67 to 5.18 µM h−1 (1M=1mol L−1). The mean value (2.69 µM h−1) was higher than that in dew water collected on a Teflon board and higher than the mean value in rain water published previously. Of the total OH radical formation rate observed in dew water on the pine needles, 16.4 % was estimated to originate from N (III) (NO2− and HNO2) and 24.6 % was estimated to originate from NO3−. There were other sources of OH radical photochemical formation in dew water on the pine needles besides photolysis of NO2− and NO3−.

Uncertainties in predicted ozone concentrations due to input uncertainties for the UAM-V photochemical grid model applied to the July 1995 OTAG domain

The photochemical grid model, UAM-V, has been used by regulatory agencies to make decisions concerning emissions controls, based on studies of the July 1995 ozone episode in the eastern US. The current research concerns the effect of the uncertainties in UAM-V input variables (emissions, initial and boundary conditions, meteorological variables, and chemical reactions) on the uncertainties in UAM-V ozone predictions. Uncertainties of 128 input variables have been estimated and most range from about 20% to a factor of two. 100 Monte Carlo runs, each with new resampled values of each of the 128 input variables, have been made for given sets of median emissions assumptions. Emphasis is on the maximum hourly-averaged ozone concentration during the 12–14 July 1995 period. The distribution function of the 100 Monte Carlo predicted domain-wide maximum ozone concentrations is consistently close to log-normal with a 95% uncertainty range extending over plus and minus a factor of about 1.6 from the median. Uncertainties in ozone predictions are found to be most strongly correlated with uncertainties in the NO2 photolysis rate. Also important are wind speed and direction, relative humidity, cloud cover, and biogenic VOC emissions. Differences in median predicted maximum ozone concentrations for three alternate emissions control assumptions were investigated, with the result that (1) the suggested year-2007 emissions changes would likely be effective in reducing concentrations from those for the year-1995 actual emissions, that (2) an additional 50% NOx emissions reductions would likely be effective in further reducing concentrations, and that (3) an additional 50% VOC emission reductions may not be effective in further reducing concentrations.

Influence of environmental parameters on the accuracy of nitrogen dioxide passive diffusion tubes for ambient measurement.

Two studies at three sites in the UK provided confirmation that systematic positive bias in NO2 diffusion tube measurement occurred because of changes to "within-tube" chemistry, rather than eddy diffusion at the mouth of the tube. In the first study in Cambridge, UK, sampler overestimation for 1 and 2 week exposures was compared to corresponding time-averaged monitor measurements (NO-NO2-NOx, O3) and weather variables. Noninearity between sampler and monitor NO2 measurements was interpreted in terms of spatial and temporal variations in relative and absolute availability of NO, NO2 and O3 at the site. A maximum overestimation occurred for an exposure mean NO2/NOx approximately 0.5. The separate contributions of reduced NO2 photolysis and eddy diffusion were compared in Study II using samplers of two materials, acrylic and quartz, and of different lengths (40, 55, 71 and 120 mm) at three sites: Norwich background, Cambridge intermediate, London kerbside. For compared sites, NO2 measured by acrylic samplers was significantly higher than for equivalent quartz samplers. For quartz samplers [NO2]mean was only just above the monitor at Norwich and London; sampler/monitor NO2 = 1.04 (P = 0.59) and 1.01(P = 0.76), respectively. For acrylic samplers the order of [NO2]mean was 40 mm > 120 mm > 71 mm > or = 55 mm. Excepting 40 mm samplers, this accords with a chemical bias where co-diffusing NO and 03 molecules in longer tubes have more time to react to form excess NO2. Bias in 40 mm samplers is discussed. Eddy diffusion is negligible for standard samplers because [NO2]mean was equivalent for 55 mm and 71 mm acrylic samplers and close to monitor NO2 for 71 mm quartz tubes. Both studies showed that sampler accuracy was dependent on location. Significantly, overestimation was greatest (approximately 3-4 ppb) where the NO2 annual mean was approximately 20 ppb, close to the UK and EU air quality standard of 21 ppb.

An efficient photolysis system for fast‐response NO2 measurements

A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground‐, and aircraft‐based field data. Focusing the output of a 200‐W short‐arc Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. At lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within ±(40 pptv+7%) for a 10‐s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.

Oxidation of Trimethylsilyl Radicals by N2O: A Mechanistic Study

The reaction of Me3Si (Me⋮CH3) radicals with N2O has been studied by analysis of the end products of the mercury-sensitized photolysis of N2O with Me3SiH. The main products found were N2, Me3SiSiMe3, Me3SiOSiMe3, and Me3SiOH. The influence on both major and minor products of reactant pressure, different scavenger and quencher molecules, temperature, and the reactor surface permits the derivation of a reaction mechanism and identification of the intermediates involved. The most important steps comprising the mechanism are oxygen abstraction from N2O by Me3Si with formation of Me3SiO and a chain propagation step, the reaction of Me3SiO and Me3SiH with formation of Me3SiOH and Me3Si. Although the precise nature of the last step is unknown, it is a multistep reaction. Me3SiO radicals also undergo combination with Me3Si radicals. Self-combination under formation of a peroxide does not occur. Vibrationally excited species emerge in Si−O bond-forming processes, and a number of minor products stem from unimolecular decomposition of these excited species. The rate constant of O atom abstraction from N2O (reaction 5) relative to Me3Si radical combination (reaction 4) is given by k5/k41/2 = (6.5 ± 1.3) × 10-12 cm3/2 s-1/2 at room temperature. An activation energy of EA(5) = 23 ± 1 kJ mol-1 is obtained from experiments in the temperature range 295−520 K.

Photodissociation of IO (355 nm) and OIO (532 nm): Quantum Yields for O(3P) and I(2PJ) Production

The photodissociation of IO and OIO was investigated using 2-color, pulsed laser photolytic generation and dissociation of these species, combined with the detection of product atoms by resonance fluorescence. IO radicals were generated in the reactions of O(3P) with I2, O(3P) with CF3I, and I(2PJ) with O3 and were detected by absorption spectroscopy. The self-reaction of IO was used as the source of OIO, which was also detected by absorption spectroscopy. The quantum yield of O(3P) formation following the photolysis of IO at 355 nm was measured relative to NO2 photolysis at the same wavelength and was found to be 0.91 ± . O(3P) was not detected in the photolysis of OIO at 532 nm, enabling us to place an upper limit to the quantum yield of 7 × 10-3 at this wavelength. I(2PJ)was detected in the 532-nm photolysis of OIO but was formed only at very high laser fluence, presumably in a sequential two-photon process. These results show that OIO is probably stable with respect to photolysis in its strong visible...

Rate Coefficient for the Reaction: O + NO2 + M → NO3 + M

The rate coefficient for the reaction O(3P) + NO2 + M → NO3 + M (M = N2), k1b, was measured over the pressure range 20−800 Torr (N2) between 220 and 296 K. Pulsed laser photolysis of NO2 at 352 nm was used to produce O atoms and transient long-path diode laser absorption at 662 nm was used to detect NO3 produced in the reaction. The pressure and temperature dependence of the measured rate coefficient is reproduced by the expression k1b(T,M) = [k0(T)[M]/(1 + (k0(T)[M]/k∞(T))]Fcx, where x = {1 + [log(k0(T)[M]/k∞(T))]2}-1 and k0(T) = k0(300) (T/300)-n, k∞(T) = k∞(300) (T/300)-m with k0 = 3 × 10-31 cm6 molecule-2 s-1, n = 1.75, k∞ = 3.75 × 10-11 cm3 molecule-1 s-1, m = 0, and Fc = 0.6. This fit does not yield unique values for k∞ or k0 but are presented here merely to obtain k1b(T,M) for atmospheric modeling purposes. The reported values are significantly larger than the current recommendation for stratospheric modeling. Our larger values do not significantly increase the role of the title reaction in the Ear...

Photolysis of NO2 excited below the dissociative limit

The intermediate LIF spectra were observed and compared in the NO2 (λexc>400 nm) and ClONO2 (λexc=308 nm) photolyses. Both spectra could be assigned to the same transient. Transient fluorescence spectra were recorded in the 12 500–16 400 cm−1 range at λexc=436.45 nm. Analysis shows that the observed intermediate spectra can be attributed to the NO3 radical. The NO3 lifetime variations in the presence of CF4, CF3Cl, CF2Cl2, CFCl3, and CCl4 were also studied. It was shown that these compounds only influence the photolysis yield of NO3, without affecting its lifetime. The magnetic field influence on the discrete and total NO2 fluorescence, and on the NO3 radical yield were also measured using λexc=436.45, 458.13, 478.93, and 489.12 nm.

Constraining the atmospheric N2O budget from intramolecular site preference in N2O isotopomers

Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse gas in the troposphere and it also controls ozone concentration in the stratosphere through nitric oxide production. One way to trace the geochemical cycle of N2O is by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15). Here we report the intramolecular distribution of 15N within the linear NNO molecule, determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer. This revealed a preference for 15N at the central N position, or alpha-site, within N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly throughout the atmosphere. In the troposphere, low alpha-site preference indicates local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer signatures, which then mixes with background N2O. In the stratosphere, on the other hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution of 15N is a parameter that has the potential to increase significantly the resolution with which sources and sinks of N2O can be identified and quantified in the atmosphere.

Measurement of the isotopic fractionation of 15N14N16O, 14N15N16O and 14N14N18O in the UV photolysis of nitrous oxide

The isotopic analysis of atmospheric nitrous oxide (N2O) has become a valuable tool in the investigation of its sources, sinks, and its atmospheric cycle. In particular the considerable isotopic enrichment accompanying stratospheric photolysis of N2O, its dominant atmospheric sink process, provides a key isotope signal in the construction of a global N2O isotope budget. Here we present the first measurements of the individual fractionation constants for 15N14NO, 15ε1 = 10.9±1.7‰ and 14N15NO, 15ε2 = 35.7±0.5‰ during ultraviolet photolysis at 193 nm, along with the 18O fractionation constant, 18ε = 17.3±0.5‰. Consistent results were obtained over a wide range of experimental conditions. The observed position‐dependent 15N fractionation confirms theoretical predictions and provides a unique signature of N2O that has been processed in the stratosphere, adding a new dimension to an isotope‐based description of the atmospheric N2O budget.

Nonlocal thermodynamic equilibrium vibrational, rotational, and spin state distribution of NO(ν = 0, 1, 2) under quiescent atmospheric conditions

A nonlocal thermodynamic equilibrium (non‐LTE) model for the NO(ν ≤ 2, J, S) state distributions for altitudes from ground to 200 km for nonauroral conditions is presented. The model includes (1) vibrational excitation due to collisions of NO with O and O2, (2) excitation due to photolysis of NO2 and due to NO2 + O→ NO + O2 in the stratosphere, and (3) excitation due to N(4S,2 D) +O2 → NO + O in the thermosphere. Spontaneous emission, induced emission, and absorption of tropospheric and solar radiation are included. Intravibrational spin and rotational relaxations are included by means of an exponential power gap law approach. This model improves upon previous work mainly through its completeness. It provides non‐LTE distributions of the three spin, rotational, and vibrational degrees of freedom covering the whole atmosphere up to the upper thermosphere. All previous knowledge of non‐LTE excitation processes of NO has been incorporated. The most important results of the model are (1) the stratospheric daytime vibrational distribution departs from local thermodynamic equilibrium (LTE) due to NO(ν≥1) production by NO2 photolysis; (2) the rotational and spin distributions of the NO(ν = 1, 2) are in non‐LTE above ∼110 km; (3) a 90% propensity to conserve the spin orbit state in intravibrational NO+O collisional relaxation was determined by comparison of the model results to cryogenic infrared radiance for shuttle (CIRRIS‐1A) data. The implications of the non‐LTE model results to remote sensing of stratospheric NO abundances from spectrally resolved 5.3 μm limb radiances are discussed. The neglect of thermospheric rotational/spin non‐LTE as well as the neglect of NO(ν>1) production by NO2 photolysis map into errors in the modeled spectra of 20–25%.

Experimental and theoretical study of the temperature and pressure dependences of the recombination reactions O+NO2(+M)→ NO3(+M) and NO2+NO3(+M)→N2O5(+M)

The recombination reactions O + NO2(+M) → NO3(+M) and NO2 + NO3(+M) → N2O5(+M) were studied at temperatures of 300 and 400 K, and at pressures of the bath gas M = N2 between 1 and 900 bar. Oxygen atoms were generated by laser flash photolysis of N2O at 193 nm, NO3 radicals were monitored by light absorption at 578 nm. The measured fall-off curve of the reaction O + NO2(+M) → NO3(+M) could be well represented by limiting low pressure rate constants k3,0 = (1.3 ± 0.3) × 10−31 (T/300 K)−1.5 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k3,∞ = (2.3 ± 0.2) × 10−11 (T/300 K)0.24 cm molecule−1 s−1, and fall-off broadening factors of Fc = 0.71 exp(−T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k3,0, Fc and k3,∞ led to consistency with the experimental data. There is strong evidence that, besides the electronic ground state NO3(2A′), the first excited electronic state NO3(2E′) also contributes to the observed recombination reaction. The measured fall-off curve of the reaction NO2 + NO3(+M) → N2O5(+M) was represented by limiting low pressure rate constants k5,0 = 3.6 × 10−30(T/300 K)−5.0 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k5,∞ = (1.9 ± 0.3) × 10−12(T/300 K)0.2 cm3 molecule−1 s−1 and Fc = 0.38 exp(−T/4900 K). A theoretical analysis of these values is also presented.

Are Primary Quantum Yields of NO2 Photolysis at λ ≤ 398 nm Smaller than Unity?

Primary quantum yields Φ of NO

Production of SiO and Si(3P) Atom in the Reaction of Silane with O(1D)

The mechanism of SiO production in the reaction of O(1D) with SiH4 is reinvestigated, where O(1D) is produced by the 193 nm photolysis of N2O and nascent distribution of SiO (v = 0−8) is determined by using a laser-induced fluorescence (LIF) detection system. Improvement of the reliability of the data is attained by using a simple tunable optical parametric oscillator system, since this system can cover the progression of the LIF spectrum over a wide range of wavelengths within a single scan. Nascent vibrational distribution of SiO is approximately expressed by a Boltzmann distribution of about Tv = 5200 K. Also, the Si(3P) atom is identified as a direct product of this reaction, although the branching fraction is estimated to be only 3 × 10-4 of the total products. The mechanism of the formation of SiO is discussed by comparing the observed nascent vibrational distribution with that calculated by a quasi-static theory for the multiple-step unimolecular decomposition of chemically activated silanol.

Biogenic volatile organic compound emissions in central Africa during the Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) biomass burning season

The recent aircraft and ground‐based Experiment for the Regional Sources and Sinks of Oxidants (EXPRESSO) campaign in central Africa studied atmospheric trace gases and aerosols during the biomass burning season. Isoprene, emitted from vegetation, was the most abundant nonmethane hydrocarbon observed over the forest and savanna, even though intense biomass burning activity was occurring several hundred kilometers to the north. The isoprene flux, measured directly from midmorning to noon by a relaxed eddy accumulation technique, was approximately 890 μg isoprene m−2 h−1 from the tropical rain forest and semideciduous forest landscapes and 570 μg isoprene m−2 h−1 from transitional and degraded woodland landscapes. Model estimates derived from satellite landscape characterization coupled with leaf enclosure emission measurements conducted during EXPRESSO compared well with these measured fluxes. Isoprene concentrations and fluxes were used to determine the oxidant balance over the forest and savanna. Radiative transfer calculations indicate that the observed strong vertical gradient of the NO2 photolysis rate coefficient could be explained by the presence of substantial amounts of absorbing aerosols, probably from biomass burning. Chemical (box) model simulations of the planetary boundary layer (PBL), constrained by measured isoprene emission fluxes and concentrations, show that this suppression of photolytic radiation lowers OH concentrations by about a factor of 2 relative to aerosol‐free conditions. Consequently, the direct contribution of PBL photochemistry to ozone production, especially from biogenic isoprene, is small.

Sensitivity of photolysis rates and ozone production in the troposphere to aerosol properties

Aerosol impacts on NO2 photolysis rates and ozone production in the troposphere are studied by applying a modern sensitivity analysis technique “ADIFOR” on a coupled transport/chemistry/radiative transfer model. Four representative types of tropospheric aerosol (rural, urban, maritime, and desert) are evaluated in terms of loading strength and radiative characteristics. The effects of relative humidity (RH), aerosol vertical loading profile, and NOx (NO + NO2) emission are also studied. The presence of absorbing aerosols in the boundary layer is found to inhibit near‐ground ozone formation and to reduce ground level ozone by up to 70% in polluted environments. The presence of strongly scattering aerosols may increase ozone concentration in the lower boundary layer, but their effects vary with season, NOx, nonmethane hydrocarbon emission (NMHC), and temperature. Ozone production in the upper troposphere can be either enhanced or weakened, depending on the scattering and absorbing ability of aerosol particles and availability of NOx. In the lower troposphere, NO2 photolysis and ozone production rates are most sensitive to urban aerosol, followed by rural, then desert, and finally, maritime aerosol. As expected, NMHC, and NOx emissions also are found to have a large influence on O3 formation.

Comparison of modeled and observed values of NO2 and JNO2 during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission

Stratospheric measurements of NO, NO2, O3, ClO, and HO2 were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO2 and NO are in steady state through NO2 photolysis and reactions involving O3, ClO, BrO, and HO2. By combining observations of O3, ClO, and HO2, observed and modeled values of the NO2 photolysis rate coefficient (JNO2), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO2 and JNO2. An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (±50 to 70%). These results suggest that photochemistry controlling the NO2/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + O3 → NO2 + O2 would aid future analyses of these or similar atmospheric observations.