Photolysis Of Formaldehyde Research Articles

Ozone photochemistry in an oil and natural gas extraction region during winter: simulations of a snow-free season in the Uintah Basin, Utah

Abstract. The Uintah Basin in northeastern Utah, a region of intense oil and gas extraction, experienced ozone (O3) concentrations above levels harmful to human health for multiple days during the winters of 2009–2010 and 2010–2011. These wintertime O3 pollution episodes occur during cold, stable periods when the ground is snow-covered, and have been linked to emissions from the oil and gas extraction process. The Uintah Basin Winter Ozone Study (UBWOS) was a field intensive in early 2012, whose goal was to address current uncertainties in the chemical and physical processes that drive wintertime O3 production in regions of oil and gas development. Although elevated O3 concentrations were not observed during the winter of 2011–2012, the comprehensive set of observations tests our understanding of O3 photochemistry in this unusual emissions environment. A box model, constrained to the observations and using the near-explicit Master Chemical Mechanism (MCM) v3.2 chemistry scheme, has been used to investigate the sensitivities of O3 production during UBWOS 2012. Simulations identify the O3 production photochemistry to be highly radical limited (with a radical production rate significantly smaller than the NOx emission rate). Production of OH from O3 photolysis (through reaction of O(1D) with water vapor) contributed only 170 pptv day−1, 8% of the total primary radical source on average (primary radicals being those produced from non-radical precursors). Other radical sources, including the photolysis of formaldehyde (HCHO, 52%), nitrous acid (HONO, 26%), and nitryl chloride (ClNO2, 13%) were larger. O3 production was also found to be highly sensitive to aromatic volatile organic compound (VOC) concentrations, due to radical amplification reactions in the oxidation scheme of these species. Radical production was shown to be small in comparison to the emissions of nitrogen oxides (NOx), such that NOx acted as the primary radical sink. Consequently, the system was highly VOC sensitive, despite the much larger mixing ratio of total non-methane hydrocarbons (230 ppbv (2080 ppbC), 6 week average) relative to NOx (5.6 ppbv average). However, the importance of radical sources which are themselves derived from NOx emissions and chemistry, such as ClNO2 and HONO, make the response of the system to changes in NOx emissions uncertain. Model simulations attempting to reproduce conditions expected during snow-covered cold-pool conditions show a significant increase in O3 production, although calculated concentrations do not achieve the highest seen during the 2010–2011 O3 pollution events in the Uintah Basin. These box model simulations provide useful insight into the chemistry controlling winter O3 production in regions of oil and gas extraction.

Radical Quantum Yields from Formaldehyde Photolysis in the 30 400–32 890 cm–1 (304–329 nm) Spectral Region: Detection of Radical Photoproducts Using Pulsed Laser Photolysis–Pulsed Laser Induced Fluorescence

The relative quantum yield for the production of radical products, H + HCO, from the UV photolysis of formaldehyde (HCHO) has been measured using a pulsed laser photolysis–pulsed laser induced fluorescence (PLP–PLIF) technique across the 30,400–32,890 cm(–1) (304–329 nm) spectral region of the Ã(1)A2–X̃(1)A1 electronic transition. The photolysis laser had a bandwidth of 0.09 cm(–1), which is slightly broader than the Doppler width of a rotational line of formaldehyde at 300 K (0.07 cm(–1)), and the yield spectrum shows detailed rotational structure. The H and HCO photofragments were monitored using LIF of the OH radical as a spectroscopic marker. The OH radicals were produced by rapid reaction of the H and HCO photofragments with NO2. This technique produced an “action” spectrum that at any photolysis wavelength is the product of the H + HCO radical quantum yield and HCHO absorption cross section at the photolysis wavelength and is a relative measurement. Using the HCHO absorption cross section previously obtained in this laboratory, the relative quantum yield was determined two different ways. One produced band specific yields, and the other produced yields averaged over each 100 cm(–1). Yields were normalized to a value of 0.69 at 31,750 cm(–1) based on the current recommendation of Sander et al. (Sander, S. P.; Abbatt, J.; Barker, J. R.; Burkholder, J. B.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation No. 17; Jet Propulsion Laboratory: Pasadena, CA, USA, 2011). The resulting radical quantum yields agree well with previous experimental studies and the current JPL recommendation but show greater wavelength dependent structure. A significant decrease in the quantum yield was observed for the 5(0)(1) + 1(0)(1)4(0)(1) combination band centered at 31,125 cm(–1). This band has a low absorption cross section and has little impact on the calculated atmospheric photodissociation rate but is a further indication of the complexity of HCHO photodissociation dynamics.

Hydrogen oxide photochemistry in the northern Canadian spring time boundary layer

[1] Measurements of OH and HO2 concentrations were made at the surface of the eastern coast of the Hudson Bay during the COBRA campaign from February 18th to March 8th 2008. Diurnally averaged OH and HO2 concentrations peaked at 1.16 (±1.02) × 106 molecule cm−3 and 1.42 (±0.64) × 108 molecule cm−3 respectively. A box-model, constrained to supporting observations, is used to access the radical budget in this cold, northerly environment. Formaldehyde (HCHO) photolysis is found to be the dominant daytime radical source, providing 74% of the observed HOx. A considerable (>80% of the total source) surface HCHO source is required to reconcile the model and observed HCHO concentrations. Model simulations also suggest significant roles for the heterogeneous loss of HO2 and for halogen chemistry in the cycling of HO2 to OH. The formation of HO2NO2 is identified as an important radical reservoir, reducing HOx concentrations during the day and enhancing them at night. This impacts both local oxidizing capacity and reduces local ozone production by approximately 30%. The sensitivity of the local chemistry to uncertainties in these processes is explored. The majority of these processes are not currently represented in global chemistry models.

Isotope effect in the formation of H<sub>2</sub> from H<sub>2</sub>CO studied at the atmospheric simulation chamber SAPHIR

Abstract. Formaldehyde of known, near-natural isotopic composition was photolyzed in the SAPHIR atmosphere simulation chamber under ambient conditions. The isotopic composition of the product H2 was used to determine the isotope effects in formaldehyde photolysis. The experiments are sensitive to the molecular photolysis channel, and the radical channel has only an indirect effect and cannot be effectively constrained. The molecular channel kinetic isotope effect KIEmol, the ratio of photolysis frequencies j(HCHO→CO+H2)/j(HCDO→CO+HD) at surface pressure, is determined to be KIEmol=1.63−0.046+0.038. This is similar to the kinetic isotope effect for the total removal of HCHO from a recent relative rate experiment (KIEtot=1.58±0.03), which indicates that the KIEs in the molecular and radical photolysis channels at surface pressure (≈100 kPa) may not be as different as described previously in the literature.

Pressure dependence of the deuterium isotope effect in the photolysis of formaldehyde by ultraviolet light

Abstract. The pressure dependence of the relative photolysis rate of HCHO vs. HCDO has been investigated for the first time, using a photochemical reactor at the University of Copenhagen. The dissociation of HCHO vs. HCDO using a UVA lamp was measured at total bath gas pressures of 50, 200, 400, 600 and 1030 mbar. The products of formaldehyde photodissociation are either H2 + CO (molecular channel) or HCO + H (radical channel), and a photolysis lamp was chosen to emit light at wavelengths that greatly favor the molecular channel. The isotope effect in the dissociation, kHCHO/kHCDO, was found to depend strongly on pressure, varying from 1.1 + 0.15/−0.1 at 50 mbar to 1.75±0.10 at 1030 mbar. The results can be corrected for radical channel contribution to yield the kinetic isotope effect for the molecular channel; i.e. the KIE in the production of molecular hydrogen. This is done and the results at 1030 mbar are discussed in relation to previous studies at ambient pressure. In the atmosphere the relative importance of the two product channels changes with altitude as a result of changes in pressure and actinic flux. The study demonstrates that the δD of photochemical hydrogen produced from formaldehyde will increase substantially as pressure decreases.

Impacts of formaldehyde photolysis rates on tropospheric chemistry

AbstractA global chemistry transport model is employed to investigate the impact of recent laboratory determinations of photolysis parameters for formaldehyde on concentrations of tropospheric trace gases. Using the new laboratory data, the photolysis of formaldehyde is a more significant removal pathway. HOx levels are increased with the greatest changes towards the top of the troposphere and the poles, making formaldehyde a more significant source of upper tropospheric HOx than previously thought. Global totals of ozone and secondary organic aerosol increase with the rise in ozone being more significant at higher solar zenith angles. Copyright © 2010 Royal Meteorological Society

Measurement of Atmospheric Hydroxyacetone, Glycolaldehyde, and Formaldehyde

A method has been modified and optimized for the measurements of hydroxyacetone as well as formaldehyde and glycolaldehyde, based on aqueous scrubbing using a coil sampler followed by DNPH derivatization and HPLC analysis. Derivatization equilibrium and kinetics were studied to optimize the hydroxyacetone-DNPH derivative yield. It was found that the low sensitivity of hydroxyacetone by this method is due to a relatively small equilibrium constant for the hydroxyacetone-DNPH derivatization reaction, and thus it can be improved by increasing DNPH reagent concentration. In a medium containing 500 microM DNPH and 50 mM HCl, the derivatization reaches equilibrium within 30 min. An online reagent purification procedure using a DNPH-saturated Sep-Pak C-18 cartridge effectively removed hydrazone impurities in the DNPH reagent solution, and a sample preconcentration procedure using a C-18 guard column greatly enhanced the sensitivity and lowered the detection limits. The lower detection limits of the system under optimized conditions are 30, 9, and 36 pptv for hydroxyacetone, glycolaldehyde, and formaldehyde, respectively, with a sampling/analysis cycle time of 30 min. The method has been successfully deployed at a rural site in Pinnacle State Park in Addison, NY, for a 5 week period during the summer of 1998. The ambient concentration means (medians) were 372 (332), 301 (323), and 2040 (2030) pptv for hydroxyacetone, glycolaldehyde, and formaldehyde, respectively. A late-afternoon maximum and an early morning minimum were observed in the diurnal concentration distributions of all three carbonyl compounds. Good correlations among the three carbonyl compounds suggest that they originated from a common source, i.e., photochemical oxidation of biogenic hydrocarbons. Formaldehyde photolysis accounted for about 23% of the total radical photoproduction, whereas contributionsfrom hydroxyacetone and glycolaldehyde photolysis were insignificant because of the much slower photolysis and lower concentrations of these compounds.

Atmospheric oxidation capacity in the summer of Houston 2006: Comparison with summer measurements in other metropolitan studies

Both similarities and differences in summertime atmospheric photochemical oxidation appear in the comparison of four field studies: TEXAQS2000 (Houston, 2000), NYC2001 (New York City, 2001), MCMA2003 (Mexico City, 2003), and TRAMP2006 (Houston, 2006). The compared photochemical indicators are OH and HO 2 abundances, OH reactivity (the inverse of the OH lifetime), HO x budget, OH chain length (ratio of OH cycling to OH loss), calculated ozone production, and ozone sensitivity. In terms of photochemical activity, Houston is much more like Mexico City than New York City. These relationships result from the ratio of volatile organic compounds (VOCs) to nitrogen oxides (NO x ), which are comparable in Houston and Mexico City, but much lower in New York City. Compared to New York City, Houston and Mexico City also have higher levels of OH and HO 2, longer OH chain lengths, a smaller contribution of reactions with NO x to the OH reactivity, and NO x -sensitivity for ozone production during the day. In all four studies, the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) are significant, if not dominant, HO x sources. A problematic result in all four studies is the greater OH production than OH loss during morning rush hour, even though OH production and loss are expected to always be in balance because of the short OH lifetime. The cause of this discrepancy is not understood, but may be related to the under-predicted HO 2 in high NO x conditions, which could have implications for ozone production. Three photochemical indicators show particularly high photochemical activity in Houston during the TRAMP2006 study: the long portion of the day for which ozone production was NO x -sensitive, the calculated ozone production rate that was second only to Mexico City's, and the OH chain length that was twice that of any other location. These results on photochemical activity provide additional support for regulatory actions to reduce reactive VOCs in Houston in order to reduce ozone and other pollutants.

Modeling ozone formation from industrial emission events in Houston, Texas

It is now generally accepted that industrial emission events are occurring from chemical processing facilities in Houston, Texas with daily frequencies and significant temporal variability. These events have been reported to last from hours to days, and a large fraction are made up of four highly reactive volatile organic compounds (HRVOCs): ethene, propene, butenes, and 1,3-butadiene. The Texas Commission on Environmental Quality (TCEQ) has targeted industrial sources of HRVOCs by imposing annual and hourly limits and creating a market-based HRVOC emissions cap and trade (HECT) program. The HECT program uses the Maximum Incremental Reactivity (MIR) scale to calculate the magnitude needed to trade between HRVOCs. The work reported here used the TCEQ regulatory model to evaluate the HECT program's use of the MIR scale by simulating a series of hypothetical but observational-based industrial emission events for different VOC species. The magnitude of each release was adjusted based on the MIR scale under the assumption that this would give the same increase in ozone. The regulatory model, however, predicted that o-xylene caused the largest increase in ozone. In every simulation, ozone production was directly related to the amount of hydroxyl radicals produced from the photolysis of formaldehyde and other aldehydes. The sensitivity of ozone production to these hydroxyl radical sources appeared regardless of whether the industrial emission event plume encountered high sources of NO x . The MIR scale was developed for an average urban atmosphere and its failure in equating ozone reactivity here may be due to the extreme levels of NO x and VOC seen in event emissions in Houston.

Hydrogen isotope fractionation in the photolysis of formaldehyde

Abstract. Experiments investigating the isotopic fractionation in the formation of H2 by the photolysis of CH2O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in the H2 produced is 500(±20)‰ with respect to the parent CH2O. We also observed that complete photolysis of CH2O under atmospheric conditions produces H2 that has virtually the same isotope ratio as that of the parent CH2O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH2O+hν → CHO+H) as compared to the molecular channel (CH2O+hν → H2+CO) of the photolysis of CH2O in order to balance the relatively small isotopic fractionation in the competing reaction of CH2O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH2O. When CH2O is in photochemical steady state, the isotope ratio of the H2 produced is determined not only by the isotopic fractionation occurring during the photolytical production of H2 (αm) but also by overall fractionation for the removal processes of CH2O (αf), and is represented by the ratio of αm/αf. Applying the isotopic fractionation factors relevant to CH2O photolysis obtained in the present study to the troposphere, the ratio of αm/αf varies from ~0.8 to ~1.2 depending on the fraction of CH2O that reacts with OH and that produces H2. This range of αm/αf can render the H2 produced from the photochemical oxidation of CH4 to be enriched in D (with respect to the original CH4) by the factor of 1.2–1.3 as anticipated in the literature.

Global budget of molecular hydrogen and its deuterium content: Constraints from ground station, cruise, and aircraft observations

The distribution and atmospheric budgets for molecular hydrogen and its deuterium component δD are simulated with the GEOS‐Chem global chemical transport model and constrained by observations of H2 from the NOAA Climate Monitoring and Diagnostics Laboratory network and δD observations from ship and ground stations. Our simulation includes a primary H2 source of 38.8 Tg a−1 (22.7 Tg a−1 from fossil and biofuels, 10.1 Tg a−1 from biomass burning, 6.0 Tg a−1 from the ocean) (where a is years) and a secondary photochemical source from photolysis of formaldehyde of 34.3 Tg a−1. The simulated global tropospheric mean H2 is 525 ppbv, with a tropospheric burden of 141 Tg and tropospheric lifetime of 1.9 a. Uptake by enzymes in soils accounts for 75% of the H2 sink, with the remainder due to reaction with OH. The model captures the observed latitudinal, vertical, and seasonal variations of H2. For δD we find that a photochemical source signature from methane and biogenic volatile organic compound oxidation of 162‰ yields a global mean atmospheric δD of 130‰, consistent with atmospheric observations. The model captures the observed latitudinal gradient in δD, simulating a 21‰ greater enrichment in the Southern Hemisphere because of the predominance of isotopically depleted fossil fuel emissions in the Northern Hemisphere. We find that stratospheric‐tropospheric exchange results in 37‰ enrichment of tropospheric δD. Our simulation provides new simultaneous constraints on the H2 soil sink (55 ± 8 Tg a−1), the ocean source (6 ± 3 Tg a−1), and the isotopic signature for photochemical production (162 ± 57‰).

Analysis of Quantum Yields for the Photolysis of Formaldehyde at λ > 310 nm

Experimental quantum yields of the photolysis of formaldehyde at lambda > 310 nm are combined with absolute and relative rate calculations for the molecular elimination H2CO --> H2 + CO (1), the bond fission H2CO --> H + HCO (2), and the intramolecular hydrogen abstraction H2CO --> H ... HCO --> H2 + CO (3) taking place in the electronic ground state. Temperature and pressure dependencies of the quantum yields are analyzed with the goal to achieve consistency between experiment and modeling. Two wavelength ranges with considerably different properties are considered: 340-360 nm, where channel 1 competes with collisional deactivation of excited molecules, and 310-340 nm, which is dominated by the competition between the formation of radical and molecular products. The close relation between photolysis and pyrolysis of formaldehyde, such as analyzed for the pyrolysis in the companion paper, is documented and an internally consistent treatment of the two reaction systems is provided. The quantum yields are modeled and represented in analytical form such that values outside the available experimental range can be predicted to some extent.

Atmospheric variation of nitrous acid at different sites in Europe

Environmental context. Nitrous acid (HNO2) is an important source of the hydroxyl radical (OH•), the most important daytime oxidising species that contributes to the formation of ozone as well as of other secondary pollutants in the troposphere. Understanding the sources and sinks of HNO2 is of crucial interest for accurately modelling the chemical composition of the troposphere and predicting future trace gas concentrations. Abstract. Nitrous acid and several other atmospheric components and variables were continuously measured during complex field experiments at seven different suburban and rural sites in Europe. HNO2 is mainly formed by heterogeneous processes and is often accumulated in the nighttime boundary layer. Our results confirm that the photolysis of HNO2 is an important source of the hydroxyl radical, not only in the early morning hours but also throughout the entire day, and is often comparable with the contribution of ozone and formaldehyde photolysis. At all research sites unexpectedly high HNO2 mixing ratios were observed during the daytime (up to several hundred ppt, or pmol mol–1). Moreover, surprisingly, the HNO2 mixing ratio at the three mountain sites often showed a broad maximum or several distinct peaks at midday and lower mixing ratios during the night. Assuming a quickly established photo-equilibrium between the known significant gas phase reactions, only a few ppt HNO2 should be present around noon. The ratio of known sources to sinks indicates a missing daytime HNO2 source of 160–2600 ppt h–1 to make up the balance. Based on these values and on production mechanisms proposed in the literature we hypothesise that the daytime mixing ratio levels may only be explained by a fast electron transfer onto adsorbed NO2.

Corrigendum to: Atmospheric variation of nitrous acid at different sites in Europe

Environmental context. Nitrous acid (HNO2) is an important source of the hydroxyl radical (OH.), the most important daytime oxidising species that contributes to the formation of ozone as well as of other secondary pollutants in the troposphere. Understanding the sources and sinks of HNO2 is of crucial interest for accurately modelling the chemical composition of the troposphere and predicting future trace gas concentrations. Abstract. Nitrous acid and several other atmospheric components and variables were continuously measured during complex field experiments at seven different suburban and rural sites in Europe. HNO2 is mainly formed by heterogeneous processes and is often accumulated in the nighttime boundary layer. Our results confirm that the photolysis of HNO2 is an important source of the hydroxyl radical, not only in the early morning hours but also throughout the entire day, and is often comparable with the contribution of ozone and formaldehyde photolysis. At all research sites unexpectedly high HNO2 mixing ratios were observed during the daytime (up to several hundred ppt, or pmol mol-1). Moreover, surprisingly, the HNO2 mixing ratio at the three mountain sites often showed a broad maximum or several distinct peaks at midday and lower mixing ratios during the night. Assuming a quickly established photo-equilibrium between the known significant gas phase reactions, only a few ppt HNO2 should be present around noon. The ratio of known sources to sinks indicates a missing daytime HNO2 source of 160-2600 ppt h-1 to make up the balance. Based on these values and on production mechanisms proposed in the literature we hypothesise that the daytime mixing ratio levels may only be explained by a fast electron transfer onto adsorbed NO2.

Rotational analysis of the HCO [formula omitted] and [formula omitted] bands

We report laser-induced fluorescence spectra of the B ˜ 2 A ′ - X ˜ 2 A ′ transition of the formyl radical, HCO, produced by photolysis of jet-cooled formaldehyde. Two spectral regions were studied: from 38 615 to 38 712 cm −1, which corresponds to the overlapping 0 0 0 and 3 1 1 transitions, and from 37 535 to 37 640 cm −1, corresponding to the overlapping, and 3 1 0 and 3 2 1 transitions. Over 170 transitions in the 3 1 1 band and 80 transitions in the 3 2 1 band were assigned and fit using a non-linear least squares routine to provide rotational, centrifugal distortion and spin–rotation constants in the HCO X ˜ 2 A ′ ( 0 , 0 , 2 ) and B ˜ 2 A ′ ( 0 , 0 , 1 ) states.

Nitrous acid in the urban area of Rome

Nitrous acid (HNO 2) and a large variety of other components were simultaneously measured in the city centre of Rome (Italy) during the NITROCAT ground based field experiment in May/June 2001. The highest HNO 2 concentrations were found under high-pressure conditions with high nocturnal atmospheric stability and high values of pollutants. After night time formation and accumulation up to 2 ppb HNO 2 were observed. The measurements confirm that during the first hours after sunrise, when hydroxyl radical (OH) production rates from other sources (photolysis of ozone and formaldehyde (HCHO)) are slow, HNO 2 photolysis is the most important primary OH source in the lowest part of the troposphere; up to 1–4×10 7 OH cm −3 s −1 were estimated for that time from this source. This contributes considerably to the initiation of the photochemistry for the day. The unexpected high daytime concentrations of few hundred ppt observed by DOAS as well as by the two in situ wet collection techniques (wet denuder/IC, coil sampling/HPLC) possibly influence ozone chemistry during the entire day. The heterogeneous on-surface production of HNO 2 (and consequently of HNO 3) provides also a new-type acidity formation influencing directly the biosphere and the materials. About 20% of the total nitrite was found on atmospheric aerosols. The HNO 2 measurements agree well for the different in situ measurement techniques and the spatial integration DOAS simultaneously performed over several weeks in the real atmosphere and during reaction chamber experiments.

Evaluation of two MCM v3.1 alkene mechanisms using indoor environmental chamber data

Photo-oxidation experiments on propene-NO x -air and 1-butene-NO x -air mixtures were performed in the CSIRO indoor environmental chamber. These data were used to test the alkene sub-mechanisms from the Master Chemical Mechanism version 3.1 (MCM v3.1). A comparison of measured and modelled propene-NO x data showed that the mechanisms required the inclusion of O( 3P)+parent alkene reactions in order to adequately model the results over a wide range of VOC/NO x ratios. Sensitivity studies were performed on propene-NO x and 1-butene-NO x simulations to determine the effect of uncertainties in the chamber parameters and key photolysis rates on the O 3, and NO x profiles. The low VOC/NO x propene simulations were sensitive to the formaldehyde (HCHO) photolysis channel that produces HO 2 radicals and to the presence of O( 3P) reactions. For the higher VOC/NO x propene simulations, the O 3, and NO x results were more sensitive to small changes in the initial HONO and NO 2 concentrations.

Relative Tropospheric Photolysis Rates of HCHO, H13CHO, HCH18O, and DCDO Measured at the European Photoreactor Facility

The relative photolysis rates of HCHO, H13CHO, HCH18O, and DCDO were studied in pseudo-natural tropospheric conditions in July 2003 at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. The photolytic decay of HCHO, H13CHO, and HCH18O is measured relative to DCDO by long path FT-IR detection during the course of about 3 h of sunlight. The relative photolysis rates obtained are as follows: JH13CHO/JHCHO = 0.894 +/- 0.006, JHCH18O/JHCHO = 0.911 +/- 0.011, and JDCDO/JHCHO = 0.597 +/- 0.001. The errors represent 1sigma and do not include possible systematic errors. The atmospheric implications of the large isotope effects in the photolysis of formaldehyde are discussed.

Theory of Multichannel Thermal Unimolecular Reactions. 2. Application to the Thermal Dissociation of Formaldehyde

The thermal dissociation of formaldehyde proceeds on three channels, the molecular-elimination channel H2CO --> H2 + CO (1), the radical-forming bond-fission channel H2CO --> H + HCO (2), and the bond-fission-initiated, intramolecular-hydrogen-abstraction channel H2CO --> H...HCO --> H2 + CO (3) which also forms molecular products. The kinetics of this system in the low-pressure range of the unimolecular reaction is shown to be governed by a subtle superposition of collisional channel coupling to be treated by solving a master equation, of rotational channel switching accessible through ab initio calculations of the potential as well as spectroscopic and photophysical determinations of the threshold energies and channel branching above the threshold energy for radical formation which can be characterized through formaldehyde photolysis quantum yields as well as classical trajectory calculations. On the basis of the available information, the rate coefficients for the formation of molecular and radical fragments are analyzed and extrapolated over wide ranges of conditions. The modeled rate coefficients in the low-pressure range of the reaction (neglecting tunneling) over the range 1400-3200 K in the bath-gas Ar in this way are represented by k0,Mol/[Ar] approximately 9.4 x 10(-9) exp(-33,140 K/T) cm3 molecule(-1) s(-1) and k0,Rad/[Ar] approximately 6.2 x 10(-9) exp(-36,980 K/T) cm3 molecule(-1) s(-1). The corresponding values for the bath-gas Kr, on which the analysis relies in particular, are k0,Mol/[Kr] approximately 7.7 x 10(-9) exp(-33,110 K/T) and k0,Rad/[Kr] approximately 4.1 x 10(-9) exp(-36 910 K/T) cm3 molecule(-1) s(-1). While the threshold energy E0,2 for channels 2 and 3 is taken from spectroscopic measurements, the threshold energy E0,1 for channel 1 is fitted on the basis of experimental ratios k0,Rad/k0,Mol in combination with photolysis quantum yields. The derived value of E0,1(1) = 81.2 (+/-0.9) kcal mol(-1) is in good agreement with results from recent ab initio calculations, 81.9 (+/-0.3) kcal mol(-1), but is higher than earlier results derived from photophysical experiments, 79.2 (+/-0.8) kcal mol(-1). Rate coefficients for the high-pressure limit of the reaction are also modeled. The results of the present work markedly depend on the branching ratio between channels 2 and 3. Expressions of this branching ratio from classical trajectory calculations and from photolysis quantum yield measurements were tested. At the same time, a modeling of the photolysis quantum yields was performed. The formaldehyde system so far presents the best characterized multichannel dissociation reaction. It may serve as a prototype for other multichannel dissociation reactions.

NO<sub>2</sub> and HCHO photolysis frequencies from irradiance measurements in Thessaloniki, Greece

Abstract. An empirical approach for the retrieval of nitrogen dioxide (NO2) and formaldehyde (HCHO) photolysis frequencies from measurements of global irradiance is presented in this work. Four months of synchronous measurements of actinic flux and global irradiance performed in Thessaloniki, Greece by a Bentham spectroradiometer were used to extract polynomials for the conversion of global irradiance to photolysis frequencies [J(NO2) and J(HCHO)]. The comparison of these photolysis frequency values to the corresponding values calculated by spectral actinic flux measurements, showed a ratio very close to unity for all J's with a standard deviation of 12% (2σ) for J(NO2) and 6% (2σ) for J(HCHO). Additional sets of polynomials were also extracted to allow determination of J(NO2) by spectroradiometers with lower upper wavelength limits such as single and double Brewer spectroradiometers within acceptable uncertainty (corresponding ratio was 1 and standard deviation was 12% (2σ) for the method that can be used with double Brewers and 20% for the method that can be used for single Brewers). The validity of the method under different atmospheric conditions was also examined by applying the polynomials to another set of actinic flux and global irradiance measurements performed in May 2004, in Buchhofen, Germany. In this case, comparing J values extracted from the polynomials to those calculated from actinic flux, showed equivalent results, demonstrating that the method can also be applied to other measurement sites.