Photodissociation Rate Coefficients Research Articles

An analytical formulation of isotope fractionation due to self-shielding

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO2, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO2. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

The efficiency of photodissociation for molecules in interstellar ices

Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The best-fit value for the solid/gas photodissociation coefficient ratio was found to be ~0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is a proper inclusion of cosmic-ray induced desorption. Observations sampling gas with total extinctions in excess of ~22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.

The critical radiation intensity for direct collapse black hole formation: dependence on the radiation spectral shape

It has been proposed that supermassive black holes (SMBHs) are originated from direct-collapse black holes (DCBHs) that are formed at z ≳ 10 in the primordial gas in the case where H2 cooling is suppressed by strong external radiation. In this work, we study the critical specific intensity Jcrit required for DCBH formation for various radiation spectral shapes by a series of one-zone calculations of a collapsing primordial-gas cloud. We calculate the critical specific intensity at the Lyman–Werner (LW) bands |$J_{\rm LW,21}^{\rm crit}$| (in units of 10−21 erg s−1 Hz−1 sr−1 cm−2) for realistic spectra of metal-poor galaxies. We find that Jcrit is not sensitive to the age or metallicity for the constant star formation galaxies with |$J_{\rm LW,21}^{\rm crit}=1300-1400$|⁠, while Jcrit decreases as galaxies become older or more metal-enriched for the instantaneous starburst galaxies. However, for the young (the age < 100 Myr) and/or extremely metal poor (Z < 5 × 10−4 Z⊙) instantaneous starburst galaxies, such dependence is not strong and |$J_{\rm LW,21}^{\rm crit}= 1000-1400$|⁠. We also find that Jcrit is solely determined by the ratio between the H− and H2 photodissociation rate coefficients, |$k_{\rm H^-,pd}/k_{\rm H_2,pd}$|⁠, with which we develop a formula to estimate Jcrit for a given spectrum. The higher value of Jcrit for the realistic spectra than those expected in the literature significantly reduces the estimated DCBH number density nDCBH. By extrapolating the result of Dijkstra, Ferrara & Mesinger, we obtain nDCBH ∼ 10−10 cMpc−3 at z = 10, which is roughly consistent with the observed number density of high-redshift SMBHs nSMBH ∼ 10−9 cMpc−3 at z ∼ 6, considering large uncertainties in the estimation.

Solar eclipse-induced variations in solar flux, j(NO2) and surface ozone at Kannur, India

Surface ozone is mainly produced by the photodissociation of nitrogen dioxide (NO2) by solar UV radiation. Subsequently, solar eclipses provide one of the unique occasions to explore the variations in the photolysis rate of NO2 and their significant impact on the production of ozone at a location. This study aims to examine the diurnal variations in the photodissociation rate coefficient of NO2, (j(NO2*)), and mixing ratios of surface ozone and NO X * (NO + NO2*) during the solar eclipse that occurred on 15 January 2010 at Kannur (11.9°N, 75.4°E, 5 m amsl), a tropical coastal site on the Arabian Sea in South India. This investigation was carried out on the basis of the ground level observations of surface ozone and its prominent precursor NO2*. The j(NO2*) values were estimated from the observed solar UV-A flux data. A sharp decline in j(NO2*) and surface ozone was observed during the eclipse phase because of the decreased efficiency of the ozone formation from NO2. The NO2* levels were found to increase during this episode, whereas the NO levels remained unchanged. The surface ozone concentration was reduced by 57.5%, whereas, on the other hand, that of NO X * increased by 62.5% during the solar eclipse. Subsequently a reduction of *% in the magnitude of j(NO2*) was found here during the maximum obscuration. Reductions in solar insolation, air temperature and wind speed were also observed during the solar eclipse event. The relative humidity showed a 6.4% decrease during the eclipse phase, which was a unique observation at this site.

Photolysis of Trichloronitromethane (Chloropicrin) under Atmospheric Conditions

Abstract An experimental investigation on the photolysis of the pesticide chloropicrin, (trichloronitromethane, CCl3NO2), under atmospheric conditions was carried out at the outdoor European Photoreactor, (EUPHORE), in Valencia, Spain. The photodissociation rate coefficient, J obs(CCl3NO2), was determined directly under sunlight conditions during spring and summer months. Values in the range J obs(CCl3NO2) = (3.9–5.1)×10−5 s−1 were obtained, corresponding to photolysis lifetimes of 7.1–5.4 hours. Absorption cross-sections for chloropicrin were determined over the wavelength range 260–370 nm, and together with the measured solar flux intensity, were used to calculate the maximum photolysis rate for chloropicrin, J max. Comparison of the observed photolysis rate with the calculated maximum photolysis rate showed that the effective quantum yield of photodissociation, Φ = J obs(CCl3NO2)/J max, was 0.94±0.08. Photolysis of chloropicrin in air or nitrogen gave phosgene as the major carbon-containing product with a yield close to unity based on the loss of chloropicrin. The product yield data were shown to be consistent with a mechanism in which the photolysis channel produces a CCl3 radical and NO2. Kinetic studies on the reactions of hydroxyl radicals and ozone with chloropicrin suggest that, as expected, loss of CCl3NO2 by reaction with these species will be negligible under atmospheric conditions compared to photolysis. Photolysis of chloropicrin in air in the presence of isopropanol gave significant yields of ozone and is consistent with the generation of Cl atoms and NOx following the photodissociation of CCl3NO2. The atmospheric implications of the use of chloropicrin as a pesticide are discussed.

Sensitivity of photodissociation rate coefficients and O3 photochemical tendencies to aerosols and clouds

We examine the sensitivity of the daily integrated photodissociation rate (DIPR) coefficients of O3(O1D) and NO2, the daily averaged O3 photochemical tendency, and OH concentration to variations of clouds and of four aerosol components: black carbon (BC), mineral dust, sea salt, and sulfate. Clouds and BC aerosols are found to be the most important. A comparison between the sensitivity study and some representative observations showed that the global average cloud reduction of DIPR at the surface level is ∼20% and it is ∼30% in storm track zones. At the surface level the average negative BC impact is ∼10% in urban areas and could reach ∼40% in some heavily polluted urban areas. Mineral dust aerosol, which is the next most important, can reduce the photochemistry at the surface level by over 17% in some seasons over the desert or along its long‐range transport paths. The negative impact of sulfate aerosols is around 2% at the surface level, and the impact is as positive as 4% at the top of the sulfate aerosol layer in some urban areas. BC, sulfate, and mineral dust all have much smaller impacts away from their source regions. The impact of sea‐salt aerosol is generally less than 1%. While the fractional impacts on O3 production and destruction and OH concentration do not depend much on NOx, the magnitude of the impact on O3 chemical tendency and OH concentration depends strongly on the concentration of NOx: When NOx is very low, the impacts are also very small even if DIPRs are strongly affected. Different mixing states of absorbing and scattering aerosol components, external, coating, and internal, are studied. It is found that with the same amount of each component, the external mixing state produces the weakest impact while the internal mixing state produces the strongest impact. Coating has almost the same impact as internal mixing. There is very little synergy between cloud and absorbing aerosols when clouds are located above the aerosol. The aerosol absorption is strengthened when clouds and aerosol are located in the same layer. The synergy is strong when clouds are located below the absorbing aerosol, where the impact of absorbing aerosol dominates.

Photolysis of chloral under atmospheric conditions.

The photolysis of chloral under atmospheric conditions was studied at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photodissociation rate coefficient, J(chloral), was measured directly under different sunlight conditions during April 1999. Values in the range of J(chloral) = (4.61-6.11) x 10(-5) s(-1) were obtained, yielding an average value of J(chloral)/J(NO2) = (6.15 +/- 0.62) x 10(-3). This corresponds to a photolysis lifetime of 4.5-6 h under conditions appropriate to the solar flux during summer months and confirms that atmospheric photolysis is the major degradation pathway for chloral. The overall quantum efficiency of photolysis under atmospheric conditions was determined to be 1.00 +/- 0.05. The atmospheric photolysis of chloral produced phosgene, CO, and Cl atoms with molar yields of 0.83 +/- 0.04, 1.01 +/- 0.05, and 1.18 +/- 0.06, respectively. The product yield data are consistent with a mechanism in which the primary photolysis channel produces a Cl atom and a CCl2CHO radical. The latter species is converted to the oxy radical OCCl2CHO,which decomposes by both C-C and C-Cl bond fission. A chemical mechanism for the photolysis of chloral by sunlight is proposed, and the atmospheric implications are discussed.

Global and seasonal variations of O3 and NO2 photodissociation rate coefficients

The global and seasonal variations in the photodissociation rate coefficients (J values) of the reactions of ozone forming O(1D) (J(O1D)) and nitrogen dioxide forming O(3P) (J(NO2)) were investigated systematically with a focus on the troposphere. The corresponding mechanisms were studied through sensitivity modeling experiments of temperature, surface albedo, aerosols, ozone column, and clouds. The one‐dimensional radiative transfer model PHODIS was applied with Total Ozone Mapping Spectrometer (TOMS) aerosol optical depth and surface reflectivity, observation‐based three‐dimensional ozone distributions, and other numerically simulated climate variables. Our results showed that under clear‐sky conditions, the zonal averaged J(O1D) and J(NO2) peaked in magnitude at 3 × 10−5 and 8 × 10−3 s−1, respectively, in the region of 30°S to 30°N with small latitudinal and vertical variations, and decreased precipitously at 30°/60° in the winter/summer hemisphere. J(O1D) exhibited a large meridional and vertical gradient as a result of high sensitivity to temperature and ozone. The predominant effect of surface albedo led to significant longitudinal variation in J(NO2) and a poleward increase beyond 60° of the summer hemisphere. Surface J(O1D) changes were regressed as a second‐degree polynomial function of changes in temperature (±20°K). The impact of low and middle clouds on J values was stronger than high clouds by a factor of 2 to 3 because of thick optical depth. In addition, the cloud‐induced changes in J values calculated by PHODIS differed by up to 20% from those parameterized in the Regional Acid Deposition Model (RADM) and Intermediate Model for the Global and Annual Evolution of Species (IMAGES). Regional photochemical model simulations showed that an arbitrary increase/decrease of 20% above/below the cloud base (750 m) in J(O1D) resulted in decreases up to 6% in the ozone concentration below 200 m at noon and late afternoon and increases ubiquitously elsewhere.

Neutral sodium tails in comets

In the last few years, after a new type of tail composed only of neutral sodium atoms had been discovered in Comet Hale-Bopp, much attention has been given to the release and distribution of sodium in comets. It was not the first time that a sodium tail had been observed in a comet, but the amount of data collected from Comet Hale-Bopp in general and on the sodium emissions in particular is much larger than for any other comet. Several authors tried to identify the sources of the sodium and explain its distribution. However, no clear solution how neutral sodium atoms can be correlated with gas and dust particles has resulted. The presence of a sodium tail superimposed on the dust tail suggests a strong relation of sodium with the dust emitted by the nucleus. Furthermore, analysis of the dust data in the coma and tail provides important insights in the chemical and physical processes relevant to dust particles. In order to investigate whether release of sodium atoms from a Na-bearing molecule may form an distributed source, we have calculated photodissociation rate coefficients (inverse lifetimes) and excess energies for several Na-bearing molecules. The calculations are based on measured cross sections near the dissociation threshold combined with cross sections at short wavelengths from the separated atom approximation for several Na-bearing molecules in the solar radiation field. Sodium can be used as a tracer of mechanisms working on other elements that are more difficult to observe. Wide interest in the sodium emissions stimulated reanalysis of wide-field images of Comet Hyakutake. It was found that this comet also had a neutral sodium atom tail, showing that Na tails might be more common than previously thought. Our preliminary conclusion is that the sodium in the fast, narrow tail may also originate from fragmenting dust in the inner coma.

Estimation of surface actinic flux from satellite (TOMS) ozone and cloud reflectivity measurements

Recently, a method has been proposed to estimate ultraviolet (UV) spectral irradiances at the Earth's surface, from satellite‐based measurements of ozone and cloud reflectivity. Here, we extend this method to the determination of spectral actinic fluxes needed for the calculation of photodissociation rate coefficients. It is shown that the modification of actinic flux by clouds is closely approximated by the cloud reduction factor applicable to the irradiance, if considering averages over area or time instead of instantaneous point data. The conversion of irradiance estimates into actinic flux estimates introduces additional errors, but for most practical situations these fall below 10 to 20% in the UV spectral region, where the relevant photolytic reactions have their maximum sensitivity. Based on these results, large satellite‐derived data sets (e.g., ozone column and cloud reflectivity from the Total Ozone Mapping Spectrometer, TOMS) can be utilized to estimate near‐surface photolysis rate coefficients on a global scale.

Investigation and evaluation of the NO x/O 3 photochemical steady state

Measurements of O 3, NO, NO 2, the photodissociation rate coefficient of NO 2 ( j(NO 2)), and temperature made during the WAOWE’93 and ’94 (Weybourne Atmospheric Observatory Winter Experiment) campaigns on the north Norfolk coast were used to evaluate the photochemical steady state (PSS) as a method of deriving peroxy radical (HO 2+RO 2) concentrations. Highly polluted air masses containing elevated levels of NO x (NO+NO 2) were encountered during WAOWE’93. The PSS ratio, ( j(NO 2)[NO 2])/( kNO+O 3 [NO] [O 3]), was found to equal unity within experimental error which resulted in good agreement between predicted and measured O 3 concentrations and, in addition, implied the absence of peroxy radicals. Small deviations from the PSS ratio in the apparent absence of peroxy radicals indicated the presence of a temperature-dependent perturbation. Such a process could lead to artefact or enhanced PSS-derived peroxy radical concentrations at temperatures above 2°C. During the WAOWE’94 campaign, the PSS ratio reached values of up to two and calculated peroxy radical concentrations were found to be positively correlated with [O 3] and negatively correlated with NO x , indicating that the observed perturbations from the PSS were at least partly caused by peroxy radicals. However, the levels of peroxy radicals predicted by the PSS were up to two orders of magnitude higher than those calculated using a simple zero-dimensional model. Previous studies of the NO x /O 3 PSS are evaluated in conjunction with the results obtained in this study. It is suggested that the PSS method may be used as a text-book case in limited conditions to estimate levels of O 3, NO 2, NO or j(NO 2), but is subject to a variety of potential interferences which invalidate its widespread use as a method for deriving atmospheric peroxy radical concentrations.

Actinic fluxes in broken cloud fields

Photochemical processes in the atmosphere are driven by solar ultraviolet radiation. The photodissociation rate coefficients of atmospheric species are determined by the actinic flux, which is defined as 4π times the mean ultraviolet intensity. Because of the presence of clouds the actinic flux can change drastically throughout the atmosphere. Therefore clouds have large effects on photodissociation rate coefficients. At cloud top, photodissociation rate coefficients can be 300% higher than in clear sky conditions. We use Monte Carlo simulations to investigate the reflectance, the transmittance, and the actinic flux for cloud fields at various degrees of cloudiness. Scattering processes in the clouds are due to cloud particles only. We do not take absorption of radiation into account. The atmosphere outside the clouds is assumed to be completely transparent. The simulated reflectance and transmittance of plane‐parallel cloud fields and in broken cloud field conditions reproduce the results of previous model studies within statistical uncertainties. The results of actinic flux calculations for plane‐parallel cloud fields agree with the results obtained with a doubling‐adding algorithm. Horizontal and vertical actinic flux profiles in broken cloud fields are studied for various solar zenith angles and for different cloud optical thicknesses. The aim of the present model study is to obtain insight into the effect of broken cloud fields on the actinic flux.

A note on the use of the two‐stream delta‐scaling approximation for calculating atmospheric photolysis rate coefficients

The delta‐scaling approximation is widely used in radiative transfer models to improve the accuracy of calculated irradiances under strongly forward scattering conditions. At completion of the scaled radiative transfer calculations, the inverse transformation (or descaling) is applied to partition the total downward irradiance, which is assumed invariant with respect to the scaling/descaling transformation, into its diffuse and direct components. However, this invariance does not apply to the total downward actinic flux, and ambiguities arise when the actinic flux components are calculated from either scaled or descaled components of the irradiance. The problem is particularly severe in two‐stream models because of the lack of angular resolution. We use a realistic example (the calculation of the photodissociation rate coefficient, or J value, for the photolysis reaction NO2 → NO + O(3P)) to show in particular that, for high Sun and low surface albedo, the use of descaled J values leads to large overestimation of rather constant or modest increases of J values below a thin cloud or aerosol layer. Even larger overestimations of J values are found near the top of a thick cloud layer at high Sun when descaling is used. Use of scaled values appears preferable under most practical conditions, although they, too, suffer from inaccuracies generally associated with two‐stream methods.

Photodissociation in the atmosphere of Mars: Impact of high resolution, temperature‐dependent CO2 cross‐section measurements

We have investigated the impact of high resolution, temperature‐dependent CO2 cross‐section measurements (Lewis and Carver, 1983) on calculations of photo dissociation rate coefficients in the Martian atmosphere. Since the cross‐section measurements exhibit large variations on a 10–20 Å scale, the calculations are sensitive to the wavelength resolution. We find that the adoption of 50 Å intervals for the purpose of computational efficiency result in errors in the calculated values for photodissociation of CO2, H2O and O2 which are generally ≤10% but as large as 20% in some instances. These are acceptably small errors, especially considering the uncertainties introduced by the large temperature dependence of the CO2 cross‐section. The inclusion of temperature‐dependent CO2 cross‐sections is shown to lead to a decrease in the diurnally averaged rate of CO2 photodissociation as large as 33% at some altitudes, and increases of as much as 950% and 80% in the photodissociation rate coefficients of H2O and O2, respectively. The actual magnitude of the changes depends on the assumptions used to model the CO2 absorption spectrum at temperatures lower than the available measurements, and at wavelengths longward of 1970 Å. Given our current understanding, these results minimize or even reverse the sense of the CO2 chemical stability problem.

Actinometer and Eppley radiometer measurements of the NO2 photolysis rate coefficient during the Mauna Loa Observatory photochemistry experiment

Measurements of the apparent first‐order rate coefficient for NO2 photodissociation (jNO2) were made using a chemical actinometer during the month of May 1988 at the Mauna Loa Observatory, Hawaii. Simultaneous measurements of the ultraviolet irradiance (E), obtained with an Eppley radiometer, allowed extensive testing of the semi‐empirical relationships between E and jNO2 proposed by Madronich (1987a). More than 3700 simultaneous measurements of jNO2 and E were obtained for solar zenith angles ranging from 4–90 degrees, and for different sky conditions (including clear skies, partial cloud cover, arid valley clouds below the horizon). For overhead clear skies, the NO2 photodissociation rate coefficient derived from Eppley radiometer data, here denoted j′, was in good agreement with actinometric measurements, j′/jNO2=1.01±0.05(1σ). The actinometer‐radiometer relationship holds reasonably well even when low‐lying valley clouds are present. For the periods of overhead intermittent clouds, the j′values track jNO2 values well, but the observed ratio shows significantly more scatter and the average is somewhat less than unity: 0.93 ± 0.09 (1σ). Measurements taken with and without upward scattered and reflected radiation show that valley clouds can contribute to the jNO2 values for the conditions encountered during the Mauna Loa Observatory Photochemistry Experiment.

Impact of recent total ozone changes on tropospheric ozone photodissociation, hydroxyl radicals, and methane trends

Trends in total atmospheric ozone from 1979–1989 were analyzed to derive the corresponding changes in the tropospheric photodissociation rate coefficient J1 for the reaction O3 + hv (λ &lt; 330 nm) → O2 + O(1D). Zonally and annually integrated trends in J1 are +3.8±2.9 % per decade in the Northern hemisphere and +4.1±2.7 % per decade in the Southern hemisphere. Changes in [OH] due to the J1 trends may have contributed to the slowing of atmospheric methane increases observed in recent years.

Atmospheric photochemistry of alkyl nitrates

Photoabsorption spectra of gaseous ethyl nitrate, n-propyl nitrate, 2-propyl nitrate, n-butyl nitrate and 3-methyl-1-butyl nitrate were quantitatively measured in the laboratory at room temperature for λ ⩾ 185 nm. The spectra are continuous and exhibit low- and high-intensity absorption bands. The maximum for the high-intensity band occurs at 188 ± 2 nm. Atmospheric photodissociation rate coefficients for alkyl nitrates were estimated in the range 0 – 50 km and were found to have a strong dependence on altitude. Photodissociation lifetimes were on the order of days and hours for the proposphere and stratosphere respectively. Compared with reaction with OH, photodissociation is the dominant removal process for alkyl nitrates throughout the atmosphere.

Numerical integration errors in calculated tropospheric photodissociation rate coefficients

Tropospheric photodissociation rate coefficients (J values) were calculated for NO2, O3, HNO2, CH2O, and CH3CHO using high spectral resolution (0.1 mm wavelength increments), and compared to the J values obtained with numerically degraded resolution (Δλ=1, 2, 4, 6, 8, and 10 nm, and several commonly used nonuniform grids). Depending on the molecule, substantial errors can be introduced by the larger increments. Thus for Δλ=10 nm, errors are less than 1% for NO2, less than 2% for HNO2, +6.5% to -16% for CH2O, -6.9% to +24% for CH3CHO, and -24% to +110% for O3. The errors for CH2O arise from the fine structure of its absorption spectrum, and are prevalently negative (underestimate of J). The errors for O3, and to a lesser extent for CH3CHO, arise mainly from under-resolving the overlap of the molecular action spectrum and the tropospheric actinic flux in the wavelength region of stratospheric ozone attenuation. The sign of those errors depends on whether the actinic flux is averaged onto the grid before or after the radiative transfer calculation. In all cases studied, grids with Δλ≤2 nm produced errors no larger than 5%.

Photodissociation in the atmosphere: 1. Actinic flux and the effects of ground reflections and clouds

The actinic flux must be distinguished from other radiometric quantities such as the irradiance. This distinction shows that (1) a fraction of the recent atmospheric chemistry literature contains improperly calculated rates of photodissociation, and (2) photodissociation rate coefficients inside clouds, and particularly inside cloud droplets, can frequently exceed the clear‐sky values, in contrast to current usage in cloud chemistry models. Both of these findings are traceable to the ∼2 cos θ factor incurred in the actinic flux (but not in the irradiance) when the solar beam is scattered and diffused into nearly isotropic light.

Intercomparison of NO 2 photodissociation and U.V. Radiometer Measurements

A theoretical estimate of the ratio of direct to diffuse solar radiation is used to derive a simple correlation between u.v. irradiance measurements and atmospheric values of the NO 2 photodissociation rate coefficient, J. This correlation permits a critical evaluation of the performance of actinometers which measure J. Most reported J measurements in the troposphere agree to better than ± 5% for clear skies and to ± 20% for overcast skies, but photodissociation rates measured at the University of North Carolina outdoor smog chamber are not consistent with a majority of other studies. It is shown that calibrated standard u.v. radiometers may be used routinely, in place of the more complex actinometers, to obtain reasonably accurate estimates of J.