Atmospheric Photolysis Rate Research Articles

UV‐visible absorption cross sections of bromine nitrate determined by photolysis of BrONO2/Br2 mixtures

Experiments on the determination of absorption cross sections of bromine nitrate by photolysis of BrONO2/Br2 mixtures were carried out using a UV‐visible absorption spectrometer. The relatively stable end products of photolysis were identified as Br2 and N2O5. NO3 was observed as intermediate product in the photolysis of BrONO2/Br2. Absorption cross sections for BrONO2 at 298 K and 230 K were determined between 208 and 500 nm. The results are in good agreement with a recent independent study. Calculations of atmospheric photolysis rates of BrONO2 as a function of solar zenith angle and surface albedo were undertaken using the PHOTOGT program. The latter are in the range between 0.5×10−3 s−1 and 4.0×10−3 s−1, indicating that during daylight BrONO2 is rapidly photolyzed.

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.

UV–VIS absorption cross-sections and atmospheric lifetimes of CH2Br2, CH2I2 and CH2BrI

The UV–VIS absorption spectra of CH2Br2, CH2I2 and CH2BrI have been measured over the wavelength range 215–390 nm using a dual-beam diode array spectrometer. The spectra consist of broad continuous absorption bands. CH2Br2 exhibits its maximum cross-section of σ=2.71(±0.16)×10-18 cm2 molecule-1 at λ=219 nm. The magnitude of the peak cross-sections for the iodine-containing molecules above λ=210 nm are σ=1.62(±0.10)×10-18 cm2 molecule-1 at λ=248 nm and σ=3.78(±0.23)×10-18 cm2 molecule-1 at λ=288 nm for CH2I2, and σ=5.67(±0.34)×10-18 cm2 molecule-1 at λ=215 nm and σ=2.34(±0.14)×10-18 cm2 molecule-1 at λ=267 nm for CH2BrI. The temperature dependence of the absorption cross-sections was investigated over the temperature range 348–250 K. A decline in the cross-sections with decreasing temperature was observed in the tail of the spectra. At the peaks the opposite effect was observed. All three gases have been found in the atmosphere and the atmospheric photolysis rates of CH2Br2, CH2I2 and CH2BrI were calculated as a function of altitude and solar zenith angle using the measured cross-sections. Model calculations show that, during sunlit hours, CH2I2 and CH2BrI will be photolysed within minutes and hours, respectively. The photolysis of CH2Br2 is much slower and reaction with the OH radical is the dominant atmospheric loss process.

UV absorption cross sections for SO3

Room temperature gas phase absorption cross sections for SO3 have been measured over the wavelength range 195 to 330 nm using a diode array spectrometer. The SO3 spectrum is continuous with weak diffuse vibrational band structure in the 225 to 295 nm region. Atmospheric photolysis rate calculations show that photolysis of SO3 in the 190 to 230 nm actinic window is important in determining the sulfur partitioning in the upper stratosphere.

Temperature dependence of UV absorption cross sections and atmospheric implications of several alkyl iodides

The ultraviolet absorption spectra of a number of alkyl iodides which have been found in the troposphere, CH3I, C2H5I, CH3CH2CH2I, CH3CHICH3, CH2I2, and CH2ClI, have been measured over the wavelength range 200–380 nm and at temperatures between 298 and 210 K. The absorption spectra of the monoiodides CH3I, C2H5I, CH3CH2CH2I, and CH3CHICH3 are nearly identical in shape and magnitude and consist of single broad bands centered near 260 nm. The addition of a chlorine atom in CH2ClI shifts its spectrum to longer wavelengths (σmax at 270 nm). The spectrum of CH2I2 is further red‐shifted, reaching a maximum of 3.85×10−18 cm2 molecule−1 at 288 nm and exhibiting strong absorption in the solar actinic region, λ>290 nm. Atmospheric photolysis rate constants, J values, have been calculated assuming quantum efficiencies of unity for different solar zenith angles as a function of altitude using the newly measured cross sections. Surface photolysis rate constants, calculated from the absorption cross sections measured at 298 K, range from 3×10−6 s−1 for CH3I to 5×10−3 s−1 for CH2I2 at a solar zenith angle of 40°.

UV absorption cross-sections and atmospheric photolysis rates of CF3I, CH3I, C2H5I and CH2ICl

The absorption cross-sections of trifluoromethyl iodide (CF 3 I), methyl iodide (CH 3 I), ethyl iodide (C 2 H 5 I) and chloroiodomethane (CH 2 ICl) have been determined over the wavelength range 235–400 nm, with a spectral resolution of 0.6 nm (FWHM), using a diode array spectrometer. The spectra consist of a broad continuous absorption band with maximum cross-sections of (6.0±0.1)×10 -19 cm 2 molecule -1 for CF 3 I at 267 nm, (1.09±0.02)×10 -18 cm 2 molecule -1 for CH 3 I at 258 nm, (1.18±0.04)×10 -18 cm 2 molecule -1 for C 2 H 5 I at 258 nm and (1.21±0.07)×10 -18 for CH 2 ICl at 270 nm. The temperature dependence of the cross-section was investigated over the range 333–243 K. A decline in the cross-section with decreasing temperature at wavelengths longer than ca. 280 nm was observed in all cases, the decrease being most pronounced in the long wavelength tail of the absorption band. At wavelengths shorter than ca. 270 nm the cross-section increased with decreasing temperatures; the effect, however, was significantly smaller (5–10%), being most pronounced around the band maximum. The temperature dependence was parametrised in order to calculate the atmospheric photolysis rate as a function of altitude, latitude and season. Model calculations show that during sunlit hours the iodides will be rapidly photolysed with tropospheric photodissociation lifetimes of ca. 1 day for CF 3 I, several days for CH 3 I and C 2 H 5 I and several hours for CH 2 ICl.

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.

UV/visible and IR absorption cross sections of BrONO2

The absorption cross sections of BrONO2 between 200 and 500 nm were measured over the temperature range 298 to 220 K using a diode array spectrometer. The BrONO2 absorption cross sections are weakly dependent on temperature at wavelengths < 450 nm (less than 10% change between 298 and 220 K) but decrease rapidly with decreasing temperature at wavelengths > 450 nm; at 480 nm the cross section decreased by ∼35% in going from 298 K to 220 K. Our room temperature absorption cross sections are in good agreement with the measurements of Spencer and Rowland (1978) over the common wavelength range of the measurements, 200 to 390 nm. We show that wavelengths longer than 390 nm must be included in the calculation of the BrONO2 atmospheric photolysis rate. We also show that photolysis of BrONO2 could be a significant atmospheric loss process for odd oxygen (due to halogen chemistry) below about 25 km. The infrared absorption cross sections for the BrONO2 band centered at 803.3 cm−1 were also measured. The integrated band strength was (2.7±0.5) × 10−17 cm2 molecule−1 cm−1.

Temperature dependent ultraviolet‐visible absorption cross sections of N2O and N2O4: Low‐temperature measurements of the equilibrium constant for 2NO2 ⇌ N2O4

A study of the temperature dependence of the ultraviolet‐visible absorption cross sections for NO2 has been made in the temperature range 213–298 K and between 310 and 570 nm using a diode array spectrometer. Analysis of the experimental data allowed the simultaneous measurement of the NO2 and N2O4 cross sections and the equilibrium constant for the dimerization of NO2. The absorption cross sections were measured at a spectral resolution of 0.54 nm full width at half maximum (FWHM) and are presented for use in the analysis of differential absorption spectra of atmospheric NO2. Averaged cross section data for NO2 are presented for use in photolysis rate calculations. Values of cross sections for N2O4 and equilibrium constant for the association of NO2 are presented in the range 213 to 263 K. No temperature effect was observed on the overall shape of the NO2 absorption spectrum or on the averaged cross section values used for calculating atmospheric photolysis rates. Changes in the fine structure of the NO2 spectrum with temperature are observed and this has important implications for measurements of atmospheric NO2 made by ultraviolet‐visible absorption techniques.

The temperature dependence of the UV—visible absorption cross-sections of NOCl

The absorption cross-sections for NOCl were measured between 350 and 650 nm and between 223 and 343 K using a photodiode array spectrometer system. This study represents the first measurement of the temperature dependence of the NOCl UV—visible absorption spectrum and will provide a useful set of data for the calculation of the atmospheric photolysis rate of NOCl and for the use of NOCl as a gas-phase actinometer.

ChemInform Abstract: Temperature‐Dependent Formaldehyde Cross Sections in the Near‐Ultraviolet Spectral Region.

ChemInformVolume 21, Issue 37 Physical Organic Chemistry ChemInform Abstract: Temperature-Dependent Formaldehyde Cross Sections in the Near-Ultraviolet Spectral Region. C. A. CANTRELL, C. A. CANTRELL Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorJ. A. DAVIDSON, J. A. DAVIDSON Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorA. H. MCDANIEL, A. H. MCDANIEL Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorR. E. SHETTER, R. E. SHETTER Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorJ. G. CALVERT, J. G. CALVERT Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this author C. A. CANTRELL, C. A. CANTRELL Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorJ. A. DAVIDSON, J. A. DAVIDSON Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorA. H. MCDANIEL, A. H. MCDANIEL Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorR. E. SHETTER, R. E. SHETTER Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this authorJ. G. CALVERT, J. G. CALVERT Atmos. Chem. Div., Natl. Cent. Atmos. Res., Boulder, CO 80307-3000, USASearch for more papers by this author First published: September 11, 1990 https://doi.org/10.1002/chin.199037035AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume21, Issue37September 11, 1990 RelatedInformation

Temperature dependence of the atmospheric photolysis rate coefficient for NO2

Direct measurements were made of the temperature dependence of the photolysis rate coefficient of NO2 (j1) at temperatures from −70° to 30° C in sunlight for relatively cloudless summer days in Boulder, Colorado. The ratio of the J1 values (J1(30°C)/j1(T°C) for T = −20° and −70° C are, respectively: 1.046±0.040 and 1.070±0.031 (95% confidence limits). The J1 ratios were independent of solar zenith angle (22° < Z < 47°). Theoretical estimates of the temperature dependent J1 ratios based upon recently reported cross section (σ) and quantum yield (ø) data are more consistent with these experimental measurements than those based upon the currently accepted σ and ø data.