Ultraviolet Photolysis of HCHO: Absolute HCO Quantum Yields by Direct Detection of the HCO Radical Photoproduct

详细信息    查看全文

• 作者：Paula Gorrotxategi Carbajo ; Shona C. Smith ; Anne-Louise Holloway ; Carina A. Smith ; Francis D. Pope ; Dudley E. Shallcross ; Andrew J. Orr-Ewing
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：December 4, 2008
• 年：2008
• 卷：112
• 期：48
• 页码：12437-12448
• 全文大小：255K
• 年卷期：v.112,no.48(December 4, 2008)
• ISSN：1520-5215

文摘

Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, Φ_HCO, have been measured for the tropospherically relevant range of wavelengths (λ) between 300 and 330 nm. The HCO photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution 0.0035 nm) measurements of absorption cross-sections of HCHO, σ_HCHO(λ), and relative HCO quantum yields. Absolute Φ_HCO values were measured at seven wavelengths, λ = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl₂. These Φ_HCO measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for Φ_HCO. The absolute Φ_HCO(λ) determinations and previously measured σ_HCHO(λ) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, Φ_HCO(λ)σ_HCHO(λ), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0° solar zenith angle (SZA) increasing to 33% at high SZA (80°). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental Φ_HCO(λ)σ_HCHO(λ) data are available for the wavelength range 302.6−331.0 nm (at 294 and 245 K and under 200 Torr of N₂ bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of Φ_H2+CO(λ)σ_HCHO(λ) for the molecular (H₂ + CO) photofragmentation channel, produced using the measured Φ_HCO(λ) σ_HCHO(τ) values, are also provided at 0.1 and 1.0 nm resolution.