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

Abstract

Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, Phi(HCO), have been measured for the tropospherically relevant range of wavelengths (lambda) 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 approximately 0.0035 nm) measurements of absorption cross-sections of HCHO, sigma(HCHO)(lambda), and relative HCO quantum yields. Absolute Phi(HCO) values were measured at seven wavelengths, lambda = 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 Cl(2). These Phi(HCO) measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for Phi(HCO). The absolute Phi(HCO)(lambda) determinations and previously measured sigma(HCHO)(lambda) 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, Phi(HCO)(lambda)sigma(HCHO)(lambda), 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 degrees solar zenith angle (SZA) increasing to 33% at high SZA (80 degrees). 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 Phi(HCO)(lambda)sigma(HCHO)(lambda) data are available for the wavelength range 302.6-331.0 nm (at 294 and 245 K and under 200 Torr of N(2) bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of Phi(H(2)+CO)(lambda)sigma(HCHO)(lambda) for the molecular (H(2) + CO) photofragmentation channel, produced using the measured Phi(HCO)(lambda) sigma(HCHO)(tau) values, are also provided at 0.1 and 1.0 nm resolution.

Impacts of formaldehyde photolysis rates on tropospheric chemistry

“…A high-resolution UV radiation model was used by Gorrotxategi Carbajo et al (2008) to calculate J (HCO) increases of 25-30 (±15)% between 300 and 330 nm. Table I shows that the radical channel has become a more significant removal route (increase of 4.6%), and overall photolysis is a more dominant removal process (62.0-63.4%).…”

“…The first two runs used data from NASA-JPL recommendations in 1992 (DeMore, 1992) and 2006 (Sander et al, 2006). The third-run used data from Sander et al (2006), but substituted with the data of Gorrotxategi Carbajo et al (2008) and Smith et al (2006) over the wavelength range from 300 to 330 nm. These runs will subsequently be referred to as JPL92, JPL06 and CarJPL06, respectively.…”

“…Measurements at a spectral resolution of 0.0032 nm have been conducted by Smith et al (2006) showing a greater peak of formaldehyde cross-section in the range 300-340 nm. Direct detection of the HCO radical has been conducted by Gorrotxategi Carbajo et al (2008) allowing for absolute quantum yield [ (λ)] determination for the radical channel of photolysis. These studies thus provide improved quantification of the parameters, which determine the atmospheric photolysis rate of formaldehyde, and which are likely to influence the NASA-JPL and IUPAC recommendations.…”

“…We performed three simulations using formaldehyde photochemical data from DeMore (1992), Sander et al (2006) and Gorrotxategi Carbajo et al (2008). The percentages of formaldehyde removal via photolysis are calculated to be 60.9, 62.0 and 63.4%, respectively.…”

Cavity Enhanced Spectroscopy: Applications Theory and Instrumentation

“…The quantum yield of reaction (19) was determined as 0.62 by monitoring the production of the formyl radical using CRDS as formaldehyde was photolyzed with light of 304 nm. (46) Similar experiments have been performed via CES detection of a stable molecular product. The subject of many examples of this type involves the photolytic release of bromine, which is important as a catalytic destroyer of ozone in the stratosphere.…”