The tropospheric photochemistry of formaldehyde

Lead Research Organisation: University of Bristol
Department Name: Chemistry


The atmosphere of Earth is mostly composed of nitrogen (N2) and oxygen (O2) gases, but there are many more complicated and reactive chemical compounds present at very low concentrations that have a considerable impact on the properties of the atmosphere. It is well known, for example, that carbon dioxide and methane are important greenhouse gases. The hydroxyl radical (OH) is present at tiny concentrations (typically 1 OH radical for every 25,000,000,000,000 other molecules of air) but is the main chemical species in the atmosphere that oxidises organic compounds such as methane and other hydrocarbons to form CO2 and water. This oxidation is similar to the chemistry that goes on when a flame burns natural gas, but occurs at much lower temperatures in the atmosphere (down to as low as -50oC at altitudes of about 10 km). Atmospheric chemists thus need to be confident that they have identified all possible sources of the OH radical in order to understand the chemistry of the atmosphere, and how pollutants such as organic compounds are oxidised and removed from air. Formaldehyde molecules, with the chemical formula HCHO, are formed from the complicated processes that follow from reaction of methane and other organic molecules with OH, and can absorb ultraviolet (UV) radiation from the sun. With the energy it gains from this UV light, formaldehyde can split of a hydrogen atom (to form H + HCO) or can break up into a molecule of hydrogen (H2) and one of carbon monoxide (CO). The second process has very little effect on the chemistry of the atmosphere, but both H atoms and HCO radicals react quickly with oxygen in air to make OH (and another related reactive species denoted as HO2). This so-called 'photochemistry' of formaldehyde, meaning chemistry caused by absorption of light, is thus very important for influencing the concentration of OH in the atmosphere, but is poorly understood because of the complicated way in which formaldehyde absorbs UV light and dissociates into atomic or molecular fragments. This project will explore this photochemistry using one UV laser as a source of well-characterised UV light of precisely known energy and wavelength (for visible light, different wavelengths correspond to different colours), and a second laser to measure how much HCO is formed. In addition, we will measure how strongly the formaldehyde molecules absorb (i.e., remove) different wavelengths of UV light; this information is important if we want to find out how much formaldehyde is actually in any particular region of the atmosphere, whether using a satellite or a ground-based apparatus to observe the atmosphere and thus to make the measurement. The measurements of formation of HCO and absorption of UV by HCHO will be made in our lab more precisely and directly than any previous studies of formaldehyde photochemistry, and over a range of temperatures and pressures of N2 and O2 to simulate the conditions in the atmosphere from the Earth's surface up to an altitude of 10 km (the start of the stratosphere). We will make the measurements across the ultraviolet up to the region of the UV bordering on the violet and blue end of the visible spectrum. These wavelengths will cover the range of UV from the sun that reaches the Earth's surface and which is absorbed by formaldehyde molecules. The results of the measurements will be fed into computer programs designed to simulate the chemistry taking place in the Earth's atmosphere, and we will thus learn about what differences formaldehyde photochemistry makes to formation of OH radicals, and to removal of low-level pollutants such as hydrocarbons and other organic molecules.


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Description Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis have been measured for the tropospherically relevant range of wavelengths 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. The quantum yield measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations. The new data and and previously measured HCHO UV absorption cross sections 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 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 I 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 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 alongside a publication in the Journal of Physical Chemistry A, with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets for the H2+CO molecular photofragmentation channel were also determined at 0.1 and 1.0 nm resolution.
Exploitation Route Yes - incorporation in models of atmospheric chemistry. Inclusion of data in global models and assessment of impact of HCHO photochemistry on global atmospheric OH levels (with Prof D.E. Shallcross).
Sectors Environment

Description Absorption cross sections and quantum yields have been incorporated into atmospheric chemistry models by other researchers.
First Year Of Impact 2008
Sector Environment
Impact Types Societal