The tropospheric photochemistry of formaldehyde

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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.