NPTC · New Perspectives in Tropospheric Chemistry

New Perspectives in Tropospheric Chemistry

The last two hundred years have seen new anthropogenic emissions dramatically change the chemical composition and chemistry of the troposphere, creating an incredibly diverse set of atmospheric conditions based on location and level of human population. Alkenes are common to pristine environments (biogenic alkenes like isoprene); polluted or urban areas (smaller alkenes like ethene) and indoor environments (terpenes from cleaning products). The removal of alkenes from the troposphere primarily occurs through reaction with ozone, generating a wide range of molecular products. Despite the abundance of alkenes in the atmosphere, with yearly emissions on the order of 850 Tg, the atmospheric ozonolysis of alkenes is still relatively poorly understood. Only recently was a key intermediate in this ozonolysis process, a carbonyl oxide known as a Criegee intermediate (CI), detected directly. Criegee intermediates are very important species in our atmosphere, and thus to society. At night-time their unimolecular decomposition is a dominant source of hydroxyl radicals, often coined an atmospheric detergent, helping remove trace pollutants. CI reactions with other trace atmospheric constituents (bimolecular reactions) can generate a wide range of important tropospheric chemicals. They can also generate aerosols, which influence the radiative forcing of the atmosphere, transport chemicals around the atmosphere and can be detrimental to our health. In order to continue in the evaluation of the tropospheric impact of CIs, this project proposes to use advanced spectroscopy and computational chemistry to answer several key questions that still remain: Can Criegee intermediates be detected under a range of atmospheric conditions, e.g. varying temperature and pressure? Absorption spectroscopy will be employed to detect CIs under conditions consistent with those found within the atmosphere. We will develop new instrumentation to assist with this procedure. How do size and chemical complexity of Criegee intermediates influence their tropospheric chemistry? The fate of these intermediates can vary based on their size, chemical complexity, and where they are generated in the Earth's atmosphere. We will investigate a range of important atmospheric intermediates, from small CIs formed in urban environments, through to larger intermediates formed from the ozonolysis of alkenes emitted from foliage. We will develop relationships between CI size and shape with atmospheric reactivity. How do Criegee intermediates facilitate aerosol nucleation? Criegee intermediates have recently been implicated in the formation of aerosol particles in both indoor and outdoor environments. Many CI reactions with trace atmospheric gases lead to aerosol formation. We will begin to investigate this nucleation procedure, and investigate the use of UV and IR spectroscopy to detect the early stages of aerosol nucleation.

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