In Situ monitoring of NO3 radicals in a atmospheric chamber by cavity ring down spectroscopy

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry


Atmospheric chambers have a vital role to play in the elucidation of reaction mechanisms for the troposphere. The highly instrumented reactor for atmospheric chemistry (HIRAC) constructed in the School of Chemistry under grant NE/C513493/1 is a 2 m3 stainless steel chamber capable of making measurements over a wide range of conditions. Uniquely for such a chamber it can measure OH and HO2 radical concentrations using a laser based technique. The principal behind HIRAC is to measure as many reactants, intermediates and products as possible in order to constrain the chemical models used to interpret and understand the results. All measurements are subject to potential systematic errors and therefore we consider it important to initially measure using two or more complimentary techniques (e.g. FTIR and GC). This is especially true for radical species, where the short lifetimes make measurements particularly taxing. We have demonstrated an ability to detect and monitor OH and HO2 (ACPD 2007, 7, 10687) and now seek to extend our measurement capability to the NO3 radical, an important nighttime oxidant. It is proposed to measure NO3 using cavity ring down spectroscopy (CRDS). This is an extremely sensitive technique, that unlike laser induced fluorescence (LIF), yields absolute concentrations. With CRDS we should have a detection limit of ~1 pptv, well below typical nighttime NO3 concentrations. Once constructed the CRDS system will be compared with a broadband cavity enhanced absorption spectrometer (BB-CEAS) via a collaboration with Dr Steve Ball and LIF (adapting our current system from OH detection). We will also test the technique by determining rate coefficients for the reaction of NO3 with ethanal, a reaction that is well characterised. Finally, we will apply the technique to the determination of the rate coefficients and product distributions to the reaction of NO3 with alkenes under atmospheric conditions. The rates of reaction are relatively fast and determination of the rate coefficient requires knowledge of the absolute NO3 concentrations (an advantage of CRDS over BB-CEAS and LIF). Products will be examined by GC and FTIR; knowledge of the temporal dependence of NO3 will be vital in constraining the chemical model used to extract quantitative branching ratios.


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Tamsin Malkin (2009) PhD Thesis

Description The objective of this small grant was to construct, deploy and test a cavity ring-down spectroscopy system for the detection of the NO3 radical in an atmospheric chemistry chamber at the University of Leeds.

The NO3 radical plays an important role in the nighttime oxidation of organic compounds in the atmosphere and hence it is important that we understand its chemistry and reactivity. Many NO3 reactions have been studied by the relative rate technique, where NO3 is not directly observed. Direct observation allows application of a range of complementary methodologies to be deployed to study the kinetics of the reaction. When studying reactive species, it is important to use a variety of different techniques to help identify systematic errors.

For this project the cavity ring-down system was successfully constructed and tested in the Leeds HIRAC chamber. Sensitivity was good with a detection limit of 1.5 E8 molecule cm-3 (16 s averaging).
Exploitation Route Relevant for other researchers wishing to detect NO3 Work was written up in the PhD thesis of Dr Tamsin Malkin (University of Leeds 2010)
Sectors Environment

Description The Cavity Ring Down Spectroscopy system developed under this grant has been deployed in the HIRAC chamber and the results obtained contributed to the PhD thesis of Tamsin Malkin. The experience gained in cavity work will allow the deployment of other cavity based methods in the HIRAC chamber. In a wider perspective this work contributes to our understanding of atmospheric chemistry and hence links to policy associated with air quality (and hence health) and climate change.
First Year Of Impact 2010
Sector Environment
Impact Types Policy & public services