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

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.