Development of cavity enhanced absorption spectrometer for nocturnal NOx sink quantification

Abstract:

At night, nitrate radical(NO3) and dinitrogen pentoxide(N2O5), which is in thermal equilibrium with NO3, contribute a significant portion of the overall NOx (NOx = NO + NO2) removal in the troposphere via their oxidation of volatile organic compounds(VOCs) and hydrolysis reactions. In order to investigate the strength of NOx sinks especially at night, it is important to assess not only the abundance of these reactive nitrogen oxides but also their loss rates. Even with its importance, the direct measurement of NO3 reactivity, defined as pseudo-first-order loss rate of NO3 radicals, also same as the inverse of the NO3 lifetime is very sparse in terms of temporal and spatial coverage. In this respect, we developed a three-channel broadband cavity enhanced absorption spectrometer for simultaneous measurement of NO3 reactivity as well as NO3 and N2O5.

Our instrument’s working principle is based on the measurement of light attenuation by NO3 radical in a high finesse-optical cavity where LED(light-emitting diode) is used as a broadband light source. Novel designed cage system enables fine adjustment for optical layout as well as robust performance especially for the extreme measurement circumstances via insensitive responses with respect to vibration and pressure changes. For N2O5 channel, the preheating tube is installed before sample injection into the cell so as to fully dissociate N2O5 to NO3, and therefore, the sum of N2O5 and NO3 is measured. For NO3 reactivity channel, synthetically generated NO3 radicals are injected at the tip of the inlet and a regular NO flushing scheme for NO3 loss measurement was optimized. Characteristics of the instrument such as cavity loss, effective sample length, and finest conditions for duty cycles in addition to the detection limit were investigated. The wall loss effects of the system as well as the optimized temperature of the preheated tube in N2O5 channel were also analyzed. Lastly, lab-scale feasibility test results for NO3 reactivity measurement with respect to isoprene will be presented.