Total Reactive Nitrogen Flux Monitor Using CAPS NOx Detection
Authors: Onasch Timothy B , Roscioli J. Rob , Lund Matt , Shorter Joanne , Freedman Andrew 
Published: February 2023

We are in the process of developing a comparatively inexpensive, robust, cavity attenuated phase shift (CAPS)&#8208;based monitor to measure total reactive nitrogen (NR) emission and deposition rates. Total reactive nitrogen comprises both particle (ammonium and nitrate based) and gas phase species such as ammonia, nitrogen oxides, and various organo&#8208;nitrogen compounds. The monitor will be capable of partitioning the emission/deposition rates between particle and gas allowing it to adapt sampling modes to optimize the operation of the monitor.The monitor comprises a Total Reactive Atmospheric Nitrogen Converter (TRANC) (Marx et al. 2012), the output of which is coupled to a CAPS NOx monitor. The TRANC thermally decomposes and oxidizes both non&#8208;refractory atmospheric particles into their gas phase constituents, and any volatile nitrogen-containing gases to NO + NO2. It does so by passing the sample through an oven operating at >900 &#186;C which has been shown to effectively operate at the residence times (i.e., flow rates) required for emission/deposition rate measurements such as eddy covariance (Ammann et al. 2012).The output of the TRANC is sent to a CAPS NOx monitor which uses photolytically produced ozone to oxidize NO to NO2 with >98% efficiency without further oxidation to NO3. The resultant NO2 is measured using the Cavity Attenuated Phase Shift (CAPS) technique at 405 nm, a wavelength free from interference from the ozone in the flow (Kebabian, et al. 2007; Kebabian, et al., 2008; Roscioli, et al., 2022). The CAPS NOx monitor has been shown to have a sensitivity (2s, 1s) of less than 0.2 ppb and to be highly linear over a range of 0-1000 ppb.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Conversion efficiencies for gas phase ammonia and a range of nitrogen-containing particulate species are shown to be within 10% of unity. Aerosolized particles were size selected with an Aerodynamic Aerosol Classifier; equivalent gas phase concentrations were calculated by measuring particle concentrations with a condensation particle counter and assuming a per particle mass.&#160; Within experimental error, gas phase ammonia and ammonium and nitrate containing particles were completely converted to gas phase NO + NO2. This holds true over a range of particle flow rates.&#160;This work is supported by NASA, the U.S. Department of Energy and the U.S. Department of Agriculture, all under the Small Business Innovation Research program.Kebabian, P.L., W.A. Robinson and A. Freedman (2007) Rev. Sci. Instrum., 78, 063102.Kebabian, P.L., E.C. Wood, S.C. Herndon, and A. Freedman (2008) Environ. Sci. Technol., 42:6040-6045.Ammann, C., Wolff, V., Marx, O., Br&#252;mmer, C., and Neftel, A. (2012). Biogeosciences 9 (11):4247&#8211;4261. doi:10.5194/bg-9-4247-2012.Marx, O., Br&#252;mmer, C., Ammann, C., Wolff, V., and Freibauer, A. (2012). Atmospheric Meas. Tech. 5 (5):1045&#8211;1057. doi:10.5194/amt-5-1045-2012.Roscioli, J., T.B. Onasch, and A. Freedman (2022) in preparation.