Characterization of a catalyst-based conversion technique to measure total particulate nitrogen and organic carbon and comparison to a particle mass measurement instrument.

The chemical composition of aerosol particles is a key aspect in determining their impact on the environment. For example, nitrogen-containing particles impact atmospheric chemistry, air quality, and ecological N deposition. Instruments that measure total reactive nitrogen (Nr = all nitrogen compounds except for N₂ and N₂O) focus on gas-phase nitrogen and very few studies directly discuss the instrument capacity to measure the mass of N_r-containing particles. Here, we investigate the mass quantification of particle-bound nitrogen using a custom N_r system that involves total conversion to nitric oxide (NO) across platinum and molybdenum catalysts followed by NO-O₃ chemiluminescence detection. We evaluate the particle conversion of the N_r instrument by comparing to mass-derived concentrations of size-selected and counted ammonium sulfate ((NH₄)₂SO₄), ammonium nitrate (NH₄NO₃), ammonium chloride (NH₄Cl), sodium nitrate (NaNO₃), and ammonium oxalate ((NH₄)₂C₂O₄) particles determined using instruments that measure particle number and size. These measurements demonstrate N_r-particle conversion across the N_r catalysts that is independent of particle size with 98±10% efficiency for 100-600 nm particle diameters. We also show efficient conversion of particle-phase organic carbon species to CO₂ across the instrument's platinum catalyst followed by a nondispersive infrared (NDIR) CO₂ detector. However, the application of this method to the atmosphere presents a challenge due to the small signal above background at high ambient levels of common gas-phase carbon compounds (e.g., CO₂). We show the Nr system is an accurate particle mass measurement method and demonstrate its ability to calibrate particle mass measurement instrumentation using single-component, laboratory-generated, N_r-containing particles below 2.5 μm in size. In addition we show agreement with mass measurements of an independently calibrated online particle-into-liquid sampler directly coupled to the electrospray ionization source of a quadrupole mass spectrometer (PILS-ESI/MS) sampling in the negative-ion mode. We obtain excellent correlations (R² =0.99) of particle mass measured as N_r with PILS-ESI/MS measurements converted to the corresponding particle anion mass (e.g., nitrate, sulfate, and chloride). The N_r and PILS-ESI/MS are shown to agree to within ~6% for particle mass loadings of up to 120 μgm^-3. Consideration of all the sources of error in the PILS-ESI/MS technique yields an overall uncertainty of ±20% for these single-component particle streams. These results demonstrate the N_r system is a reliable direct particle mass measurement technique that differs from other particle instrument calibration techniques that rely on knowledge of particle size, shape, density, and refractive index. [ABSTRACT FROM AUTHOR]