Characterization of a catalyst-based conversion technique to measure total particulate nitrogen and organic carbon and comparison to a particle mass measurement instrument
Authors: Stockwell Chelsea E. , Kupc Agnieszka , Witkowski Bartłomiej , Talukdar Ranajit K. , Liu Yong , Selimovic Vanessa , Zarzana Kyle J. , Sekimoto Kanako , Warneke Carsten , Washenfelder Rebecca A. , Yokelson Robert J. , Middlebrook Ann M. , Roberts James M. 
Published: May 2018

Abstract. 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 Nr-containing particles. Here, we investigate the mass
quantification of particle-bound nitrogen using a custom Nr 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 Nr 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 Nr-particle conversion across the
Nr 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, Nr-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 Nr with PILS–ESI/MS measurements converted to the
corresponding particle anion mass (e.g., nitrate, sulfate, and chloride). The
Nr and PILS–ESI/MS are shown to agree to within ∼ 6 %
for particle mass loadings of up to 120 µg m−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 Nr 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.