Supporting Text Describing the Lewes Field Site and the Measurement Principles of
NAMS and TDCIMS
Field Measurement Site
Field measurements were conducted at the Hugh R. Sharp Campus of the University of Delaware
in Lewes, Delaware, USA (38o 47’ 02” N, 75o 09’ 39” W) from 23 July to 31 August 2012
[Bzdek et al., 2013]. This site hosted an earlier campaign to measure gas phase sulfur emissions
[Luther and Stecher, 1997; Stecher et al., 1997] as well as a campaign to study nanoparticle
chemical composition during NPF in the autumn of 2007 [Bzdek et al., 2011]. The field site is
located 800 m south of the Delaware Bay, which is at the outlet of the Delaware River to the
Atlantic Ocean, and the site is 3 km west of the Atlantic Ocean. A large salt marsh sits adjacent
(<50 m) to the west of the site.
Nanoparticle Mass Spectrometric Measurements
Nanoparticle composition was measured by two complementary methods. The first is the
Nano Aerosol Mass Spectrometer (NAMS), which gives quantitative elemental composition of
individual nanoparticles in the 10-30 nm size range. NAMS has been described in detail
elsewhere [Bzdek et al., 2013; Pennington and Johnston, 2012; Wang and Johnston, 2006; Wang
et al., 2006]. Briefly, particles are drawn in through an inlet, focused, size-selectively trapped in
a digital ion trap, and irradiated with a high energy pulsed laser beam to quantitatively convert
all molecular species to multiply charged, positive atomic ions. These ions are then mass
analyzed by time-of-flight. Deconvolution of overlapping signal intensities was accomplished by
the method of Zordan et al. [2010]. Nanoparticle chemical composition was averaged such that a
minimum of 20 particles was included in the average, as this number of particles simultaneously
maximizes time resolution while minimizing uncertainty from variations in the dynamics of the
laser plume [Klems and Johnston, 2013]. NAMS provides elemental abundances to within 10%
of expected values for elements commonly observed in atmospheric aerosol (including but not
limited to C, O, N, S, and Si) [Zordan et al., 2010]. Within the measurement uncertainty, there is
no molecular dependence on the measured elemental abundances. Note that NAMS does not
provide a quantitative measure of H, so no interpretation of signal arising from H is performed.
Each NAMS elemental mole fraction is the abundance of the reported element relative to the
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total abundance of all elements quantitatively measured by NAMS. For this campaign, NAMS
was set to analyze the composition of 18±3 nm mobility diameter particles. Measurements of
particle composition around 20 nm diameter provides information on the species important to
particle growth from 10-20 nm diameter, as most of the nanoparticle mass was added to the
particle as it grew from 10 nm to 20 nm diameter.
The second method by which nanoparticle composition was measured is the Thermal
Decomposition Chemical Ionization Mass Spectrometer (TDCIMS) [Smith et al., 2004; Voisin et
al., 2003]. TDCIMS provides the molecular composition of bulk nanoparticulate samples. This
system uses a low resolution electrostatic classification technique [McMurry et al., 2009] to
collect nanoparticles on a metal filament and then resistively heats the filament and analyzes the
desorbed gas using a chemical ionization time-of-flight mass spectrometer. Positive and negative
ion mass spectra were measured sequentially in order to detect particulate bases and acids,
respectively. For each run, nanoparticles with a peak mobility diameter of 30 nm and a halfwidth at half-maximum of 10 nm were collected for 30 minutes. The actual distribution of
particle sizes collected depends on the ambient particle size distribution, due to multiple charging
of collected particles [McMurry et al., 2009]. When there is a strong growth mode at or above 30
nm, this mode dominates the collected particle mass, but in the absence of such a mode the
collected particle mass is skewed to larger sizes.
References
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