Recent insights into the formation and chemical composition of atmospheric nanoparticles from the
Atlanta Aerosol Nucleation and Realtime Characterization Experiment (Atlanta-ANARChE)
J. N. Smith1, K. F. Moore1, A. K. Ghimire2, D. Voisin3, F. L. Eisele1,4, and P. H. McMurry2
1National
I.
Center for Atmospheric Research, Atmospheric Chemistry Division, P.O. Box 3000, Boulder, CO 80307, 2Mechanical Engineering Dept., University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455,
3Université de Provence, Marseille, France, 4also at Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332
For Atlanta-ANARChE, the TDCIMS was configured to size-select ambient aerosol
before they were analyzed. This task was accomplished using three sets of unipolar
chargers and nano-DMAs working in parallel, which delivered a flow of 22 l/min of
charged ambient aerosols to the TDCIMS electrostatic precipitator. A fourth nano-DMA
and ultrafine aerosol counter were used to monitor aerosol in the exhaust flow to
calculate total mass collected. The inlet system is efficient for small particles:
measurements showed that 4% of 4 nm diameter aerosols could be charged, sizeselected, and delivered to the TDCIMS.
The Atlanta Aerosol Nucleation and Real-time Characterization Experiment (AtlantaANARChE) was a DOE-sponsored field study that took place at an urban site in Atlanta
from July 22 to September 4, 2002. This study investigated both nucleation and ultrafine
particle growth immediately after the gas-to-particle conversion process. One unique
feature of Atlanta-ANARChE was the deployment of a new instrument capable of
performing real-time measurements of the chemical composition of 6-20 nm diameter
particles at ambient concentrations. The instrument, called the Thermal Desorption
Chemical Ionization Mass Spectrometer, or TDCIMS, is result of a collaborative effort
between NCAR and the University of Minnesota. We present here the principle of
operation for the TDCIMS and the results from calibration studies of laboratorygenerated (NH4)2SO4 aerosol. Following this, we focus on the measurements of aerosol
ammonium and sulfate performed on four separate days in which nucleation was
observed during Atlanta-ANARChE. The measurements indicate that the aerosols in the
6-15 nm diameter size range are composed primarily of ammonium sulfate.
We performed continuous measurements of ambient aerosols during the period of
August 1-29, 2002. Due to the limitations imposed by selected ion monitoring with a
quadrupole mass spectrometer, a maximum of 6 ions of the same polarity could be
monitored during a single aerosol sampling. Of these 6 ions, one of the major reagent
ions was always selected (e.g., O2- in the negative ion spectrum and H3O+ in the
positive ion spectrum). In addition, ions that were observed at high concentrations in
every sample were selected for continuous monitoring, i.e., NH4+ in the positive
spectrum and HSO4- in the negative spectrum. Other compounds were also monitored
in an attempt to identify all species present in the aerosols.
roof
5 sl/m
Instrument
N2
N2 overfills swagelok T,
which is open to atm
15 sl/m sheath
A. Thermal Desorption Chemical Ionization Mass Spectrometry (TDCIMS)
Measurement Principle
33 sl/m total
aerosol flow
TDCIMS inlet
Figures 9 and 10 show the fraction of aerosol mass that can be explained as
ammonium sulfate for ambient aerosols that were monitored for their sulfate content
before and during nucleation events on 23 August and 25 August, respectively. Data
from 23 August are characterized by ratios greater than one, however this effect may be
related to the width of the DMA transmission window as discussed also in Section V).
These data also exhibit greater uncertainty due to inherent difficulties in measuring
aerosol sulfate. When these uncertainties are taken into account, all data points pass
through the ratio of one, indicating that these aerosols can be described as ammonium
sulfate.
In the analysis that follows, we compare the corrected peak area (integrated area of
aerosol peak with background peak subtracted, describe in Section II-B) for the ambient
measurement to that obtained for laboratory-generated ammonium sulfate aerosols
which were analyzed using identical TDCIMS operating parameters. By normalizing
each measurement by the collected aerosol mass, we can answer the question:
Based on the integrated peak areas, how much of the ambient aerosol can be
described as ammonium sulfate?
nano-DMA
operating in
underpressure mode
9 sl/m
excess
VI. TDCIMS measurements of
ultrafine aerosol sulfate
In spite of our best efforts at searching for other compounds in the aerosols, the
TDCIMS measurements showed that the aerosols were composed almost entirely of
ammonium and sulfate. In Sections V and VI we present the results these
measurements during four days of the study.
UMinn supplied
unipolar charger
V
II.
IV. Measurement overview
III. Experimental arrangement
Abstract
triple quadrupole mass spectrometer
M
M
Figure 1 shows a schematic of the instrument. The TDCIMS can be divided into three
parts: the aerosol charger, the electrostatic precipitator and the chemical ionization
mass spectrometer. The charger, shown in figure 2, is built in-house and modeled after
the unipolar charger developed by Chen and Pui (J. Nanoparticle Res., 1999). Ambient
aerosols are first charged, and can be optionally size-selected using a differential
mobility analyzer (DMA) located directly downstream of the charger. Charged particles
are then introduced into the electrostatic precipitator: a cylindrical chamber that contains
a collection wire that is biased to a high voltage (usually 4000 V) and located on the
axis of the chamber (figure 3). The voltage polarity is chosen so that these charged
particles cross flow streamlines, into a clean buffer gas that isolates the collection wire
from contamination from the ambient gas, and are collected on the tip of the wire. Once
a sufficient amount of particles are collected, usually 1-10 pg over a period of 5-10 min,
the wire is transferred into the ion source region of the chemical ionization mass
spectrometer. A current pulse is applied to the wire to resistively heat it to thermally
desorb the aerosols. The third part of the instrument is the chemical ionization mass
spectrometer, and consists of the ion source region, a de-clustering collision cell, and a
mass spectrometer. The ion source consists of an Am241 radioactive source at
atmospheric pressure that emits a particles that ionize the buffer gas mixture to form
H3O+, O2- and CO3- as the primary stable ions. These ions react with the compounds
evaporated from the aerosol to ionize them, and electrostatic lenses direct these ions
into the collision cell where the ions are de-clustered from neutral compounds that may
be present in the gas, most commonly water. The ions are detected using selected ion
monitoring in a triple quadrupole mass spectrometer.
1.5 sl/m
3025A counter
chemical ionization mass spectrometer
collection
wire (NiCr)
ion source
(Am241)
NCAR supplied
Figures 7 and 8 show the fraction of aerosol mass that can be explained as ammonium
sulfate for ambient aerosols that were monitored for their ammonium content before and
during two nucleation events on 17 August and 18 August, respectively. Points that fall
above the ratio of one may indicate additional sources for ammonium, but are more
likely caused by sampling a proportionally greater number of large particles within the
transmission window of the DMAs. Additional analysis of particle size distributions
performed during TDCIMS sampling is expected to remedy the latter effect, if present.
During nucleation events, and during time periods that extend to several hours before
and after events, aerosols have ratios close to one, indicating from these ammonium
measurements that the aerosols can be described as ammonium sulfate.
Figure 9. Particle size distributions (top) and the fraction of the
ambient aerosol mass from ammonium sulfate (bottom) based on
aerosol sulfate measured on 23 August 2002. Nucleation event is
indicated by yellow bar, and legend shows monitored particle
diameter.
The site of the study was the Jefferson Street
location of the 1999 SOS Atlanta SuperSite
Experiment in midtown Atlanta, GA. (above).
The instrument was housed in a research
trailer (right) provided by Georgia Power. In
the photo at right the three sets of chargers
and nano-DMA’s can be seen with copper
sample tubing penetrating the ceiling.
calibration aerosols
(electrospray aerosol generator)
aerosol
inlet
V. TDCIMS measurements of
ultrafine aerosol ammonium
Figure 6. Schematic of TDCIMS configuration for Atlanta-ANARChE.
aerosol size classifier
(differential mobility analyzer)
collision cell
triple quadrupole mass spectrometer
linear actuator
V
cryotrap
ambient
aerosol
aerosol charger
15 sl/m
N2
exhaust
wire sheath inlet
scanning mobility particle sizer
electrostatic precipitator
900 l/s turbopump
900 l/s turbopump
aerosol counter
(condensation nucleus counter)
Figure 10. Particle size distributions (top) and the fraction of the
ambient aerosol mass from ammonium sulfate (bottom) based on
aerosol sulfate measured on 25 August 2002. Nucleation events are
indicated by yellow bars. All measurements performed on 15 nm
particles.
1600 l/s turbopump
Figure 1. Schematic of the TDCIMS, with optional
elements shown in dashed boxes.
sample flow
Po210 strips
1" OD tube
10.625
delrin rods (4 total)
Figure 2. Schematic of the unipolar charger
ion source region
electrostatic precipitator
sheath exhaust
900 scc/m
ion source flow
130 scc/m
ion source
Am241
aerosol exhaust
9.7 sl/m
charged aerosol
8 sl/m sample flow
2.5
2.0
1.5
1.0
0.5
0.0
sheath air
ultrapure N2
2 sl/m
NiCr collection wire
mass spec.
aperture
−
sheath air
tube
isolation flow
600 scc/m
HSO4 ion count (kHz)
0.8 cm
Figure 3. Diagram of the electrostatic precipitator and
ion source region, showing typical values for flows and
collection wire location
2.5
2.0
1.5
1.0
0.5
0.0
Figure 4 (left). Recorded ion
abundance vs. elapsed time
during sample
a
desorption/analysis of
ammonium sulfate aerosols.
(a) Ammonium (18 amu)
with collection voltage
turned on, (b) Ammonium
0
50
0
50
trace from filament in
time from start of desorption pulse (s) collection position with no
collection voltage applied,
c
d
(c) bisulfate ion (97 amu)
with collection voltage
b
turned on (arrows indicate
two peaks in trace), (d)
Bisulfate trace from filament
in collection position with no
collection voltage applied.
a
b
0
50
0
50
time from start of desorption pulse (s)
Voisin et al., Aerosol Sci. Technol., 2003
NH4+ peak area
(x1000 counts)
2.375
Figure 7. Particle size distribution (top) and the fraction of the ambient
aerosol mass from ammonium sulfate (bottom) based on aerosol
ammonium measured on 17 August 2002. Nucleation event is
indicated by yellow bar, and legend shows monitored particle
diameter.
Key for particle size distributions
(all size distributions are courtesy of H. Sakurai, Univ. of Minn.)
6
4
VII. Conclusions
2
0
0
2
4
6
8
10 12
collected aerosol mass (pg)
1. We compared the ammonium and sulfate peak area vs. collected mass for ambient
aerosols during nucleation events to calibration ammonium sulfate aerosols. Both
agree to within uncertainty, suggesting that aerosols from these nucleation events
are mostly ammonium sulfate.
14
25
HSO4- peak area
(x1000 counts)
charging region
A typical measurement sequence is as follows: the filament is first heated to 400°C for one minute to clean it before
starting a new analysis; after waiting for the filament to cool, it is then set to its collection position and a collection voltage
applied to it for a given time; the collection voltage is turned off and the filament slid into the analysis position, where it is
heated to 100°C for 60 s to evaporate the aerosol. Total analysis time is 2.5 min added to the time allowed for aerosol
collection. Figure 4 shows typical ion peaks for ammonium sulfate aerosol: peaks can be seen for NH4+ (18 amu, figure
4a), and HSO4- (97 amu, figure 4c) when the filament is heated in the ionization chamber. Small “background peaks” can
also be observed (figures 4b,d), from contaminated air which contacts the filament. These peaks are subtracted from
those in figures 2a,c to arrive at the chemical compounds present only in the aerosols. Plots of integrated ion peak area
versus collected aerosol mass for laboratory-generated ammonium sulfate aerosol are shown in figure 5. Although these
calibrations were performed using (NH4)2SO4, other tests have shown that it can sensitively quantify a variety of
inorganic (e.g., nitrates) and organic (e.g, aldehydes, amines) compounds.
8
+
ion production region
excess flow out
NH4 ion count (kHz)
honeycomb
.188" cell diam, 2" long
B. Operation and calibration with laboratory-generated ammonium sulfate aerosol
20
15
2. Ultrafine-tandem differential mobility analyzer data (acquired by H. Sakurai, Univ. of
Minn., but not reported here) show growth factors at 90% RH of 1.4-1.5 during
these nucleation events, suggesting salts with low insoluble organic content.
10
5
0
0
1
2
3
4
5
collected aerosol mass (pg)
3. We continuously monitored in the positive and negative ion mode for high molecular
weight compounds, but saw very little.
6
Figure 5. Instrument response as a
function of collected mass of ammonium
sulfate aerosols for (top) NH4+, from 14
nm aerosols, and (bottom) HSO4-, from 10
nm aerosols.
Figure 8. Particle size distribution (top) and the fraction of the ambient
aerosol mass from ammonium sulfate (bottom) based on aerosol
ammonium measured on 18 August 2002. Nucleation event is
indicated by yellow bar, and legend shows monitored particle
diameter.
Acknowledgements
Financial Support by the DOE through grant No. DE-FG02-98R62556