Atmospheric aerosol was collected in La Jolla, CA and separated
into 1 μm and 180 nm size cutoffs. The two subsets of data were
then analyzed with Fourier transform infrared (FTIR) spectroscopy in
order to analyze differences in organic functional group compositions
for the two separate size cutoffs. The results show clear
differences in organic functional group composition between the size
cuts. The 180 nm cutoff showed an increased percentage of alkane
groups when compared to the 1 μm cutoff, while some of the results
varied in other organic functional group compositions such as,
alcohols, organic acids, amines, and carbonyls.
Both urban and natural cycles and reaction have
effects on the atmosphere, and in order to understand their
characteristics chemical analysis must be carried out (Russell et. Al
2009). Aerosol particles come in a variety of sizes ranging from
coarse to fine aerosol particles (Jacobson et al. 2000).
Depending on the method used to collect aerosol particles, certain
sizes can be collected separately for size-resolved chemical
analysis. By using a sharp-cut cyclone (1 μm cutoff) and Brenner
impactor (180 nm cutoff) side by side, each with different size
cutoffs, aerosol particles can be collected and sorted simultaneously
by size into two separate subsets of data. Through this sorting,
the two different sets of aerosol can be analyzed for chemical
In this experiment, a sharp-cut cyclone, 1 μm size cutoff, and a
multistage Brenner impactor, 180 nm size cutoff, were used in parallel
to collect aerosols within their respected size regions on Teflon
filters. The samples were collected in La Jolla, California on
six separate days, October 10th, 16th, 17th, and 18th as well as
November 1st and 4th of the year 2013. To determine the best time
resolution needed to collect 180 nm particles above the detection limit
for the FTIR, the duration of the samples were varied from
The cyclone and Brenner impactor were installed behind a dryer
and before Teflon filters to be collected simultaneously. To
reach the appropriate size cutoffs, the cyclone was run at 16.7 SLPM,
with 7.0 LPM traveling through the Teflon filter, while the impactor
was run at 46 SLPM, with 10.0 SLPM traveling through the filter.
Once collected, the filters were placed in a clean room and allowed to
reach equilibrium with the environment at 20°C and 55% RH. After
the filters equilibrated for 24 hrs, the samples were analyzed by
Fourier transform infrared spectroscopy for organic functional
groups. Using a method from the Russell group, an algorithm was
applied to baseline the spectra and quantify organic functional groups
(alkane, alcohol, carboxylic acid, carbonyl and amine) (Russell et al.
2009). The composition was calculated as total mass (μg m-3) and mass
percentages of alcohols, organic acids, carbonyls, amines, and alkane
groups for each filter.
When available, meteorological data (including wind speed and
wind direction) was also taken from the Scripps Pier weather station
Figure 1 - Normalized spectra, total mass, and average organic
functional group mass percentages for aerosol sampled with a cyclone
(PM1, 1 μm) and Brenner impactor (180 nm) in October 2013. Pie charts
represent functional group percentages: alcohol (pink), alkane (blue),
carbonyl (light blue), amine (orange), and carboxylic acid (green).
Figure 2 – Wind rose plot showing the direction of wind on the two
sampling days (10/10 and 10/18) and provides prospective of the
location. The widths of the orange and blue sections represent
the standard deviation of the wind direction.
Apparent differences in the chemical compositions are seen
between the different size cuts. The differences are evident in the
overall shape of the normalized spectra and average organic functional
group composition. By viewing the masses of the PM1 and 180 nm
filters, the PM1 filter had samples 2-10x the mass of the 180 nm
By comparing the weather data from Scripps Pier on October 10th
and 18th with organic functional group composition between the size
cuts, some interesting conclusions about source regions are reached
(Figure 2). On October 10th, 2013 the wind had an average
direction of 238O from true north, meaning the wind was coming from the
ocean and free of anthropogenic aerosol sources. As a result, the
filters had a significant alcohol composition (>45%) in both the 1μm
and 180 nm size cuts, which is consistent with marine aerosol (Russell
et al. 2010). Marine aerosols are formed through natural
processes, such as evaporation and bubbles caused by wave action
(Finlayson-Pitts and Pitts 382). There is a difference, however,
in the alcohol percentage between the 1μm and 180nm cutoffs, with the
180nm level containing 15% less alcohol groups than 1μm aerosol.
This relationship would suggest that the marine aerosols are less
present in the 180nm cutoff.
In contrast to the primarily marine source, October 18th has a
significantly smaller alcohol percentage at both the PM1 and 180nm
levels. When analyzing the wind direction, it was found that the
wind was coming from a northern direction, 341O from true north.
North of La Jolla lies several major urban communities including the
Los Angeles basin, Riverside, and Oceanside. The increased
composition of carboxylic acid and alkane groups are in agreement with
urban, or anthropogenic aerosol sources. Urban aerosol consists
of a wide variety of groups that are created from combustion reactions
used in manufacturing plants and gas powered vehicles, which includes
enhanced alkane functional groups and typically strong ammonium
signatures (LIU et al. 2012). Comparing both the 1μm and the
180nm aerosol from October 18th, there is a clear difference in alkane
composition between the size cuts, with 180nm containing more alkane
functional groups (45%). A similar difference was seen in the
October 10th sample, suggesting that the 180nm size bin contains a
larger percent of alkane than it’s 1μm counterpart.
Another relationship amongst the samples is that the ammonium
absorbance was higher in 180nm samples than the 1μm samples. This
trend can be explained because fossil fuel combustion creates ammonium
functional groups. The reason the ammonium aerosol is found in
the 180nm size bin is due to the fact fresh combustions create small
particles (Liu et al. 2012).
When only the 180nm samples are compared, the composition is
relatively invariant. Williams et. al (2002) compared the
residence time of aerosol particles as a function of time. It was
found that the 100-200nm particles exhibited the highest residence time
in the lower atmosphere. Since the particles had a residence time
of around 2 days, the chemical composition observed in this experiment
should not have drastic changes (Williams et al. 2002). The
consistency between the accurate samples suggests this is in agreement
with the experiment. PM1 particles have a shorter residence time
due to their size, and therefore the PM1 size cut off is more
susceptible to variability, also in agreement with the data.
V. Error Analysis/Conclusion
The first inconsistency with the experiment can be seen by
comparing the total organic mass acquired on the filters. The 180
nm samples should have a lower mass in comparison to the 1μm
filter. The samples from 10/16 and 11/1 have masses that are too
similar between the size cuts, within 0.1 μm. The 180 nm impactor
could be experiencing a bounce effect, where larger aerosols bounce off
impaction stages and proceed to the downstream, smaller stages.
Bounce could eventually lead to these larger particles being
accidentally sampled. As a result, bounce can lead to an error in
the total organic mass, as well as the organic composition.
Recognizing this possible error, a future experiment would include more
sets of filters spread across more days. In order to reduce the
bounce effect, Vasoline could be added to the first stage to act as an
adhesive. However, this would introduce more error to the system
because Vasoline is an organic and would influence the results.
In order to limit bounce and conserve the data, the films within the
impactor will be replaced.
Finlayson-Pitts, Barbara and Pitts, J.: Chemistry of the Upper
and Lower Atmosphere. San Diego: Academic Press, 2000. pp. 382-383.
Jacobson M. C., H.-C. Hansson, K. J. Noone, and R. J. Charlson. Organic
Atmospheric Aerosols. 2000, Reviews of Geophysics pp. 267-294.
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Rubitschun, C., Surratt, J., Sheesley, R., and Scheller, S.: Secondary
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Russell, L. M., Takahama, S., Liu, S., Hawkins, L. N., Covert, D. S.,
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