Geophysical Research Letters
Supporting Information for
Direct atmospheric evidence for the irreversible formation of aqueous secondary organic
aerosol (aqSOA)
Marwa M. H. El-Sayed1, Yingqing Wang1, and Christopher J. Hennigan1
1. Department of Chemical, Biochemical and Environmental Engineering, University of
Maryland, Baltimore County Baltimore, MD, USA.
Contents of this file:
Text S1 to S3
Figures S1 to S4
Tables S1 to S4
S1 - Methods
S2 - Results
S3 - References
1
S1 - Methods
Figure S1 shows a schematic of the experimental setup to concurrently measure WSOCg,
WSOCp, and WSOCp,dry. The system was operated on a 14-min sampling cycle, which provided
an integrated measurement of WSOCg, WSOCp, and WSOCp,dry during each cycle. WSOCg was
measured using a mist chamber (MC) according to the method of Hennigan et al. [2008]. Gases
WSOCp
channel
PM2.5
cyclone
Environmental Enclosure
PILS
Carbon
denuder
WSOCp,dry
Diffusion Dryer
WSOCp - Ambient channel
Distribution valve
Mist
chamber
PM filter
WSOCg
channel
Liquid Waste
TOC
Analyzer
Data Acquisition
Figure S1: Schematic of the experimental measurement system for
WSOCg, WSOCp, and WSOCp,dry.
were sampled at 28.2 L min-1 through a 47 mm quartz fiber filter for particle removal. The MC
used an initial collection volume of 10 mL DI water (> 18.2 MΩ). The overall 14-min MC
2
sampling cycle consisted of three phases: 1) 5 minutes of sampling ambient air; 2) 4 minutes in
which the collected sample was analyzed on-line by a Total Organic Carbon Analyzer (TOC,
Model 900 Turbo, GE Analytical); and 3) 5 minute rinsing protocol to remove residual sample
and wash the MC in preparation for the next ambient sample. The aqueous sample collected in
the MC was sent to the TOC Analyzer via an 8-port distribution valve (Valco Instruments). The
rinsing procedure and sample delivery to the TOC were automated using high-precision syringe
pumps (TriContinent Scientific), which also controlled the distribution valve positioning. The
active MC sampling was started/stopped using an automated valve (Swagelok) in between the
MC and vacuum pump – this valve was also actuated by the syringe pumps.
WSOCp was measured according to the method of Sullivan et al. [2006]. A PILS
sampled soluble particles downstream of a PM2.5 cyclone and parallel plate carbon denuder. The
overall 14-min PILS sampling cycle consisted of three phases: 1) 5-min ‘ambient’ PILS sample
sent to TOC Analyzer for WSOCp determination; 2) 5-min ‘dried’ PILS sample sent to TOC
Analyzer for WSOCp,dry determination; 3) 4-min MC sample analysis time in which the PILS
sample was directed to waste through the distribution valve. The PILS sample was modulated
between the dry and ambient channels using an automated 3-way stainless steel valve (Brechtel
Manufacturing). During the ‘ambient’ PILS sampling, the air sample was directed through
copper tubing (1/2” O.D.), while the ‘dry’ sample was directed through a silica gel dryer.
Particle losses through the 3-way valve and the dryer assembly were characterized prior to the
study: differences between the dry and ambient channels were less than 5% (using total particle
number measured with a CPC (TSI model 3772)) – no corrections were applied to the data.
3
The TOC Analyzer was operated in Turbo mode, which provides a measurement of the liquid
sample every 4 seconds. Thus, the TOC measurements were averaged to achieve a single 5-min
integrated WSOCp and WSOCp,dry measurement for each 14-min sample cycle. Likewise, the
aqueous MC sample analyzed for 4 minutes in the TOC was averaged to provide a single
measurement of WSOCg as well.
Consistent with the results of Sullivan et al. [2004], laboratory testing prior to field
deployment revealed that the predominant background TOC signal was contributed by the PILS
transport water and the MC sampling water, respectively. Thus, dynamic blanks were measured
each time the DI water reservoir was filled – approximately every other day during the study
period. To conduct dynamic blank measurements, the system was run in the same manner as it
was during active sampling, but the vacuum pumps were turned off. For the PILS, the dynamic
blanks represent any background TOC contributed by the DI water, sample tubing, PILS
components (impactor and debubbler), peristaltic pump, and the distribution valve. For the MC,
the dynamic blanks represent any background TOC contributed by the DI water, sample tubing,
residual MC sample, syringe pumps, and the distribution valve. Approximately three of the 14min sample cycles were carried out for each dynamic blank: these were averaged to provide a
single background TOC concentration for the PILS and MC that carried forward until the next
dynamic blank measurements. These background TOC concentrations were subtracted from the
ambient PILS and MC measurements to calculate the WSOC concentrations. The standard
deviation of the dynamic blank measurements was also used to quantify method limits of
detection (LOD), using a 3σ LOD definition. The LODs were quantified based on the liquid
TOC levels which were converted into ambient air LOD levels using the PILS and MC sample
flow rates.
4
A critical aspect of this experiment was the use of the silica gel dryer in the ‘dried’ PILS
sample path. The relative humidity downstream of the dryer was not monitored in real time
during ambient sampling. Instead, a number of tests were performed to confirm the efficiency of
the dryer under different ambient conditions. The dryer was exchanged daily, and its continued
efficiency at the end of the 1-d sampling was confirmed with both color-indicator of the silica gel
and direct laboratory measurements of efficiency through an exchanged dryer after 24 hours of
use (Omega, RH-USB). Table S1 shows the results from laboratory testing of the dryer
efficiency during the September study.
Table S1: Comparison of ambient RH and RH sampled through the silica gel dryer in laboratory
testing.
RH-through dryer
Ambient RH (%)
Mean ± 1σ (%)
66
46.0 ± 0.7
73
42.3 ±1.9
81
42.5 ± 0.5
90
42.2 ± 1.2
5
S2 - Results
Figure S2: Nighttime a) ΔWSOCp/ΔCO vs. Temperature and b) ΔWSOCg/ΔCO vs.
Temperature. Grey circles represent individual nighttime measurements, while box and whiskers
6
represent median (red dot), 25th and 75th percentiles (lower and upper boxes), and 5th and 95th
percentiles (lower and upper whiskers) for each Temperature range. Numbers at the top/bottom
of the graphs represent the number of individual measurements within each Temperature bin.
Figure S3: Daytime relationship between Fp and relative humidity (Fp = the fraction of WSOC
in the particle phase: Fp = WSOCp / (WSOCp +WSOCg)). Grey circles represent individual
daytime measurements, while box and whiskers represent median (red dot), 25th and 75th
percentiles (lower and upper boxes), and 5th and 95th percentiles (lower and upper whiskers) for
each RH range. Numbers at the bottom of the graph represent the number of individual
measurements within each RH bin.
7
Figure S4: Examples of individual periods where simultaneous increases in Fp and RH were
observed. A total of 5 examples are shown (including Figure 2b), but at least 10 such events
were encountered during the September 2014 measurement period.
Table S2: Summary statistics of online ambient measurements in Baltimore.
Median
Mean
Standard Deviation
WSOCg*
4.04
4.51
2.35
*
WSOCp
1.17
1.25
0.59
WSOCp,dry*
1.19
1.29
0.58
*
OC
1.61
1.91
1.05
EC*
0.32
0.39
0.30
RH (%)
65.7
65.9
13.7
Temperature (°C)
19.5
19.9
3.9
**
CO
0.50
0.50
0.11
*Units in µg-C m-3 **Units in ppm
8
Range
0.88- 15.86
0.24 - 3.72
0.30 - 3.68
0.52 – 12.40
0.05 – 2.50
32.6 – 92.5
10.2 – 31.9
0.04 – 1.89
Table S3: Statistical comparison of WSOCp and WSOCp,dry measurements for daytime periods at
different RH levels.
a
WSOCp
WSOCp,dry
Difference
Average ± 1σ
Average ± 1σ
Test Statistic
statistically
RH bin
(µg-C m-3)
(µg-C m-3)
n
(T value)
significant?
> 80%
1.26 ± 0.71
1.33 ± 0.71
126
0.69
No
70-80%
1.44 ± 0.63
1.48 ± 0.62
173
0.66
No
60-70%
1.42 ± 0.56
1.47 ± 0.54
264
0.90
No
< 60%
1.33 ± 0.56
1.37 ± 0.56
578
1.17
No
All
1.36 ± 0.59
1.40 ± 0.58
1141
1.75
No
a
Based on two-sample t-test for comparing means with different variances. If the value of the
computed t statistic is less than the critical t-value, then the WSOCp and WSOCp,dry means are
not statistically different (null hypothesis is accepted). At the 95% confidence level (α = 0.025
for two-tailed test), the critical t-value is 1.960 for n > 120 [Devore, 1995].
Table S4: Statistical comparison of WSOCp and WSOCp,dry measurements for nighttime periods
at different RH levels
a
WSOCp
WSOCp,dry
Difference
Average ± 1σ
Average ± 1σ
Test Statistic
statistically
RH bin
(µg-C m-3)
(µg-C m-3)
n
(T value)
significant?
> 80%
1.45 ± 0.85
1.51 ± 0.85
236
0.79
No
70-80%
1.14 ± 0.42
1.19 ± 0.41
341
1.44
No
60-70%
1.02 ± 0.41
1.05 ± 0.39
396
1.13
No
< 60%
1.02 ± 0.39
1.04 ± 0.39
155
0.33
No
All
1.15 ± 0.56
1.19 ± 0.55
1128
1.71
No
a
Based on two-sample t-test for comparing means with different variances. If the value of the
computed t statistic is less than the critical t-value, then the WSOCp and WSOCp,dry means are
not statistically different (null hypothesis is accepted). At the 95% confidence level (α = 0.025
for two-tailed test), the critical t-value is 1.960 for n > 120 [Devore, 1995].
9
S3 - References:
Devore, J. L. (1995), Probability and Statistics for Engineering and the Sciences – 4th Ed.,
Wadsworth, Inc., Belmont, CA.
Hennigan, C. J., M. H. Bergin, J. E. Dibb, and R. J. Weber (2008), Enhanced secondary organic
aerosol formation due to water uptake by fine particles, Geophys. Res. Lett., 35(18),
doi:10.1029/2008gl035046.
Sullivan, A. P., R. E. Peltier, C. A. Brock, J. A. de Gouw, J. S. Holloway, C. Warneke, A. G.
Wollny, and R. J. Weber (2006), Airborne measurements of carbonaceous aerosol soluble
in water over northeastern United States: Method development and an investigation into
water-soluble organic carbon sources, J. Geophys. Res., 111(D23),
doi:10.1029/2006JD007072.
Sullivan, A. P., R. J. Weber, A. L. Clements, J. R. Turner, M. S. Bae, and J. J. Schauer (2004), A
method for on-line measurement of water-soluble organic carbon in ambient aerosol
particles: Results from an urban site, Geophys. Res. Lett., 31, L13105,
doi:10.1029/2004GL019681.
10