Introduction
Mercury (Hg) has been classified as a persistent, bioaccumulative toxin (PBT)
(UNEP, 2013), and deposition from the atmosphere is considered the dominant
pathway by which Hg enters remote ecosystems (Lindberg et al., 2007). In some
areas, scavenging by precipitation controls atmospheric Hg removal processes,
such as in the southeastern United States of America (USA), where
precipitation amounts are high (Prestbo and Gay, 2009). However, wet
deposition concentrations are not necessarily correlated with precipitation
amounts > 81 mm, and deposition has not decreased with emission
reductions as coal combustion facilities in the region have implemented
control technologies (Prestbo and Gay, 2009; MDN, 2014). For example,
fluxes at Naval Air Station Pensacola Outlying Landing Field (OLF) in Florida were 17.1 µg m-2 in 2012 and 21.0 µg m-2 in 2014 (MDN,
2014). A contributing factor to wet deposition in the Gulf Coast area may be
related to high atmospheric convection during thunderstorms and scavenging of
gaseous oxidized Hg (GOM) from the free troposphere (Nair et al., 2013), as
well as down-mixing of air with high GOM from the free troposphere (Gustin et al.,
2012).
An additional concern is that the Tekran®
system measurement currently used to quantify GOM does not equally quantify
all GOM forms, and has interferences with water vapor and ozone (cf. Ambrose
et al., 2013; Gustin et al., 2013; Huang et al., 2013; Lyman et al., 2010,
2016; McClure et al., 2014). Since GOM is considered an important form that
can be rapidly removed from the atmosphere due to high water solubility
(Lindberg et al., 2007), it is important to understand both atmospheric
concentrations and chemistry (i.e., specific chemical compounds). Use of the
University of Nevada Reno Reactive Mercury Active System (UNRRMAS) that
collects GOM on nylon membranes in tandem with cation exchange membranes has
indicated that there are different chemical compounds in the air and
concentrations are 2 to 13 times higher than previously thought at locations
in the western USA (Huang et al., 2013; Gustin et al., 2016).
Mercury has been studied in Florida for many years, initially because of the
high concentrations measured in fish and Florida panthers (Dvonch et al.,
1999; Gustin et al., 2012; Marsik et al., 2007; Pancras et al., 2011;
Peterson et al., 2012). Long-term GEM and GOM concentrations as measured by
the Tekran® system have declined; however,
PBM concentrations increased after 2009 (Edgerton, unpublished data),
suggesting the atmospheric chemistry has changed. Peterson et al. (2012) and
Gustin et al. (2012) suggested, based on detailed assessment of passive
sampler and Tekran® system collected Hg
data, criteria air pollutants, and meteorology, that at three locations in Florida
(OLF, Davie, and Tampa) different GOM compounds were present, and these were
generated by in situ oxidation associated with pollutants generated by mobile
sources, indirect and direct inputs of Hg from local electricity generating
plants, and direct input of Hg associated with long-range transport. At OLF,
background deposition was equal to that associated with mobile sources, and a
significant component was derived from long-range transport in the spring.
Long-range transport has been reported for OLF in the spring (Weiss-Penzias
et al., 2011; Gustin et al., 2012). Long-range transport of ozone is a very
common event in the spring in the western United States (see special issues on ozone: Gertler and Bennett,
2015; Lefohn and Cooper, 2015).
In this work, GOM collected using the UNRRMAS and the Aerohead dry-deposition measurement method (Lyman et al., 2007, 2009) were analyzed along
with Tekran® Hg and criteria air pollutant
data to understand GOM chemistry and dry deposition at OLF, located ∼ 15 km NW of Pensacola, Florida.
Sampling site and point sources (NEI 2011) map. Cluster trajectories
for daytime (11:00–13:00) and nighttime (01:00–03:00). The x and y axes represent longitude and latitude.
GOM dry-deposition fluxes were calculated using deposition velocities
determined using a multi-resistance model with ambient air GOM concentrations
from the Tekran® system (multiplied by a
factor of 3 due to bias in the Tekran®
system; cf. Huang and Gustin, 2015), and compared to those obtained using
Aerohead data. The chemistry of GOM compounds was identified. Results were used
to estimate dry-deposition velocities for the GOM compounds observed. The
hypothesis for this work was that since GOM compounds can vary spatially and
temporally, due to different compounds produced by different sources and
processes, this will result in different dry-deposition velocities and dry-deposition fluxes.
Methods
Field site
The sampling site was located at OLF (30.550∘ N, 87.374∘ W,
44 m a.s.l. – above sea level). The closest major Hg emission source is a
coal-fired power plant (Plant Crist) northeast of the site (Fig. 1). This
area has been used for atmospheric Hg research in previous studies (Caffrey
et al., 2010; Lyman et al., 2009; Gustin et al., 2012; Peterson et al., 2012;
Weiss-Penzias et al., 2011). OLF is a coastal site (∼ 25 km away from
Gulf of Mexico) influenced by sea breezes especially during the summer
(Gustin et al., 2012). Based on cluster analyses of data from 1 year at
this location, ∼ 24 % of the air is derived from the marine
boundary layer during the day and 60 % at night (Fig. 1).
Sampling methods
Aerohead samplers for determination of dry deposition were deployed bi-weekly
from June 2012 to March 2014. UNRRAMS samples were taken bi-weekly from
March 2013 to March 2014. Atmospheric Hg concentrations, including GEM, GOM,
and PBM, were measured using a Tekran®
system (model 2357/1130/1135, Tekran®
Instrument Corp., Ontario, Canada) that was operated with 1 h sampling and
1 h desorption with detection limits of 0.1 ng m-3,
1.5 pg m-3, and 1.5 pg m-3,
respectively.
Reactive Hg (GOM + PBM) concentrations were measured using the UNRRAMS
with three sets of two in-series 47 mm cation-exchange membranes (ICE450, Pall
Corp., MI, USA). Three sets of nylon membranes (0.2 µm,
Cole-Parmer, IL, USA) were also deployed to assess Hg compounds in the air
(see Pierce and Gustin, 2017, for schematic). Cation exchange membranes have
been demonstrated to quantitatively measure specific compounds of GOM in the
laboratory; however, they may not measure all compounds (Gustin et al., 2015, 2016). These membranes have also been shown to retain
compounds loaded for 3 weeks (Pierce and Gustin, 2017). Nylon membranes do
not retain GOM compounds quantitatively, and retention during transport needs
to be tested (Huang et al., 2013; Gustin et al., 2015, 2016). Nylon membrane
retention is impacted by relative humidity that might limit uptake of
specific forms. Criteria air pollutants and meteorological data, including
CO, SO2, O3, PM2.5, NO, NO2, NOy, temperature,
relative humidity, wind speed, wind direction, pressure, solar radiation, and
precipitation were available at this site for the sampling period. See
Peterson et al. (2012) for detailed information on collection of these
measurements.
Aeroheads and membranes were prepared at UNR, packed in a thermal isolated
cooler, and shipped back and forth between the laboratory and site. Samples
were stored in a freezer (-22 ∘C) at UNR until analyzed.
Cation-exchange membranes were digested and analyzed following EPA Method
1631 E (Peterson et al., 2012), and nylon membranes were first thermally
desorbed, and then analyzed using EPA Method 1631 E (Huang et al., 2013).
Cation-exchange membrane blanks for Aerohead and UNRRAMS were
0.40 ± 0.18 (n=42), 0.37 ± 0.26 (n=77) ng, respectively, and
for nylon membranes used in the active system blanks were 0.03 ± 0.03
(n=69) ng. Therefore, method detection limits (MDLs, 3σ) for a 2-week
sampling time (336 h) was 0.13 ng m-2 h-1 for dry deposition. For the active membrane system, the Hg amount on the back-up
filters and blanks were not significantly different (cation-exchange
membrane: 0.4 ± 0.3 vs. 0.4 ± 0.3 ng; nylon membrane:
0.03 ± 0.03 vs. 0.02 ± 0.02 ng); therefore, the back-up filters
were included in the calculation of the bi-weekly blanks. The bi-weekly MDLs
(336 h) for active systems with cation-exchange and nylon membranes were
2–68 pg m-3 (mean: 24 pg m-3) and
0.01–14.6 pg m-3 (mean: 2.1 pg m-3), respectively. Bi-weekly
MDL was calculated from 3 times the standard deviation of bi-weekly blanks.
The MDL was calculated for each period of sampling, due to the fact this can
vary based on treatment of the membranes, the time samples are prepared for
deployment, deployment at the field site, and handling once returned to the
laboratory. The membranes may also vary by material lot. All samples were
corrected by subtracting the blank for the corresponding 2-week period.
Data analyses
Hourly Tekran®, criteria air pollutants,
and meteorological data were managed and validated by Atmospheric Research
& Analysis, Inc (see Peterson et al., 2012). These were then averaged into
2-week intervals to merge with the membrane measurements.
Overall seasonal average of criteria air pollutants, GEM, PBM, GOM
(measured using three different methods) concentration, GOM dry deposition
(DD), and meteorological data at OLF.
2012
2013
2014
Summer
Fall
Winter
Spring
Summer
Fall
Winter
March
Ozone [ppb]
30 ± 15
30 ± 12
29 ± 11
38 ± 12
24 ± 12
26 ± 11
27 ± 10
35 ± 12
CO [ppb]
143 ± 38
161 ± 35
167 ± 41
165 ± 36
139 ± 35
156 ± 33
167 ± 35
183 ± 33
SO2 [ppb]
0.3 ± 0.4
0.6 ± 1.5
0.4 ± 0.5
0.3 ± 0.5
0.2 ± 0.3
0.4 ± 0.5
0.7 ± 1.2
0.3 ± 0.4
NO [ppb]
0.3 ± 0.7
0.3 ± 0.7
0.3 ± 0.8
0.2 ± 0.5
0.3 ± 0.7
0.3 ± 0.8
0.4 ± 0.8
0.2 ± 0.5
NO2 [ppb]
2.4 ± 2.4
3.0 ± 2.7
3.0 ± 3.1
2.0 ± 2.3
2.2 ± 2.1
3.1 ± 2.9
3.2 ± 3.0
2.3 ± 2.8
NOy [ppb]
3.6 ± 2.9
4.3 ± 3.1
4.3 ± 3.6
3.1 ± 2.8
3.2 ± 2.5
4.4 ± 3.3
4.2 ± 3.4
3.6 ± 3.1
GEM [ng m-3]a
1.2 ± 0.1
1.2 ± 0.1
1.3 ± 0.1
1.2 ± 0.2
1.1 ± 0.1
1.0 ± 0.1
1.2 ± 0.3
1.2 ± 0.1
GOM [pg m-3]a
0.6 ± 1.3
1.1 ± 2.8
1.0 ± 2.2
2.9 ± 5.1
0.5 ± 1.0
1.1 ± 2.1
1.3 ± 2.5
2.0 ± 3.6
PBM [pg m-3]a
2.4 ± 2.6
3.6 ± 3.8
7.3 ± 8.7
5.9 ± 6.8
2.3 ± 2.0
2.9 ± 2.3
4.9 ± 5.3
4.0 ± 3.4
GOM [pg m-3]b
–
–
–
43 ± 110
24 ± 57
14 ± 18
17 ± 23
24 ± 15
GOM [pg m-3]c
–
–
–
4 ± 10
0.4 ± 1.3
1.2 ± 1.1
0.6 ± 0.6
0.6 ± 0.5
GOM DD [ng m-2 h-1]
0.24 ± 0.20
0.17 ± 0.12
0.15 ± 0.06
0.40 ± 0.23
0.20 ± 0.13
0.13 ± 0.18
0.20 ± 0.50
0.14 ± 0.04
WS [m s-1]
2.1 ± 1.2
2.1 ± 1.0
2.8 ± 1.7
2.9 ± 1.8
2.0 ± 1.1
2.1 ± 1.1
2.5 ± 1.3
2.5 ± 1.5
TEMP [∘C]
26 ± 3
19 ± 6
14 ± 6
18 ± 6
26 ± 3
20 ± 7
11 ± 7
14 ± 5
RH [%]
83 ± 14
76 ± 18
79 ± 19
73 ± 21
84 ± 13
77 ± 17
76 ± 23
78 ± 21
SR [W m-2]
230 ± 302
193 ± 271
121 ± 199
266 ± 304
210 ± 278
175 ± 255
129 ± 212
182 ± 278
Precipitation [mm]
637
186
385
223
1010
254
357
183
a Tekran® data.
b Cation-exchange membrane data.
c Nylon membrane data.
In previous studies, scaling factors similar to HNO3 (α=β=10) were used to calculate oxidized Hg dry-deposition velocity (Marsik et
al., 2007; Castro et al., 2012); however, Lyman et al. (2007) used the
effective Henry's Law constant, and half-redox reactions in neutral solutions
of HgCl2, and indicated HONO might better represent the chemical
properties of oxidized Hg rather than HNO3. Huang and Gustin (2015a)
indicated that due to limited understanding of oxidized Hg chemical
properties, no single value can be used to calculate oxidized Hg dry
deposition, because α and β would change with different GOM
compounds. Here, dry deposition was calculated using the multiple resistance
model of Lyman et al. (2007) using both α=β=2, 5, 7, and 10.
Back trajectories were calculated using the Hybrid Single Particle Lagrangian
Integrated Trajectory (HYSPLIT 4.9) model with Eta Data Assimilation System (EDAS) 40 km, 1000 m starting
height. For day and nighttime analyses, starting times were local standard
time (LST) 11:00–13:00 and 100–300 h, 72 h simulations. For a
high-concentration event analyses, trajectories were started for each day at
00:00, 04:00, 08:00, 12:00, 16:00, and 20:00 LST. Overall, the uncertainties
of back trajectories calculated from HYSPLIT are 20 % of the air-parcel
traveling distance (Draxler, 2013; Gebhart et al., 2005; Stohl, 1998; Stohl
et al., 2003). Back trajectories for the entire sampling time were analyzed
using cluster analysis (Liu et al., 2010).
Sigmaplot 14.0 (Systat Software Inc, San Jose, CA, USA), and Minitab 16.0
(Minitab Inc., PA, US) were used to do t tests and correlation analyses.
Comparisons were considered significantly different and correlations were
considered significant when p < 0.05.
Results and discussion
Overall measurements
Similar to previous work at this location (Gustin et al., 2012), O3 was
highest in the spring. CO concentrations were high in winter, due to a low
boundary layer and biomass burning, and low in summer (Table 1). Observations
from the three GOM sampling methods (Tekran®,
and nylon and cation exchange membranes) showed higher GOM concentrations in
spring relative to other seasons (Table 1). Concentrations of GOM measured by
cation-exchange membranes in the active system were significantly (p value
< 0.05, paired-t test) higher than those measured by
Tekran® KCl-coated denuder and nylon
membranes, both of which have been reported to be influenced by relative
humidity (Huang and Gustin, 2015b; Gustin et al., 2015). Mean cation-exchange
membrane concentrations were higher than
Tekran®-derived GOM by 14, 48, 11, and 13
times in the spring, summer, fall, and winter, respectively.
Nylon membranes collected higher GOM concentrations than those measured by
the Tekran® in spring 2013 when the
humidity was low. Overall, air concentrations measured by the
Tekran® system in this study were similar
to those measured at OLF in 2010 (Peterson et al., 2012). Particulate-bound
Hg had the same seasonal trend as GOM, but higher concentrations.
Understanding the oxidants present in air is important for understanding
potential GOM compounds. Oxidants to consider include O3, halogenated
compounds, and sulfur and nitrogen compounds (cf. Gustin et al., 2016).
Since the active system is currently limited to a 2-week sampling period,
they are useful for understanding the specific compounds that might be
present, and this in turn can be used to understand sources.
Potential GOM compounds
Standard desorption profiles for GOM compounds obtained by Huang et
al. (2013) and Gustin et al. (2015) are compared to those obtained at OLF
(Fig. 2). Compounds used as standards included HgBr2,
HgCl2, HgN2O6 ⚫ H2O, HgSO4, and HgO.
HgCl2 and HgBr2 have been identified as being released from
permeation tubes (Lyman et al., 2016); however, the exact N and S compounds
are not known. During 10 periods the nylon membranes (collected in
triplicate) collected a significant amount of GOM based on their bi-weekly
detection limit (Fig. 2), and the desorption profiles varied. Although data
are limited, we have observed similar thermal desorption compounds in other
studies (i.e., Huang et al., 2013; Gustin et al., 2016). For example, in the
marine boundary layer in Santa Cruz, California, based on the additional
curves in Gustin et al. (2015), Hg–nitrogen and sulfur compounds were
observed. At the Reno Atmospheric Mercury Intercomparison Experiment
(RAMIX) site, Nevada, Huang et al. (2013) reported
HgBr2 / HgCl2 compounds; this is due to free troposphere inputs
at this site (Gustin et al., 2013). At a highway-impacted site Huang et
al. (2013) reported similar patterns to that in Gustin et al. (2016) that
included Hg–nitrogen and sulfur compounds, and unknown compounds that
generated a high residual tail in the profile. This indicates that similar
chemical forms are being collected, and is supported by work described below,
and that the compounds are not being generated on the membranes. Lack of
generation on membranes has also been shown to be the case in a limited study
(Pierce and Gustin, 2017). In addition to our work, HgBr2 and HgCl2
were reported to occur in Montreal, Canada (Deeds et al., 2015).
Desorption profiles from nylon membranes with standard materials
in laboratory investigation (top) and field measurements. Each whisker is 1
standard variation, and only present in the desorption peak. Note the
Hg–nitrogen compound in the permeation tube was HgN2O6 ⚫ H2O. The y axis indicates the percentage released at each temperature.
Seven distinct patterns of release were observed from membranes collected at
OLF during thermal desorption. One had a high residual tail that does not
match our standard profiles; however, this was also observed in Nevada (Gustin et
al., 2016). These occurred on 2 and 9 April and 21 May 2013. This suggests that in spring there
is a compound that is unknown based on current standard profiles. Based on
our methylmercury profile, generated using methylmercury added as a liquid to
membranes and presented in Gustin et al. (2015), it is possible this could be
some organic compound. A nitrogen-based compound was found on 21 May 2013
based on the desorption profile. A pattern occurred on 19 March and
19 November 2013, and this corresponded to HgBr2 / HgCl2 with
some residual tail that is again some compound not accounted for.
Patterns observed on 7 May and 27 August 2013 corresponded to a Hg–nitrogen-based compound with a residual tail. The fifth pattern occurred on
14 January 2014, and 24 September 2013 was associated with HgSO4 and the
error bars are small. Data collected on 22 October 2013
was noisy and had subtle peaks that correspond with HgO, a nitrogen-based
compound, and a high residual tail. It is interesting to note that the
19 November 2013 profile was similar to
HgCl2.
Previous studies reported consistent desorption profiles from three sites in
Nevada and California without significant point sources (Huang et al., 2013).
Huang et al. (2013) presented desorption profiles from a highway,
agriculture, and marine boundary layer site. Profiles from the marine
boundary layer and agriculture-impacted site did not show clear residual
tails at 185 ∘C, but these were observed at the highway-impacted
site. At OLF, a significant amount of GOM (15–30 %) was released after
160 ∘C. This and previous work implies that we are missing one or
more GOM compound(s) (Fig. 2) in our permeation profiles. Interestingly, a
peak was found in the 9 April 2013 sample at the GEM release temperature;
this is not due to GEM absorption as demonstrated by Huang et al. (2013), and
was also observed in Nevada (Gustin et al., 2016), suggesting an additional
unidentified compound. This information indicates GOM compounds at OLF varied
with time, and this variation is due to complicated Hg emission sources and
chemistry at this location (cf. Gustin et al., 2012).
At OLF, GOM composition on the nylon membrane was more complicated than that
collected at rural sites in the western USA (cf. Huang et al., 2013; Gustin
et al., 2016); however, similar complexity was observed at a highway location
in Reno, Nevada (Gustin et al., 2016). Desorption curves from the nylon
filters collected at rural locations in Nevada were in the range of the
standard GOM compounds that have been investigated (Huang et al., 2013;
Gustin et al., 2016). Curves with multiple peaks in this study imply that
there were at least seven GOM compounds collected on the nylon membranes.
Measured and modeled GOM dry-deposition fluxes.
Tekran® data (correction factor of 3)
were used with multiple resistance models (α=β=2 and 10).
Tentative GOM compounds were determined using the results from nylon
membrane desorption.
Dry-deposition measurements
Dry deposition of GOM measured by Aerohead samplers ranged from 0 to
0.5 ng m-2 h-1, and 83 % of GOM dry deposition was higher
than the detection limit (0.13 ng m-2 h-1). Higher GOM dry
deposition was observed in spring relative to winter (ANOVA one-way rank,
p value < 0.01); GOM dry deposition was slightly lower in summer and
fall (not statistically different) relative to the spring due to high wet
deposition and scavenging processes during these seasons. The pattern in GOM
seasonal dry deposition was similar to that reported by Peterson et
al. (2012). However, GOM dry-deposition rates were significantly higher in
this study than 2010 values (0.2 vs. 0.05 ng m-2 h-1). This is
due to the correction of 0.2 ng m-2 h-1 applied in Peterson et
al. (2012) to account for contamination of the Aerohead that has been
demonstrated to be unnecessary (Huang et al., 2014). Although the highest GOM
dry deposition measured using the Aerohead sampler and GOM concentrations
measured using the UNRRAMS were observed in spring 2013, the value in March
2014 was relatively low. In March 2014, atmospheric conditions were more
similar to winter than spring, with low temperatures and high CO
concentrations. These results are different from those calculated using
Tekran® measurements that suggest low GOM
concentrations and high deposition velocities, and this is because the
denuder measurements are biased low.
Modeled GOM dry-deposition fluxes were calculated using GOM concentrations
measured by the Tekran® system that were
multiplied by a factor of 3 (cf. Huang et al., 2014). In general,
measured Hg dry-deposition fluxes were similar to those modeled simulations
with GOM dry deposition α=β=2 during winter, spring, and fall
(see below; Fig. 3). Measured Hg dry deposition was significantly higher than
modeled results (both α=β=2 and 10) in summer and early fall
(Fig. 3). This indicates that there are compounds of GOM in the summer that are
poorly collected by the denuder, and this also can help explain the higher
wet deposition measured during this season (Prestbo and Gay, 2009). Highest
deposition was measured during the spring, when the input from long-range
transport is greatest (Gustin et al., 2012). Figure 3 shows the disparity
that occurs by season, and compares model and measured values. For example in
spring a=b=10 significantly overestimates deposition, while in the summer
and early fall measured deposition is greater than modeled values.
Temporal variation of GOM concentrations (mean ± standard
deviation, bi-week average); outlined rectangle indicates a polluted event
with high Hg, CO, and ozone concentrations. Data are missing for 3 weeks
because it was not collected. Tekran® data are presented when > 75 % of
the data were available and membrane data are shown when above the method
detection limit.
Modeled (multiple-resistance model) and measured (surrogate
surfaces) GOM dry deposition (ng m-2 h-1); GOM concentrations used
to calculate for modeled results are from the
Tekran® data and corrected by compounds'
corresponding ratios from Gustin et al. (2015, 2016). Model resistance for the unknown compound was calculated using the Tekran® data multiplied by 3. The tentative GOM compounds are
identified from nylon membrane results.
Start date
Tentative GOM
Measured GOM
Modeled GOM
Modeled GOM
Modeled GOM
Modeled GOM
compound
dry deposition
dry deposition
dry deposition
dry deposition
dry deposition
flux
α=β=2
α=β=5
α=β=7
α=β=10
12 Mar 2013
HgBr2
0.50 ± 0.06
0.34
0.49
0.54
0.58
26 Mar 2013
unknown
0.40 ± 0.11
0.34
0.47
0.52
0.56
30 Apr 2013
Hg(NO3)2
0.50 ± 0.13
1.21
1.67
1.81
1.95
14 May 2013
Hg(NO3)2
0.40 ± 0.09
1.19
1.69
1.88
2.07
20 Aug 2013
Hg(NO3)2
0.15 ± 0.07
0.10
0.14
0.16
0.17
12 Nov 2013
HgCl2
0.08 ± 0.03
0.11
0.16
0.17
0.19
7 Jan 2014
HgSO4
0.19 ± 0.03
0.18
0.24
0.27
0.29
Because of the low GOM concentrations and influence of humidity on the nylon
membrane measurements (Huang and Gustin, 2015b), GOM compounds were
identified only in one summertime sample as
HgN2O6 ⚫ H2O. During this time, measured GOM dry
deposition was ∼ 6 times higher than both modeled results, and
considering the Tekran® correction factor
of 3, membrane-based HgN2O6 ⚫ H2O dry-deposition
flux was ∼ 18 times higher than the
Tekran®-model-based value. Gustin et
al. (2015) indicated HgN2O6 ⚫ H2O collection
efficiency on cation-exchange membrane in charcoal scrubbed air was
∼ 12.6 times higher than on the Tekran® KCl-coated denuder.
However, in May 2013, two samples were dominated by a profile similar to the
Hg nitrogen-based compound with lower measured-to-modeled ratios (2.1–6.0 with
Tekran® correction factor). This might be
due to ambient air GOM chemistry being dominated by a compound with a
different dry-deposition velocity, less interference on the denuder surface,
or parameters in the dry-deposition scheme. In May, GOM concentrations
measured by the Tekran® were higher than in
summer due to lower wet deposition and mean humidity (Table 1). Therefore,
despite the fact that GOM collection efficiency associated with the Tekran®
and nylon membranes are impacted by environmental conditions, this
demonstrates the presence of different compounds in the air. The dry-deposition scheme needs Henry's Law constants for determining the scaling
factors for specific resistances for different compounds (Lyman et al., 2007;
Zhang et al., 2002).
Lin et al. (2006) stated that the dry-deposition velocity of HgO is 2-times
higher than that for HgCl2, due to the different Henry's Law constant.
The Henry's Law constants for HgCl2, HgBr2, and HgO presented in
previous literature (Schroeder and Munthe, 1998) have high uncertainty, for
how these calculations were done is not clear (S. Lyman, Utah State
University, personal communication, 2015), and the constants for
HgN2O6 ⚫ H2O and HgSO4 are unknown. Some
researchers considered that GOM is similar to HNO3(α=β=10),
and some treated GOM as HONO (α=β=2) (Castro et al., 2012; Lyman
et al., 2007; Marsik et al., 2007); however, using the parameters of
HNO3 could overestimate GOM dry-deposition velocities due to the
differences of effective Henry's law constants (HgCl2:
∼ 106 HNO3: ∼ 1013 M atm-1).
If the ratios (HgBr2 : 1.6, HgCl2 : 2.4, HgSO4 : 2.3,
HgO : 3.7, and HgN2O6 ⚫ H2O : 12.6) of GOM
concentrations measured by the Tekran®
vs. cation-exchange membranes for different GOM-permeated compounds
(Gustin et al., 2015; Huang et al., 2013) are used to correct
Tekran® GOM data in this study, modeled GOM
dry deposition (Fig. 3) is not correlated with measurements. For example, on
9 March and 19 November 2013 (Fig. 3), GOM was dominated by HgBr2 and
HgCl2. Dry deposition of HgBr2 from Aerohead measurements and
modeling were close to α=β=10; however, modeled and measured
HgCl2 dry deposition were matched as α=β=2. Average
deposition velocity for α=β=2 was 0.78 cm s-1, and for
α=β=10 is 1.59 cm s-1, if we assume the model is right.
There were three samples that were identified as Hg–nitrogen-based compounds
using nylon membranes; however, the ratios of measurement and modeling
HgN2O6 ⚫ H2O dry deposition were inconsistent over
time. In spring, all modeled HgN2O6 ⚫ H2O dry-deposition values were much higher than measured values; however, in summer,
measured and modeled HgN2O6 ⚫ H2O dry deposition
were similar as α=β=5 (Table 2). If you assume the dry-deposition measurements made by the surrogate surfaces are accurate then this
demonstrates that there are different forms that occur over time, and these will
have different deposition velocities, as suggested by Peterson et al. (2012).
Elevated pollution event
In spring 2013, there was a time period when concentrations of O3,
CO, and all Hg measured were high (Fig. 4). Figure 5 shows that during this time, air
masses traveled west to east across the continent. The air movement pattern
is similar to that found in Gustin et al. (2012) for OLF Class 2 events which
had low SO2 concentrations. During this 4-week period, air parcels
traveling to OLF were in the free troposphere and descended to the surface
(Fig. 5). Although there are coal-fired power plants in the upwind area
within a 500 km range (Fig. 1), the low SO2 concentrations and
elevated CO, O3, and GOM values were not from fossil fuel combustion.
Gustin et al. (2012) also indicated that free troposphere air impacted OLF.
The first few endpoints for these trajectories indicate that air parcels entered
North America at > 1000 m a.g.l.; therefore, there was transport of some
air measured during this time from the free troposphere. Ozone concentrations
were also similar to those measured in Nevada in the free troposphere at this
time (Gustin et al., 2014). It is important to note that the back
trajectories are only for 72 h and the ones that subsided to surface levels
in the midwestern USA were traveling fast. This is a common event in the spring that
represents free troposphere and/or stratosphere transport into the western USA and Florida (Gertler and Bennett, 2015; Lefohn and Cooper, 2015;
Weiss-Penzias et al., 2011; Gustin et al., 2012).
Results of gridded frequency distribution (top panel); light color
indicates less endpoints in a grid. Altitude of 72 h trajectories (bottom
panel) during the polluted event (12 March–2 April 2013); light color of
dots on left panel represents low altitude.
The chemical composition of this event suggests potential input from Asia, as
previously suggested, for three locations in Florida in the spring by Gustin
et al. (2012). During this time, based on thermal desorption profiles,
HgBr2 was measured initially and then the following profiles obtained
showed a gradual increase in GOM with increasing temperature with a high
residual tail. This would suggest initial subsidence of air from the
stratosphere and/or troposphere (cf. Lyman and Jaffe, 2012) followed by a mixture of
polluted air as observed in the western USA (cf. VanCuren and Gustin, 2015).