Aerodynamic gradient measurements of the air–surface exchange of gaseous
elemental mercury (GEM) were undertaken over a 40 ha alpine grassland in
Australia's Snowy Mountains region across a 3-week period during the late
austral summer. Bi-directional GEM fluxes were observed throughout the study,
with overall mean value of 0.2 ± 14.5 ng m-2 h-1 and mean
nocturnal fluxes of -1.5 ± 7.8 ng m-2 h-1 compared to diurnal
fluxes of 1.8 ± 18.6 ng m-2 h-1. Deposition velocities ranged
from -2.2 to 2.9 cm s-1, whilst ambient GEM concentrations
throughout the study were 0.59 ± 0.10 ng m-3. Cumulative GEM fluxes
correlated well with 24 h running mean soil temperatures, and one
precipitation event was shown to have a positive impact on diurnal emission
fluxes. The underlying vegetation had largely senesced and showed little
stomatal control on fluxes. Nocturnal atmospheric mercury depletion events
(NAMDEs) were observed concomitant with O3 depletion and dew formation
under shallow, stable nocturnal boundary layers. A mass balance box model was
able to reproduce ambient GEM concentration patterns during NAMDE and
non-NAMDE nights without invoking chemical oxidation of GEM throughout the
column, indicating a significant role of surface processes controlling
deposition in these events. Surface deposition was enhanced under NAMDE
nights, though uptake to dew likely represents less than one-fifth of this
enhanced deposition. Instead, enhancement of the surface GEM gradient as a
result of oxidation at the surface in the presence of dew is hypothesised to
be responsible for a large portion of GEM depletion during these particular
events. GEM emission pulses following nights with significant deposition
provide evidence for the prompt recycling of 17 % of deposited mercury, with
the remaining portion retained in surface sinks. The long-term impacts of any
sinks are however likely to be minimal, as cumulative GEM flux across the
study period was close to zero.
Introduction
International support for a legally binding agreement on the control of
mercury in the environment, beginning in 2003, recently culminated in the
2013 United Nations Environmental Programme (UNEP) Minamata Convention on
Mercury . This convention includes provisions for
emissions reductions; technology sharing; public awareness; and enhanced
mercury monitoring in human populations, wildlife and the environment. Such
monitoring is essential in assessing the efficacy of emissions reduction
measures and improving the understanding of the dynamics of the global
mercury cycle . These dynamics – including methylation,
oxidation/reduction, dry/wet deposition and emission/re-emission – all have
implications for the transport and environmental legacy of previously emitted
mercury . The burden of this legacy is likely to be
felt to varying degrees by all nations as, due to its long atmospheric
lifetime, mercury is known to be distributed across hemispheric scales
, and environmental reactions to changes
in atmospheric loading may take place on the order of years to centuries
. These concerns point to the
necessity of an enhanced research effort to better understand the
complexities of mercury biogeochmical cycling .
Although atmospheric mercury is known to be transported across hemispheric
scales, it is understood that it can undergo deposition to, and rapid
re-emission from, terrestrial surfaces – a process referred to as prompt
recycling . The impact of this process has
been estimated to contribute 256–1400 Mg a-1 to worldwide emission
of mercury from terrestrial surfaces, based on 20 % prompt re-emission of
deposited mercury .
Evidence for this process came from isotopic measurements
, where newly deposited mercury was
found to be more susceptible to re-emission to the atmosphere on a scale of
days to months. Data from the Mercury Experiment to Assess Atmospheric Loadings in Canada and the US (METAALICUS) study suggest between 5 and 40 %
of mercury newly deposited over terrestrial surfaces is promptly recycled
. Further evidence has
since been compiled during observations taken during atmospheric mercury
depletion events (AMDEs), which are characterised by the rapid oxidation of
gaseous elemental mercury (GEM) to the more reactive gaseous oxidised mercury
(GOM) and subsequent deposition to the surface and references
within. Flux studies by and
suggested net GEM emission from the surface following
AMDEs, though others have seen very little subsequent GEM emission, or even
net deposition to the surface .
Further evidence for prompt recycling using surface GEM flux measurements is
difficult to establish, as it is generally not possible to distinguish
between mercury emitted from long-term stores and mercury re-emitted
following atmospheric deposition .
Despite the global nature of atmospheric mercury pollution both long-term
monitoring and air–surface exchange research of this powerful neurotoxicant
are weighted heavily towards industrialised countries in the Northern
Hemisphere . There is a lack of
atmospheric mercury data taken within the Southern Hemisphere and in
particular for Australia. Modelling efforts by suggest
that anthropogenic mercury emissions throughout Australia are around 10 to
20 Mg a-1, compared with 95 to 285 Mg a-1 emitted/re-emitted
from natural sources. As patterns of wet and dry deposition of mercury are
known to exhibit regional variability due to anthropogenic emissions
, Australia provides a valuable opportunity for
atmospheric mercury biogeochemical research as it is relatively unimpacted by
anthropogenic sources. Atmospheric dry-deposition rates across remote regions
of Australia were estimated by to range between 20 and
70 µg m-2 a-1, based on background GEM pool
concentrations of 1.2 ng m-3. Available empiricisms for such modelling
efforts however are derived from Northern Hemisphere observations that may
not apply to Australia's unique ecosystems . Furthermore, recent evidence has shown
that background atmospheric GEM pool concentrations in the Southern
Hemisphere are considerably lower than those in the Northern Hemisphere
, which may mean that delivery of atmospheric mercury to
ecosystems in the Southern Hemisphere is currently overestimated. A recent
review of mercury air–surface exchange research by
found only one Australian mercury flux study and
showed there was a general lack of flux data taken over vegetated surfaces
globally.
This study was undertaken over a grassland in the Snowy Mountains region of
Australia's alpine country, during the austral summer. The site was chosen as
it is the location of an ongoing OzFlux greenhouse gas net ecosystem exchange
site. The micrometeorological aerodynamic gradient method was employed to
obtain a high-time-resolution flux time series that did not disturb the
natural ecosystem studied. This study furthers our understanding of
Australian mercury cycling by providing additional background ambient and
air–surface exchange data, and a comparison with similar existing flux
datasets undertaken in alpine regions elsewhere. Further, we provide
additional data on the process of prompt recycling observed during this
study.
MethodsSite description
This study took place on Nimmo Plains (36∘12′57′′ S
148∘33′10′′ E, elevation 1340 m a.s.l.) in Australia's Snowy
Mountains region, bordering the eastern boundary of Kosciuszko National Park.
Measurements were taken between 28 January and 18 February (DOY 28 to 49).
Nimmo Plains is a ∼ 40 ha region of reasonably level terrain (within
20 m elevation change) and the site of an ongoing OzFlux ecosystem research
site. Vegetation immediately within the Kosciuszko National Park consists of
dry sclerophyll forests that extend for a distance of at least 5 km. On the
plain, vegetation consists predominantly of grasses of various
Poa spp. that had largely senesced prior to this study. These grasses had been
trampled and grazed by cattle to an average height of approximately 2 cm
prior to the study taking place. Rainfall over the preceding 3 months
(NDJ) was in the lowest 20th percentile since 1907, resulting in dry
conditions. The plain is located 23 km to the north and 51 km west of the
towns of Jindabyne (918 m a.s.l.; 2011 population: 1727) and Cooma
(800 m a.s.l.; 2011 population: 6301) . There are no
known significant natural or anthropogenic mercury sources in the region.
Substrate characterisation
Substrates were characterised using samples taken in each of the four
cardinal directions at distances 10, 30 and 50 m from the flux sampling
tower. Surface vegetation, along with soils at depths of 0 to 2, 5 to 10 and
10 to 20 cm were sampled using clean equipment and stored in double-sealed
plastic bags. Vegetation and soil samples were dried at 105 ∘C for
24 h, and soil samples were sieved into 2 to 19 mm (granules),
63 µm to 2 mm (sand) and < 63 µm (silt/clay) size
fractions. Total mercury (THg) contents of soil and vegetation samples were
determined in triplicate using a DMA-80 direct mercury analyser (Milestone
Inc., Shelton, CT, USA) and US EPA Method 7473. Instrument precision was
verified to within 5 % using five repetitions of 100 µL of a
100 ppb Hg standard between each run of 60 replicates. Accepted sample boat
blank concentrations were < 0.002 ng THg. Soil organic carbon (SOC) and
soil inorganic carbon (SIC) contents were determined according to the
loss-on-ignition method described by . SIC values were
multiplied by a correction factor of 1.36 under the assumption that it
existed predominantly as carbonate . Soil pH was
determined using a 1 : 5 soil / water suspension and CyberScan pH 300
(Eutech Instruments, Vernon Hills, IL, USA).
Flux instrumentation and methods
Eddy covariance flux measurements of sensible and latent heat, carbon dioxide
(CO2) and momentum were taken at a height of 2.42 m using a CSAT-3
sonic anemometer (Campbell Scientific, Logan, UT, USA) and LI-7200
closed-path, infrared gas analyser (Li-Cor Biosciences, Lincoln, NE, USA).
Samples were collected at 20 Hz using data capture software developed
in-house using LabVIEW (National Instruments, Austin, TX, USA).
Post-processing of eddy covariance fluxes and quality control were undertaken using
Li-Cor EddyPro 5.2.1. The convention of positive values representing fluxes
upward from the surface is used herein. The planar fit method of
was applied across the entire dataset in order to
rotate sonic anemometer data into mean wind coordinates, and ogive analyses
were prepared across the longest continuous measurement period (5.4 days)
according to in order to determine an appropriate flux
averaging period. Quality control flags were calculated for each averaging
period using the scheme of , and fluxes given a flag 2
were discarded from further analyses. Additionally, flux values beyond the
mean ±3 standard deviations were deemed outliers and removed. Tests for
significance were performed using Student's two-sample t test, and use of
the ± symbol hereafter signifies 1 standard deviation. Unless otherwise
stated, significance is assessed at p<0.05.
GEM fluxes were determined using the method and assumptions outlined in
. Flux calculations were undertaken according to
Eq. (), where C(zi) represents GEM concentration at
height zi, u* friction velocity, k=0.40 the von Kármán
constant, d the zero-plane displacement height and ψ(zi) the
integrated universal similarity functions as given by
.
F=ku*(C(z2)-C(z1))ln(z2-d)ln(z1-d)-(ψ(z2-ψ(z1))
GEM gradients were constructed from samples taken at two heights (z1=0.48 m and z2=0.73 m) and quantified using a Tekran 2537B (Tekran
Instruments, Knoxville, TN, USA), with detection limit of 0.01 ng m-3
and reference volumes reported at 1 atm and 0 ∘C. Density
corrections due to water vapour were undertaken according to .
Sample air was drawn from the sample inlets through a 0.2 µm
polytetrafluoroethylene (PTFE) filter and a single PTFE tube of length 14 m
by a PTFE pump drawing at 10 L min-1.
The 2537B sub-sampled from this flow through an additional 0.2 µm
PTFE filter. Switching of sampling between the intakes took place every
10 min (2 × 2537B samples) and was achieved using a PTFE solenoid
valve controlled by the LabVIEW software. The random error in the gradient
induced by the cumulative electronic and sampling delays following each
switch of the solenoid was calculated to be 3 %. Calibration of the 2537B
was undertaken 19 times throughout the study using the internal mercury
permeation source. Verification of the permeation source was undertaken in
the lab before and after the study using manual injection of mercury vapour
to within 2 %.
Ancillary data
In addition to GEM fluxes, ambient GEM (at 3.1 m height) was quantified with
a Tekran 2537A sampling at 1 L min-1 through an unheated 5 m PTFE
tube and 2 × 0.2 µm PTFE filters. Calibration of the 2537A
was undertaken every 23 h using the internal mercury permeation source,
verified in the same manner as the 2537B. Comparison of the two 2537
instruments in the lab showed good agreement (< 4 % systematic
difference) across a range of concentrations spanning 0 to 12 ng m-3.
Ambient ozone (O3) was sampled at the same height using a Thermo
Scientific 49i (Thermo Fisher Scientific, Waltham, MA, USA) sampling through
a separate, unheated 5 m PTFE tube. Incoming and outgoing solar and
terrestrial radiation were measured with a Campbell Scientific CNR1 net
radiometer positioned at 1.5 m. Soil temperature was measured 2 cm beneath
the surface with two Campbell Scientific TCAV temperature-averaging probes
and soil volumetric water content at a depth of 5 cm with a Campbell
Scientific CS615. Soil and radiation data were collected and stored using a
Campbell Scientific CR3000 data logger.
Back trajectories and mixed-layer heights (MLHs) were modelled using NOAA's
Single Particle Lagrangian Integrated Trajectory Model
(HYSPLIT; ; ; ) with GDAS
0.5∘ data as inputs. Aerodynamic (ga) and bulk canopy
(gc) conductances were calculated using
Eqs. () and (), by rearranging the
Penman–Monteith equation in the same manner as
.
Here Δ is the slope of the vapour pressure curve at temperature T,
γ the psychrometric constant, Rn net radiation, G soil
heat flux, λE latent heat flux, ρa air density,
cp specific heat of air at constant pressure, Da vapour
pressure deficit and z0 the roughness length; the subscripts m and v
represent momentum and water vapour, respectively.
We followed the method of and modelled dew depth on
the basis of the surface energy balance methodology described in
. For each time step i of length Δt, the
depth of dew (Di+1) was calculated using
Di+1=Di+EiΔt if Di+1≥0Di+1=0 if Di+EiΔt<0,
where E is the dew flux density calculated from Eqs. (4)–(7) in
, derived from the surface energy balance using
Penman's substitution. Surface temperatures were calculated from CNR1
outgoing longwave-radiation data using the Stefan–Boltzmann equation and an
emissivity of 0.95 .
A simple box model was employed in order to investigate the influence of
observed nocturnal GEM deposition. This model took MLH from HYSPLIT outputs
H and imposed a constant entrainment velocity w=0.005 m s-1
between the mixed layer and free troposphere. No chemical production or
destruction of GEM was incorporated in the model, as this would have led to
speculation regarding unknown concentrations of important oxidative and
reductive species. The model was started for each day at sundown (net
shortwave < 20 W m-2), and the GEM concentration in the free-tropospheric
Cft was set as the observed mean GEM concentration across the
previous hour. The modelled GEM concentration C was then calculated using
Eq. ().
Ci+1=Ci+(Fi+1+(Cft-Ci)w)⋅ΔtH
60 min averaged meteorological variables and ambient GEM/O3
during the campaign. Colours for each variable match those on the axis label.
Shading represents nocturnal periods (net shortwave < 20 W m-2).
Observations of dew and fog are denoted above the upper panel by “D” and
“F”, respectively.
Results and discussionObservations and trendsSite characterisation
Substrate sampling confirmed background
< 100 µg kg-1; THg levels, with
vegetation shown to have average concentrations of
18 ± 3 µg kg-1 (n=12). THg content in the soil was
found to be 48 ± 9 µg kg-1 (n=36) and did not vary
significantly with depth or size fraction. Soils were predominantly sandy,
with silt and clay making up 7 % of mass below 2 mm, and mildly acidic
(pH = 4.9). No soil material was found to have a diameter greater than
19 mm. SIC was spatially uniform both horizontally
and vertically (6 ± 1 %, n=36), whereas SOC
varied with depth, from 15 ± 2 % (n=12) in the upper
2 cm to 8 ± 1 % (n=12) in the 10–20 cm layer. Fibrous root
matter was observed to extend to a depth of around 10 cm.
Meteorological observations
Weather over the study period was dominated by high-pressure systems,
punctuated by weak troughs that brought patchy cloud and minor showers
(Fig. ). Temperatures ranged between 2 and
28 ∘C, with an average overnight minimum of 6 ∘C and
average daily maximum of 22 ∘C. Winds were primarily from the
higher, forested Kosciuszko range to the west, peaking at 8 m s-1,
though nights were predominantly calm and stable. Following the calmest of
these nights, fog events were witnessed and/or dew was observed. As no
instrumental measurements of either fog or dew took place, such events were
manually noted with the first observations of the day, taken at sunrise.
These observations are noted in Fig. . Nights following
which both dew modelling and observations at sunrise confirmed the existence
of dew are hereby termed “dew nights” (n=8), and those where the
existence of dew may or may not have been indicated from modelling but manual
observations could not corroborate this are hereby “non-dew nights” (n=10). Measurements had failed during two of the nights, and therefore there
were insufficient data to undertake dew modelling.
GEM concentrations
GEM concentrations throughout the study were 0.59 ± 0.10 ng m-3.
Diurnal GEM concentrations were slightly higher than the mean and less
variable at 0.63 ± 0.08 ng m-3. Nocturnal GEM concentrations
were 0.54 ± 0.10 ng m-3. Wind direction and HYSPLIT analyses
showed that there was no influence from significant GEM sources. These values
are lower than annual mean sea-level measurements recently reported across
the Southern Hemisphere .
Analysis of the systematic uncertainty of the 2537 system by
suggests this can be on the order of 0.1 ng m-3.
With this uncertainty taken into consideration, GEM concentrations observed
over the study were significantly (p< 0.0001) lower than for all four
sites operating in the same year Cape Point, Amsterdam Island, Cape
Grim, Troll;. This comparison is only against annual means
for these sites and cannot account for any seasonal or regional variation.
More recent observations of ambient GEM in Sydney (33∘45′55′′ S,
151∘07′04′′ E; elevation: 59 m) over the period
February–September 2015 gave a mean value of 0.64 ng m-3.
Linear correlation with environmental correlates using all available data
was generally stronger for ambient GEM than for GEM fluxes
(Table ). The strongest significant relationship was
with specific humidity (r=0.51), followed by O3 (r=0.49) and air
temperature (r=0.47). For diurnal data, correlation was maintained with
specific humidity (r=0.52), though it decreased for O3 (r=0.37) and
air temperature (r=0.26). Peak diurnal O3 followed a similar trend
to peak MLH (Fig. b), whilst diurnal GEM changed very
little, further confirming the absence of local sources. For nocturnal data,
correlation with both O3 (r=0.41) and air temperature (r=0.50)
were higher than for diurnal data. Nocturnal wind speed was also
significantly correlated (r=0.56), as was specific humidity (r=0.60).
Each of these variables showed significant decreases on some nights, though
not on others. These decreases will be discussed further in
Sect. .
Correlations between hourly averaged flux and measured environmental
variables. The top right triangle represents nocturnal data (n=102); the
bottom left triangle represents diurnal data (n=128). The bottom two rows
represent correlations using all data (n=230). Bold type represents
significance at p<0.05. Abbreviations are as follows: GEM_f: GEM
flux (ng m-2 h-1); CO2_f: CO2 flux
(µmol m-2 h-1); WS: wind speed (m s-1);
u*: friction velocity (m s-1); SWnet: net shortwave
radiation (W m-2); AirT: air temperature (∘C);
SoilT: soil temperature (∘C); SpHum: specific humidity
(kg kg-1); VWC: volumetric water content (m3 m-3);
H: sensible heat flux (W m-2); LE: latent heat flux
(W m-2).
GEM_fGEMCO2_fO3WSu*SWnetAirTSoilTSpHumVWCHLEGEM_f–0.020.260.040.020.000.070.120.020.150.06-0.06-0.05GEM-0.20–0.300.410.560.450.050.500.590.600.05-0.030.04CO2_f-0.05-0.03–0.020.230.160.100.180.140.170.13-0.400.01O30.080.37-0.24–0.700.580.240.670.700.19-0.440.04-0.09WS0.020.21-0.170.73–0.860.310.690.720.36-0.100.01-0.02u*-0.02-0.160.180.290.52–0.470.680.650.32-0.080.15-0.29SWnet0.190.14-0.090.460.390.21–0.360.280.03-0.120.44-0.26AirT0.160.26-0.180.810.750.280.65–0.850.47-0.17-0.07-0.18SoilT0.020.290.050.520.620.200.180.60–0.48-0.290.06-0.12SpHum-0.160.520.04-0.17-0.29-0.38-0.22-0.34-0.25–0.060.050.07VWC0.070.020.42-0.29-0.34-0.16-0.15-0.31-0.130.36–-0.070.05H0.230.27-0.200.620.650.090.740.760.58-0.17-0.23–0.02LE0.280.230.090.500.500.150.720.650.54-0.140.070.84–GEM_f–-0.060.010.150.100.050.240.210.06-0.090.060.270.29GEM-0.06–0.110.490.430.150.300.470.440.510.040.340.33GEM fluxesQuality control
Ogive analyses on wind, temperature, CO2 and H2O showed that most
scalar fluxes were convergent within 20 min and that low-frequency
contributions at this site had little influence on fluxes, up to 150 min
(Table ). Though each scalar has its own unique sources and
sinks , these results were taken to suggest that
low-frequency contributions to GEM fluxes at this site were also minimal. The
averaging period was set at 60 min, as this gave some smoothing to both flux
and environmental data, allowing for greater comparison between the two that
was not achievable using 20 min averaging. No further averaging or smoothing
of GEM flux data was applied. Following the application of quality control
protocols, GEM flux values were obtained for 87 % of the study period;
39 % of flux values fell below the theoretical detection limit
Eq. 8, . These were not removed as doing so would
have resulted in an artificial increase of the observed mean see
also.
Results from ogive analyses. Definitions of cases are given in
and here describe comparisons between 20 and 150 min
covariance averaging.
Bi-directional GEM fluxes were observed during the study and were normally
distributed with a mean of 0.2 ± 14.5 ng m-2 h-1
(Fig. ). The range of GEM fluxes was between -52.9 and
54.7 ng m-2 h-1, with generally greater magnitudes throughout
the day than at night. Mean nocturnal fluxes were significantly different
to diurnal fluxes, at -1.5 ± 7.8 ng m-2 h-1 compared to
1.8 ± 18.6 ng m-2 h-1. Over the entirety of the study, the
net cumulative GEM flux was close to zero (Fig. a) as net
deposition to the surface seen in the first 7 days was balanced by net
emission throughout the remaining period. Low biological productivity in the
underlying vegetation, suggested by manual observations, was confirmed by
observed CO2 fluxes that were largely positive with net cumulative
CO2 flux across the period of 1.7 mol m-2
(Fig. a). Both GEM and CO2 fluxes were bi-directional
and showed high variability on sub-diel timescales
(Fig. b). The mean GEM deposition velocity calculated using
all data was 0.0 ± 0.7 cm s-1 (positive values indicate
deposition to the surface) and ranged from -2.2 to 2.9 cm s-1. Mean
nocturnal and diurnal deposition velocities were opposite in sign
(direction), though not significantly different at
0.1 ± 0.4 cm s-1 and -0.1 ± 0.9 cm s-1,
respectively.
(a) Histogram of GEM flux values. Yellow bars represent
diurnal data; blue bars represent nocturnal data. Red line is Gaussian fit to all data.
(b) Box plots for all, diurnal and nocturnal GEM flux data. Red
lines represent medians, blue boxes inter-quartile range (IQR), whiskers
1.5 ⋅ IQR and
red crosses outliers.
Mean GEM, GEM flux and deposition velocity values (± 1
standard deviation, range in parentheses) for various studies undertaken over
background THg substrate in sub-alpine grasslands during summer periods. Values
taken from a,
b and
c. Values obtained using
d modified Bowen ratio method and 30 min averaging,
e aerodynamic gradient and 30 min averaging,
f aerodynamic gradient and 20 min averaging,
g aerodynamic gradient and 60 min averaging.
SiteAmbient GEMGEM fluxDeposition velocityng m-3ng m-2 h-1cm s-1Seebodenalp, Switzerlanda1.65 ± 0.01-1.9 ± 0.2d0.03 ± 0.003Fruebuel, Switzerlandb1.20 ± 0.20-4.3 (-27 to 14)e0.10 ± 0.16Neustift, Austriab1.22 ± 0.20-2.1 (-41 to 26)e0.05 ± 0.16Big Meadows, United Statesc1.28 (0.98 to 1.50)2.5 (-124.8 to 82.4)f0.05 (-3.1 to 1.9)Nimmo, Australia0.59 ± 0.100.2 ± 14.5g0.002 ± 0.7(< 0.01 to 0.91)(-52.9 to 54.7)(-2.2 to 2.9)Comparison with other flux studies
Patterns of net diurnal emission and nocturnal deposition were reported by
and for
summertime periods over subalpine grasslands in Fruebuel, Switzerland, and Big
Meadows, USA (Table ). The range of flux values
observed here is larger than those reported by
yet lower than those reported by
. Determining the significance of these differences
however is difficult, as flux averaging periods differed across each of the
studies, as well as application of temporal smoothing. The mean deposition
velocity is within the range of uncertainty of those reported at Fruebuel and
Neustift, Austria . Greater variation in
deposition velocities was seen here, albeit with a similar range to those
seen at Big Meadows . The mean deposition velocity
was significantly lower than that reported for Seebodenalp, Switzerland
.
Comparability of deposition velocities was maintained despite a mean ambient
GEM concentration between 50 and 64 % lower than those reported at the Northern
Hemisphere sites. It is not reported in these papers whether the underlying
vegetation had senesced, and so the relative control on fluxes by vegetation
may differ across sites. concluded that there was no
relationship between their observed GEM fluxes and stomatal conductance.
Seasonal studies by these authors
as well as modelling
studies such as have shown increased deposition of
GEM to the surface during vegetation growing seasons, and we therefore expect
that deposition velocities at Nimmo would similarly change throughout the
year as biological activity increases. Further discussion of deposition
velocities across different ecosystems is given in . The
agreement between deposition velocities obtained here and elsewhere over
similar conditions provides evidence towards the robustness of such an
approach to estimating GEM deposition.
(a) Cumulative GEM and CO2 fluxes across the study
period, along with precipitation and 24 h running means of soil temperature
at 2 cm depth. Colours match those of axis labels. Observations of dew and
fog are denoted above by “D” and “F”, respectively. (b) 60 min
GEM and CO2 fluxes over the study period.
Environmental correlates
Factors that have been identified as controls on GEM surface exchange in the
literature include soil moisture , precipitation
, radiation ,
temperature , vegetation
and GEM/oxidant concentrations
. Linear correlations between GEM fluxes
and environmental data were generally weak (Table ).
The strongest significant relationship using all available data was with
latent heat flux (r=0.29), followed closely by sensible heat flux (r=0.27), net solar radiation (r=0.24) and air temperature (r=0.21). A
similar result was seen for diurnal data, whilst for nocturnal data the only
significant correlation was with CO2 flux (r=0.26).
Soil volumetric water content (VWC) was low (< 0.07; wilting point: 0.08)
for most of the study and had no significant linear relationship with GEM
fluxes. The precipitation event on DOY 45 was the only to impact VWC, raising
it from 0.06 to 0.08. The two largest maximum diurnal cumulative GEM
emissions (cumulative GEM flux from sunrise to peak value) of 128.2 and
112.1 ng m-2 occurred on the days following this event
(Fig. a), as VWC receded down to 0.07. This is consistent
with the laboratory results of , who
hypothesised that evaporation of soil water helps mobilise mercury adsorbed
to soil matter upwards to the air–surface interface. In their study, a pulse
of GEM emission was observed immediately after precipitation events,
attributed to expulsion of soil GEM from within the pore space. Such an
immediate pulse was not observed during DOY 45, likely due to the small
amount of precipitation in this event and its minimal impact on the
already-dry soil.
Solar radiation and temperature have been independently shown to influence
GEM fluxes from soils via photo-reduction and volatilisation of stored
mercury , though their inherent
interconnectedness makes it difficult to resolve relative influence in the
field. showed that inclusion of surface air
temperature in parameterisations of soil GEM emissions gave closer
reproduction of seasonal observations than with radiation alone. Shading by
overlying vegetation may also reduce the influence of solar radiation on soil
GEM emission . Linear correlation between GEM flux
and net radiation was similar to that with air temperature, though GEM flux
showed low correlation with soil temperature. The low linear correlation
between GEM flux and soil temperature compared to scalar fluxes of heat may
suggest that turbulent atmospheric processes (quasi-laminar and turbulent
diffusion), rather than in-soil processes, represent the larger influence on
GEM fluxes. found stronger relationships between
aerodynamic variables and GEM fluxes when measured by micrometeorological
methods than when measured with flux chambers, suggesting the choice of
measurement may influence these relationships.
however similarly observed stronger relationships with atmospheric variables
and GEM fluxes measured using dynamic flux chambers over bare background THg
soil. The stronger relationship with air temperature than with soil
temperature only held across sub-diel timescales; comparing cumulative GEM
flux to 24 h averaged temperature data (Fig. a) resulted
in a much stronger correlation with soil temperature (r=0.62,
p< 0.0001, n=443) than with air temperature (r=0.36,
p< 0.0001, n=443).
Median diel composite CO2 fluxes were almost consistently positive both
during the day and at night (Fig. b), excepting a notable
period between 1 and 3 h after sunrise during which median net CO2
release switched to uptake before reverting back to release. Overnight GEM
fluxes were close to zero with little variance relative to daytime
(Fig. a); however during the day two peaks of GEM emission
can be seen, the largest coinciding with solar noon, whilst the smaller peak
occurred shortly after sunrise and coincides with an increase in bulk canopy
conductance of water vapour. The cessation of this early pulse coincides with
the switch to CO2 uptake that may be related to uptake of GEM by the
underlying vegetation. However, the correlation between the timing of the
early GEM pulse and the brief increase in bulk canopy conductance suggests
that this pulse is likely related to evaporation of dew. The bimodal pattern
of GEM fluxes has been observed previously by others
e.g. and attributed to co-deposition of mercury with dew overnight,
with subsequent re-volatilisation of GEM as the dew is evaporated from the
surface. The linear correlation of GEM fluxes with latent heat fluxes in this
study similarly provides evidence for this explanation of the bimodal GEM
flux pattern.
(a) Diel composite of GEM flux values. Black line
represents median values, blue shading inter-quartile range (IQR), and blue lines
5th and 95th percentiles. Grey shading represents nocturnal periods.
(b) As above for CO2 fluxes (solid black line, green shading)
and bulk canopy conductance for water vapour (broken black line, magenta
shading).
Nocturnal atmospheric mercury depletion events
Both GEM and O3 concentrations, as well as air temperature and wind
speed, were intermittently observed to show significant decreases
(Fig. b), twice to below instrument detection limits for
GEM. These nocturnal atmospheric mercury depletion events (NAMDEs) occurred
exclusively overnight, with concentrations returning to pre-depletion levels
after sunrise. Depletion was most pronounced during the calm, stable dew
nights. Patterns of depletion differ for GEM and O3, with O3
exhibiting initially rapid depletion followed by slower removal, mirroring
decreases in atmospheric turbulence (linear correlation with
σw/u* for dew nights; r=0.86, p< 0.0001, n=86).
GEM concentrations show a more linear rate of depletion through to sunrise,
when both GEM and O3 levels increase with the turbulent breakup of the
nocturnal boundary layer. These differences in patterns are particularly
pronounced on DOY 45, 46 and 48 and are similar in character to the depletion
events reported by . Similar nocturnal depletion events
have been reported elsewhere e.g. and differ from the better-known polar
AMDEs and reference within, as the former take
place in the absence of sunlight and photolytic reactions. HYSPLIT
trajectories showed no distinct source pattern for NAMDE nights, suggesting
that the observed phenomena are due to local interactions and not the result
of long-range transport of depleted air masses such as those observed by
, and
.
Nocturnal composites of cumulative GEM flux, ambient GEM and modelled
ambient GEM/dew were composed to further explore NAMDEs, demarcating between
dew and non-dew nights. Figure confirms that depletion
of GEM occurred exclusively during dew nights, highlighting dew or fog as a
potential sink of GEM. Field studies of mercury concentrations in fog water
e.g. are rare, though
concentrations up to 435 ng L-1 have been reported
. When estimating conservatively using this concentration
and a fog droplet density of 0.05 g m-3, only 0.02 ng m-3 of
GEM depletion can be explained by fog uptake. Field measurements of mercury
in dew are also limited, though they have generally shown that it represents
only a small sink for atmospheric mercury. , from
the first such measurements, calculated values of mercury deposition that
ranged between 1.2 and 9.6 ng m-2 per dew event. THg concentrations
similar to those from Malcolm and Keeler have since been reported by
and .
reported GEM depletion events coincident with their dew mercury measurements
and concluded, based on a mass balance approach, that mercury uptake to dew
could explain < 1 % of observed depletion. From the combined dew
measurements of , and
a reasonable estimate of dew mercury concentration
is 6.3 ± 4.9 ng L-1 (n=27). Applying this concentration to
the median modelled dew depth (0.13 mm) gives a cumulative nocturnal
deposition to dew of 0.82 ng m-2.
(a) Nocturnal composites for dew nights. Solid black line
represents median cumulative GEM flux, blue shading inter-quartile range
(IQR), and blue lines 5th and 95th percentile. Black broken line represents
ambient GEM, magenta shading IQR, and magenta lines 5th and 95th percentile.
Green line and error bars represent median modelled ambient GEM and IQR.
Yellow lines and error bars represent median modelled dew depth and IQR. Note
the change in scaling on the right axis. (b) As above but for non-dew
nights.
observed nocturnal depletion of GEM, concurrent with
measurements of GOM and nitrate (NO3). They showed evidence of NO3
assisting in the oxidation of GEM to GOM overnight, noting a strong
correlation between NO3 and GOM, and weak correlations between GOM and
other measured variables such as O3 and wind speed. ,
in chemical modelling of their observed depletion events, attributed 80 %
of chemical oxidation of GEM to NO3, whilst also considering oxidation
by O3 and OH. The presence of cattle prior to the study may have
provided an additional source of NO3 at the surface
, though without measurements of nitrogen species
this remains speculative. Further, this source would be available during both
dew and non-dew nights and cannot alone explain differences in the composite
ambient GEM patterns. Higher relative humidity during dew nights relative
to non-dew nights (Fig. ) is likely to have a limiting
influence on oxidation of GEM by NO3 due to enhanced depletion of the
radical ; noted
a significant anti-correlation between relative humidity and NO3, as
well as between relative humidity and GOM. They also highlighted possible
evidence of GEM oxidation by O3, albeit only at levels above 47 ppb,
higher than the nocturnal concentrations observed here.
note that O3 oxidation represented the largest uncertainty in their
modelling estimates, with differences in oxidation of 86 % attributable
to varying rate constants from the literature.
In the absence of adequate measurements of potentially important chemical
species, a simple box model was utilised in order to explore the relative
influence of observed surface deposition (Eq. ). The outputs
from this model show that patterns of depletion and non-depletion can largely
be explained by surface deposition, though these outputs are strongly
dependent on the modelled mixed-layer height, which represents the largest
uncertainty in the model. The median height during dew nights (50 m) was
58 % lower than for non-dew (120 m), consistent with surface stability
measurements. , in their modelling efforts, estimated that
dry deposition to the surface could account for 70 % of their observed
depletion events, whilst investigated GEM depletion for a
range of surface deposition and turbulent diffusivity values under a boundary
layer of height 100 m. They showed that a deposition flux of
7.3 ng m-2 h-1 and turbulent diffusivity of 0.1 to
1 cm s-1 could reduce GEM concentrations within a stable boundary
layer from 1.56 to 0 ng m-2. Turbulent diffusivity values during dew
nights were similar to those seen by (median
2 cm s-1), whilst deposition fluxes were generally smaller (median
3.1 ng m-2 h-1). Free-tropospheric GEM concentrations however
were also considerably smaller, as were modelled boundary layer heights.
GEM fluxes were significantly different between dew and non-dew nights, with
the former showing consistent net deposition, whilst the latter showed fluxes
not different from zero. The median cumulative nocturnal GEM flux for dew
nights was 22.6 ng m-2 lower than for non-dew. Care must be taken
here, as under low-turbulence conditions atmospheric gradients may be
enhanced, leading to an overestimation of surface fluxes
. This however is the case for both dew and non-dew
nights, where the integral turbulence characteristic is consistently < 1.
This result points to an additional GEM sink at the surface that is present
during dew nights only. The most likely candidate is chemical oxidation of
GEM to the more reactive GOM, much as in the case of AMDEs, as both the
solubility and deposition velocity of this form of mercury are higher than
for GEM . We hypothesise, based on the
greater observed GEM deposition and results of the box model, that any
oxidation is taking place largely at the surface, leading to an enhanced GEM
gradient. Complex surface chemistry may be taking place in the presence of
high humidity and liquid water, such as the enhanced oxidation of GEM by
O3 observed by . Other oxidation pathways are also
possible, and we therefore recommend consideration of the chemical processes
taking place at the surface in future investigations.
Once in oxidised form, mercury can be more readily taken into dew
, soil
or vegetation via foliar uptake
, where it can become
associated with cuticular membranes . In their isotopic
investigations, observed that 66 % of
wet-deposited mercury was bound within vegetation, suggesting this may be a
significant sink of GOM formed near the surface. Figure
shows that, following the significant deposition during dew nights, an
emission pulse is observed shortly after sunrise. This initial pulse, with
median value of 3.9 ng m-2, represents 17 % of total nocturnal
deposition. This pulse is not observed following non-dew nights, showing that
the bimodal shape of GEM fluxes discussed in
Sect. is not the regular diel pattern.
Instead, this suggests that co-deposition of mercury with dew overnight does
play a role in the initial emission pulse as suggested by others; however the
estimated 0.82 ng m-2 co-deposition represents only 4 % of total
mercury deposition. As such, we further hypothesise that the majority of this
morning GEM emission pulse is due to prompt recycling of mercury, likely
volatilised from GOM created near the surface and deposited overnight. This
percentage of promptly recycled mercury is within the range observed by
and , with the remaining
deposited mercury likely stored in vegetation and soils. The long-term impact
of any additional sink is however likely to be minimal, as evidenced by the
near-zero cumulative GEM flux over the study period.
Conclusions
GEM flux measurements were undertaken over a mid-latitude alpine grassland
region of Australia during a 3-week period in the late austral summer. The
micrometeorological aerodynamic gradient method was employed, providing
high-time-resolution fluxes and facilitating evaluation of controlling factors.
Both deposition and emission fluxes were observed, with a cumulative flux
close to zero over the study period. Nocturnal GEM fluxes were
-1.5 ± 7.8 ng m-2 h-1 compared to diurnal fluxes of
1.8 ± 18.6 ng m-2 h-1; this equated to deposition
velocities of 0.1 ± 0.4 cm s-1 and
-0.1 ± 0.9 cm s-1, respectively. These deposition velocities
are mostly within the range of uncertainty of others reported for alpine
grasslands during summer periods in Europe and North America. This is an
important result towards justification of applying known deposition
velocities over similar surfaces in global atmospheric mercury models, as
background atmospheric mercury pool concentrations in the Southern Hemisphere
are now understood to be lower than previously believed. Worldwide,
air–surface GEM exchange studies across multiple seasons are rare yet
required in order to reduce uncertainty in chemical transport model
parameters and resolve the importance of terrestrial systems as sources or
sinks of atmospheric elemental mercury.
Direct linear correlations between GEM fluxes and other environmental
parameters were generally weak, with measures of temperature, radiation and
heat fluxes (both sensible and latent) showing the strongest relationships.
Soil temperature did not prove to be strongly related to raw GEM flux values;
however it showed a moderately strong relationship when averaged over a running
24 h period and compared to cumulative GEM flux. Soil moisture was below the
wilting point for the majority of the study and had little observable impact
on GEM fluxes; however during the drying period following one precipitation
event diurnal GEM emission fluxes were enhanced. The vegetation at the site
had largely senesced and showed little overall stomatal control on GEM
fluxes. A bimodal pattern of diurnal GEM emission was observed, the larger
peak of which coincided with solar noon, whilst the smaller coincided with a
brief increase in bulk canopy conductance of water vapour shortly after sunrise. Correlation
between GEM and latent heat fluxes suggests that the latter pulse is
attributable to release of co-deposited mercury as GEM during evaporation of
dew.
Nocturnal atmospheric mercury depletion events are also reported here,
concomitant with depletion of O3 and formation of dew, under calm,
stable boundary layers. Modelling of dew depth confirmed manual observations
that showed dew formation during NAMDEs only and that uptake of mercury to
dew represents only 4 % of nocturnal mercury deposition. Other
researchers have also reported NAMDEs with no definitive explanation; however
there is evidence to suggest that surface deposition plays a large role. Here
we investigated the role of surface deposition using observed flux data in a
simple mass balance box model. This model reproduced ambient GEM patterns for
nights both with and without observed depletion, though with high uncertainty
in the modelled boundary layer heights. GEM deposition fluxes were also
enhanced during NAMDEs, leading us to hypothesise an enhancement of the
near-surface GEM gradient due to oxidation of GEM to GOM and subsequent
deposition. Early-morning GEM emission pulses representing 17 % of
nocturnal mercury deposition were observed following NAMDEs only. We further
hypothesise that this pulse is due to prompt recycling of mercury deposited
during these nights, with the remaining deposited mercury retained in
vegetation and soil. As cumulative GEM fluxes over the 3-week period were
close to zero, the influence of any additional sink would extend over
reasonably short time periods. The growing literature on mercury emission and
deposition behaviour is building greater evidence towards prompt recycling
behaviour of GEM in the global mercury cycle, with significant impacts on our
understanding of the legacy of anthropogenic perturbations to this cycle.
Data availability
Data are available upon request from the corresponding
author.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors would like to thank Robert Simpson and Mark Adams for providing
soil and precipitation data, and Mark Cohen for his assistance with HYSPLIT
modelling. Edited by: Aurélien
Dommergue Reviewed by: two anonymous referees
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