Introduction
Ammonia (NH3) is the most prevalent alkaline gas in the atmosphere and
has important implications for both climate and air quality (Seinfeld and
Pandis, 2006). For instance, NH3 partitions to acidic fine particulate
matter (PM2.5, aerosol with diameter < 2.5 µm) to form
particulate-phase ammonium (NH4+), which alters aerosol properties
such as hygroscopicity (Petters and Kreidenweis, 2007) and scattering
efficiency (Martin et al., 2004). High atmospheric loadings of PM2.5 can
lead to adverse health effects (Pope et al., 2002) as well as reduced
visibility. NH3 is primarily emitted to the atmosphere through
agricultural activities (e.g. fertilization, animal husbandry) in addition to
natural sources (e.g. soil, vegetation, oceans, volcanoes, wildfires) and
other anthropogenic sources (vehicles and industry) (Reis et al., 2009).
Deposition of atmospheric NHx (≡ NH3 + NH4+)
can cause eutrophication and soil acidification in sensitive ecosystems
(Krupa, 2003). Hence, there is great interest in being able to accurately
model sources, sinks, and reservoirs of NHx.
A common feature in the diurnal cycle of atmospheric NH3 mixing ratios
is a morning increase or “spike” that typically occurs around
07:00–10:00 LT. Frequently observed in many
environments, current hypotheses to explain the morning NH3 increase
include dew evaporation (Gong et al., 2011; Wentworth et al., 2014; Wichink
Kruit et al., 2007), plant and/or soil emissions (Bash et al., 2010; Ellis et
al., 2011), mixing down of NH3-rich air during the break-up of the
nocturnal boundary layer (Walker et al., 2006), and automobile emissions
during morning rush hour (Gong et al., 2011; Löflund et al., 2002; Nowak
et al., 2006; Whitehead et al., 2007). Several field studies have indicated
that NH3 desorption from microscopic water films on leaf surfaces can
yield significant fluxes (Burkhardt et al., 2009; Flechard et al., 1999;
Neirynck and Ceulemans, 2008; Sutton et al., 1998); therefore, it is
reasonable to suggest that macroscopic dew droplets have the same effect.
Wentworth et al. (2014) observed a larger morning increase following nights
with high relative humidity (RH, a surrogate for dew) and Wichink Kruit et
al. (2007) found increasing upward fluxes as soon as the canopy began to dry
as measured by a leaf wetness sensor.
Dew generally forms during calm, clear nights when radiative cooling of the
surface favours the condensation of water (Richards, 2004). Formation
typically starts shortly after sunset and lasts until sunrise when surface
heating and a drop in RH initiate evaporation. Over the last 5 decades,
several dozen studies have characterized dew composition and have found that
NH4+ is a ubiquitous constituent of dew and, in some environments,
can be the most abundant cation (e.g. Polkowska et al., 2008; Wagner et al.,
1992; Yaalon and Ganor, 1968; Yadav and Kumar, 2014). Average [NH4+]
reported in dew ranges from 25 µM (Lekouch et al., 2010) to
1600 µM (Yadav and Kumar, 2014). The composition of dew is
primarily controlled by dissolution of water-soluble gases (e.g. NH3,
HNO3, CO2, SO2) and deposition of coarse mode particles
(larger than PM2.5 but smaller than 10 µm in diameter)
(Takeuchi, 2003).
Field-scale models typically allow NH3 to only deposit on leaf cuticles
(i.e. it cannot desorb) and use an empirically derived function of RH and
cuticle acidity to calculate a cuticle deposition velocity. This
parameterization accounts for enhanced deposition to acidic water films on
leaf surfaces. There are only a handful of field-scale NH3 models that
allow for desorption of NH3 from drying water films on leaf cuticles
(Burkhardt et al., 2009; Flechard et al., 1999; Neirynck and Ceulemans,
2008; Sutton et al., 1998). Three of these studies (Flechard et al., 1999;
Neirynck and Ceulemans, 2008; Sutton et al., 1998) compared models with and
without cuticle desorption and found that allowing for NH3 emission
from water films on cuticles agrees better with observed fluxes during the
morning. Other field-scale measurements attribute discrepancies between
measured and modelled morning fluxes to NH3 emission during the drying
of canopies (e.g. Bash et al., 2010; Walker et al., 2013; Wyers and Erisman,
1998). Most larger-scale (regional or global) chemical transport models
(CTMs) still employ highly simplified deposition schemes for NH3 using
look-up tables for deposition velocity and canopy resistance terms (Wesely,
1989). In other words, they treat deposition and emission of NH3
independently despite abundant field evidence that these processes are often
highly coupled. However, some recent studies have successfully incorporated
a bi-directional NH3 exchange framework into regional and global CTMs
(Bash et al., 2013; Wichink Kruit et al., 2012; Zhu et al., 2015).
Although most NH3 surface–air exchange studies account for enhanced
deposition to microscopic water films and several even model NH3
desorption, far fewer have attempted to assess the role that macroscopic dew
has on influencing NH3 fluxes. This partly stems from the inherent
difficulty in measuring dew amount, composition, and pH. Only a few NH3
surface–air exchange studies have attempted to measure dew composition and
pH, doing so by manually collecting enough individual droplets in pipettes
to perform bulk analyses (Bash et al., 2010; Burkhardt et al., 2009; Walker
et al., 2013). To constrain dew amount, Burkhardt et al. (2009) used an
empirically derived relationship to approximate water film thickness from a
leaf wetness sensor. Walker et al. (2013) estimated dew amount by difference
in water mass between wet and dried leaves. Both studies acknowledge the
large uncertainties associated with these methods for estimating total dew
amount in the canopy. Regardless, Walker et al. (2013) estimated a maximum
flux of 17.6 ng NH3 m-2 s-1 from dew evaporation in a
fertilized corn canopy. One key assumption in this calculation is that all of
the NH4+ present in dew is released as NH3 during evaporation.
Few studies have examined the fate of semi-volatile solutes (e.g.
NH4+ / NH3, NO2- / HONO, acetate/acetic acid) in rain, cloud, fog, or dew during droplet evaporation. Takenaka
et al. (2009) studied the chemistry of drying aqueous salts in a series of
lab experiments and found that the fraction of “volatile” anions (which
they operationally defined as NO2-, acetate, and formate) remaining
on the surface as a salt upon evaporation depends on the relative equivalents
of “non-volatile” cations (Na+, K+, Mg2+, and Ca2+) and
“non-volatile” anions (Cl-, NO3-, and SO42-). The
fraction of volatile anion (X-) that is released during evaporation (as
HX(g)) can then be predicted using the following equation (Takenaka
et al., 2009):
Frac(X)=[X]i-(Σcations-Σanions)[X]i,
where Frac(X) is the fraction of the initial anion released to the
atmosphere during evaporation, [X]i is the initial equivalents of
“volatile” anion X, and Σcations and Σanions are the sums
of “non-volatile” cations and anions respectively. The authors performed
numerous experiments for NO2-, acetate, and formate under a wide
range of solute concentrations and pH values and found that Eq. (1) was
consistently able to predict the fraction of each constituent liberated
during evaporation of aqueous salt solutions. Although not the focus of the
work, Takenaka et al. (2009) also performed some evaporation experiments on
solutions containing NH4+ and were able to predict Frac(NH3)
with an analogous equation:
Frac(NH3)=[NH4+]i-(Σanions-Σcations)[NH4+]i,
where [NH4+]i is the initial ammonium concentration in the
solution.
Few field studies have simultaneously quantified both dew and atmospheric
composition. He et al. (2006) observed HONO emission from a drying forest
canopy and performed lab studies to show that, on average, ∼ 90 %
of NO2- was released as HONO during droplet evaporation. Rubio et
al. (2009, 2012) found positive correlations between formaldehyde, phenols,
and HONO in dew and the atmosphere. However, none of these studies, or those
mentioned earlier for NH3, accurately measured dew amount (in
g m-2) on the surface, so the relative abundances of the analyte in the
dew and gas phase could not be reliably calculated.
Therefore, great uncertainty exists regarding the influence of dew on
boundary layer composition, particularly with respect to NH3 mixing
ratios. Motivated by the paucity of data on dew–atmosphere NH3 fluxes,
as well as uncertainties about the origin(s) of the frequently observed yet
currently unexplained morning NH3 spike, the specific goals of this
study are as follows:
Determine the fate of NH4+ during dew evaporation (Sect. 3.1). What
is the ratio of NHx released as NH3 vs. NH4+ remaining
on the surface as a non-volatile salt? What factor(s) govern this ratio?
Simultaneously quantify dew amount, NH3 mixing ratio, and dew
composition at the onset of evaporation at a field site (Sect. 3.2).
Calculate the relative abundance of NH4+ in dew and
NH3 in the boundary layer, as well as NH3 fluxes from dew
evaporation (Sect. 3.3). Is dew a significant night-time sink or reservoir
for NH3? Is NH3 release from dew an important morning source?
Evaluate whether NH3 is also released during rain evaporation (Sect. 3.4).
Assess the impact of dew evaporation for other water-soluble gases
(Sect. 3.5).
Materials and methods
Drying chamber
A drying chamber was used to determine the fraction of NH4+ lost as
NH3 during droplet evaporation and was based on the set-up used by
Takenaka et al. (2009). The set-up consists of a zero air cylinder (AI Z300,
AirGas) and mass flow controller which deliver zero air at a controlled flow
rate into a drying chamber (URG-2000-30H, URG Corp.) containing droplets of
synthetic dew. Downstream of the drying chamber is an annular denuder
(URG-2000-30, URG Corp.) coated with a phosphorous acid solution
(10 g H3PO3 in 100 mL deionized water and 900 mL HPLC grade
methanol) to capture any NH3 emitted during dew drying.
At the beginning of each experiment, 26 droplets (20 µL each) of
synthetic dew were deposited in the drying chamber and dried over the course
of several hours by exposure to a flow of 2 L min-1 of zero air.
Immediately after the last droplet had dried, the residue remaining in the
chamber was extracted twice using two separate 10 mL aliquots of deionized
water (18.2 MΩ cm-1) and vigorous washing. The second aliquot
always contained < 10 % of each analyte relative to the first aliquot.
The annular denuder was extracted by adding 10 mL of deionized water and
rotating for 10 min. Concentrations of ions in all three extracts were
quantified using ion chromatography (IC) systems (DX-500, Dionex Inc.) and an
isocratic elution scheme
(1.8 / 1.7 mM Na2CO3 / NaHCO3 solution for anions
and 0.020 mM methanesulfonic acid solution for cations). The pH of the dew
was determined using a commercial pH metre (Orion Model 250A, Thermo
Scientific). The fraction of each analyte remaining in the salt residue was
then calculated, as well as the fraction of ammonium lost as NH3 based
on the total NHx amount measured in the three aliquots.
Experimental parameters (composition, pH, and drying time) were varied to
determine the factor(s) responsible for the fraction of NH3 that is
released from dew as it dries. Synthetic dew was prepared by dissolving salts
in deionized water to the desired concentration. All salts were reagent
grade, obtained from Sigma Aldrich, and used without further purification. The
pH was then adjusted with either concentrated acid (HCl) or base (NaOH). A
total of nine different synthetic dew were prepared to mimic ambient dew
composition reported from previous studies (e.g. Lekouch et al., 2010;
Takenaka et al., 2003; Yadav and Kumar, 2014). The pH and concentrations of
the nine synthetic dew are listed in Table S1 in the Supplement.
Synthetic dew was deposited as 20 µL droplets, which corresponds to
a hemispherical diameter of ∼ 4.25 mm. Takeuchi et al. (2002) found
that the diameter of most dew droplets range from 0.8 to 1.0 mm in diameter;
however, applying such small droplets would bring the concentration of the
extracts below detection limit. In order to maintain solute concentrations
relevant to ambient dew, but generate sufficient signal for analysis, it was
necessary to use 20 µL droplets. The impact of larger droplet size
on NH3 liberation was tested by performing several drying experiments on
four 140 µL drops (∼ 8.1 mm in diameter). These larger
droplets had no effect on the fraction of NH3 emitted relative to the
20 µL droplets.
Field site
Ambient measurements of dew composition, dew volume and gas-phase NH3
were obtained at a field site situated on the eastern edge of Rocky Mountain
National Park (RMNP) in Northern Colorado (40.2783∘ N,
105.5457∘ W; 2784 m a.s.l.) from 28 May to 31 August 2015. The
field site is remote with the nearest town (Estes Park, CO, population
∼ 6000) located approximately 14 km north. This site is also used by
the Interagency Monitoring of Protected Visual Environments (IMPROVE) and EPA
Clean Air Status and Trends Network (CASTNet) programs for air quality
monitoring and has been the location of extensive studies on nitrogen
deposition (Beem et al., 2010; Benedict et al., 2013a) and atmospheric
reactive nitrogen (Benedict et al., 2013b). The field site is a grassland
clearing approximately 150 m in diameter surrounded by a mixed aspen and
pine forest (average summertime maximum leaf area index of 1.5). In addition,
excessive nitrogen deposition at RMNP has been linked to ecological impacts
including changes in diatom assemblages (Baron, 2006; Wolfe et al., 2003) and
shifts in a dry alpine meadow community (Bowman et al., 2012). Recently,
Nanus et al. (2012) suggested that the critical load for nitrogen deposition
(a value beyond which negative ecological impacts are observed) has been
exceeded in ∼ 21 % of the Rocky Mountains. The existing body of
knowledge regarding reactive nitrogen at RMNP makes this site ideal to
examine how dew–atmosphere interactions affect NH3 in the boundary layer
as well as its deposition.
Atmospheric measurements
NH3 was measured using a Picarro G1103 Analzyer, a cavity ring-down
spectroscopy instrument. The inlet line was 3.56 cm diameter Teflon tubing
located approximately 2.5 m above ground level. The entire length of the
0.61 m inlet line was insulated and heated to 40 ∘C to minimize
wall losses. A filter (Picarro P/N S1021) was placed on the end of the inlet
to prevent particles from entering the instrument. The filter was also
heated,
which may have caused NH4NO3 to volatilize from the filter or air
stream, resulting in an overestimation of the ammonia concentration. However,
a previous study at the site found that, on average, only a small fraction of
particulate NH4+ exists as NH4NO3 during the summer
(Benedict et al., 2013b). Furthermore, the same study found that NH3 was
the majority of the NHx (≡ NH3 + NH4+)
loading. Hence, it is unlikely that there is a large interference from
NH4NO3 volatilization.
Calibrations were performed twice during the field deployment using MKS mass
flow controllers, a certified 2 ppm NH3 cylinder (AirGas), and a zero
air source (Teledyne Zero Air Generator Model 701). The calibration gas was
split between the Picarro and a phosphorous acid (10 % w/v) coated
denuder to act as a check of the concentration. The denuder was sampled at
2 L min-1 and the total volume was recorded using a dry gas metre. The
concentration determined by the denuder was used as the “true”
concentration in the calibration curve.
Meteorological measurements were made at the site by a 10 m tower operated
by the National Park Service. Measurements are reported at 1 h intervals for
solar radiation, temperature, wind speed, wind direction, standard deviation
of the wind direction over the period, relative humidity, and rainfall.
Dew measurements
Ambient dew samples at RMNP were gathered using a dew collector with a design
similar to Guan et al. (2014). The collector was built in-house and consists
of a wooden base that supports a 7 cm thick polystyrene foam block with an
area of 48 × 60 cm. The top surface of the polystyrene block is
covered by a 0.2 mm thick polytetrafluoroethylene
(Teflon®) sheet. The
Teflon® sheet is parallel to the ground at
a height of 30 cm. During the night the
Teflon® sheet undergoes radiative cooling
while the polystyrene insulates the sheet from below. This results in dew
formation on the Teflon® surface which can
be manually collected into clean sample bottles the following morning using a
pre-cleaned scraper and funnel. The emissivity of
Teflon® is 0.94 (Baldridge et al., 2009)
and is very similar to that of vegetation (0.95) (Guan et al., 2014).
The dew collector was deployed before dusk on nights that had a forecast
favourable for dew formation (high relative humidity, light winds, and clear
skies). The Teflon® surface was cleaned
immediately before deployment in a two-step process: (1) splashing
∼ 1 L of deionized water across the surface, followed by (2) squirting
∼ 30 mL of deionized water on the surface and scraping it off using a
plastic scraper. The latter step was repeated 10 times, and the tenth rinse
was collected and used as a field blank for dew collected the following
morning. Prior to dew collection, the funnel and scraper were rinsed 10 times
with deionized water. This cleaning procedure proved sufficient and is
similar to prior studies using a similar collector (e.g. Okochi et al., 2008;
Wagner et al., 1992). Dew samples were collected into 15 mL polypropylene
sample bottles in order to minimize headspace during transport and storage.
When rain had occurred during the night, then rain samples were also collected
off of the dew collector in a similar fashion the following morning. Rain
samples were unambiguously identified using data from the dewmeter described
below.
Chemical analyses of all dew samples were performed within 6 h of
collection, with the exception of one sample which was stored at
4 ∘C and analysed 48 h later. The concentration of ions (Na+,
NH4+, K+, Mg2+, Ca2+, Cl-, NO2-,
NO3-, SO42-, PO43-, acetate, formate, and oxalate) in
dew samples was determined through ion chromatography and pH was measured
with a pH metre, as outlined in Sect. 2.1. The total organic carbon (TOC) and
inorganic carbon (IC) were quantified with a commercial TOC analyser
(TOC-VCSH, Shimadzu Corp.) equipped with a total nitrogen (TN)
analyser (TNM-1, Shimadzu Corp.) for quantification of TN. Concentrations of
analytes in ambient dew samples were background corrected by subtracting the
volume-weighted concentration in the tenth rinse collected the prior evening,
which is likely an upper bound for the background signal given that some
volatile solutes will be scavenged from the air during application and
collection of the rinse.
It was also necessary to quantify the volume of dew (Vdew) that
formed each night. The dew collector is not suitable since Vdew
obtained from the collector is not necessarily representative of
Vdew that forms naturally on the grassland canopy at RMNP. Numerous
methods and instruments exist to measure Vdew; for instance, the
cloth-plate method (Ye et al., 2007), lysimeter-related instruments (Grimmond
et al., 1992; Price and Clark, 2014), and eddy-correlation techniques (Moro
et al., 2007). Although there is no standard method to measure
Vdew, Richards (2004) provides a detailed overview of various
techniques that have been used to collect and quantify dew.
For this study, we constructed a dewmeter similar to that of Price and
Clark (2014). The design consists of a circular collection tray (diameter of
35 cm) that is attached to the top of an analytical balance (HRB 3002, LWC
Measurements). The balance has a resolution of 0.01 g and a maximum load of
3000 g. The tray contains artificial turf that is intended to be
representative of the grass at the RMNP field site during the early part of
the growing season. The balance was contained in a weatherproof box with a
hole cut in the lid to accommodate the tray/turf. The mass on top of the
balance was recorded to a laptop at a rate of 5 Hz so that the mass of dew
was continuously monitored as it formed and evaporated. The data were
averaged to 10 min to achieve better signal-to-noise ratio.
Price and Clark (2014) performed an extensive characterization of the
dewmeter and compared dew formation/evaporation on co-located dewmeters
containing real turf and artificial turf. The authors found that
Vdew and the dew deposition rate were identical between the two
turfs. In other words, the radiative properties and surface area of
artificial turf sufficiently mimic real turf such that artificial turf can be
used as a surrogate to quantify Vdew and its temporal evolution.
The advantage of using artificial turf is that there are no changes in mass
due to evapotranspiration during the daytime. The dewmeter is also capable of
quantifying rainfall and its evaporation. However, if the rainfall is too
intense (≥ 2 mm) then the tray becomes flooded and must be replaced
with a dry tray/turf.
Flux calculation
NH3 fluxes from dew evaporation were calculated using the following
equation:
FNH3=[NH4+]⋅Vdewtevap⋅Frac(NH3)⋅17 031,
where FNH3 is the average emission flux (in
ng m-2 s-1) during dew drying, [NH4+] is the
concentration of ammonium in dew (in µM), Vdew is the
volume of dew in the canopy (in L m-2), tevap is the time it
takes for dew to evaporate (in s), Frac(NH3) is the fraction of
NH4+ in the dew that is released as NH3, and 17 031 is to
convert µmol to ng. It is important to note that Eq. (3) yields the
average FNH3 during evaporation and cannot account for any
variations in FNH3 over the evaporation period. The dewmeter was
used to record Vdew and tevap, whereas sample from the
dew collector was used to quantify [NH4+] and calculate
Frac(NH3). The dewmeter is automated and was deployed continuously from
22 June until 31 August (and intermittently between 27 May and 21 June),
whereas the dew collector requires manual cleaning and collection and so was only
deployed when forecasts were favourable for dew formation.
Results and discussion
Fraction of NH3 that evaporates from drying dew
We tested the validity of Eq. (2) by performing a series of drying
experiments similar to Takenaka et al. (2009) but specifically targeting
conditions relevant for dew (i.e. composition and drying time). Takenaka et
al. (2009) used solutions in the mM range with drying times of ∼ 9 h,
whereas natural dew is typically less concentrated (µM range) and
usually dries within a few hours. The composition of synthetic dew (Table S1
in the Supplement) and drying time (∼ 2.5 h) in this work is a better
representation of natural dew.
Figure 1 shows the measured Frac(NH3) vs. predicted Frac(NH3)
from an updated form of Eq. (2) (see below for details) for the nine
synthetic dew. Drying experiments were performed three times per dew
composition, and error bars in Fig. 1 denote the standard deviation between
experiments. The amount of NHx (≡NH4+ + NH3)
recovered was always within 20 % of the amount of NH4+ added at
the beginning of the experiment. There is good agreement between the measured
and predicted Frac(NH3), which is mostly consistent with the findings of
Takenaka et al. (2009) with a few key differences: (1) the majority of
acetate and formate remained as a salt after evaporation, (2) HCO3-
was an important constituent in the anion balance, and (3) the pKa of each
substance must be considered. Although acetic acid, formic acid, and carbonic
acid are relatively volatile, the conjugate bases can (and do) form
non-volatile salts upon evaporation when there is an excess of cations.
Furthermore, if the pH is near or less than the pKa of the acids then a
significant fraction will be neutral (protonated) and unable to form a salt.
Hence, we update the definition of Σanions in Eq. (2) to include
acetate, formate, and bicarbonate (also reflected in Fig. 1), which yield
much better agreement in predicted vs. measured Frac(NH3).
Fraction of NH3 liberated during drying experiments vs. the
fraction predicted according to an updated Eq. (2) to include acetate,
formate, CO32-, and HCO3- in the anion balance. Excluding
these anions significantly reduces the correlation. Error bars represent
±σ from three experiments per synthetic dew. The dashed line is the
1 : 1 line.
Total organic carbon (TOC), total nitrogen (TN), inorganic carbon
(IC), pH, the ratio of measured to predicted [NH4+] in dew, and
parameters pertinent to NH3 flux calculations in the field dew samples.
Date
TOC
IC
TN
pH
Frac(NH3)
Vdew
tevap
Flux
[NH4+]meas : [NH4+]eqm
(mg C L-1)
(mg C L-1)
(mg N L-1)
(mL m-2)
(s)
(ng m-2 s-1)
28 May
0.65
0.52
0.05
5.46
1.0
79.8
6000
2.4
0.02
1 Jun
2.05
1.21
0.32
5.65
0.68
97.0
6600
4.9
0.08
23 Jun
6.10
0.58
0.61
5.35
1.0
167.2
10 800
7.3
0.02
27 Jun
6.13
0.59
0.62
5.70
0.85
195.6
9000
11.0
0.05
28 Jun
9.69
0.56
0.95
5.16
1.0
161.6
8400
17.9
0.04
29 Jun
5.27
0.19
0.46
4.83
1.0
60.9
3000
7.3
0.01
30 Jun
6.71
0.22
0.32
4.99
1.0
163.4
7800
3.3
0.01
4 Jul
6.78
0.23
1.40
5.32
1.0
206.8
16 800
2.5
0.02
19 Jul
6.53
0.11
1.47
5.85
1.0
188.2
24 600
1.0
0.08
29 Jul
10.04
0.31
2.59
5.80
1.0
92.2
8400
5.4
0.09
10 Aug
7.54
0.38
0.80
5.34
1.0
96.9
7200
6.9
0.07
11 Aug
7.28
0.17
0.85
4.67
0.74
108.4
14 400
4.2
0.02
Avg
6.23
0.42
0.85
5.19
0.94
134.8
10 250
6.2
0.04
Since ion chromatography quantifies the total amount of each species (i.e.
both charged and neutral forms) it is necessary to use pH and the acid
dissociation constant (Ka) for each species to calculate the ionic
fraction of each. Furthermore, Takenaka et al. (2009) recommend including
carbonate/bicarbonate in the ion balance for field samples. The authors did
not account for CO2 equilibria since their lab experiments were
performed under strict CO2-free conditions, whereas our synthetic dew
samples had sufficient exposure to lab air to equilibrate with atmospheric
CO2 (∼ 500 ppm in the lab) as verified by subsequent inorganic
carbon measurements (Sect. 2.4). Hence, we calculated the amount of
HCO3- and CO32- in synthetic dew using pH and carbonate
equilibria assuming PCO2 = 500 ppm. Charge imbalance
calculated in Eq. (2) is a result of CO2 dissolving (or outgassing when a
large quantity of bicarbonate/carbonate salt was added) as well as the
addition of HCl or NaOH.
Dew parameters
A total of 12 dew samples for chemical analysis were collected at RMNP over
the study period. The equivalent concentrations of ions are given in Fig. 2
and TOC, IC, TN, pH, and Frac(NH3) in Table 1. Average values of
[NH4+] in dew found in the literature span several orders of
magnitude ranging from 25 µM in coastal Croatia (Lekouch et al.,
2010) to 1600 µM in urban India (Yadav and Kumar, 2014). Dew at
RMNP is at the lower end of this range with median
[NH4+] = 28 µM. In general, the concentrations of all
species in RMNP dew are lower than most previous studies (e.g. Singh et al.,
2006; Takenaka et al., 2003; Wagner et al., 1992). This is due to the
remoteness of RMNP resulting in low levels of coarse mode aerosol and
water-soluble gases which tend to control the composition of dew via
deposition and dissolution (Takeuchi, 2003; Wagner et al., 1992). The
dominant cations in dew at RMNP are Ca2+ and NH4+. The former is
likely from the deposition of coarse mode soil and/or dust particles and the
latter from gas-phase dissolution of NH3. Acetate and formate are the
major anions and may be the result of dissolution of acetic and formic acid
(Wagner et al., 1992) and/or the products of aqueous-phase oxidation of
semi-volatile organics (SVOCs, e.g. aldehydes) which has been observed in
cloud and fog water (Herckes et al., 2007, 2013; Munger et al., 1989). The
area surrounding the field site is heavily forested and the boundary layer is
likely rich in biogenic SVOCs, which could explain the high TOC content in the
dew (average = 6.23 mg C L-1). The ability for dew to act as a
medium for aqueous-phase oxidation of SVOCs is outside the scope of this
paper but warrants further investigation.
Ionic composition (in µN) of ambient dew collected at
RMNP.
The average pH of dew at RMNP was 5.19 (median = 5.34), which is on the
lower range of what has been reported for dew. For instance, Yaalon and
Ganor (1968) and Xu et al. (2015) found median dew pH of 7.7 and 6.72 in
Jerusalem and Changchun, China, respectively, whereas Pierson et al. (1986)
reported an average dew pH of 4.0 at a rural site in Pennsylvania in a region
containing several coal-fired power plants. Given the remoteness of RMNP and
low ionic concentrations, CO2 dissolution plays an important role in
governing dew pH. Acidic dew are considered to enhance deposition of
NH3 and hinder that of certain weakly acidic gases (e.g. SO2,
organic acids) (Chameides, 1987; Okochi et al., 1996). In addition, the
average summertime NH3 mixing ratio at RMNP is about a factor of 3
higher than that of HNO3 (Benedict et al., 2013b), which is roughly the
same ratio as NH4+ : NO3- in dew measured in this study.
Figure 2 reveals a persistent ion imbalance for ambient dew samples. On
average, about 25 % more anion is needed to achieve ion balance with the
measured cations. This implies that some anions are unaccounted for in the
system. Possible explanations include (1) longer chain organic acids (e.g.
succinate, maleate, malonate, and pyruvate) and/or (2) silicates from
wind-blown dust.
Equation (2) was used to calculate Frac(NH3) for ambient dew samples
(average = 0.94). Only 3 of the 12 samples had a Frac(NH3)
less than 1 meaning that, in most cases, all of the NH4+ present is
predicted to volatilize as NH3 during dew evaporation. It is important
to note that acetate, formate, and HCO3- were included in the ∑anion budget in contrast to Takenaka et al. (2009). Had the aforementioned
anions not been included in the Frac(NH3) calculation then all dew
samples would have Frac(NH3) = 1.
The high Frac(NH3) has an important implication for N deposition:
NH3 that is dry deposited onto a surface wetted with dew does not
necessarily contribute to N deposition. In other words, NH3 deposited
into dew overnight should not necessarily be counted towards the total
N-deposition budget for a given ecosystem. The consequence of this
implication likely extends beyond RMNP and merits additional field
measurements of dew to calculate Frac(NH3) in other environments (e.g.
agricultural, urban, and rural). To our knowledge, this is the first field
study to quantify the extent to which NH4+ is released as NH3
during dew evaporation. Additional research is needed to examine the effects
of (1) salts already present on vegetative surfaces on dew composition,
(2) dew transfer from leaf to soil prior to evaporation, and (3) different
canopies (e.g. forest, tall grass) on the amount and timing of dew
accumulation and evaporation.
Dew–atmosphere NH3 fluxes
In this section we examine how the formation and evaporation of dew impacts
NH3 in the boundary layer. Figure 3 shows time series (from 19:00 to
11:00 the following day) of dew mass (g m-2), air temperature
(∘C), and NH3 mixing ratio (ppbv) on four separate nights with
dew. One feature common to all four panels is the increase of NH3 at the
onset of dew evaporation followed by a plateau or decrease of NH3 once
the surface had dried completely. The features in Fig. 3 are representative
of the other 29 nights in which dew formed during the study period (27 May to
31 August). It should be noted that in Fig. 3c and d the start of the
morning NH3 increase is slightly delayed from the onset of dew
evaporation. This may be attributed to canopy growth over the course of the
campaign – during May and June (Fig. 3a and b) the grassland canopy was
relatively short (∼ 5 cm) and roughly the same height as the
artificial turf on the dewmeter. However, during July (Fig. 3c) and August
(Fig. 3d) the canopy had grown significantly (up to ∼ 30 cm), providing
significant shade to lower parts of the grass such that dew finished
evaporating off the dewmeter prior to complete drying of the canopy. This
would also cause an underestimation of dew amount by the dewmeter towards the
end of the measurement period.
Dew accumulation (blue, g m-2), NH3 mixing ratio (orange,
ppbv), and air temperature (red, ∘C) overnight on
(a) 22 June, (b) 27 June, (c) 21 July, and
(d) 9 August 2015. The black line in (b) is the best fit
for the NH3 mixing ratio to an exponential decay function (see Eq. 4)
between 20:00 and the onset of dew evaporation.
The consistent timing between dew evaporation and the increase in NH3
mixing ratio is strong evidence that dew evaporation and the early morning
NH3 increases are linked, but other phenomena must be considered. For
instance, it is well known that NH3 emissions from plant stomata and
soil are heavily temperature dependent and increase at higher temperatures
(Massad et al., 2010; Sutton et al., 2013; Zhang et al., 2010). However,
NH3 decreases after dew evaporation ceases despite a continued increase
in temperature, suggesting that this morning increase is not from stomata or
soil emissions. Another possible explanation is reduced deposition after dew
evaporation since wet canopies provide a lower resistance to deposition for
water-soluble gases (e.g. NH3) relative to dry canopies (Fowler et al.,
2009; Neirynck and Ceulemans, 2008); however, this scenario requires other
continuous source(s) of NH3. If this were the mechanism responsible for
morning NH3 increases then one would expect a plateau in NH3 after
canopy drying. However, Fig. 3a, b, and d all show NH3 decreases after
dew evaporation. In addition, RMNP is sufficiently remote that morning
NH3 increases cannot be from rush-hour traffic or industrial sources.
It is also useful to consider the behaviour of NH3 on mornings without
dew. Of the 72 nights during which the dewmeter was deployed and functioning,
there was night-time rain on 23 of the nights and no surface wetness
(neither rain nor dew) at sunrise on 16 nights. Typically, dew formation
began around 20:30 and it had completely evaporated by 09:00 the following
morning. Figure 4 compares NH3 mixing ratios from 04:00 to 11:00 on
mornings with dew (Fig. 4a) and without dew or rain (Fig 4b). The clear
morning NH3 increase only happens on mornings with dew, further
supporting the hypothesis that dew evaporation has a significant influence on
near-surface NH3 mixing ratios. The traces in Fig. 4 are coloured
according to the average NH3 mixing ratio the previous night (from 19:00
to 21:00). The magnitude of the morning increase is related to the amount of
NH3 present the previous night suggesting that most of the NH4+
in dew is a result of NH3 dissolution. This is additional evidence that
NH3 deposited in dew overnight at RMNP is recycled back to the
atmosphere the following morning upon evaporation and should not be counted
towards total N deposition. In other words, the dew acts as a temporary
reservoir for atmospheric ammonia and the cycle of dew formation and
evaporation has a strong influence on boundary layer NH3 concentrations.
Time series of NH3 mixing ratio (in ppb) from 04:00 to 11:00 on
(a) mornings with dew and (b) mornings with no surface
wetness. Traces are coloured according to the average NH3 mixing ratio
measured the previous night between 19:00 and 21:00.
Table 1 shows the calculated NH3 fluxes from dew during evaporation
(average = 6.2 ng m-2 s-1) as well as the relevant
parameters required for flux calculations (tevap, Frac(NH3),
and Vdew). To our knowledge, only two studies to date have reported
NH3 fluxes in a non-fertilized grassland. Wichink Kruit et al. (2007)
used the aerodynamic gradient method to measure a daily average summertime
NH3 flux of 4 ng m-2 s-1 in a field in the Netherlands,
whereas Wentworth et al. (2014) inferred a daily average soil emission flux
of 2.6 ng m2 s-1 during August in a rural field near Toronto,
Canada, using simultaneous soil and atmospheric measurements and a simple
resistance model. In the context of these previous studies over the same land
type, the dew-related NH3 fluxes at RMNP are significant. Furthermore,
it is likely that dew-related NH3 fluxes would be substantially larger
at the other field sites given that NH3 mixing ratios were a factor of
3–10 higher which would result in higher dew [NH4+].
It is likely that during some periods the emission/deposition footprint of
the atmospheric and dew measurements extends beyond the grassland clearing
and into the surrounding forest. While we did not find that the overnight
loss rate of ammonia depended on dew amount, the deposition rate of ammonia
likely depends on surface type, so estimates of moles of NH3 deposited
per m2 from the dew collector may not be representative of the
surrounding forest. Upslope and downslope flow conditions could also explain
some of the variability in nocturnal NH3 since the latter is prevalent
during the night-time and delivers cleaner air from the west of RMNP.
Subsequent work should be performed to examine the representativeness of
grassland dew measurements to the larger surrounding ecosystem.
For the 12 dew samples listed in Table 1, a simple calculation was performed
to estimate the moles of NH4+ contained in dew relative to the moles
of NH3 in the boundary layer. Particulate NH4+ is not considered
due to its low mass loadings at RMNP (Benedict et al., 2013b). The
µmol m-2 of NH4+ in dew at the onset of evaporation
was calculated by multiplying Vdew by dew [NH4+]. One
inherent assumption is that [NH4+]dew on the collector is
representative of the dew on the dewmeter. An equivalent mole loading (also
in µmol m-2) of NH3 in the boundary layer was calculated
by first converting the measured mixing ratio from ppbv to
µmol m-3 and then multiplying by an assumed boundary layer
depth of 150 m. The average ratio of
NH4,dew+ : NH3,BL is 1.6 ± 0.7 for the
12 dew samples collected. In other words, on a per mole basis there is nearly
double the NH4+ in dew than there is NH3 in a 150 m deep
boundary layer. Unfortunately, there are no measurements at RMNP that allow a
better constraint of the boundary layer height. Assuming a smaller (larger)
boundary layer height would increase (decrease) the
NH4,dew+ : NH3,BL ratio.
The measured loss of NH3 (in ppbv) during dew nights was used to
estimate the sink of NH3 (in µmol m-2) between the onset
of dew formation and evaporation. This loss was estimated in a similar
fashion as above, assuming (1) 150 m nocturnal boundary layer, (2) no
reactive sinks (e.g. NH4NO3 formation), (3) no exchange with the
free troposphere, and (4) no influence from horizontal advection (i.e.
upslope/downslope flow) on NH3. Figure 5 shows a correlation plot of
estimated NH3 lost on dew nights vs. the observed NH4+
accumulated in dew. The good correlation and near-unity slope (0.71) show
that there is approximate mass closure between NH3 lost overnight and
NH3 sequestered by dew. Although these calculations are simplistic it is
evident that, on average, dew sequesters a significant portion (estimated at
nearly two-thirds) of NH3 over the course of the night. Subsequent
studies on dew–atmosphere interactions should include measurements of
boundary layer height so a more thorough mass balance calculation can be
performed.
Estimated NH3 lost overnight assuming a 150 m boundary layer
vs. measured NH4+ accumulated in dew by the onset of evaporation. The
red line is the best fit line (forced through the origin) and the dashed grey
line is the 1 : 1 line.
The loss rate of NH3 on dew nights vs. dry nights was examined by
fitting the NH3 mixing ratio to an exponential decay function between
20:00 and 09:00 (or dew evaporation) on the 46 nights in Fig. 4. The fit
function used was
[NH3]t=[NH3]sunsete-kt+[NH3]overnight,
where [NH3]t is the mixing ratio of NH3 at time t,
[NH3]sunset is the mixing ratio at 20:00,
[NH3]overnight is the plateau in nocturnal NH3 mixing
ratio, and k is an empirical fit parameter representing the apparent
first-order loss rate constant of NH3. An example of the fit is shown by
the black trace in Fig. 3b.
The average NH3 loss rate constant on dew nights was
1.33 ± 0.5 × 10-4 s-1 compared to
1.35 ± 0.3 × 10-4 s-1 on dry nights. In other
words, there is no significant difference in the rate of NH3 loss on dew
vs. non-dew nights. This implies that dew does not actually enhance NH3
deposition under these conditions, suggesting that the aerodynamic and
quasi-laminar resistances dominate over surface resistances. The average
nocturnal wind speed on dew nights was lower than on dry nights
(1.3 m s-1 vs. 2.2 m s-1). Lower wind speeds typically result
in a higher Ra and Rb. It is possible that increased
aerodynamic and quasi-laminar resistances on dew nights are partially
compensated for by a lower surface resistance due to dew, such that the
overall canopy resistance is similar on dew nights and dry nights. Average
nocturnal wind direction was from the northwest (i.e. downslope flow) on both dew
nights (307∘) and dry nights (313∘). The average nocturnal
maximum for RH was 75 % on dew nights and only 53 % on dry nights.
The lower wind speeds and higher RH on dew nights are consistent with the
meteorological conditions favourable for dew formation.
Deposition of NH3 on dry nights could be to either leaf cuticles and/or
soil pore water. However, it is not possible to unambiguously attribute the
nocturnal NH3 loss solely to deposition. Enhanced downslope flow of
cleaner air on dry nights cannot be ruled out as a contributor to nocturnal
NH3 loss. Since NH3 deposition is independent of dew amount, there
could be a large discrepancy between [NH4+] for dew on the dewmeter
vs. the dew collector if Vdew is significantly different on the two
surfaces. However, the campaign averages of Vdew on the dewmeter
(Table 1) are within 10 % of dew volume obtained off the collector
(data not shown) so [NH4+] is likely similar for dew on both
platforms.
Since most of the NH4+ in dew volatilizes and the presence of dew
does not affect NH3 deposition overnight, the net impact is a reduction
in the overall removal of NH3. As a result, the atmospheric lifetime and
range of NH3 transport will be extended.
Potential Influence from rain evaporation
Numerous studies have reported rapid increases of near-surface NH3
within 1–2 h after some rain events (e.g. Cooter et al., 2010; Walker et
al., 2013; Wentworth et al., 2014). Given the findings discussed in the
previous section, one possible explanation is the emission of NH3 from
drying rain droplets. However, unlike dew, some difficult-to-predict fraction
of rain will permeate through the soil, thus preventing or delaying the
release of NH3. Nonetheless, we attempt to qualitatively explore this
hypothesis by examining the Frac(NH3) of four rain samples collected at
RMNP as well as the behaviour of NH3 during rainfall evaporation. Rain
samples were collected with the same procedure used to collect dew, which
differs from the usual method of capturing precipitation via an automated
precipitation bucket (e.g. Benedict et al., 2013a). The precipitation bucket
is normally equipped with an O ring and lid to prevent dry deposition and
dissolution of water-soluble gases when it is not precipitating. However, precipitation on the dew collector surface was left exposed and its
composition is influenced by dry deposition and gas-phase dissolution until
it was collected at the onset of evaporation.
Table S2 in the Supplement gives the concentration of ions measured in rain
samples. In general, concentrations of ions are comparable between dew and
rain samples, with the exception of NH4+, SO42-, and
NO3-, which are a factor of 2–4 times more concentrated in rain
samples. The enhancement of these species in rain may reflect additional
in-cloud and below-cloud scavenging of gases (NH3, HNO3, and
SO2) and PM2.5 aloft. Another possibility is that rain generally
forms during upslope conditions which coincide with more polluted air masses
from east of RMNP, whereas dew typically forms during downslope (cleaner)
conditions. Numerous studies have compared dew composition to rain
composition and, in general, have found that concentrations are enhanced in
dew relative to rain (e.g. Polkowska et al., 2008; Wagner et al., 1992).
However, Pierson et al. (1986) reported dew composition to be similar to, but
more dilute than, rain at a rural site in Pennsylvania.
Table S3 shows the TOC, IC, TN, pH, and calculated Frac(NH3) for the four
rain samples. Rain samples were more acidic (average pH = 4.54) than dew
samples (average pH = 5.19). The average Frac(NH3) for rain samples
was 0.66 suggesting that, on average, roughly two-thirds of NH4+
contained in precipitation on surfaces should be liberated as NH3 upon
evaporation. This could pose a significant flux of NH3 to the boundary
layer; however, since the fraction of rain that remains on surfaces after
rainfall where it can readily evaporate is not constrained, only an upper
estimate on NH3 fluxes from drying rain can be calculated
(21.2 ± 13 ng m-2 s-1). This value was calculated in same
manner as the dew samples and assumes all rainfall evaporates.
Figure 6 shows time series of rain accumulation (g m-2), air
temperature (∘C), and NH3 mixing ratio (ppbv) on 4 separate
days with observed rainfall. The rain accumulation was measured with the
dewmeter; 1000 g m-2 of accumulation is equivalent to 1 mm of
rainfall. Rainfall in excess of 2000 g m-2 flooded the collection tray
and could not be reliably recorded by the dewmeter. On 24 June (Fig. 6a)
there were three light rainfalls at 15:00, 16:00, and 19:00. The first event
at 15:00 was accompanied by a rapid decrease in NH3 likely due to
scavenging by rain droplets; however, this was not observed for the other two
rainfalls that day. For the second rain event in Fig. 6a (at 16:00) a
substantial increase in NH3 (from 0.5 to 1.5 ppbv) was observed during
evaporation and is consistent with NH3 liberation from evaporating rain.
However, evaporation of the other rain events on 24 June (Fig. 6a) as well as
those on 27 June (Fig. 6b) and 11 July (Fig. 6c) is not associated with
concomitant increases in NH3, implying that these rain evaporation
events did not release NH3. The evaporation of a more substantial
rainfall on 13 August (Fig. 6d) is associated with a temporary rise in
NH3 until evaporation ceases at sundown. The instances of rain
evaporation not associated with NH3 increases could be due to rain with
a low Frac(NH3), an insignificant amount of NH4+ in the rain,
more atmospheric dilution than dew mornings due to higher turbulence, and/or
significant rain penetration into the soil.
Rain accumulation (blue, g m-2), NH3 mixing ratio
(orange, ppbv), and air temperature (red, ∘C) during the afternoon and
evening on a) 24 June, (b) 27 June, (c) 11 July, and
(d) 13 August 2015; 100 g m-2 is equivalent to 0.1 mm of
rain.
The results from Fig. 6 are consistent with previous literature showing
NH3 increase immediately following only some rainfall events (Cooter et
al., 2010; Walker et al., 2013; Wentworth et al., 2014). The timing of some
rain evaporation events with NH3 increases, as well as the high
Frac(NH3) (average = 0.66) of the four measured rain samples,
suggests it is possible for rain evaporation from surfaces to be a
substantial source of NH3. Neirynck and Ceulemans (2008) reported
NH3 increases concomitant with a drying forest canopy (after rainfall)
as measured by a leaf wetness sensor.
Currently, all NH4+ collected in precipitation samples is counted
towards N deposition. However, if a fraction of NH4+ in rainfall is
emitted as NH3 during evaporation then N deposition could be
overestimated. At RMNP, wet deposition of NHx and dry deposition of
NH3 account for 35 and 18 % respectively of total reactive
nitrogen deposition to the site (Benedict et al., 2013a). This budget does
not take into account any re-emission of NH3 from drying rain. This
budget also does not explicitly account for ammonia uptake or emission during
dew formation and evaporation. A more extensive suite of dew and rainfall
measurements is necessary to quantify the impact of evaporation on annual
N-deposition budgets at RMNP.
Implications for other Gases
Other water-soluble gases with similar or larger effective Henry's law
constants (KHeff) to NH3 are likely influenced by dew
and rain evaporation as well, provided that the relative abundance of
counterions allows for volatilization during evaporation.
KHeff is the equilibrium constant for describing
gas-aqueous partitioning and accounts for chemical equilibria in solution.
Since acid–base equilibria are pH dependent, then the KHeff
for acidic and basic species is also pH dependent (Sander, 2015).
KHeff of NH3 was calculated for the 12 dew samples
using data from Sander (2015) to determine the temperature-dependent Henry's
law constant (KH) and from Bates and Pinching (1950) for the
temperature-dependent acid dissociation constant (Ka) of
NH4+ required for the calculation of KHeff. During
the study, dew KHeff spanned 2 orders of magnitude and
ranged from 4.5×105 to 2.7×107 M atm-1. These
high values are indicative of the high water solubility of NH3 at the
observed pHs and temperatures. Chameides (1987) used a simple resistance
model to show that deposition of gas-phase species with
KHeff > 105 M atm-1 to wetted surfaces
(i.e. dew) will be limited by the aerodynamic resistance since the surface
resistance is negligible for such highly water-soluble species. In other
words, it is likely that dew will be a significant night-time sink for other
trace gas species with KHeff > 105 M atm-1
since the dissolution into dew is controlled by aerodynamic processes
independent of the identity of the gas.
Table 1 shows the ratio of [NH4+] measured in dew to the
concentration predicted from equilibrium calculations using
KHeff and measured NH3 mixing ratio at the onset of
evaporation. The average ratio is low (0.04), consistent with a significant
aerodynamic resistance that prevents NH4+ saturation in dew droplets
overnight.
It has been suggested that dew can act as a reservoir for phenol,
nitrophenols, formaldehyde, and HONO based on observations of these species in
dew in Santiago, Chile (Rubio et al., 2009, 2012). Zhou et al. (2002) found a
correlation between high night-time RH (a surrogate for dew formation) and
HONO increases the following morning coincident with a decrease in RH. A
follow-up study (He et al., 2006) confirmed aqueous solutions mimicking dew
can release > 90 % of NO2- as HONO upon evaporation and
observed similar HONO pulses during canopy drying at a rural forest site in
Michigan. Indeed, there is some evidence in the literature that water-soluble
gases (primarily HONO) exhibit a similar behaviour to NH3 during dew
formation and evaporation observed in this study.
Table 2 shows the calculated KHeff (at 10 ∘C) for
common water-soluble gases that could be influenced by dew
formation/evaporation. This table is by no means exhaustive, but it highlights
the important role dew may have as a night-time reservoir and morning source
for gases other than NH3. Formic acid (HCOOH), acetic acid
(CH3COOH), nitrous acid (HONO), and nitric acid (HNO3) all have
increasing KHeff with increasing pH since a more basic
solution will promote dissociation of the acid into its conjugate base. The
average pH of dew at RMNP (∼ 5.2) is likely sufficiently acidic for
HONO to experience a surface resistance
(KHeff ≪ 105 M atm-1), which would limit
its transport across the dew–air interface. This is consistent with the low
average [NO2-] (0.2 µM) in dew at RMNP, although this might
simply reflect low HONO mixing ratios at the remote RMNP site.
KHeff of NH3 and other water-soluble gases at
10 ∘C and various pHs.
Gas
pH
KHeff (M atm-1)
NH3 (ammonia)
4.5
2.1×107
6
6.7×105
7.5
2.1×104
HCOOH (formic acid)
4.5
1.1×105
6
2.8×106
7.5
8.9×107
CH3COOH (acetic acid)
4.5
1.9×104
6
2.3×105
7.5
7.0×106
HONO (nitrous acid)
4.5
1.3×103
6
3.9×104
7.5
1.2×106
HNO3 (nitric acid)
4.5
5.3×1012
6
1.7×1014
7.5
5.3×1015
Future field studies on these species should include simultaneous
measurements of dew composition, dew amount, and gas-phase mixing ratios to
determine whether dew is an important night-time reservoir and morning
source. The latter will be dependent on the fraction of gas released upon dew
evaporation, which requires further investigation specific to each gas. Based
on the findings in this work and Takenaka et al. (2009) it is likely that
acidic semi-volatiles (e.g. acetic acid, formic acid, HONO) will be retained
as salts during dew evaporation at RMNP due to the excess of cations.
Conclusions
Laboratory experiments involving synthetic dew were performed to determine
the factor(s) controlling the fraction of NH4+ released as NH3
upon dew evaporation. Results were mostly consistent with Takenaka et
al. (2009), who found that the amount of NH3 that volatilized from drying
aqueous solutions is governed by the relative abundances of NH4+ and
excess “non-volatile” anions (Σanions - ∑cations). However, our findings suggest that
acetate, formate, and HCO3- should also be counted towards the anion
budget. Hence, the Frac(NH3) released from a drying dew sample can be
predicted given the ionic composition and pH.
A dewmeter (for dew amount, deployed continuously from 22 June to 31 August)
and dew collector (for dew composition, deployed successfully on 12
occasions) were set up at a remote field site in Colorado. Dew was relatively
dilute compared to previous studies and had an average [NH4+] of
26 µM and pH of 5.2 at sunrise. Simple calculations revealed that
dew can act as a significant night-time reservoir of NH3. At the onset
of dew evaporation there was, on average, roughly twice as much NH4+
in dew as NH3 in the boundary layer. Furthermore, the observed NH3
loss overnight was roughly equivalent to the amount of NH4+ that
accumulated in dew by sunrise. Dew composition was used to calculate an
average Frac(NH3) of 0.94, suggesting that the vast majority of NH3
sequestered in dew overnight is emitted during evaporation shortly after
sunrise. Mornings with dew experience a large increase in NH3 coincident
with dew evaporation. Once the dew has completely evaporated, NH3 mixing
ratios either plateau or decrease. Fluxes of NH3 from dew averaged
6.2 ± 5 ng m-2 s-1 during evaporation and were calculated
using measured [NH4+], Vdew, tevap, and
Frac(NH3). These fluxes are substantial compared to previously reported
fluxes in non-fertilized grasslands (Wentworth et al., 2014; Wichink Kruit et
al., 2007). Mornings without any surface wetness (neither dew nor rain) never
experienced a sharp increase in NH3. Dew-related NH3 fluxes are
likely much more substantial in urban and agricultural areas where NH3
and [NH4+] in dew are significantly higher than at RMNP.
Morning increases of NH3 frequently observed at RMNP (and other sites)
are very likely the result of NH3 emissions during dew evaporation. This
hypothesis is supported by (1) coincident timing of morning NH3
increases/decreases at the start/completion of dew evaporation, (2) lack of
NH3 morning increase on every non-dew morning, (3) significant NH3
fluxes calculated from dew, (4) relative abundances of NH4+ in dew
and NH3 in the boundary layer, and (5) approximate mass balance closure
between NH3 lost overnight and NH4+ accumulated in dew. The
phenomenon of dew “recycling” atmospheric NH3 could lead to an
overestimation of NH3 dry deposition in some ecosystems since dew formed
overnight can take up much of the near-surface ammonia and then release most
of it again in the morning upon evaporation. Such phenomena are generally not
considered in current models of NH3 dry deposition. In addition,
nocturnal loss rates of NH3 were unaffected by the presence of dew. Our
results suggest the net effect of dew is to reduce the overall removal of
NH3 and prolong its atmospheric lifetime as long as the dew composition
yields a high Frac(NH3).
Similar behaviour (coincident timing of NH3 increases and evaporation)
was occasionally observed for rain. Analysis of four rain samples yielded an
average Frac(NH3) of 0.66, suggesting NH3 can be released from
evaporation of rain in RMNP as well. However, due to the limited number of
samples and lack of constraint for amount of rain sequestered below ground it
is currently impossible to be even semi-quantitative about potential NH3
fluxes from rain evaporation. This uncertainty merits further research since
NHx wet deposition does not account for re-release of NH3 from
evaporation. Subsequent studies should also examine (1) the role of
biological processes on surface water composition (e.g. stomatal exchange,
modification via microbes) and (2) influence of guttation (leaf exudate) on
surface–air NH3 exchange.
Additional field measurements quantifying NH3 release from dew and rain
evaporation are needed to determine how relevant these phenomena are for
modulating NH3 mixing ratios and N deposition in different environments
(e.g. urban, rural, agricultural). Although the majority of NH4+ in
dew was released back to the atmosphere at RMNP, this is not necessarily the
case at other locations. For instance, environments with HNO3 deposition
exceeding NH3 deposition to dew would cause a low (or zero)
Frac(NH3). In addition, a tall canopy can recapture near-surface
NH3 emissions and might modulate emissions from dew drying in the lower
canopy (Walker et al., 2013). Regardless, the ability for dew to act as a
morning source of NH3 is currently absent from atmospheric models, with
the exception of a few field-scale models based on the work of Flechard et
al. (1999). The observations from this study suggest dew imparts a large
influence on boundary layer NH3; hence, future work should also focus on
developing model parameterizations for NH3 uptake during dew formation
and release from evaporating dew.
To our knowledge, this is the first study to quantitatively examine the
influence of dew on any water-soluble gas by simultaneously measuring dew
amount, dew composition, and atmospheric composition. Although NH3 is the
focus of this work, gases with similar KHeff
(> 105 M atm-1) might be influenced by dew formation and
evaporation in a comparable manner. Such species include, but are not limited
to, acetic acid, formic acid, HONO, and HNO3. Methodology similar to this
study should be used to conduct quantitative field studies for the
aforementioned species to better understand the dynamic influence of dew on
boundary layer composition.