Wintertime ammonium nitrate (NH4NO3) pollution events
burden urban mountain basins around the globe. In the Salt Lake Valley of
Utah in the United States, such pollution events are often driven by the
formation of persistent cold-air pools (PCAPs) that trap emissions near the
surface for several consecutive days. As a result, secondary pollutants
including fine particulate matter less than 2.5 µm in diameter
(PM2.5), largely in the form of NH4NO3, build up during these
events and lead to severe haze. As part of an extensive measurement campaign
to understand the chemical processes underlying PM2.5 formation, the
2017 Utah Winter Fine Particulate Study, water-soluble trace gases and
PM2.5 constituents were continuously monitored using the ambient ion
monitoring ion chromatograph (AIM-IC) system at the University of Utah
campus. Gas-phase NH3, HNO3, HCl, and SO2 along with
particulate NH4+, Na+, K+, Mg2+, Ca2+,
NO3-, Cl-, and SO42- were measured from 21 January
to 21 February 2017. During the two PCAP events captured, the fine
particulate matter was dominated by secondary NH4NO3. The
comparison of total nitrate (HNO3+ PM2.5 NO3-) and
total NHx (NH3+ PM2.5 NH4+) showed NHx was
in excess during both pollution events. However, chemical composition
analysis of the snowpack during the first PCAP event revealed that the total
concentration of deposited NO3- was nearly 3 times greater
than that of deposited NH4+. Daily snow composition measurements
showed a strong correlation between NO3- and Ca2+ in the
snowpack. The presence of non-volatile salts (Na+, Ca2+, and
Mg2+), which are frequently associated with coarse-mode dust, was also
detected in PM2.5 by the AIM-IC during the two PCAP events,
accounting for roughly 5 % of total mass loading. The presence of a
significant particle mass and surface area in the coarse mode during the
first PCAP event was indicated by size-resolved particle measurements from
an aerodynamic particle sizer. Taken together, these observations imply that
atmospheric measurements of the gas-phase and fine-mode particle nitrate may
not represent the total burden of nitrate in the atmosphere, implying a
potentially significant role for uptake by coarse-mode dust. Using the
NO3- : NH4+ ratio observed in the snowpack to estimate the
proportion of atmospheric nitrate present in the coarse mode, we estimate
that the amount of secondary NH4NO3 could double in the absence of
the coarse-mode sink. The underestimation of total nitrate indicates an
incomplete account of the total oxidant production during PCAP events. The
ability of coarse particles to permanently remove HNO3 and influence
PM2.5 formation is discussed using information about particle
composition and size distribution.
Introduction
Episodes with high particulate matter (PM) pollution occur frequently in
urban air basins across the globe during winter months when a stable
boundary layer persists for multiple days, including in the mountain valleys in
the western USA
(Baasandorj
et al., 2017; Bares et al., 2018; Green et al., 2015; Silcox et al., 2012;
Whiteman et al., 2014), the Po Valley in Italy
(Bernardoni
et al., 2017; Vecchi et al., 2018), and the Sichuan
(Tian et al., 2019) and Twin-Hu
(Gao et
al., 2019) basins in China. These mountain basins, including the Salt Lake
Valley (SLV) in northern Utah, experience strong temperature inversions that
develop into persistent cold-air pools (PCAPs), which suppress vertical
mixing and trap emissions within a shallow boundary layer
(Lareau
et al., 2013; Whiteman et al., 2014). Under these conditions, mass loadings
of fine particles smaller than 2.5 µm (PM2.5) often reach values
of 60–80 µg m-3 in the western USA, which are far above the US
National Ambient Air Quality Standard (35 µg m-3, 24 h average)
(Lareau
et al., 2013; Silcox et al., 2012; Whiteman et al., 2014). Along with
environmental impacts such as haze and localized deposition, mass loadings
of PM2.5 that exceed this standard have been associated with increased
risk of mortality, especially cardiopulmonary or cardiovascular disease
mortality
(Pope et
al., 2003, 2017). However, despite significant societal concerns about the
impact on human health from these PM2.5 pollution episodes and their
common occurrence around the world, the major chemical processes that drive
PM formation in these regions are still uncertain.
Processes influencing the concentrations of NH3, NOx,
HNO3, and NH4NO3. Pathways that occur during the day are shown
in dashed grey arrows, while pathways that occur during the night are shown
in black. The rate of uptake of HNO3 to coarse particles, ≥ PM1, composed of CaCO3 and/or NaCl is unknown.
Several studies conducted in northern Utah have shown that PM2.5 during
these PCAP episodes is predominantly composed of ammonium nitrate
(NH4NO3), accounting for roughly 60 %–80 % of the total dry particle mass (Baasandorj
et al., 2017; Franchin et al., 2018; Hansen et al., 2013; Kelly et al.,
2013; Kuprov et al., 2014; Long et al., 2002, 2003; Mangelson et al., 1997).
NH4NO3 formation is thermodynamically favorable under conditions
with low temperatures and high relative humidity based on equilibrium
partitioning with gas-phase ammonia (NH3) and nitric acid (HNO3)
(Mozurkewich,
1993; Nowak et al., 2010; Seinfeld and Pandis, 2006), illustrated in the
right-hand portion of Fig. 1.
NH3 emissions are often associated with agricultural activities and
waste disposal; however, in recent years automotive emissions and industrial
processes have become increasingly important sources in urban areas
(Behera
et al., 2013; Bishop et al., 2010, 2016; Livingston et al., 2009; Nowak et
al., 2012; Roth et al., 2019; Suarez-Bertoa et al., 2014; Sun et al., 2017).
In contrast, HNO3 is formed in the atmosphere and is a major sink of
nitrogen oxides, which are emitted primarily through fossil fuel combustion.
The two dominant mechanisms highlighted in Fig. 1 that lead to HNO3
occur either (1) from oxidation of NO2 via reaction with OH (in orange),
which is photochemically driven during the day, or (2) through heterogenous
uptake of N2O5 (in black), which typically occurs at night. Gas-phase HNO3 and NH3 and particulate NH4NO3 are removed
from the atmosphere through dry and wet deposition under typical atmospheric
conditions. However, under PCAP conditions, the stably stratified boundary
layer reduces convective mixing, which in turn allows pollutants to
accumulate. In addition, HNO3 can also be lost through heterogenous
reaction with dust and sea salt components, such as CaCO3 and NaCl
(Beichert
and Finlayson-Pitts, 1996; Dasgupta et al., 2007; Fenter et al., 1994; Liu
et al., 2008a). When the loss of HNO3 to deposition is suppressed under
PCAP conditions, the loss to reactive uptake to airborne dust and sea salt
can potentially become more important.
To formulate effective control strategies that reduce wintertime PM2.5
in northern Utah, measurement campaigns have been conducted to improve our
understanding of fine particulate formation during these PCAP episodes (Baasandorj
et al., 2017; Bares et al., 2018; Hansen et al., 2010; Kelly et al., 2013;
Kuprov et al., 2014; Lareau et al., 2013; Malek et al., 2006; Mangelson et
al., 1997; Silcox et al., 2012; Whiteman et al., 2014). Several analyses of
the PM build-up in cold-air pool events in the SLV have been published, all
based on measurements from one or several ground sites
(Baasandorj
et al., 2017; Kuprov et al., 2014; Long et al., 2003; Silcox et al., 2012).
Long-term PM2.5 composition measurements in the SLV are made using
filter samples
(Kelly
et al., 2013; Kuprov et al., 2014; Long et al., 2003; Mangelson et al.,
1997; Silcox et al., 2012; Whiteman et al., 2014), with PM2.5
speciation carried out on filter extracts in accordance with the US Environmental
Protection Agency (EPA) Speciation Trends Network (STN) protocol. Kuprov
et al. (2014) were the first to report the gas and PM2.5 composition
during pollution events in northern Utah using an ambient ion monitoring ion chromatograph (AIM-IC) system; however, this study only measured
anionic species, providing HNO3 and particle nitrate (pNO3-)
concentrations at an hourly resolution. Ambient NH3 concentrations were
monitored using a chemiluminescence-based NH3 analyzer, but particulate
ammonium (pNH4+) had to be inferred based on the assumption that
both pNO3- and particulate sulfate (pSO42-) were in the
form of their respective ammonium salts. Particle mass fractions of crustal
and carbonaceous material in PM2.5 were calculated on 24 h average
measurements of Al, Si, and elemental and organic carbon in fine particles from
the NCore program (US EPA, 2010).
Recent research in the SLV has focused on determining whether ammonium
nitrate mass loading is most sensitive to reductions in NH3 or
HNO3 to inform PM2.5 reduction strategies
(Kuprov et al.,
2014). Baasandorj et al. (2017) recently proposed that PM2.5 forms
overnight in the upper layers of the PCAP and mixes down to the surface,
enhancing the total PM2.5 loading experienced in the SLV. This
explanation of stratified particulate nitrate chemistry throughout PCAP
events suggests a complex coupling of a HNO3-limited surface layer with
a NH3-limited elevated layer
(Baasandorj et al., 2017).
Aircraft measurements during the 2017 Utah Winter Fine Particulate Study
(UWFPS) provided the first detailed vertically resolved chemical composition
of these PCAP episodes in the SLV. Analysis presented in McDuffie et al. (2019) confirms that the formation of NH4NO3 during these
pollution events is largely limited by HNO3 but with more frequent
periods approaching NH3-limited conditions in the upper boundary layer
over time
(McDuffie et
al., 2019). Regimes of both NH3-limited and HNO3-limited
conditions are also consistent with an aerosol thermodynamic model
sensitivity study of the same data by Franchin et al. (2018), who found
simulations of non-refractory PM1 mass were sensitive to reductions in
both total ammonium and total nitrate
(Franchin et al., 2018).
Box model studies of the odd oxygen budget during UWFPS by Womack et al. (2019) assumed that NH4NO3 formation during PCAP episodes is
HNO3-limited, and found full episodes tend to be NOx-saturated. In
conditions where pNO3- production is HNO3-limited, it is
often assumed that reduction in NOx would directly lead to reductions
in HNO3. However, Womack et al. (2019) showed the production of
HNO3, and therefore, NH4NO3 formation, is sensitive to
changes in VOC concentrations emphasizing the complexity of chemical
processes involved in PM formation under these conditions.
The balance of factors influencing HNO3 formation are still being
understood. Modelling results from both McDuffie et al. (2019) and Womack et
al. (2019) predict that up to ∼ 50 % of HNO3
production is from the heterogeneous N2O5 pathway in the residual
layer. This is in line with ground-based measurements of N2O5 by
Baasandorj et al. (2017) highlighting the importance of nighttime pathway
(Baasandorj et al., 2017). Less
attention has been paid to the possibility that alternate sinks for
HNO3 may also significantly limit the amount of nitrate that can
contribute to PM2.5 formation.
Here, results are presented from ground site observations in Salt Lake City
from a moderately elevated site at the edge of the SLV during the UWFPS
campaign in January and February 2017. Atmospheric concentrations of
inorganic trace gases and water-soluble PM2.5 constituents were
measured by an AIM-IC, equipped with both cation and anion ion
chromatographs. During PCAP episodes, we observed the formation and build-up
of NH4NO3 over several days. Between the hours of 08:00 and 18:00 mountain standard time (MST)
on several PCAP days, we also observed the presence of elevated levels of
non-volatile cations in PM2.5. To investigate the role of
mineral–coarse-mode aerosols in limiting the availability of HNO3 during
these events, the final section presents estimates of HNO3 lifetime
against reactive uptake onto mineral–coarse-mode dust based on aerodynamic
particle sizer (APS) data and estimates of the total nitrate budget using
the chemical composition of the snowpack measured during UWFPS.
MethodsSite description
The SLV (1300 km2) is a mountain basin in northern Utah surrounded by
steep mountain ranges with the Great Salt Lake sitting to the northwest. The
Wasatch Mountains (peak at 3636 m a.s.l.) span the entire eastern border of the
valley with the Oquirrh Mountains (peak at 3235 m a.s.l.) to the west. There is
a narrow passage in the southern Traverse Range (1878 m a.s.l.) that creates a
small opening into the Utah Valley. Salt Lake City sits at the base of the
valley (1288 m a.s.l.), making a vast elevation difference between the
metropolis and the surrounding mountains. Measurements were conducted from
the rooftop of the William Brown Building 33 m above ground level at the
University of Utah (UU) located 40∘45′58.7′′ N, 111∘50′51.6′′ W, in Salt Lake City. The UU site is located 155 m above the valley
floor along the northeast sidewall of the SLV. The site is impacted by local
traffic that enters the university on the north side of campus.
Instrumentation
Hourly averages of PM2.5 chemical composition and gas-phase precursors
were measured using an online continuous AIM-IC instrument (Model 9000D, URG
Corp, Chapel Hill, NC). Briefly, gases and particles are sampled from the
same airflow by passing through a short inlet with elements for size
selection (PM2.5 impactor), gas collection (wet parallel plate
denuder), and particle capture (saturation chamber) at a flow rate of 3 L min-1. Aqueous solutions containing analytes originally in the gas and
particle phases are separately transferred through 22 m inlet lines into 5 mL glass syringes that are subsequently analyzed by ion chromatographs (ICs) following hour-long
collection periods. More in-depth details about the AIM-IC system, including
important adaptations from the standard commercial system, can be found in
Markovic et al. (2012). During the UWFPS campaign, the components of the
inlet assembly were housed in a small weatherproof box 2 m above the roof of
the UU site and the IC systems were housed in a laboratory below. Standard
calibrations of the ICs were performed offline using mixed ion standard
solutions before and after the measurement period. Charged species
(Na+, NH4+, K+, Mg2+, Ca2+, NH3, amines,
Cl-, NO2-, NO3-, and SO42-) were measured
using a Dionex ICS-2000 equipped with concentrator columns (TCC-ULP1 and
TAC-ULP1), guard columns (CG17 and AG19), and 4 mm analytical columns (CS17
and AS19). Gradient elution methods, using methanesulfonic acid (MSA) and
potassium hydroxide (KOH), were carried out using electrolytically
regenerated suppressors to reduce the influence of the eluent in the
conductivity detection. Carbonate salts are never quantified in the anion IC
due to the natural presence of carbonate in the distilled deionized water
reservoir that sustains the IC systems. Background measurements of the
entire AIM-IC system were conducted by overflowing the sampling inlet using
high-purity zero air to determine the method zero and detection limits of
each species. The limits of detection (LODs), based on 3σ of the
background, for trace gases were found to be 150, 20, 40, and 10 ppt for NH3, HCl, HNO3, and SO2, respectively. The LODs for
PM2.5 chemical constituents were 0.2, 0.1, 0.1, 0.04, and 0.4 µg m-3, for pNH4+, pNa+, pK+, pMg2+, and
pCa2+ and 0.03, 0.03, and 0.01 µg m-3 for pCl-, pNO3-, and pSO42-.
Traces gases and PM2.5 constituents measured as cations are reported
from 21 January to 21 February 2017. Due to technical difficulties, anionic
constituents measured from 21 January to 8 February were compromised.
Therefore, PM2.5 anionic components are only reported from 13 to 21 February and the trace gases measured as anions during the former period may
be less reliable.
Continuous measurements of total PM2.5 mass concentrations were also
captured at the site using the Filter Dynamics Measurement System (FDMS) with
a tapered element oscillating microbalance (TEOM) ambient particulate monitor (FDMS
TEOM 1400ab, Thermo Fisher Scientific) provided by the Utah Division of Air
Quality (UDAQ). Ambient air was sampled at ∼ 1.5 m above the
roof through a ∼ 7 m long 1/4′′ o.d. PFA line with a
volumetric flow rate of 20 L min-1. A custom-made inertial PM impactor was
connected to the entrance of the sampling inlet to remove coarse PM and
water from the sample flow. A critical orifice was installed downstream of
the PM impactor to restrict the sample flow, maintaining a pressure below 200 mbar. The air sample was sent through an additional filter to remove
particles larger than 2.5 µm. An aerodynamic particle sizer (APS; TSI
Inc, Model 3321) was operated from 26 January to 10 February to size and
detect particles with an aerodynamic diameter of between 0.54 and 19.81 µm based on the time of flight of the particle between two (633 nm) He-Ne
lasers. This instrument was attached to an aerosol inlet that included a 2.54 cm diameter stainless-steel tube connected to a stainless-steel rain cap
with approximately a 10 cm diameter. Aerosol mass was calculated using the
Aerosol Instrument Manager (AIM) software for the APS Model 3321. The
particle density used for calculating the aerosol mass concentration was 1.0 g cm-3, and particles were assumed to be spherical. The APS was attached
to this inlet system via a custom-made stainless-steel pickoff. To improve
aerosol transmission through the inlet, an external pump was attached to the
system. The pump pulled at approximately 7.2 L min-1. The calculated
transmission shows a 50 % cutoff at approximately 7 µm for this
inlet system (Skiles et al., 2018).
Snow sampling and analysis
The inorganic chemical composition (Na+, NH4+, K+,
Mg2+, Ca2+, NH3, Cl-, NO2-, NO3-,
and SO42-) of the snowpack was monitored throughout the campaign
during periods of snow cover. The snowpack composition and depth were
measured daily within a 2 m radius around the AIM-IC inlet atop UU using a
clean graduated 5 cm diameter beveled snow corer. Samples were collected in
triplicate along with a field blank to account for any contamination during
sampling. Collected samples were melted in sealed containers. The resulting
snowmelt solution was measured for total melt volume and pH (Hach
sensION+ PH1 Portable pH Meter). Snowmelt samples were
then filtered through 0.25 µm PTFE filters (Whatman, VWR
International) before analysis utilizing ion chromatography.
Aerosol thermodynamic modelling using ISORROPIA
Thermodynamic modelling was used to examine the expected gas and particle
partitioning of semi-volatile constituents based on the local meteorological
conditions and the observed concentrations of gas and particle constituents.
These models carry out bulk calculations to estimate the gas-phase and
particle-phase composition of inorganic species, in which all particles are
assumed to have the same chemical composition. To examine how the presence
of non-volatile particle components observed during this period may affect
nitrate partitioning, ISORROPIA v2.1 (Nenes et al., 1998; Fountoukis and
Nenes, 2007) was used. Model calculations were run in forward mode, with
observed total ammonia (NH3+pNH4+), particulate sulfate
(pSO42-), total nitrate (HNO3+pNO3-), total
chloride (HCl +pCl-), and non-volatile particulate species (pK+,
pNa+, pCa2+, and pMg2+), along with ambient relative humidity
and temperature, as inputs. Model runs were conducted using the metastable
state option, which prevents solid formation by forcing all particulate
components to remain in an aqueous state.
Results and discussionTrace gas and PM<inline-formula><mml:math id="M231" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2.5</mml:mn></mml:msub></mml:math></inline-formula> composition in the Salt Lake Valley
The AIM-IC measurement period included two PCAP pollution episodes, 27 January to 3 February 2017 (episode 1) and 13 to 17 February 2017 (episode 2), defined by the meteorological conditions that stabilize the boundary
layer and by the total observed PM2.5 mass, separated by a relatively
clean period from 5 to 12 February 2017. The PCAP episodes can be clearly
seen in the build-up of PM2.5 mass and the inorganic constituents
measured by the AIM-IC as shown in Fig. 2. Pollution periods with a total 24 h
average PM2.5 mass > 17.5 µg m-3 are deemed PCAP
periods, while periods with a 24 h average PM2.5 mass ≤ 2 µg m-3 are considered clean periods, as previously defined by Whiteman et al. (2014). The
pollution period from 27 January to 3 February 2017 (episode 1) was the most
persistent and severe during the UWFPS measurement campaign.
(a) The time series of NH3 and pNH4+ observed by the AIM-IC and total PM2.5 mass loading (µg m-3) measured by the TEOM from 23 January to 19 February 2017. (b) The AIM-IC observations of inorganic gases HCl, HNO3, and SO2 and PM2.5 components pCl-, pNO2-, pNO3-, and pSO42- present in
PM2.5. (c) The AIM-IC observations of the non-volatile inorganic cations pNa+, pK+, pMg2+, and pCa2+ measured in
PM2.5 for the entire campaign period. The two PCAP pollution episodes
captured during the campaign are emphasized by the dotted black line. Time
is expressed in mountain standard time (MST).
Both pollution events are marked in Fig. 2, while the full suite of
inorganic gases and PM2.5 components was measured by the AIM-IC only
from 8 to 19 February 2017. The increase in total PM2.5 mass at the
onset of episode 1 on 27 January coincides with a rise in pNH4+,
seen in Fig. 2a, while there is an initial drop in ambient NH3
concentration before it plateaus at a mixing ratio of ∼ 1 ppb as
the PCAP progresses. The HNO3 and pNO3 data, depicted in Fig. 2b
for the second pollution event, confirm the major water-soluble component of
PM2.5 in the SLV is NH4NO3, as previously reported
(Hansen
et al., 2010; Kelly et al., 2013; Kuprov et al., 2014; Long et al., 2003).
Compared to the pCl- and pSO42- mass loadings measured during
that event, Fig. 2b shows the total mass of pNO3- is an order of
magnitude larger. Particularly at midday, the majority of the PM2.5
mass is NH4NO3. It is important to note the organic fraction of
PM2.5 was not measured by the AIM-IC. Based on TEOM total mass and
AIM-IC-measured mass fractions, the AIM-IC captured 75 % of the total
PM2.5 mass.
The distinction between PCAP episodes 1 and 2 is important to note because
the stability of the cold-air pool and its persistence influences the
intensity of PM2.5 build-up in the SLV. Episode 1 was the most severe,
persisting for 9 d and allowing PM2.5 daily averages to reach the
highest observed through the measurement period, a maximum hourly average of
60 µg m-3. The episode occurred a few days after a storm
deposited roughly 22 cm of snow across the SLV and Wasatch front. Ambient
temperatures remained below freezing, between 263 and 273 K, sustaining the
snowpack throughout the event. This in turn increased the surface albedo,
limiting the strength of convection that can disrupt the stable boundary
layer (Whiteman et al., 2014). Winds during this episode were light,
averaging 1 m s-1, with gusts under 7 m s-1 during the most stable
and calm cold pool period from 31 January to 2 February.
The second PCAP observed during the campaign was much shorter-lived with
moderate cold pool conditions only present from 13 through 17 February. This
episode was distinctly different than episode 1, having no snow cover and much
warmer conditions, with temperatures ranging from 270 to 284 K. The winter
snowpack in the surrounding mountain range showed signs of spring melt,
which extended the surface area of exposed ground. Winds were slightly
stronger, averaging 1.6 m s-1 with stronger gusts averaging around 8 m s-1 caused by stronger convection. The hourly PM2.5 average at the
UU site peaked at 28 µg m-3, and elevated PM levels only persisted for
2 to 3 full days, so the intensity and duration of episode 2 was about half
that of episode 1.
The presence of non-volatile cations (pCa2+, pMg2+, pK+, and
pNa+), seen in Fig. 2c, was also observed during PCAP events. Of all the
non-volatile cations observed, only potassium increases with the same
pattern as the increase in pNH4+ and total PM2.5 mass
measured by the TEOM. Potassium salts are often found in primary aerosols
formed from biomass or biofuel burning
(Pósfai
et al., 2003; Rissler et al., 2006). Wood burning is still a commonly used
source of heat during the winter months in the SLV and could be the source
of pK+ observed during the severe PCAP episode 1. Aerosol mass
spectrometer (AMS) data from Franchin et al. (2018) showed traces of
levoglucosan, a known marker for wood combustion, during the pollution
episode. However, levoglucosan was typically measured at close to background
values. Overall, the data reported from AMS measurements indicated the
organic material from wood combustion is not a dominant mass fraction on
the regional scale
(Franchin et al., 2018).
Recent radiocarbon analysis from ground sites in the SLV indicates that fossil
fuels were the dominant sources of carbonaceous aerosols during winter,
contributing to 88 % (80 % to 98 %) of the black carbon in aerosols and
58 % (48 % to 69 %) of the organic carbon in aerosols
(Mouteva et al., 2017).
Similarly, the AIM-IC data also suggest that pK+ salts generally
compose a small fraction of the total PM2.5 mass.
The alkali and alkaline metal components pCa2+, pMg2+ and
pNa+ are commonly found in larger primary particles often emitted from
the lofting of mineral dust and salts. These species are often found in
mineral aerosols, which typically range from 0.1 to more than 10 µm,
partially contributing to PM2.5 (Zender,
2003). The coarse-mode fraction of these primary particles, with diameters
greater than 2.5 µm, has sufficiently large settling velocities that
the particles have much shorter lifetimes in the atmosphere compared to PM2.5.
If the non-volatile cations observed in PM2.5 are present in the
largest fine particles, then their lifetime in the atmosphere is
considerably shorter than NH4NO3 found in the accumulation mode.
This may be the explanation for the more pronounced diel pattern of
pCa2+, pMg2+, and pNa+ during the PCAP periods. It is also
possible that because vertical mixing is suppressed during PCAP episodes,
the UU site is in contact with air from the mineral dust source region to a
measurable degree only at midday. During the clean periods, the
concentrations of these ions measured by the AIM-IC are very close to their
respective detection limits. pNa+, pCa2+ and pMg2+ PM2.5
components peak at midday during pollution events, suggesting similar and/or
co-located sources. Concentrations of pMg2+, which is an effective
tracer for mineral dust along with pCa2+
(Maxwell-Meier et al., 2004),
are present throughout the entire measurement period, averaging 0.02 µg m-3 and reaching a maximum of 0.15 µg m-3. It is known
that sea salt is the most common source of pNa+, so examining the
Ca-to-Na ratios can suggest the predominant source. Particularly for the SLV
region, the Great Salt Lake can also be a source of sea salt. In PCAP
episode 1, Ca-to-Na ratios were below 1, suggesting the presence of sea
salts. In contrast, in episode 2 ratios of Ca-to-Na were above 1, suggesting
the sources of these particulate components differ between the two PCAP
periods. Due to the heavy snowfall during episode 1, road salt was
continuously applied to all major roads and walkways throughout the UU
campus, which may act as a local source of Na salts during periods of heavy
traffic. The lack of snow cover during the latter episode and higher
temperatures may suggest some influence of mineral dust from the surrounding
ground surface areas, where sediments and weathered minerals are often
Ca-rich
(Chesselet
et al., 1972; Kassomenos et al., 2012; Kuo et al., 2005). Recent studies
have found the Great Salt Lake is quickly receding, suggesting the exposed
the lake bed could act as a source of dust and sea salt
(Hahnenberger
and Nicoll, 2012; Skiles et al., 2018; Wurtsbaugh et al., 2017). However,
the bulk PM2.5 chemical composition measured by the AIM-IC is not
detailed enough to unambiguously identify the source of non-volatile cations
observed during PCAP events.
Estimating HNO<inline-formula><mml:math id="M356" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">3</mml:mn></mml:msub></mml:math></inline-formula> loss to coarse-mode particles
Gas-phase HNO3 exists in equilibrium with semi-volatile nitrate in
PM2.5, and the extent of gas-to-particle partitioning depends on
temperature, relative humidity, and NH3, via the formation of
NH4NO3(s) in dry particles or its influence on particle acidity in
deliquesced particles. At the same time, HNO3 may undergo net uptake by
primary particles when it reacts with their constituents, for example
CaCO3 or NaCl, to form non-volatile nitrate salts. The rate of
HNO3 loss can be impacted by the composition of the reactive salt and
the reactive surface area available (Seinfeld and Pandis, 2006).
During the two PCAP episodes, we observed elevated midday concentrations of
pNa+, pCa2+, and pMg2+ in PM2.5 using the AIM-IC.
Inferences from aerodynamic particle sizing
Coarse-mode particles (diameters > 2.5 µm) were not
speciated during the UWFPS campaign. There are very few reports of coarse-mode nitrate in winter conditions that could offer an approximation of how
much HNO3 may have been taken up by coarse-mode particles in the SLV.
The study of Hansen et al. (2010) is the only one commenting on coarse-mode nitrate in
northern Utah region, in Lindon, ∼ 50 km located south of the SLV,
during January to February 2007. The authors observed high mass loadings, up to 80 µg m-3 of coarse aerosols (PM10–PM2.5), during PCAP
events, based on measurements from two GRIMM optical particle counters
(Hansen et
al., 2010). The authors did not report any quantification of the chemical
components of the coarse aerosols collected by integrated filter samples,
although they commented that the nitrate levels were relatively low.
(a, b) The total fine-mode (PM2.5) and coarse-mode (> PM2.5) surface area calculated in µm2 cm-3 from APS data. (c) The particle size distribution as a function of time, 28 January to 4 February, with particle size displayed along the y axis and color
contours representing mass that is normalized by size bin (dM/dlogDp). The
population of particles that the AIM-IC captures is below the labelled
PM2.5 line.
(a) Wind speed and temperature (∘C) measured at UU. The coarse particle (PM10–PM2.5) mass loading in µg m-3 as a function of time, 28 January to 4 February 2017, at three separate ground-based sites: Rose Park (b) shown in green and Hawthorne Elementary (c) shown in black, which are EPA monitoring sites, and the UU site (d) in blue.
To obtain information about the coarse-mode loadings during PCAP events in
UWFPS, APS data collected during the pollution episode from 28 January to 3 February are displayed in Fig. 3. The particle size bins are colored by mass
normalized to the size bin (dM/dlogDp), clearly showing elevated midday
burdens of coarse-mode particles, which become more pronounced during the
intensive cold-air pool period from 30 January to 2 February 2017. This is
consistent with the midday increases in coarse-mode mass observed across
the SLV, shown in Fig. 4. The coarse fraction of PM10 measured at Hawthorne
Elementary (the air quality regulatory site at the base of the SLV operated by UDAQ) shows a similar trend to the APS mass loading of
PM10–PM2.5 observed at UU. In contrast, the Rose Park site
(another UDAQ site), which is close to Great Salt Lake, exhibits moderate
increases in coarse-mode mass compared to UU and Hawthorne. This difference
is more evident during the calmest period of the pollution event, when the
lowest wind speeds were observed, which suggests the sources of coarse
particles are highly localized during PCAP events. An analysis of coarse
particles by Li et al. (2013) found only moderate correlation between PM10–2.5 mass loadings and wind speed at several sites across the USA
(Li et al., 2013).
The time series of the total surface area of PM larger than 2.5 µm in
µm2 cm-3 (Fig. 3) shows the available coarse-mode surface
area exceeds 1500 µm2 cm-3 for several hours on each of the
3 most severe pollution days, 30 January to 1 February 2017. For
comparison, the total surface area of the fine-mode particles is also shown,
which is roughly 3 times the maximum surface area of the coarse
fraction but with less diurnal variability. However, the fine-mode aerosol
nitrate is assumed to be in equilibrium with gas-phase HNO3, in
contrast to the coarse-mode surface area, which could be viewed as a
reactive sink allowing net uptake. The midday increases in coarse-particle
surface area, therefore, could represent an important permanent sink for
HNO3 if the particles are composed of reactive salts such as CaCO3
or NaCl.
A lower limit to the lifetime of HNO3 with respect to uptake by the
coarse aerosol surface can be calculated to infer the potential importance
of mineral–coarse aerosol particles during these heavy pollution events. The
approximate first-order removal rate of HNO3 due to the heterogeneous
reaction with coarse aerosols, khet, depends on its average
molecular speed, c; the surface area density of coarse aerosols, SA; and
the uptake coefficient, γ, given in the following expression
(Ammann
et al., 2013; Crowley et al., 2010; Kolb et al., 2010; Tang et al., 2017):
khet=0.25⋅c⋅SA⋅γ.
The variation in mineralogy of coarse-mode particles can influence the
efficiency in HNO3 reactive uptake via the uptake coefficient, γ. This is represented in the range of reported uptake coefficients of
HNO3 to CaCO3 (γ=0.07 at RH = 50 %) and NaCl
(γ=0.11 at RH = 55 %) (Liu et al., 2008a; Saul et al., 2006).
The average RH at peak coarse-mode loading was 54 %; therefore, the
reported uptake coefficients close to this RH value were used in our
calculations. Literature reports have shown RH significantly enhances the
reaction probability of HNO3 uptake with mineral dust and sea salts
(Liu
et al., 2008a; Saul et al., 2006; Vlasenko et al., 2006). The detailed
kinetics of HNO3 reacting with CaCO3 particles in a flow reactor
by Liu et al. (2008a) found low uptake values of γ≤0.003 at 10 % RH, while at 80 % RH, γ can be ≥ 0.21. This agrees with
reports from Vlasenko et al. (2006), in which HNO3 uptake on dust
aerosols containing ∼ 5 % CaCO3 was also enhanced by an
increase in RH. Saul et al. (2006) also found RH affects the uptake
of HNO3 onto pure NaCl showing a maximum γ of 0.12 at 50 %
RH, while at 85 % RH, γ is 0.05. They also observed an increase in
reactivity with the presence of MgCl2 in NaCl powder. The greater
reactivity of MgCl2 is consistent with the hygroscopic character of
magnesium salts that facilitate the increase in adsorbed water at the
particle surface, which can in turn increase the effective uptake of
HNO3.
Predicted HNO3 loss rate to coarse-particle surface area
concentration measured by the APS from 28 January to 4 February 2017.
Because pNa+ and pCa2+ were the dominant non-volatile species that
showed common behavior with the coarse-mode mass loading, the rates of
uptake with respect to NaCl and CaCO3 were calculated to approximate
how quickly HNO3 might be sequestered. Literature values for the uptake
coefficients of HNO3 on NaCl (0.11 at 55 % RH) and CaCO3 (0.07
at 50 % RH) were used to estimate a lifetime against reactive uptake
based on the measured coarse-mode surface area
(Fenter
et al., 1994; Liu et al., 2008a; Vlasenko et al., 2006). The time series of
khet from 27 January to 4 February 2017 is presented in Fig. 5. The
effective lifetime of HNO3 against reactive uptake to CaCO3 during
episode 1 ranges from 2 to 90 min with the shortest lifetimes during
periods of peak coarse-mode loading. The lifetime against NaCl (0.11) ranges
from 1.3 to 57 min. These lifetime calculations assume that the entire
surface is reactive and do not account for any mass transfer limitations
that may occur on the particle surface. Therefore, the time range is an
estimate of the upper limit to the rate of HNO3 uptake, or how quickly
the reaction could potentially occur.
Under high-RH conditions, which often occur during PCAP events, it is also
possible coarse particles may be deliquesced, allowing exchange between
surface and bulk ions within a particle. This would result in an increased
loss of nitrate if uptake were not limited to the surface. If the total mass
of coarse particles measured during episode 1 were assumed to be composed of
Ca(NO3)2, in which carbonate has been completely displaced, that
would amount to an average of 6.2 µg m-3 of HNO3 sequestered
with a maximum of 32 µg m-3 during midday. This assumes HNO3
can react with the bulk particle components, thereby representing an upper
estimate of HNO3 mass loss to PM10. However, due to the size
cutoff of the APS, with significant transmission losses above 10 µm,
it is possible that the mass of coarse-mode particles is underestimated
under some conditions.
The calculations of the removal rate do not include changes in reactive
uptake associated with the changes in hygroscopicity of the resulting
nitrate salts formed. The presence of calcium or magnesium nitrate salts can
enhance the absorbance of water, which has been shown to increase the
relative uptake of HNO3 because it is no longer limited to the particle
surface, so the estimation of HNO3 loss can also be highly variable
(Beichert
and Finlayson-Pitts, 1996; Goodman et al., 2000). This is dependent on the
deliquescence relative humidity (DRH) of the nitrate salt formed, in which
DRH of Ca(NO3)2 is 10 % at 298 K
(Liu
et al., 2008b; Sullivan et al., 2009a, b) and NaNO3 is 81 % at
273 K (Seinfeld and Pandis, 2006). The DRH is also
temperature-dependent and tends to increase with decreasing temperature. The
temperatures during episode 1 were consistently below 273 K; therefore, it
is assumed DRH would be higher. The inference from the heterogeneous uptake
calculation suggests that the loss of HNO3 can be very rapid. The
question of how this loss in HNO3 to coarse aerosols could be shifting
the gas-to-particle equilibrium that exists between HNO3, NH3, and
NH4NO3 is still unknown. Therefore, further investigation of
time-resolved coarse-aerosol chemical composition, in addition to PM2.5
composition, is needed to better quantify how much coarse aerosols may be
limiting HNO3 availability during these PCAP pollution events and
elucidate their effect on NH4NO3 formation.
Studies from coastal sites provide evidence that the uptake of nitrate to
coarse-model aerosols can be substantial. For example, single particle
measurements of sea salt particles using an aerosol time-of-flight mass spectrometer (ATOFMS) in California found that the
chloride mole fraction could decrease from 0.3 to 0, while the nitrate mole
fraction could increase from 0 to 0.5 following exposure of sea salt to
urban air pollution (Gard et al., 1998). In
coastal Florida, measurements using an online ion chromatographic technique
showed that deliquesced sea salt particles could have more than 50 % of
the chloride content replaced by nitrate
(Dasgupta et al., 2007).
Measurements from a size-resolved integrated sampler showed that nitrate
peaked in the coarse mode, where its concentration was close to that of
chloride (on a molar basis) and half that of sodium. Size-resolved particle
composition measurements near the coast in British Columbia during the
summer showed that during the day, nitrate was only present in coarse-mode
particles (corresponding to a deficiency in chloride), whereas during the
night, nitrate could also be measured in accumulation mode particles
(Anlauf et al., 2006). In
addition, Lee et al. (2018) identify that coarse-mode nitrate particles, formed
from acid displacement, are more important in national park areas in Arizona
and Tennessee. Measurements were taken during spring and summer,
respectively, so did not have competing NH4NO3 formation; however,
they do highlight the fact coarse particle nitrate extends into the
PM2.5 regime and not all nitrate in this regime is associated with
NH4.
Evidence from snow analysis
During the campaign period, a snow event occurred prior to the first PCAP
episode (21 January), in which the snow remained on the surface until
3 February. This provided a snowpack that was subjected to the first
pollution event, providing a record of chemical composition changes caused
by PCAP exposure. A snow event that happened later in the season occurred
during clean conditions (23 February to 2 March 2017), when no PCAP was
observed, thus providing a representative example of chemical composition
changes in snow not associated with PCAP exposure.
Chemical analysis of the snowpack exposed to PCAP episode 1 reflects a
combination of cloud water composition and the atmospheric gas and particle
composition of species scavenged by snowfall or later deposited to the
snow, including particles larger than the PM2.5 cutoff monitored by the
AIM-IC. Figure 6 shows a scatter plot of the NH4+ versus
NO3- measured in the snowpack, where each data point is
representative of the averaged chemical composition of the entire snowpack
(sampling area 0.008 m2) on a single day, and in the atmosphere by the
AIM-IC (gas phase + PM2.5), where PM2.5 NO3- is
assumed to be equivalent to 90 % of PM2.5 NH4+ on a molar
basis, and by a Twin Otter aircraft (gas phase + PM1). Aerosol particle
measurements from the aircraft do not include non-volatile cations and their
associated nitrates, which the AMS does not measure (Franchin et al., 2018).
Gas-phase HNO3 measurements collected on the aircraft are from the
University of Washington high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS) instrument operated similarly to the method
described in Lee
et al. (2018). Gas-phase NH3 measurements from the Twin Otter were
taken using a quantum cascade tunable infrared differential absorption spectrometer (QC-TILDAS) instrument (Aerodyne Research Inc, MA, USA) and are described
in Moravek et al. (2019). Based on aircraft data (Franchin et
al., 2018) and AIM-IC measurements at the ground site, the sum of nitrate in
the gas phase and particle phases, or total NO3 (TNO3) per cubic metre
of ambient air, is less than the sum of ammonia in the gas phase and
particle phases, total NHx per cubic metre of ambient air. Therefore, we
might expect that there would be more ammonium depositing into the snow
compared to nitrate. However, in Fig. 6, it is clear that the deposition of
NHx is less than half that of total NO3 on a molar basis for the
highest deposition amounts. The average TNO3 : NHx molar ratio of
3 : 1 measured in the snowpack is significantly different compared to what is
observed in the atmospheric measurements. This implies that there could be
another source of nitrate being deposited to the snow that is not reflected
in the atmospheric measurements of the gas-phase and fine-mode particles.
Therefore, there is a missing portion of the overall nitrate budget that is
connected to how much NH4NO3 can be formed.
Total NO3 (pNO3-+ HNO3) with respect to total
NHx measured and predicted from AIM-IC data (grey circles), aircraft
measurements (open blue circles) per cubic metre of ambient air and snow (red
triangles) within 0.008 m2 sampling area from 21 January to 3 February
2017.
Further investigation into the composition of the snowpack reveals there is
a strong correlation between the amount of NO3- and the amount of
Ca2+ (r2=0.836), seen in Fig. 7. Both Na+ and Ca2+
are also found in the snowpack in higher abundances than NH4+,
despite being present at much lower levels in PM2.5. The larger
concentrations of pNa+ relative to pCa2+ seen in the AIM-IC
PM2.5 data are consistent with the larger concentrations of Na+
measured in the snowpack relative to Ca2+ concentrations.
Daily total micromoles of NH4, Ca, and Na vs. NO3 within
the snow from 21 January to 3 February 2017, leading into the most severe
PCAP period from 27 January to 4 February 2017.
This suggests the coarse-mode aerosols during this PCAP event were Na-rich.
Due to the large excess of micromoles of Na+ when compared to
NO3-, the amount of Cl- was examined to determine whether
Na-rich particles came in the form of NaCl salt. The snowpack showed the
highest concentrations of Cl- out of all of the inorganic anions measured,
over 10 times more than NO3- and SO42- combined. The total water-soluble ion balance measured in the snowpack
shows a slight excess of anions, partially due to the high Cl- content.
However, the concentrations of Cl- measured in the snow were
greater than what can be accounted for by observed Na+. The low
concentrations of Mg2+ in the snowpack had no noticeable correlation
with Cl-. A closer look at the daily inorganic composition changes and
decrease in snowpack height reveals Ca2+ and
NO3- exhibit similar trends throughout the lifetime of the
snowpack. This is also true for changes in K+ and Cl- total snow
column concentrations.
The proportions of non-volatile cations, where K+> Na+> Ca2+> Mg2+, found in the snow
exhibit a similar distribution to that observed in PM2.5 composition. We infer
that our measurements of these constituents in PM2.5 also reflect a
larger unmeasured contribution of these non-volatile cations in the coarse
mode. In addition, the nitrate present in the snow appears to have a strong
correlation with Ca2+, suggesting there could be a significant amount
of particle nitrate in the coarse mode that is not accounted for in
PM2.5 measurements. This underestimation of particle nitrate during
peak coarse-mode periods that occur during the day could lead to an
underestimation in the production rate of nitrate due to photochemistry.
This could impact estimates of the sensitivity of PM2.5 to NH3 and
NOx emissions reductions.
ISORROPIA analysis
The comparison of ISORROPIA model runs of predicted NH4NO3
(µg m-3) with and without non-volatile components measured by
the AIM-IC are depicted in Fig. 8 through pollution episode 2 from 13 to 19 February 2017 when both anion and cation gas and particle data were
available. ISORROPIA calculates the equilibrium concentrations of the
NH4+–SO42-–NO3-–Na+–Cl-–K+–Mg2+–Ca2+–H2O
system. Overall, the difference in model outputs for these two input
conditions demonstrates how influential the presence of non-volatiles
(pNa+, pCa2+, pMg2+, and pK+) is on the thermodynamic
equilibrium between HNO3 and its particulate counterpart
pNO3-.
Predicted concentrations (ISORROPIA) with respected to measured (AIM-IC) ambient
concentrations of NH4NO3 in µg m-3 through the second pollution event from 13 to 19 February 2017. The model was run in
three separate conditions: (1) with (red cross) and (2) without (open black
circle) the addition of pK+, pCa2+, and pMg2+ and (3) with
non-volatiles and the increase in total nitrate based on the ratio of
NH4+ to NO3- measured in the snow.
Previous studies have shown the inclusion of non-volatile cations in
thermodynamic models can more accurately reproduce gas-particle partitioning
observations by correctly reflecting the ion balance and the
ammonium : sulfate ratio (Guo et al., 2018). With
the addition of pCl-, pNa+, pCa2+, pMg2+, and pK+ in
ISORROPIA model runs, the model consistently underpredicts the
concentrations of NH4NO3 observed. This underestimation is
expected because the implicit assumption in the ISORROPIA run is that all
the non-volatile cations are associated with particle nitrate, which is
unlikely to be the case. The model also does not accommodate the presence of
carbonate, nor does the AIM-IC measure particulate carbonate, which can also
associate with non-volatile cations. Therefore, the model runs represent an
upper limit of how much nitrate could be permanently trapped in the
particulate phase. The model outputs imply that some non-zero fraction of
the nitrate in the gas + PM2.5 system is associated with non-volatile
cations. Based on the strong relationship ISORROPIA predicts between
non-volatile cations and particle nitrate, the next step is to determine how
sensitive NH4NO3 formation is to this portion of missing nitrate.
Figure 8 demonstrates how much more NH4NO3 could be formed if
HNO3 was not sequestered by the coarse particles, depicted in blue
squares. Based on measured gas-phase NH3 and pNH4+, it is
assumed there is a very little coarse-mode NH4+ in the atmosphere.
To estimate the missing nitrate budget associated with coarse particles,
assuming the total concentration of snow nitrate is derived from coarse
particles is unrealistic. Therefore, with ambient NHx measurements as a
point of reference, the total NH4+ in the snow was used to
estimate the fraction of snow nitrate potentially from coarse particles, in
the absence of any other direct coarse-mode measurements. The total nitrate
input into ISORROPIA was calculated using the NO3--to-NH4+ ratio observed in the snowpack during the first PCAP event.
This serves as an upper limit of the missing nitrate budget and the impact
this HNO3 loss has on NH4NO3 formation. As seen in Fig. 6,
the TNO3- concentration was on average 2.5 times greater than the
TNH4+ concentration measured in the snowpack. This ratio is
strictly for the concentrations measured during the PCAP event. When
averaging TNO3- : TNH4+ molar ratios for the full lifetime
of the snowpack, the average ratio is slightly greater (3). This average
ratio estimates the relative amount of TNO3- that may come from
coarse particles. This does not account for HNO3 dry deposition loss;
therefore, the ratio of NO3- to NH4+ is an overestimate.
However, with the combination of stable PCAP conditions, in which the vertical
mixing is suppressed, and relatively low ambient concentrations of HNO3
(< 1 ppb), the dry deposition velocities would be relatively small.
Therefore, this overestimate is within reason. This average molar ratio of
2.5 measured during the pollution event was applied to the total NHx
measured by the AIM-IC and used as the total nitrate input into ISORROPIA
along with pCl-, pNa+, pCa2+, pMg2+, and pK+
concentrations observed by the AIM-IC. Overall, the model predicts greater
amounts of NH4NO3 and, therefore, PM2.5 will form. Based on the
upper-limit estimates of the missing nitrate, the increased amounts of
NH4NO3 modelled also represent an upper limit of how much more PM
could potentially form. When observed NH4NO3 concentrations are
above 10 µg m-3, the model predicts roughly 2 times more
NH4NO3 mass will form when nitrate lost to coarse particles is
accounted for. The additional modelled pNO3 mass is on average 6.8 µg m-3 with a maximum of 18 µg m-3, which is within the upper-limit range of total nitrate loss predicted when assuming uptake to the
entire bulk of the aerosol particle. This simple analysis also shows how
strongly HNO3-limited the PCAP event in the SLV was, when additional
nitrate led to increases in NH4NO3. This suggests the coarse
particles have the potential to control NH4NO3 formation
indirectly by acting as a permanent sink for HNO3, thereby reducing the
amount of available HNO3 to form PM2.5. Therefore, future campaign
studies should also include coarse particle speciation, especially in
systems involving semi-volatile nitrate salts.
Conclusion
The AIM-IC measurements confirmed the dominance of NH4NO3 in
wintertime PM2.5 pollution events in the SLV. The detection of non-volatile
cations (pNa+, pK+, pCa2+, and pMg2+) in PM2.5 by
the AIM-IC suggests the presence of mineral dust and/or salt during PCAP
pollution events, which can potentially impact the availability of
HNO3. This is due to the reactive uptake of HNO3 onto NaCl- and
CaCO3-containing particles, which can be predominantly found in the
coarse mode. This can introduce a permanent sink for HNO3. Due to the
lack of coarse-mode speciation, auxiliary data were used to estimate the
amount of HNO3 potentially lost to reactive uptake. The APS surface
area data were used to calculate a lifetime for HNO3 with respect to
uptake that was on the order of minutes, depending on the uptake coefficient
used, which is influenced by particle composition (Fenter et al., 1994; Liu
et al., 2008b; Vlasenko et al., 2006). The potential presence of coarse-mode
nitrate was also supported by the strong correlation between total Ca2+
and total NO3- observed in the snowpack. Total NO3- in
the snowpack was 2.8 times larger than the total NH4+, implying
there is additional NO3- that is not observed by PM2.5pNO3- and gas-phase HNO3 measurements alone.
The equilibrium dynamics were explored by thermodynamic modelling of trace
gas and PM2.5 composition, during episode 2. Despite the discrepancies
between observed and modelled NH4NO3 concentrations during PCAP
episode 2, the inclusion of coarse nitrate, from snowpack estimates, into
the model shows that coarse particles could be limiting a significant
portion of the HNO3 being generated, preventing its reaction with NH3
to form NH4NO3. The extent to which coarse-mode aerosols are
limiting PM2.5 formation remains unanswered and warrants further
investigation of coarse-mode composition and the role these particles play
in the atmospheric chemistry during pollution events.
Data availability
Ground site and aircraft data from UWFPS can be found here: https://csl.noaa.gov/groups/csl7/measurements/2017uwfps/ (NOAA, 2017).
Author contributions
AH performed the measurements and analysis of snowpack and inorganic gas and particles and wrote the paper. JGM provided input at all stages regarding the measurements, analysis, and discussion of the results. AM collected and analyzed aircraft NH3 data. AF and AMM obtained and analyzed the aircraft-based AMS data. JCL provided TOEM and NOx instrumentation. RB collected and processed TEOM data. AGH and RCP collected and processed APS data. LG collected HR-ToF-CIMS aircraft data. BHL provided processed HR-ToF-CIMS data. SSB and MB planned and organized the UWFPS measurement campaign. All authors discussed the results and contributed to the final paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank all the members of the UWFPS campaign. The authors
would also like to thank many members from the University of Utah in the
Atmospheric Sciences department for their support. We would especially
like to thank Catherine Chachere and Lauren Zuromski for processing the APS
data. The authors acknowledge the funding support from the Michael Smith
Foreign Study Supplements Program awarded by the Natural Sciences and
Engineering Research Council of Canada. NOAA acknowledges support for Twin
Otter flights from the Utah Division of Air Quality under agreement number
16-049696.
Financial support
This research has been supported by the Natural Sciences and Engineering Research Council of Canada (Michael Smith Foreign Study Supplements Program) and the Utah Division of Air Quality (agreement no. 16-049696).
Review statement
This paper was edited by Ryan Sullivan and reviewed by two anonymous referees.
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