Introduction
Nocturnal processing of nitrogen oxides, NOx (= NO +
NO2), can strongly influence daytime air quality (Dentener and Crutzen,
1993; Brown et al., 2006c). At night, once photochemical reactions shutdown,
NOx reacts with ozone (O3) to form nitrate radical (NO3) and
dinitrogen pentoxide (N2O5) (Reactions R1 through R3a).
N2O5 can react heterogeneously with airborne particles to form
either nitric acid (HNO3) (Reaction R4a) or, in the presence of
particulate chloride, nitryl chloride (ClNO2) (Reaction R4b, where
YClNO2 represents the molar yield of ClNO2 with respect to
the N2O5 reacted). In the presence of basic species like ammonia
(NH3), HNO3 can be neutralized to form particulate nitrate
(NO3-(p)). NO3 radicals can alternatively react
with volatile organic compounds (VOCs), which suppresses HNO3 formation
(Reaction R3b). Much research has focused on the influence of nocturnal
NOx processing on the regional budgets of NOx and O3 and on
the oxidative capacity of the atmosphere during subsequent mornings (e.g.,
Brown et al., 2006b; Thornton et al., 2010; Wild et al., 2016). The
corresponding impact of nighttime production of
NO3-(g+p), a key nocturnal sink for NOx, on local
and regional air quality can be considerable (Lowe et al., 2015; Pusede et
al., 2016) but is less often considered in detail.
NO+O3→NO2+O2NO2+O3→NO3+O2
NO2+NO3↔N2O5NO3+VOC→products
N2O5+H2O(het)→2HNO3N2O5+Cl-(het)→YClNO2+(2-YClNO2)NO3-
The importance of nocturnal NOx chemistry to NO3-(p)
production can be especially important in the winter. Relative to summer,
nights in winter are longer, colder and more humid, and biogenic VOC
emissions tend to be smaller. This allows for a larger fraction of NO2
to be oxidized to HNO3 via the N2O5 hydrolysis pathway
(Cabañas et al., 2001; Wagner et al., 2013), and colder temperatures favor
partitioning of nitrate to the particle phase (Stelson and Seinfeld, 1982).
In winter, nighttime HNO3 production can more efficiently compete with
daytime photochemically driven production due to the low photolysis rates and
hydroxyl radical concentrations (Wagner et al., 2013; Pusede et al., 2016).
Multiday pollution events (i.e., periods with elevated particulate matter,
PM, concentrations) can occur when meteorological conditions inhibit dispersion,
as is the case with the persistent cold air pool formation often found in valley
regions (Whiteman et al., 2014; Baasandorj et al., 2017). During the daytime,
sunlight-driven convection leads to an evolution of the near-surface
temperature profile and causes the atmosphere to be reasonably well mixed up
to some height (typically less than 1 km; cf. Fig. S1 in the Supplement).
Radiative cooling in the late afternoon leads this mixed layer (ML) to
decouple and separate into a shallow, near-surface-level nocturnal boundary
layer (NBL) and a residual layer (RL) aloft, the behavior of which can be
further modified by valley flows.
Nocturnal conversion of NOx to NO3-(p) can occur
either in the NBL or the RL. Surface NO emissions can substantially limit
direct production of NO3-(p) in the NBL by titrating
O3, depending on the initial conditions. Nocturnal surface NO emissions
do not directly influence the decoupled RL, with chemical production of
NO3-(p) dependent on the NOx, O3 and particulate
matter in the mixed layer at the time of decoupling. Box and 3-D models have
been previously used to assess the contribution of nocturnal processes in the
RL to the daytime surface concentrations of particulate matter,
especially NO3-(p) (Riemer et al., 2003; Curci et
al., 2015). Yet, computational models often have difficulty in accurately
predicting surface NO3-(p) in many regions, particularly in
the winter season, despite good estimations of NOx emissions (Walker et
al., 2012; Terrenoire et al., 2015), although this is not always the case
(e.g., Schiferl et al., 2014). Here, airborne and ground measurements made
over Fresno, CA, in the San Joaquin Valley (SJV) during the wintertime 2013
DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and
VERtically resolved observations relevant to Air Quality; Appendix A)
(Crawford and Pickering, 2014) study are used to further develop our
understanding of the role that different factors play in determining
surface-level NO3-(p) concentrations.
Time series of surface PM2.5 concentration
(µgm-3) measured in Fresno during the DISCOVER-AQ campaign
for 1 h averages (light red dotted line) and for a running average (red
line, smoothed over 24 h), along with the 1 h average
NO3-(p) concentration (blue line). The vertical orange
lines indicate the days on which airborne measurements were made. The
horizontal dashed black line indicates the NAAQS 24 h standard of
35 µgm-3 for PM2.5.
Winters in Fresno are characterized by frequent multiday pollution episodes
(Chow et al., 1999; Watson and Chow, 2002), when PM2.5 (PM with
aerodynamic diameter < 2.5 µm) mass concentrations exceed the
24 h National Ambient Air Quality Standard (NAAQS) of
35 µgm-3 (Fig. 1). Fresno is one of the largest cities in
the San Joaquin Valley, which is largely an agricultural area and suffers
from some of the worst air pollution in the United States (American Lung
Association, 2014). Shallow daytime mixed layer heights and low wind speeds
in winter lead to the accumulation of pollutants across the valley (San
Joaquin Valley Air Pollution Control District, 2003). Previous observations
in the SJV region have found a buildup of NH4NO3 during pollution
episodes (e.g., Chow et al., 2008). Approximately 30–80 % of the
wintertime PM2.5 mass in this region is ammonium nitrate
(NH4NO3), with a strong diurnal variability, and most other
PM2.5 is organic matter (Chow et al., 2006; Ge et al., 2012; Young et
al., 2016; Parworth et al., 2017). During DISCOVER-AQ specifically,
NO3-(p) was found to represent 28 % of
non-refractory PM1.0 (NR-PM1; PM1.0, PM with aerodynamic
diameter < 1 µm) mass on average (Young et al., 2016).
An important role for nocturnal NO3-(p) production in
this region has been previously identified based on observations of long-term
trends, the spatial and diurnal variability in NO3-(p),
and the chemical environment in and around Fresno. For example, Watson and
Chow (2002) reported a sharp, early-morning (∼ 09:00 LT, local time)
increase in surface NO3-(p) concentrations on many days
of a severe pollution episode in 2000 and suggested that this behavior was
consistent with mixing down of nitrate-rich air from the RL aloft. Young et
al. (2016) and Parworth et al. (2017) observed similar behavior more than a
decade later during DISCOVER-AQ in 2013. Pusede et al. (2016) characterized
the relationship between long-term (multiyear) surface measurements of
wintertime NO3-(p) and NO2 in Fresno and
Bakersfield and showed that the decline in NO3-(p) in
the SJV over time (2001–2012) was predominately driven by reduced nocturnal
NO3-(p) production in the residual layer. The balance
between production, especially nighttime production, and daytime losses was
identified by them as critical to understanding the multiday buildup during
pollution events. Further, they concluded from DISCOVER-AQ aircraft
measurements that much of the NO3-(p) production was
localized over the cities given the sharp urban–rural gradients in
NO3-(p); the spatial gradients in 2013 (from Pusede et
al., 2016) seem to be sharper than gradients in 2000 (from Chow et
al., 2006), likely reflecting the increasing localization of the
NO3-(p) pollution to the urban centers as overall
NO3-(p) concentrations in the region have decreased.
Brown et al. (2006a) observed that the number concentration of accumulation
mode particles (0.32–1.07 µm) often increased above the surface
at 90 m a.g.l. compared to surface (7 m a.g.l.) measurements during
night and suggested that this was due to the growth of smaller particles into
the accumulation mode via NO3-(p) formation. They also
observed that the concentration of NO3-(p) at
90 m a.g.l. often increased at night, which is suggestive of in situ
production.
The present study builds on this literature by examining the role that aloft
nocturnal nitrate production, in concert with other processes, has in
determining surface NO3-(p) concentrations during the
DISCOVER-AQ campaign that took place in January and February 2013 in the SJV.
Our study combines aircraft and surface observations from DISCOVER-AQ
(Fig. S2). During DISCOVER-AQ, two pollution episodes were observed during
which PM2.5 concentrations were elevated (Young et al., 2016). The
analysis here focuses on the quantitative assessment of
NO3-(p) concentrations during this first episode
(14–22 January) in terms of the processes that govern the
NO3-(p) diurnal behavior; the observed behavior during
this first episode is qualitatively compared with that during the second
episode (30 January–6 February) to examine the factors that contribute to
episode-to-episode variability. On flight days, in situ measurements of the
vertical profiles of particulate and gas concentrations above Fresno (and
other SJV cities) were made three times: in the midmorning
(∼ 09:30 LT), around noon and in the midafternoon (∼ 14:00 LT).
These measurements allow for the assessment of the daytime evolution of the
vertical distribution of PM and gases as well as characterization of the
time-varying boundary layer height (BLH). They also allow for the
determination of the overnight evolution of the PM vertical distribution,
which can be used to characterize the factors that control
NO3-(p) concentrations in the RL. The influence of
processes occurring aloft on the temporal evolution of
NO3-(p) surface concentrations is quantitatively
evaluated for this case study using an observationally constrained 1-D box
model. The box model accounts for both vertical mixing (entrainment) of air
to the surface and for photochemical NO3-(p) production,
as well as NO3-(p) loss processes. Ultimately, the
observations and analysis further illustrate how daytime surface-level
NO3-(p) concentrations depend on a combination of both
nocturnal and daytime production of NO3-(p), vertical
mixing, altitude-dependent advection in the RL overnight, daytime entrainment
of clean air from the free troposphere (FT) and evaporation-driven dry
deposition. The model and observations are used to examine the relative
importance of these different pathways during the case-study episode
considered. This work adds to the existing literature by providing an
observationally based, case-study demonstration of how nocturnal processes
occurring aloft – in concert with other processes – exert a major control
over the evolution of pollution episodes within the SJV specifically and
likely in other regions as well.
Results and discussion
Vertical distribution of NO3-(p)
The concentration and vertical distribution of NO3-(p) in
the RL ([NO3-(p)]RL) in the morning serves
as the initial condition constraint on what is mixed down to the surface as
the day advances and the ML rises. Thus, knowledge of the vertical
distribution of NO3-(p) in the RL near sunrise is needed to
predict the temporal evolution of surface-level NO3-(p)
during the daytime, as will be done below. Nighttime flights were not made
during DISCOVER-AQ to allow for the characterization of the overnight evolution
of the RL. However, the early-morning (∼ 09:30 LT) vertical
profiles over Fresno allow for the characterization of the vertical structure of
most of the RL near sunrise (∼ 07:10 LT), as the surface
boundary layer height at this point is still quite shallow (∼ 50 m;
see Appendix B for a description of the mixed boundary layer height
determination method, Figs. B1–B2). Fast measurements of total NO3-
(gas + particle, NO3-(g+p)) were only available for a
subset of flights (Pusede et al., 2016), and particulate-only NO3-
measurements were not made with sufficient time resolution, less than about a
minute, to allow for robust characterization of the NO3-(p)
vertical profile. Therefore, NO3-(p) vertical profiles for
each flight during Episode 1 are estimated from in situ measurements of dry
particle scattering and the influence of water uptake on scattering, i.e.,
from the particle hygroscopicity, and are calibrated against the slower particle-into-liquid sampler
(PILS) measurements (Appendix A, Fig. A1). The derived, observationally constrained
NO3-(p) profiles based on the estimated
NO3-(p) exhibit generally good correspondence with the
sparser direct measurements of NO3-(g+p), although on 1
of the 2 days available for comparison the total NO3- somewhat
exceeds the estimated NO3-(p) below ∼ 75 m (Fig. 2).
This indicates that the estimation method is reasonable, especially since
most nitrate is expected to be in the particle phase (Parworth et al., 2017)
given the high relative total ammonium (NH3 + NH4+)
concentrations (Fig. 3). Only 4 out of 5 flight days during Episode 1
have been included in this analysis due to insufficient data on 16 January.
Vertical profiles for 2 individual flight days of particulate
nitrate concentrations estimated from in situ total particle scattering
measurements (open markers) and total nitrate (gas + particle)
concentrations measured by the TD-LIF (solid black markers) for
(a) the morning (∼ 09:30 LT) and (b) the afternoon
(∼ 14:30 LT). The solid blue lines indicate the average
NO3-(p) vertical profiles for all 4 flight days of
Episode 1 (18, 20, 21 and 22 January).
The gaseous fraction of total nitrate versus the molar ratio of
total ammonia to total nitrate (ppb) under different environmental conditions
(blue lines). The total ammonia is the sum of NH3(g) measured on
the P3-B close to ground (< 20 m a.g.l.) and NH4+(p) at
ground level measured by PILS at approximately same time. The total nitrate
is the NO3-(g+p) measured by TD-LIF close to ground
(< 20 m a.g.l.). The gray dashed arrow indicates the observed range of
molar ratio values during the campaign period. The total (gas + particle)
ammonia is shown for reference (orange line).
Over Fresno, the observed afternoon (∼ 14:30 LT)
NO3-(p) concentrations are nearly constant with altitude up
to ∼ 400 m (the daytime boundary layer height) (Fig. 2b), whereas the
early-morning NO3-(p) concentrations decrease steeply with
altitude up to ∼ 350 m (Fig. 2a). Corresponding vertical profiles for
NO, NO2, O3, relative humidity, temperature and total particle
scattering are shown in Figs. S2 (early morning) and S3 (afternoon). Like
NO3-(p), all indicate substantial differences between the
early-morning and afternoon profile shapes. This provides a strong indication
that altitude-specific processes occur overnight that lead to a reshaping of
the NO3-(p) vertical profile. At some altitudes the
NO3-(p) in the early-morning RL is greater than the
NO3-(p) measured in the previous afternoon, indicating net
production, while at other altitudes the early-morning RL
NO3-(p) is less than the previous afternoon, indicating net
loss (Fig. 2). As noted by Pusede et al. (2016), there tend to be sharp
concentration gradients in NO3-(p) and NOx between the
city and surrounding areas, with lower concentrations outside the city. Thus,
whether NO3-(p) at a given altitude increases or decreases
overnight results from the competing effects of chemical production versus
horizontal advection bringing in this (typically) cleaner air from outside
the city. (In the absence of a strong jet aloft and no convective mixing,
nighttime entrainment of cleaner FT air into the RL is expected to be
considerably slower than horizontal advection.) Like
NO3-(p), the boundary layer is reasonably well mixed with
respect to NOx, O3 and particles at the time when decoupling of the
RL occurs around 15:00 LT the previous day (Fig. S4). Box model
calculations indicate that the expected local nocturnal chemical production
of nitrate in the RL should exhibit relatively minor vertical variation due
to variations in temperature and RH alone (Fig. S5). In other words, without
advective loss or dilution processes of either NO3-(p) or
the precursor gases, it is expected that the NO3-(p)
concentration would increase to a similar extent at all RL altitudes.
The substantial changes observed in the shape of the vertical profile
overnight indicate that nighttime differential advection in the RL is a
major factor in determining the shape of the morning
NO3-(p) vertical profile during this pollution episode.
Differential horizontal advection serves to directly export
NO3-(p) from the urban area and import cleaner air from
surrounding areas. Secondarily, as NOx concentrations are also lower
outside of the Fresno urban area (Pusede et al., 2014), this differential
advection will also influence the over-city concentrations of precursors
gases (NOx and O3; Figs. S3–S4) and consequently the
altitude-specific nitrate production, with decreases likely. This is
supported by surface-level measurements of NOx and O3 made in
Fresno and in the nearby and much more rural cities of Parlier (located
35 km SE of Fresno) and Madera (located 40 km NW of Fresno). The NOx
and NO2 concentrations are higher and the O3 lower in Fresno
compared to the surrounding cities throughout the day, and the instantaneous
nitrate production rate ([NO2][O3]) is substantially higher in
Fresno in the late afternoon, when decoupling occurs (Fig. S6). The important
implication is that overnight advection both directly and indirectly alters
the vertical NO3-(p) profile and decreases the over-city
NO3-(p) concentrations in the morning, which will
consequently serves to limit the extent of localized pollution buildup during
events. The impact of overnight differential advection on reshaping the
vertical distribution of NO3-(p) has likely increased over
the last 15 years as the sharpness of the urban–rural concentration gradients
has increased (Chow et al., 2006; Pusede et al., 2016). Nonetheless, the
NO3-(p) advected from urban areas in the RL will contribute
to the regional SJV background and serve to sustain NO3-(p)
levels across the valley during pollution episodes.
(a) Vertical profile of the average nighttime
(19:00–07:00) horizontal winds over Visalia, CA (65 km SE of Fresno), and
the surface (10 m) wind in Fresno for flight days during Episode 1 (18, 20,
21 and 22 January). The length of the arrows corresponds to the wind speed
and the direction to the average wind direction, with the measurement height
indicated by the circle on the tail of the arrow. (b) Corresponding
wind roses for (b1) the surface, (b2) 125–175 m,
(b3) 225–345 m and (b4) 400–500 m. The length of each
arc corresponds to the normalized probability and the colors indicate the
wind speed (m s-1; see legend). Data are from the National Oceanic and
Atmospheric Administration, Earth System Research Laboratory, Physical
Sciences Division Data and Image Archive
(https://www.esrl.noaa.gov/psd/data/obs/datadisplay/, accessed 3 June
2017).
In the summer, transport and dispersion of pollutants has been attributed to
low-level winds (less than 500 m a.g.l.) in the SJV (Bao et al., 2008). We
suggest that a similar, but weaker, circulation may exist even in the winter,
just at much slower wind speeds, and that this advection overnight is what
leads to differential washout and the establishment of the particular
vertical NO3-(p) concentration profiles in the RL. The
concentration of NO3-(p) will likely be lowest in the
early-morning RL at altitudes where horizontal advection has the greatest
impact. Wind profiler measurements made in nearby Visalia, CA (65 km SE of
Fresno), indicate that during the night (19:00–07:00) there was local
maximum in the mean wind speed at ∼ 250 m, which is around the
altitude at which the early-morning NO3-(p)
concentration is minimum (Fig. 2a). Below 250 m there was a monotonic
increase in the nighttime mean wind speed with altitude, with very slow
speeds observed at the surface. Above 250 m the mean wind speed was
relatively constant to ∼ 450 m, above which it increased with
altitude. Explicit comparison between the vertical profiles of nighttime mean
wind speed and the estimated early-morning NO3-(p)
concentration indicates an inverse relationship (r= -0.98) between the
two (Fig. S7). This is consistent with the idea that differential advection
as a function of altitude overnight serves to shape the early-morning
concentration profiles. The wind direction at lower altitudes
(∼ 150 m) was generally more variable than those at higher altitudes
(285 or 450 m), with a general shift from more westerly at lower altitudes
(but above the surface) to more northerly near the top of the RL (Fig. 4b).
(Note that vector average wind speeds for each individual night were
calculated and then a scalar average of these night-specific vector averages
was calculated to give the episode average mean wind speeds. This averaging
process emphasizes directional consistency of the winds on a given night, but
not between nights.) The increase in NO3-(p)
concentration at ∼ 400 m a.g.l. in the early-morning profile,
especially noticeable on 21 January (Fig. S8), could result from a slowing of
the winds near the top of the RL or from enhanced recirculation of pollutants
at higher altitudes. Regardless of reason, this work indicates that the
gradient between the local (above city) and regional
NO3-(p) and precursor gases, evident in Pusede et
al. (2016), is an important factor in determining the nighttime evolution of
the RL vertical profile. Explicit characterization of the temporal evolution
of the vertical structure of NO3-(p) within the
nighttime RL would provide further insights into the altitude-specific
processes that control the shape of the early-morning profile (and thus the
concentration of NO3-(p) aloft that can be mixed to the
surface in daytime).
The difference between the concentration of NO3-(p) at
each altitude of the early-morning vertical profile and that at 15:00 LT on
the preceding afternoon (Δ[NO3-(p)]RL)
yields the net overnight NO3- production or loss in the RL. If it is
assumed that the layer with the highest NO3-(p) is not
influenced by advection, then the
Δ[NO3-(p)]RL in this layer provides an
estimate of the maximum chemical production (PNO3-). This estimate
of PNO3- is certainly a lower bound on actual nitrate formation
given the assumption of no influence of horizontal advection, and this also
does not account for produced nitrate that remains in the gas phase (although
this is likely to be small). On average, the observations indicate that
chemical production overnight in the RL leads to an approximate doubling over
the initial NO3-(p) concentration, or
10–25 µgm-3 of NO3-(p) produced over
the course of the night for this episode (Table S1 in the Supplement).
Observed day-to-day variability in PNO3- likely results from
day-to-day variations in precursor (NOx and O3) concentrations and
N2O5 reactivity, as well as limitations of the assumption of no
advection in this layer. To assess the reasonableness of this estimate of
PNO3 as a maximum production rate, values of the night-specific
average rate coefficients for N2O5 heterogeneous hydrolysis
(kN2O5) and associated uptake coefficients
(γN2O5) needed to reproduce the observed PNO3-
are back calculated based on the initial NOx, O3, and wet particle
surface area, and assuming ClNO2 formation is negligible (see Appendix C
and Table S1). The derived kN2O5 values range from 1.3 to
5.1×10-5 s-1 with corresponding γN2O5
from 2.5×10-4 to 4.8×10-4. These are smaller than
values observed under water-limited conditions in other field studies (Brown
et al., 2006c; Bertram et al., 2009) and lower than expected based on lab
experiments (Bertram et al., 2009). γN2O5 values
separately calculated from the particle composition measurements, following
Bertram et al. (2009), are larger than the above back-calculated values, with
γN2O5∼10-3, and more consistent with the
literature. This suggests that the PNO3- is, in fact, a lower
estimate and that the NO3-(p) concentration in even the
lower layers of the RL is influenced by advection. Box model calculations
using the (too low) back-calculated kN2O5 and
γN2O5 yield ∼ 15–42 % NOx conversion to
HNO3 overnight during this episode. If instead γN2O5=10-3 is used, the calculated overnight conversion is somewhat larger,
∼ 52 %. Also, if kN2O5 and γN2O5
were assumed to be sufficiently large such that they are not rate limiting the
overnight conversion would increase further to ∼ 63 %. It should be
noted that during this episode the surface O3 overnight is essentially
completely titrated away by 18:00 LT (Fig. 5). The reaction between NO2
and O3 (Reaction R1) is thus very slow and nighttime chemical
production of NO3-(p) at the surface in the NBL is
comparably small.
Diurnal profiles for ozone (blue), NO2 (brown), NO (green), and
the product of O3 and NO2 (gray) for the first pollution episode.
(a) Average diurnal profile (solid line) of surface
NO3-(p) for all days of Episode 1. The shaded area
indicates the 1σ standard deviation. The solid black line is a linear
fit (r2=0.99) to the data between 13:30 and 15:30 LT. (b) Time
series (solid blue line) of surface-level NO3-(p) during
Episode 1. The circles indicate the daytime peak values. The linear fit (red
line) to the daytime NO3-(p) peaks suggest an increase
of 1.32 µgm-3day-1.
(a) Comparison between the observed (blue circles) and
observationally constrained model-predicted (green squares) diurnal profile
of the surface NO3-(p) concentration
(µgm-3) for the 4 flight days (18, 20, 21 and 22 January,
2013) during Episode 1. Also shown is the diurnal variation in the boundary
layer height (gray), as constrained by daytime measurements. (b) The
diurnal variation in the simulated fraction of the total surface-level
NO3-(p) contributed by the initial surface-level
NO3-(p) (i.e., that at the surface level at
00:00 LT), the
NO3-(p) mixed down from the RL and
NO3-(p) produced from daytime photochemical reactions.
(c) Comparison between the simulated diurnal profile when all
processes are included (green squares, same as panel a) and when
only one NO3-(p) sink at a time is considered. The
individual sinks considered are only entrainment of free-troposphere air
(yellow crosses) or only dry deposition of HNO3 via the gas-phase pump
(orange triangles).
Model predictions of the diurnal variation in surface-level
NO3-(p) under (a–c) different assumptions
regarding the NO3-(p) concentration and vertical
variability in the early-morning RL or (d) without daytime
photochemical production of NO3-(p). In all panels the
blue curve shows the observations and the green curve shows the full
observationally constrained model results (identical to Fig. 6) for the
average of the 4 flight days in Episode 1. For (a–c), the
assumptions were as follows: (a) the
[NO3-(p)]RL is equal to zero;
(b) the [NO3-(p)]RL is constant
with altitude and equal to the NO3-(g+p) at 15:00 LT
in the previous afternoon, corresponding to a case of zero net production or loss;
(c) the [NO3-(p)]RLis constant with
altitude and equal to the maximum observed [NO3-(p)] in
the early-morning RL profile.
Vertical mixing, photochemical production and NO3-(p) sinks
The observed episode average-surface-level NO3-(p)
concentration exhibits a distinct, rapid increase starting at
∼ 08:00 LT, then peaks around 10:00–11:00 LT and decreases fairly
continuously after the peak, especially between 13:00 and 16:00 LT
(Fig. 6a). For reference, time series of NO3-(p) during
the pollution episode, along with CO, NO, NO2, O3, temperature,
surface radiation and PM1, are shown in Fig. S9. Both Young et
al. (2016) and Pusede et al. (2016) noted this increase, arguing it is a
signature of nocturnal nitrate production. Here, we provide a more detailed
examination of the specific influence of vertical mixing and nocturnal
NO3-(p) production in the RL on the observed daytime
variability in surface-level NO3-(p) using an
observationally constrained one-dimensional box model (see Appendix D for
details). In brief, the model accounts for time-dependent mixing between air
in the mixed boundary layer and the RL, daytime photochemical production of
nitrate, gas-particle partitioning of nitrate, entrainment of clean air from
the free troposphere into the ML and loss of nitrate via dry deposition to
calculate the time-dependent evolution of the surface-level
NO3-(p) concentration. The observed vertical profiles of
NO3-(p) concentrations in the RL (referred to as
[NO3-(p)]RL and taken as the observed
early-morning and noon profiles) provide a unique constraint for
understanding and quantifying the influence of vertical mixing specifically,
allowing us to expand on previous studies. The model is additionally
constrained by the surface-level concentrations of NO2 and O3, as well as
temporally varying ML height. The evolution of the daytime ML height and rate
of entrainment are determined using the Chemistry Land-surface Atmosphere
Soil Slab model (CLASS; https://classmodel.github.io/; Ouwersloot and
Vilà-Guerau de Arellano, 2013). The CLASS model is constrained by
observations of the time-dependent vertical profile measurements of
temperature, RH and other gas-phase species over Fresno and by T and RH
profiles and surface sensible heat flux measurements at nearby Huron, CA
(∼ 83 km SSW of Fresno) (Appendix B). Starting at around 08:00 LT,
the ML begins to grow vertically by entraining air from the RL. It is assumed
that air within the ML is instantaneously mixed throughout the volume. Within
the (shrinking) RL the NO3-(p) is assumed to retain the
initial profile shape until it reaches the maximum ML height observed in the
afternoon (∼ 12:30 LT). After this point entrainment of
free-tropospheric air begins. The concentration of
NO3-(p) in FT air is determined from the vertical
profile observed around noon. While entrainment of FT air also alters the
NO2 and O3 concentrations in the mixed layer, since these are
constrained by the surface observations (within the mixed layer), this is
accounted for. Photochemical production of HNO3 is calculated based on
the oxidation of NO2 by hydroxyl radicals, with wintertime
concentrations estimated to peak around [OH]=106 molecules cm-3 at noon in the region, with contributions from
O(1D) + H2O (from O3 photolysis), HONO photolysis and
CH2O photolysis (Pusede et al., 2016). The OH concentration is assumed
to scale linearly with the observed solar radiation (Fig. S10).
The average calculated daytime temporal evolution of surface
NO3-(p) from the observationally constrained box model
agrees reasonably well with the average of the surface observations from the
4 Episode 1 flight days considered (Fig. 7a). (The observed diurnal
average in Fig. 6 uses all of the days from Episode 1, whereas in Fig. 7 only
4 flight days are included. This is because the initial early-morning
NO3-(p) vertical profile is required as input to the
model.) The model predictions for the individual flight days also exhibit
generally good agreement with the NO3-(p) observations
except in the late evening, which is discussed further below (Fig. S8). Specifically,
the observationally constrained model also shows a rapid increase in
NO3-(p) beginning at 08:00 LT, a peak around
10:00–11:00 LT and a gradual, time-varying decrease through the afternoon.
Consideration of the individual processes occurring in the model demonstrates
that the vertical mixing down of [NO3-(p)]RL and
the shape of the [NO3-(p)]RL vertical profile
predominately control the morning-time evolution of the surface
NO3-(p) during this episode (Figs. 7 and 8). The
particularly steep rise in the surface-level NO3-(p) in
the morning results from the combination of the NBL height being
exceptionally shallow (only ∼ 20 m) and the
NO3-(p) in the low-altitude region of the RL being
greater than the NO3-(p) in the early-morning NBL. The
peak and turnover in surface-level NO3-(p) occurs when
even higher RL layers, where
[NO3-(p)]RL < [NO3-(p)]ML,
are entrained. In other words, the temporal evolution of the surface-level
NO3-(p) is linked to the shape of the early-morning
vertical NO3- profile. Further, it should be noted that the exact
model behavior is dependent on the timing of the CLASS-predicted boundary
layer height increase, with the initial increase and timing of the
surface-level NO3-(p) peak being particularly sensitive
to the shape of the rise between 08:00 and 10:00 LT. Nonetheless, because
the NBL is so shallow here, only ∼ 3–12 % of the daytime ML
height, the surface concentration is strongly impacted by the concentrations
in the RL and the initial (pre-08:00 LT) surface-level nitrate has control
over daytime concentrations. Thus, the model results demonstrate that the
observation of the large 10:00 LT peak in NO3-(p) is a
clear indication of the strong influence of nocturnal processes occurring
aloft – both chemical production and advection-driven local loss – on
daytime surface concentrations.
As an extreme counterexample, if there were no NO3-(p)
in the RL, mixing would have led to an initial decline in the early-morning
surface NO3-(p) (Fig. 8a). Alternatively, if the aloft
NO3-(p) concentration were assumed to be equal to that
from the previous day at 15:00 LT (and with no vertical variability), there
would not have been a sharp increase in the morning surface
NO3-(p) (Fig. 8b). Instead, there would have been a more
gradual increase from the morning into the afternoon due largely to the
increasing influence of photochemical production. This is representative of a
case in which there was neither aloft production of
NO3-(p) nor losses from advection, such that the
early-morning RL concentration was determined entirely by carryover from the
prior day; in this case the difference between the early-morning surface
concentration and that in the RL is small compared to the observations. If,
instead, the RL NO3-(p) concentration at all altitudes
had been equal to the maximum NO3-(p) observed in the RL
(no vertical gradient in the RL), then the morning peak in surface-level
NO3-(p) would have occurred later and the
NO3-(p) concentration would be substantially higher
throughout a greater fraction of the day (Fig. 8c). This is representative of
a case in which nocturnal production in the RL occurred, but where advection
did not serve to reshape the NO3-(p) vertical profile in
the RL. Clearly, export of pollution from the relatively compact Fresno urban
area to the broader region (and import of cleaner air) plays an important
role in determining the daytime surface-level concentration of
NO3-(p), multiday buildup and the population exposure
in this urban area. While it has previously been suggested that the morning
increase in surface-level NO3-(p) is indicative of
mixing down of NO3-(p) in the RL (Watson and Chow, 2002;
Pusede et al., 2016; Young et al., 2016), the current study provides an
explicit, observationally constrained demonstration of this effect and
highlights the dual roles of chemical production and advective loss in the
RL.
The time-evolving relative contributions of surface-level
NO3-(p) from the NBL, the RL and photochemical
production are individually quantifiable from the model for this episode
(Fig. 7b). As the ML rises, the relative contribution of
NO3-(p) from the RL rapidly increases, reaching
∼ 80 % at the 10:00–11:00 LT peak. After this point, the relative
contribution of NO3-(p) from photochemical production
increases continuously. By the time that decoupling of the NBL occurs
(∼ 15:00 LT), photochemically produced NO3-(p)
comprises 58 % of surface-level NO3-(p) while
NO3-(p) from the previous nights' RL still comprises
40 %; the contribution of NO3-(p) that was in the
NBL is negligible (< 2 %). Pusede et al. (2016) showed that future
decreases in NOx emissions are more likely to decrease nighttime than
daytime NO3-(p) production. The results here therefore
suggest that decreases in NO3-(p) may be more apparent,
on average, in the morning than the afternoon since the fractional
contributions of nighttime-produced versus daytime-produced
NO3-(p) shift throughout the day. However, care must be
taken when interpreting observations from individual days since the
meteorological conditions that favor observation of an early-morning increase
will not always occur (discussed further below). Since it is assumed here
that OH scales with solar radiation, the potential for enhanced production of
OH (and subsequently NO3-(p)) in the early morning via,
for example, HONO photolysis is not accounted for in the model (Pusede et al., 2016).
If this process were included, the increase in morning surface-level
NO3-(p) would be even greater than is already calculated
from mixing down of NO3-(p) in the RL. Since the
observationally constrained model already predicts a somewhat larger peak at
10:00 LT for surface-level NO3-(p) concentrations
compared to the observations, early-morning photochemical production appears
to have had a relatively limited influence on the morning surface-level
NO3-(p) compared to mixing down of nocturnal
NO3-(p) during this episode.
While vertical mixing and the shape of the NO3-(p) vertical
profile are what predominately drive the morning temporal evolution in the
surface-level NO3-(p) (especially the peak) for this
episode, the afternoon behavior, especially between ∼ 13:00 and
16:00 LT, is shaped by the balance between photochemical production and loss
via (i) dilution by entrainment of FT air and (ii) evaporation of
NO3-(p) and subsequent dry deposition of HNO3 gas,
i.e., a gas-phase pump for NO3-(p) loss. Here, the relative
importance of these loss pathways is considered. The latter process
(gas-phase pump) has been previously considered by Pusede et al. (2016) while
the former (FT entrainment) was not. Loss through dry deposition of
NO3-(p) is negligible since deposition velocities for
HNO3 (vd= 1–10 cm s-1) are much larger than for
particles (vd= 0.001–0.1 cm s-1) (Meyers et al., 1989;
Horii et al., 2005; Farmer et al., 2013; Pusede et al., 2016). These loss
mechanisms ultimately limit the extent of the pollution episode buildup.
Once the daytime model ML reaches maximum height, entrainment into the ML of
typically cleaner air from just above the ML (i.e., from the FT) occurs. The
time-evolving entrainment rates are estimated from the CLASS model
(Appendix C).
Considering the gas-phase pump, the warm (typically 290 K) and dry
(RH = 40 % or less during the campaign) afternoon conditions enhance
evaporation of NO3-(p) relative to nighttime and
early-morning conditions, thereby increasing loss through dry deposition of
HNO3 gas in the afternoon (Pusede et al., 2016). However, total ammonia
is in substantial excess (3.8–8.9 times NO3-(g+p) on a
molar basis), with thermodynamic calculations indicating that the gas-phase
fraction of NO3- is < 0.15 during the daytime and near zero at
night when it is colder and RH is higher (Fig. 3). These estimates of the
gas-phase fraction of NO3- are similar to the observational
measurements of Parworth et al. (2017), who determined the daytime and
nighttime averages during the first episode were 0.08±0.03 (1σ) and
0.04±0.05 (1σ), respectively. Importantly, the gas-phase fraction
here is substantially smaller than that estimated in Pusede et al. (2016), who
found a daytime gas-phase fraction of 0.4 (median) and a 24 h average of
0.15. Consequently, the loss of nitrate via the gas-phase pump is less than in
their analysis and suggests that the role of this pathway was likely
overestimated. The general influence of the gas-phase fraction on loss via
dry deposition is shown in Fig. S11. In general, the results indicate that
the gas-phase fraction has a strong influence on the loss of
NO3-(p) due to HNO3 deposition.
Including both FT entrainment and dry deposition, the box model reasonably
reproduces the observed afternoon decrease in surface-level
NO3-(p). This allows assessment of the relative importance
of these two loss processes by turning them off one at a time (Fig. 7c). The
calculations indicate that entrainment of clean FT air plays an important
role in the afternoon surface concentration decline. Without entrainment, the
model predicts that the afternoon NO3-(p) would be
∼ 18 % higher, leading to a double-humped daytime profile. Despite
the relatively low gas-phase fraction, the gas-phase pump also contributes to
the afternoon decline. The model results indicate that these two loss
processes contribute approximately equally to the afternoon decline. There
are, however, a few hours when the gas-phase pump is potentially of extreme
importance. When the RL decouples and the surface mixed layer becomes quite
shallow the rate of loss due to dry deposition is enhanced. This leads to a
rapid decrease in surface-level NO3-(p). Yet, the
concurrent decrease in the NBL temperature and increase in RH and NH3
enhances the partitioning of nitrate to the particle phase, thereby limiting the
impact of this rapid decline over time. (In the model here, the decoupling is
assumed to occur very rapidly while the temperature and RH changes are from
observations and occur more gradually. If the decoupling were actually slower
the influence of the gas-phase pump at this point in time would be reduced
and the modeled decrease in NO3-(p) that occurs around
15:00–17:00 LT would be less than shown.)
The model predicts that after decoupling and cooling occur the surface-level
NO3-(p) will continue to decrease at
∼ 2 % h-1 overnight via the gas-phase pump, which is similar
to the loss rate observed between midnight and 07:00 LT (Fig. 7a). If the
gas-phase pump is turned off completely (i.e., the nitric acid deposition
velocity is set to zero), there is an increase in the modeled
NO3-(p) that begins at ∼ 15:00 LT (when
decoupling occurs) and continues until 18:00 LT (Fig. 7c). This is a result
of the continual decrease in temperature and increase in RH enhancing
partitioning to the particle phase. Although not a focus of this study, on
some days, there is a sharp increase in surface-level
NO3-(p) observed in the evening, starting around
20:00 LT. While this could theoretically result from enhanced partitioning
to the particle phase at night, the timing does not match the observed
temperature and RH variations. Surface-level chemical production of nitrate
via N2O5 hydrolysis could alternatively be the source of this
increase, but given the near-zero surface-level O3 concentration due to
titration by NO the production via this pathway would be insufficient. This
evening increase is observed on many days, although with somewhat variable
timing and magnitude (Fig. S8). Thus, it may be that the evening increase
results from advection to the measurement site of air from a not-too-distant
location (given low wind speeds) that has higher surface concentrations.
Regardless, while the reason for this nighttime increase in surface
NO3-(p) remains unclear, the occurrence does not impact
the analysis of the early-morning and daytime NO3-(p)
behavior.
The cumulative impact of the nocturnal production in the RL, daytime
photochemical production and afternoon loss processes is that the
NO3-(p) concentration at ∼ 15:00 LT, the point
when decoupling of the RL occurs, is slightly higher than that at 08:00 LT
during the episode. Therefore, there is a gradual net increase (average of
1.32 µgm-3day-1) in surface-level
NO3-(p) as the episode progresses, albeit with
day-to-day variability (Fig. 6b). For comparison, the 24 h
average-surface-level NO3-(p) increases by
0.66 µgm-3day-1. While decreasing NOx emissions
and NO3-(p) production, especially nocturnal production
(Pusede et al., 2016), is the most direct and reliable route towards
decreasing surface NO3-(p) concentrations (Kleeman et
al., 2005), decreases in NH3 could theoretically also have some
influence on NO3-(p) by increasing the efficiency of the
gas-phase pump. However, this will only be the case if NH3 decreases
exceed decreases in NOx by at least a factor of 5 such that the ratio
between the two is changed substantially and the gas-phase fraction is
increased (Fig. 3). Such preferential targeting of NH3 sources is
therefore highly unlikely to be an efficient control strategy, at least for
the SJV where the total ammonia-to-nitrate ratio is large. In regions where
the NH4+(g+p) : NO3-(g+p) molar
ratio is closer to unity, the nitrate partitioning is more sensitive to
changes in this ratio and thus ammonia control could potentially prove
effective.
Panels (a, b): diurnal variation in the surface-level
particulate nitrate concentration during (a) the first episode and
(b) the second episode. The solid black lines are the average
profile over the episode and the colored lines are for individual days.
Panels (c, d, e, f): wind roses for surface-level (10 m) winds in
Fresno for the early morning (05:00–08:00 LT) during (c) Episode 1
and (d) Episode 2 and for the late morning (09:00–12:00 LT) during
(e) Episode 1 and (f) Episode 2.
Comparison between episodes
The above analysis focuses on observations made during one pollution episode,
but there was a second pollution episode observed during DISCOVER-AQ
(30 January–5 February 2013). The episode-averaged diurnal behavior of the
surface NO3-(p) concentration for this second episode
showed evidence of an early-morning increase, but the increase is not as
sharp as the first episode (Fig. 9). Additionally, the day-to-day variability
in the surface NO3-(p) was much greater during the
second episode; on some days, there was minimal evidence of an early-morning
increase but on others there was a substantial increase. The shapes of the
early-morning vertical NO3-(p) profiles (around
09:30 LT) were notably different during Episode 2 on 2 of the flight days as
well, as was the evolution of the profiles from morning to afternoon
(Fig. S12). The afternoon mixed layer heights were much higher during
Episode 2 than Episode 1, ranging from 600 to 700 m a.g.l. compared to
300–400 m a.g.l., respectively. The early-morning mixed layer heights were
also higher during Episode 2 (∼ 170 m) compared to Episode 1 (around
70 m). During Episode 1, the surface-level winds exhibit a consistent shift
in direction from easterly in the early morning (05:00–08:00 LT) to
southerly in the later morning (09:00–12:00 LT), and the mean surface-level
wind speed increased over this same period, from 0.31 to 0.82 m s-1
(Fig. 9). In contrast, during Episode 2 there was a lack of day-to-day
consistency in the surface wind direction, especially during the early
morning (05:00–08:00 LT), and there was a more substantial change in the
mean surface-level wind speed from the early morning to later morning, from
0.32 to 1.12 m s-1 (Fig. 9). The Episode 2 mean nighttime aloft wind
speeds were also overall lower and more constant with altitude, with little
variability from 150 to 400 m. However, the wind speed did increase
substantially from the surface to 150 m (Fig. S13). The aloft nocturnal winds
during Episode 2 were somewhat more variable than Episode 1 winds in terms of
the wind direction (Fig. 4 versus Fig. S13).
Overall, this increased day-to-day variability in both the surface
NO3-(p) and wind behavior, as well as a difference in the
evolution of the NO3-(p) vertical profiles from the early
morning to late morning or early afternoon in Episode 2 compared to Episode 1,
suggests that the meteorological conditions during the second episode were
generally less conducive to simple interpretation using the mixing model
discussed above. Instead, it seems that advection and export from the urban
area were of increased importance during Episode 2, both overnight and
especially in the early-to-midmorning. The contrasting behavior between the
two episodes suggests that while the observation of a sharp, early-morning
rise and peak in surface-level NO3-(p) (such as during the
first episode) might be generally considered a strong indicator of the
production of NO3-(p) in the RL, the absence of such a
feature does not preclude an important role for nocturnal production aloft.
Linking to other regions
Production of NO3-(p) in the RL can vary widely based on
initial concentrations of its precursor gases, as well as the rate of
heterogeneous uptake of N2O5 by particles. It may be that
production of NO3-(p) via the N2O5 hydrolysis
pathway may be significant in the aloft RL in other regions with similar
geographical and meteorological conditions, such as Salt Lake Valley, Utah
(Kuprov et al., 2014; Baasandorj et al., 2017). However, in valley regions
with lower NOx or O3 the nocturnal PNO3- may be lower,
thus limiting the importance of this pathway (Akira et al., 2005; Bigi et
al., 2012). Among other factors, the extent to which nocturnal
NO3-(p) formation occurs more so in the surface layer
versus in layers aloft will depend importantly on the extent of NOx
emissions at the surface (which titrate O3, suppressing particulate
nitrate formation), the absolute and relative height of the nocturnal
boundary layer (which affects the rate of HNO3 deposition and the air
volumes in which nitrate production occurs), and gradients in RH, T and
NH3 (Kim et al., 2014).
For example, Baasandorj et al. (2017) observe at their valley wall and valley
floor sites in wintertime Utah that O3 concentrations near the surface
remain well-above zero even during pollution episodes, thus allowing for
surface-level NO3-(p) formation overnight, substantiated
by direct measurements of N2O5, in addition to formation aloft.
Nitrate-specific diurnal profiles were not reported. In Shanghai, China, Wang
et al. (2009) observed in fall 2007 that both O3 and NO2 remained
elevated at night at the surface, with a concomitant increase in surface
NO3-(p). And in wintertime Seoul, Korea, Kim et
al. (2017) observed relatively limited diurnal variability in O3 and
NO2 concentrations measured at 60 m, with both remaining elevated
throughout the night. However, they did not observe any notable buildup in
NO3-(p) overnight, but did observe
an NO3-(p) increase and peak in the morning, as here. In
contrast, in Fresno the nighttime surface O3 levels during
Episode 1 were nearly zero, suppressing surface NO3-(p)
formation. This near-zero nocturnal O3 is similar to observations by
Kuprov et al. (2014) made a few years before Baasandorj et al. (2017) at one
of the same valley floor sites in Utah, reflecting year-to-year differences.
Such differences can influence the extent to which a notable increase in
NO3-(p) is observed to occur in the early morning as air
is entrained from the residual layer to the surface. This is because if
surface production and production in the residual layer are similar in
magnitude the contrast between the two will be reduced and entrainment will
appear to have a less apparent impact on the diurnal profile. However,
because the effective volume of the residual layer is typically much larger
than the nocturnal boundary layer (as is the case here), even without an
observed increase in NO3-(p) at the surface in the
morning the NO3-(p) produced in the residual layer can still
dominate the overall NO3-(p) burden during the day.
Additionally, comparison between the Baasandorj et al. (2017) observations of
late-afternoon surface NO2 and O3 (which reflect the initial
conditions within the residual layer) with the Fresno observations indicates
that differences can exist in how nocturnal production in layers aloft influences
the buildup and sustaining of PM2.5 in pollution episodes. They
observed during a strong PM2.5 episode a slow buildup of PM2.5
followed by a plateau lasting multiple days. During this period,
late-afternoon O3 concentrations decreased over time while
late-afternoon NO2 was approximately constant (in the daily average).
Consequently, the nitrate radical production rate in the residual layer, and
thus the N2O5 and NO3-(p) production rates,
decreased over time in their study. In contrast, for Episode 1 here, the
late-afternoon nitrate radical production rate increased over time across the
episode (by 0.25 µgm-3day-1), with only a moderate
decrease in the daytime O3 over time (Fig. S9). These differences
reflect the different photochemical conditions between the regions and
illustrate the coupling between the daytime photochemical conditions (i.e.,
O3 production) and nighttime NO3-(p) formation above
the surface.
Conclusion
This work combines surface and aircraft observations
made during a pollution episode in 2013 to demonstrate that in the San
Joaquin Valley (specifically Fresno, CA) production of
NO3-(g+p) in the nocturnal residual layer can play a
crucial role in determining daytime surface concentrations of particulate
NO3- in winter, when photochemical production is relatively slow and
morning boundary layers are extremely shallow. The influence of processes
occurring in the aloft RL on NO3-(p) surface concentrations
is evident in the NO3-(p) diurnal variability, specifically
the occurrence of a midmorning peak in surface-level
NO3-(p). While the midmorning peak has been previously
suggested as a signature of nocturnal nitrate production aloft (Watson and
Chow, 2002; Brown et al., 2006a; Lurmann et al., 2006; Pusede et al., 2016;
Young et al., 2016), the current study makes novel use of vertical profiles
of NO3-(p) concentrations measured multiple times on
individual days to quantitatively illustrate the importance of nocturnal
processes on surface concentrations. The analysis shows that the
NO3-(p) concentration in the morning-time mixed boundary
layer can be dominated by nocturnally produced NO3-(p);
vertical mixing in the early morning, which entrains air from the residual
layer into the surface mixed layer, has a particularly large impact on the
surface concentrations here due to the nocturnal boundary layer being
exceptionally shallow. In the afternoon, photochemically produced nitrate
contributes the majority of the total NO3-(p) burden for
the episode examined but still with a substantial contribution from
nocturnal production. The case study here illustrates that nocturnal
NO3-(p) production can play a critically important role in
the buildup and sustaining of pollution episodes in the SJV, supporting
previous suggestions made, in part, on the basis of calculated chemical
production values and an assessment of multiyear trends in the relationship
between NO3-(p) and NO2 (Pusede et al., 2016).
The current work also demonstrates that a difference exists between the shape
of the typical vertical profiles of NO3-(p) in the afternoon
and early morning over Fresno. This difference is shown to very likely result
from altitude-specific horizontal advection in the nocturnal RL leading to
differential washout of NO3-(p) and precursor gases
rather than from differences in chemical production rates. Consequently,
there is a steep vertical gradient in NO3-(p) in the
early-morning RL that, in turn, influences the temporal evolution of
surface-level NO3-(p) during the day, especially in the early
morning. Ultimately, differential advection is shown to have an important
role in limiting the maximum surface-level concentration of
NO3-(p) observed within the urban area during the day, which is a
result of the urban–rural gradients being particularly steep (Pusede et
al., 2016). Absent this overnight export of pollution from the city, nitrate
pollution would build up during pollution events to a much greater extent.
However, advection likely contributes to the buildup of
NO3-(p) throughout the valley, outside of the cities.
Daytime loss processes are also shown to help in limiting the multiday buildup of
surface-level NO3-(p). Afternoon entrainment of air from
the cleaner free troposphere into the ML (and export of mixed layer air to
the FT) is shown to be an important loss process for particulate nitrate.
Janssen et al. (2012, 2013) have similarly identified afternoon loss via FT
entrainment as an important process shaping the diurnal variability of
surface-level organic aerosol concentrations in forested areas that are
dominated by organic aerosol. Loss of NO3-(p) via dry
deposition of HNO3 and subsequent evaporation of NH4NO3 is
found to contribute to afternoon particulate nitrate loss, but the effect is
limited by the (relatively) high afternoon boundary layer and the small
gas-phase fraction of nitrate (< 0.15). However, this gas-phase pump may
have a substantial influence on the surface concentrations in the few hours
just after decoupling of the RL occurs, when the boundary layer height is low
and it is still sufficiently warm. Consistent with previous suggestions
(Kleeman et al., 2005; Pusede et al., 2016), we conclude that control
strategies for the region should focus on the reduction of concentrations of
NOx and O3 (the latter of which might require VOC controls) in the
midafternoon, specifically around the time that the RL decouples from the
surface layer, as this largely determines the production rate of nitrate in
the aloft RL.