Temporal variation of particulate matter
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The temporal variation in PM2.5 over Japan at Fukuoka, Tsushima Island,
the Goto Islands, and Tottori are presented in Fig. 1. The PM2.5
observation data are taken from the ACSA at Fukuoka and from the PM-712 at
other sites. The temporal resolution is 1 h for all observations. During the
analyzed period of 7–17 January 2015, episodic PM2.5 peaks reached
around 100 µg m-3 at Fukuoka twice. The first peak, observed
at 12:00 LT on 11 January (shown in blue in Fig. 1), reached a maximum
concentration of 86.4 µg m-3 at Fukuoka and
105.1 µg m-3 at the Goto Islands. During this first peak, the
concentration at Tsushima Island was 63.9 µg m-3, which was
lower than at the other remote island sites in the Goto Islands, and there
was no distinctive peak at Tottori. The second peak, observed at 00:00 LT on
17 January (shown in red in Fig. 1), reached a maximum concentration at
Fukuoka of 106.2 µg m-3. During this second peak, the remote
sites of the Goto Islands and Tsushima Island also recorded high PM2.5
concentrations of 104.8 and 89.1 µg m-3, respectively, and
the PM2.5 concentration reached 37.6 µg m-3 at Tottori.
In Fig. 1, we show the model results as black lines. Generally, the model
captured the observed temporal PM2.5 behavior, although it
underestimated the first peaks at Fukuoka and the Goto Islands and the second
peak at the Goto Islands. The timing of the high PM2.5 concentration was
reproduced well by the modeling system. A statistical analysis of the model
reproducibility demonstrated that all paired datasets for PM2.5 showed
good correlations between the observations and the model at the four sites in
Japan, with a correlation coefficient (R) of 0.86. The mean fractional bias
(MFB) and mean fractional error (MFE) were -42.6 and 67.4 %,
respectively, and these results satisfied the model performance criteria (MFB
≤ ± 60 % and MFE ≤+ 75 %) proposed by Boylan and
Russell (2006). Figure 1 also shows the model results of a sensitivity
simulation performed by switching off the Japanese anthropogenic emissions.
The sensitivity simulation results are shown as dotted black lines, and the
difference between the base case and the sensitivity simulation is shown in
gray, which indicates the domestic contribution of Japan. Except for Fukuoka,
there were few domestic contributions for PM2.5; therefore,
transboundary air pollution was dominant in January 2015 at remote sites in
western Japan. At Fukuoka, although domestic contributions for PM2.5
were found in some cases (from 8–10 and on 14 January), the concentration of
PM2.5 was lower compared with the two peaks. During the two episodes
when PM2.5 concentration reached around 100 µg m-3 over
Japan, the model simulation suggested that the effect of transboundary air
pollution was dominant, even at Fukuoka.
The temporal variation in SO42- and NH4+ over
Japan at Fukuoka, the Goto Islands, Tsushima Island, and Tottori from
7–17 January 2015. The blue and red shading shows the type N and S patterns
focused on in this study. The red lines indicate SO42- observations by
ACSA, and the green lines indicate NH4+ observations by the NHx monitor at
Fukuoka. The open circles are D–F pack observations at Fukuoka. The open squares are
tape filter measurements at the Goto Islands, Tsushima Island, and Tottori.
The black lines indicate the base case simulation, and the dotted black lines
indicate the sensitivity simulation in which the anthropogenic emissions
from Japan were switched off; the differences between these results, shown in
gray, represent local contributions.
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The temporal variations in PM2.5 over China at Beijing, Shanghai, and
Shenyang are shown in Fig. 2. The PM2.5 observation data are taken from
the BAMs, and the temporal resolution is 1 h. The model results of a
sensitivity simulation suggested that the impact on three Chinese sites from
Japanese anthropogenic emissions was negligible during this period. At
Beijing, there were high concentrations of PM2.5 that correspond to
the high concentration of PM2.5 found over Japan. One high
concentration was approximately 300 µg m-3 on 10–11 January and
another was around 600 µg m-3 on 16 January. These peak times were
almost 1 day before the high concentration was observed over Japan. At
Shanghai, there were two clear peaks with a PM2.5 concentration of 200 µg m-3
on 11 and 17 January. The time corresponded well to the peak
time over Japan. At Shenyang, where the local emissions from domestic
sources were dominant in winter, the temporal variation was complex compared
with Beijing and Shanghai. PM2.5 showed sharp peaks several times with
concentrations of around 300 µg m-3, whereas the model only showed
gentle peaks. An analysis of the model reproducibility showed that all for
PM2.5 paired datasets for the observations and the model at three sites
over China, R was 0.73 and MFB and MFE were -9.8 and
46.8 %, respectively; these numbers satisfy the model goal criteria (MFB ≤ ± 30 % and MFE ≤± 50 %) proposed by Boylan and Russell
(2006). The evaluation of the model performance over China supports the
discussion on downwind regions.
SNA
The temporal variations in SNA are shown in Figs. 5 and 6. In Fig. 5,
SO42- and NH4+ are shown for four sites in Japan. At
Fukuoka, the ACSA and D–F pack observations are shown for SO42-. The
NHx monitor and D–F pack results are shown for NH4+. The temporal
resolutions of the ACSA and the NHx monitor were 1 h, and those of the D–F packs
were 6–8 h depending on the samples. For the Goto Islands, Tsushima Island,
and Tottori, the PM-712 tape filter data were used. The temporal resolution
was 4 h at the Goto Islands and Tsushima Island and 1 or 0.5 days at
Tottori. In Fig. 6, NO3-, HNO3, NH3, and total ammonia
(sum of NH4+ and NH3) are shown for Fukuoka. The ACSA and D–F
pack observations for NO3- are shown, the D–F pack observations are
shown for HNO3, and the NHx monitor observations are shown for NH3 and
total ammonia. Because of a temperature change during the PM-712 sample storage,
the NO3- concentrations could have been affected by volatilization;
hence, only the NO3- analysis at the Fukuoka site was used. At
Fukuoka, the SNA concentration contributed 52 and 46 % of the
PM2.5 concentration in the first and second episodes, respectively. For
SO42- (Fig. 5; left), the concentration during the second episode
was larger than during the first episode at Fukuoka, the Goto Islands, and
Tsushima Island. At Tottori, there was no peak for the first episode for
SO42-. In contrast to SO42-, a higher NO3-
concentration was observed during the first episode instead of the second
episode (Fig. 6). NH4+ showed high concentrations during both
episodes because it is the counterion for SO42- and NO3-
(Fig. 5; right). Based on the analysis of the PM2.5 (Fig. 1) and SNA
(Figs. 5 and 6) observations, the PM2.5 concentrations were similar
during the episodes on 11 and 17 January; however, the main component of SNA
was NO3- during the first episode and SO42- during the
second episode. At Fukuoka, the relative portions of SO42-,
NO3-, and NH4+ within the PM2.5 were respectively
18, 20, and 14 % during the first episode, and 27, 6, and
14 % during the second episode. Therefore, the first episode (shown in
blue in Figs. 1, 5, 6, 7 and 8) is referred to as “type N”, and the second
episode (shown in red in Figs. 1, 5, 6, 7 and 8) is referred to as “type S”
hereafter.
The temporal variation in NO3-, HNO3, NH3, and
total ammonia at Fukuoka from 7–17 January 2015. The blue and red shading
shows the type N and S patterns focused on in this study. The red lines indicate
NO3- observations by the ACSA, the green lines indicate NH3 and total
ammonia observations by the NHx monitor, and the open circles indicate D–F pack
observations. For NH3, periods of nearly zero concentration (24 h
average of less than 1 µg m-3) are indicated by arrows. The black lines
indicate the base case simulation, and the dotted black lines indicate the
sensitivity simulation in which the anthropogenic emissions from Japan were
switched off; the differences between these results, shown in gray, represent
local contributions.
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The model results for the base case and sensitivity simulations are overlaid
with the same temporal resolution as the observations in Figs. 5 and 6. The
model tended to underestimate the SO42- concentration (Fig. 5; left);
however, the model reproduced the features of types N and S, and the
sensitivity simulation indicated the dominance of transboundary air
pollution for SO42- during the intensive observation campaign in
January 2015, even at Fukuoka. For NO3- (Fig. 6), the model
reproduced the features of the type N and S peaks well, although the model
overestimated the dip in NO3- concentration found from the evening
of 10 January to before the type N episode. The D–F pack observations
generally underestimated NO3- compared with the ACSA observations
because of the difference in the cutoff diameter between these measurement
systems. Except for the type N and S episodes, domestic contributions were
seen for NO3- from 8–10 and on 14 January. However, the sensitivity
simulation confirmed that transboundary NO3- air pollution was
dominant for types N and S. Because NH4+ is the counterion for
both SO42- and NO3-, small domestic contributions for
NH4+ were observed at Fukuoka (Fig. 5; right). This result
corresponded to the domestic contribution for NO3-. For the other
three remote sites, there were no domestic contributions for NH4+.
The behavior of SNA, gas-phase HNO3, and NH3 was analyzed
comprehensively based on the NHx monitor and D–F pack observations (Fig. 6)
to support our understanding of NO3- behavior. There are few
synergetic analyses including gas-phase behavior over the downwind region of
the
Asian continent. Peaks for gas-phase HNO3 were found for types N and S,
whereas the concentration of gas-phase NH3 was nearly zero (less than 1 µg m-3
for 24 h average) for types N and S on 8, 10,
12, and 15 January (the green arrows in Fig. 6). Type S, in particular, showed an almost
zero concentration of NH3. The concentration of total ammonia showed
distinct peaks for types N and S; therefore, the nearly zero concentration
of NH3 suggested the full conversion of NH3 to produce
NH4+ as a counterion for SO42- and NO3-. The
sensitivity simulation, in which Japanese anthropogenic emissions were
switched off, clarified the different features of related gas-phase species.
The base case simulation and sensitivity simulation were similar for
HNO3, suggesting that it originated from transboundary air pollution. A
slight increase in HNO3 in the sensitivity simulation was found from
8–10 and on 12 January (the red arrows in Fig. 6). These were the complex cases
connected to overseas and domestic emissions. If there are no Japanese
NH3 emissions, the transported HNO3 cannot produce NO3-
in Japan, and so it remains as gas-phase HNO3. The synergetic analysis
for gas-phase HNO3 and NH3 indicated that abundant HNO3 was
transported from abroad and reacted with domestic NH3, producing
NO3- from 8–10 January. Compared with these cases, domestic
HNO3 and NH3 produced NO3- on 14 January (orange arrows
in Fig. 6). The concentrations were lower than for types N and S, which were
dominated by transboundary air pollution.
Coarse-mode aerosols
Coarse-mode aerosols were also partly analyzed in this study. Because of the
effect of transboundary air pollution on HNO3 (Fig. 6), we focused on
coarse-mode NO3-. Coarse-mode NO3- is produced by
reactions of HNO3 with mineral dust or sea salt particles. In general,
mineral dust mainly has an effect in spring over East Asia (Itahashi et al.,
2010), whereas sea salt particles play an important role throughout the
year. Recently, we reported the importance of coarse-mode NO3- as
an atmospheric input in East Asian ocean regions (Itahashi et al., 2016).
Figure 7 shows the modeled and observed coarse-mode NO3-,
Na+, and Cl-. The ACSA and D–F pack observations are shown for
coarse-mode NO3-, and the D–F pack observations are shown for
coarse-mode Na+ and Cl-. During the intensive observation period
in January, coarse-mode NO3- also showed high concentrations for
types N and S of around 10 µg m-3 (on 9–10 January, around 5
µg m-3). Based on the model results and because the domestic
contribution for HNO3 was found on 14 January (Fig. 6), the domestic
contribution for coarse-mode NO3- was found only on 14 January,
but the concentration was below 1 µg m-3. Na+ and Cl-
from sea salt particles also had peaks for types N and S. Sea salt particles
are mechanically produced by high winds; therefore, these peaks generally
corresponded to high wind speeds (Fig. 4b). High winds were observed on
15 January; Na+ and Cl- peaks occurred, but the coarse-mode
NO3- concentration was close to zero. This was because there was
no HNO3 to react with NaCl from 12–15 January or the wet deposition of
coarse-mode NO3- with the precipitation from noon on 14 January to the
evening of 15 January. For coarse-mode NO3-, transboundary air
pollution was the dominant factor. This means that a large amount of
HNO3 was transported from abroad (Fig. 6), reacted with sea salt
particles over the ocean, and reached Fukuoka in the air mass.
The temporal variation in coarse-mode NO3-, Na+, and
Cl- at Fukuoka from 7–17 January 2015. The blue and red shading shows the
type N and S patterns focused on in this study. The red lines indicate
coarse-mode NO3- observations by the ACSA, and the open circles indicate
D–F pack observations. The black lines indicate the base case simulation, and
the dotted black lines indicate the sensitivity simulation in which the
anthropogenic emissions from Japan were switched off; the differences
between these results, shown in gray, represent local contributions.
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BC
To support the discussion of the domestic and transboundary contributions to
SNA, the behavior of BC at Fukuoka and the Goto Islands is shown in Fig. 8.
The sensitivity simulation would suffer from a nonlinear chemical response
if complex atmospheric chemistry were involved; hence, we focused on BC,
which is a primary aerosol. The temporal variation of BC also showed
distinctive peaks for types N and S at Fukuoka and the Goto Islands. The
model results reproduced these peaks well, and the sensitivity simulation
also suggested the dominance of transboundary air pollution for both peaks N
and S. The temporal variation at the Goto Islands showed only two peaks of
types N and S, although many short-term peaks were seen at Fukuoka. The
sensitivity simulation confirmed that domestic air pollution contributed to
these short-term peaks at Fukuoka; however, the model could not fully
capture the peaks observed on 7, 13, and 14 January. To improve the
performance of the model to capture these short-term peaks, a
higher resolution model simulation and a revision of the emission inventory
are needed. An analysis of the primary aerosol confirmed that transboundary air
pollution was dominant for types N and S in January 2015.
Consequently, the well-validated model simulation indicated that two high
PM2.5 episodes with concentrations of around 100 µg m-3
occurring over western Japan in January were dominated by NO3-
for the first peak (type N) and by SO42- for the second peak
(type S); the NH4+ concentration was high for both types. The model
sensitivity simulation clarified that these high SNA concentrations in the
type N and S episodes were dominated by transboundary air pollution. In
addition to the transport of SNA, abundant gas-phase HNO3 and
coarse-mode NO3- reacted with sea salt particles over the ocean
and were also transported to western Japan. NH3, which mainly came from
domestic emissions, showed concentrations of around zero during type N and S
events, suggesting that NH3 was depleted to neutralize SO42-
and NO3-.
Trajectory analysis
Analyzing the synergetic observations at Fukuoka and the other three remote
sites in Japan with the regional chemical transport model demonstrated that
the two PM2.5 episodic peaks were dominated by transboundary heavy
pollution, even at Fukuoka. The two peaks had different SNA compositions.
The first episode on 11 January showed a high NO3- (type N)
concentration and the second episode on 17 January was dominated by
SO42- (type S). The differences in these episodes were
investigated further by a model simulation combined with a backward trajectory
analysis. The spatial distributions of SO42- and NO3-
during type N and S patterns are shown in Figs. 9 and 10, respectively.
The temporal variation in BC at Fukuoka and the Goto Islands from
7–17 January 2015. The blue and red shading shows the type N and S patterns
focused on in this study. The magenta line indicates BC observations by
MAAP. The black lines indicate the base case simulation, and the dotted black lines
indicate the sensitivity simulation in which the anthropogenic emissions
from Japan were switched off; the differences between these results, shown in
gray, represent local contributions.
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The simulated spatial distribution of (a) SO42- and
(b) NO3- during a type N pattern. The contours shown by the white lines for
NO3- represent 40 and 60 µg m-3. The 72 h HYSPLIT
backward trajectory from Fukuoka is overlaid with orange lines and orange circles
at 12 h intervals. (Left) 21:00 LT on 10 January 2015 when the air mass left
China and (right) 15:00 LT on 11 January 2015 when the air mass reached
Fukuoka.
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The simulated spatial distribution of (a) SO42- and
(b) NO3- during a type S pattern. The contours
shown by the white lines for NO3- represent
40 and 60 µg m-3. The 72 h HYSPLIT
backward trajectory from Fukuoka is overlaid with orange lines and orange circles
at 12 h intervals. (Left) 00:00 LT on 16 January 2015 when the air mass left
China and (right) 00:00 LT on 17 January 2015 when the air mass
reached Fukuoka.
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In type N (Fig. 9; right), the model results showed that a low
SO42- concentration of less than 5 µg m-3 and a high
NO3- concentration of more than 10 µg m-3 covered
Fukuoka. The spatial distribution patterns indicated an outflow of
SO42- and NO3- from continental Asia to western Japan.
The dominance of transboundary air pollution suggested by these spatial
distributions was consistent with the model sensitivity simulation results
(Figs. 5 and 6). High-concentration regions of SO42- and
NO3- stretched from the eastern coastline of China to the East
China Sea and western Japan. The spatial distribution implied the direct
transport from continental Asia to the downwind regions. In addition, the
high-concentration region stretched from eastern China to western Japan,
which is
consistent with the corresponding PM2.5 peak on 11 January at Shanghai
and over Japan. To investigate the air mass origin for type N, the HYSPLIT
backward trajectory (Stein et al., 2015) was analyzed over 72 h starting from Fukuoka
(TN in Fig. 9; left). The backward trajectory during type N
transport suggested that the air mass originated from Shaanxi Province and
passed over Shanxi Province, southern Hebei Province, and Shandong Province, and
then reached Fukuoka. The traveling time from the coast of China to Fukuoka
was about 18 h. The distance from the coastline of China to Fukuoka is
approximately 1000 km, so the air mass speed for type N was 55.6 km h-1.
Figure 9 (left) shows the spatial distribution when the air mass was located
over China. A high concentration of NO3- of more than 60 µg m-3
occurred over the east coast of China before the air mass arrived
in Fukuoka, whereas the SO42- concentration was as high as
10 µg m-3 above the East China Sea.
In type S (Fig. 10; right), the model calculated that a high
SO42- concentration of more than 20 µg m-3 and a low
NO3- concentration of around 5 µg m-3 covered Fukuoka.
The HYSPLIT backward trajectory is shown as TS in Fig. 10. The air mass
during type S transport originated from Shanxi Province and slowly moved
over northern Henan Province and Shandong Province, and reached Fukuoka within
about 24 h. The high-concentration regions stretched from eastern China to
western Japan, which is consistent with the simultaneous PM2.5 peak at Shanghai
and over Japan. The air mass was stagnant over China compared with type N
transport. The spatial distribution when the air mass was located over China
is shown in Fig. 10 (left). For SO42-, the concentration was
higher when the air mass arrived at Fukuoka compared with that in China,
suggesting the fast production of SO42- during the transport
process. A high concentration of SO42- of 20 µg m-3
spread over the East China Sea and western Japan. A high NO3-
concentration of more than 60 µg m-3 occurred over China, similar
to type N; however, the NO3- concentration was immediately reduced
during the transport. The high NO3- concentration of more than 10 µg m-3
did not reach Fukuoka in this type S transport. Comparing
the spatial distribution of the air mass over Fukuoka (Fig. 10; right) and
China (Fig. 10; left) showed a 1-day delay in the high-concentration peaks
over Japan compared with the peak for Beijing (Figs. 1 and 2).
The backward trajectories for types N and S both showed similar transport
patterns from China and Fukuoka. However, the spatial distribution patterns
demonstrated clear differences between SO42- and NO3-
distributions for type N and S patterns. To discuss the different mechanisms
for types N and S further, the model results were analyzed along the backward
trajectories of TN and TS. The path analyses are
shown in Figs. 11 and 12, along with additional indexes. The conversion ratio
of SNA from the gas phase to the aerosol phase is an important indicator. For
SO42-, considering the concentration of the gas-phase species of
SO2, the conversion ratio of Fs is defined as follows and
calculated based on molar concentrations, shown in square brackets (Khoder,
2002):
Fs=[SO42-][SO2]+[SO42-][molmol-1].
A ratio of 0 indicates that SO42- was not produced, and a ratio of 1
indicates that SO2 was converted completely to SO42-. To confirm
the Fs analysis, the concentration of the highly reactive
hydroperoxy radical (HO2) was also analyzed. The self-reaction of
HO2 produces hydrogen peroxide (H2O2), and this is the most
effective oxidant of aqueous-phase SO2 (Pandis and Seinfeld, 1989).
Because the analyzed period was in winter, aqueous-phase reactions would be
the main pathway for producing SO42-.
The path analysis of the model results along trajectory
TN. (a) Temperature and relative humidity;
(b) SO42- (with expansion at the bottom) and SO2;
(c) HO2 concentration, (d) NO3-, HNO3,
NO, NO2, other NOy (NO3, HNO2, N2O5, and PANs), and
coarse-mode NO3-; (e) NH4+ and NH3;
(f) adjGR, Fs, and Fn′. In (b),
(d), and (e), BC and CO concentrations normalized to the
maximum value are also shown. The time axis indicates the backward time from
Fukuoka. The brown and blue bars at the bottom are schematic
images of the trajectory location over land and ocean.
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For NO3-, the conversion ratios of Fn are defined by considering
the concentration of NO2 as a precursor and calculated based on molar
concentrations (Khoder, 2002):
Fn=[HNO3]+[NO3-][NO2]+[HNO3]+[NO3-][molmol-1].
To investigate the ratio of NO3- to gas- and aerosol-phase
components analogous to the calculation for SO42-, the conversion
ratio of Fn′ is introduced:
Fn′=[NO3-][HNO3]+[NO3-][molmol-1].
The path analysis of
the model results along trajectory TS. (a) Temperature
and relative humidity; (b) SO42- (with expansion at the
bottom) and SO2; (c) HO2 concentration;
(d) NO3-, HNO3, NO, NO2, other NOy
(NO3, HNO2, N2O5, and PANs), and coarse-mode
NO3-; (e) NH4+ and NH3;
(f) adjGR, Fs, and Fn′. In (b),
(d), and (e), BC and CO concentrations normalized to the
maximum value are also shown. The time axis indicates the backward time from
Fukuoka. The brown and
blue bars at the bottom are schematics of the trajectory location over land
and ocean.
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The behavior of SNA is determined by introducing candidate indicators
according to the work of Ansari and Pandis (1998) and Pinder et al. (2008).
The gas ratio (GR) is an indicator of the sensitivity of NO3- to
changes in SO42- and NH4+ concentrations. The GR is
defined as the ratio of free ammonia to total nitrate as
GR=NH3+NH4+-2[SO42-][HNO3+NO3-][molmol-1],
where it is assumed that SO42- is fully neutralized, as indicated
by the coefficient of 2 for [SO42-]. The GR value implies the
following condition for NO3- production:
GR>1:NH3-rich0<GR<1:NH3-neutralGR<0:NH3-poor.
Here, NH3-rich means that there is sufficient NH3 to neutralize
both SO42- and NO3-, NH3-neutral means that there
is sufficient NH3 to neutralize SO42-, and NH3-poor
means that there is insufficient NH3 to neutralize SO42- or
NO3-. Free ammonia, which can form NH4NO3 in the
equilibrium process, is quantified more accurately by adjusting
[SO42-] with the degree of sulfate neutralization (DSN). The DSN
is defined as
DSN=NH4+-[NO3-][SO42-][molmol-1],
where DSN is equal to or greater than 2 if there is sufficient
NH4+. By substituting the coefficient 2 before
[SO42-] in Eq. (2) with Eq. (4), the adjusted GR (adjGR) can be
defined as
adjGR=NH3+NH4+-DSN×[SO42-][HNO3+NO3-]=NH3+[NO3-]HNO3+[NO3-][molmol-1]
by using the indexes, and the path analysis of the backward trajectories for the
model results are shown in Figs. 11 and 12 and summarized in Table 2.
The analyses are shown for the following: (a) the meteorological components of temperature
and relative humidity; (b) SO42- and SO2 with an enlarged
view for SO42-; (c) HO2 concentration; (d) NO3-,
HNO3, NO, NO2, other NOy consisting of NO3, HNO2,
N2O5, and peroxyacyl nitrates (PANs), and coarse-mode
NO3-; (e) NH4+ and NH3; and (f) adjGR, Fs, and Fn′.
The concentrations of the air pollutants were reduced by chemical reactions,
dispersion, and deposition during the transport; therefore, as an index of
the dispersion and deposition processes, BC and CO concentrations normalized to
their maximum concentrations during the transport were used in the variation
of the total concentration in (b), (d), and (e). Table 2 shows the
meteorological components and the air pollutant concentrations of each
component and total sulfate, total nitrate, and total ammonia. The indexes
were averaged over China, the transport time above the ocean, and Fukuoka.
The SNA concentration was balanced between NH4+ and SO42- with
NO3- during both types N and S.
During the type N pattern (Fig. 11), the concentration of air pollutants
increased after the air mass moved into Hebei Province (-30 h). The
relative humidity was lower than 40 % when the air mass traveled over
Shanxi Province and Hebei Province (Fig. 11a). The SO42-
concentration was around 1 µg-S m-3 during transport
(Fig. 11b and Table 2), and SO2 was dominant in the total sulfate
concentration. In this type N pattern, the concentration of the most
effective oxidant of HO2 for the SO2 aqueous-phase reaction was
smaller due to the lower relative humidity (Fig. 11c), and the conversion
ratio was less than 0.1 (Fig. 11f and Table 2). Compared with the
SO42- variation, as the trajectory passed from Shanxi Province
(-36 h) to Shandong Province (-18 h) and over the ocean (-12 h), the
NO3- and NH4+ concentrations increased. The concentrations of
NO3- and NH4+ decreased as the air mass traveled to Japan
(Figs. 11d and 11e). Over China (-30 to -18 h), gas-phase NH3 was
abundant and HNO3 was fully consumed to produce NH4NO3
(Figs. 11d and e). The excess HNO3 over the ocean contributed to
producing coarse-mode NO3- by reacting with sea salt particles as the
air mass traveled over the ocean (Fig. 11d). The adjGR was super
NH3-rich over China (Fig. 11f). After the air mass left Shandong,
HNO3 increased and NH3 was close to zero (Fig. 11d and e), so the
adjGR shifted to an NH3-neutral status (Fig. 11f). Consequently, the
proportion of NO3-, indicated by Fn′, remained around 80 %
during transport from China to Japan (Fig. 11f). The rates of the decrease in
total sulfate, total nitrate, and total ammonia were generally consistent
with the rates of decrease in normalized BC and CO, suggesting that the
budget was almost satisfied during transport (Table 2). The decrease in BC
was larger than that in CO because of wet deposition (e.g., Pan et al., 2011;
Kanaya et al., 2016).
Summary of the path analysis for types N and S.
Type
Type N
Type S
Arrival time at Fukuoka
15:00 LT, 11 Jan
00:00 LT, 17 Jan
Transport time from China
18 h
24 h
Over China
During transport
Over Fukuoka
Over China
During transport
Over Fukuoka
Temperature [∘C]
6.6
6.7
9.3
3.1
4.0
6.5
Relative humidity [%]
32.7
60.5
50.5
66.3
74.4
67.0
SO42- [µg-S m-3]
0.9
1.1
1.1
3.3
3.7
6.2
(+24.5 %)
(+25.5 %)
(+14.1 %)
(+67.2 %)
SO2 [µg-S m-3]
55.9
25.0
14.1
72.1
40.0
17.0
Total sulfate [µg-S m-3]
56.8
26.1
15.2
75.4
43.8
23.1
(-54.1 %)
(-74.7 %)
(-41.9 %)
(-73.2 %)
Fs [mol mol-1]
0.02
0.04
0.07
0.04
0.10
0.27
NO3- [µg-N m-3]
4.9
7.5
3.3
15.9
9.4
2.8
(+53.6 %)
(-42.0 %)
(-41.1 %)
(-85.6 %)
HNO3 [µg-N m-3]
0.0
2.1
1.7
1.9
6.1
5.2
Other NOy [µg-N m-3]
22.2
8.5
3.3
16.7
6.4
0.9
Coarse-mode NO3- [µg-N m-3]
0.0
1.1
1.4
0.4
0.7
1.4
Total NOy [µg-N m-3]
27.1
19.1
9.7
34.8
22.5
10.4
(-29.3 %)
(-66.2 %)
(-35.3 %)
(-77.0 %)
Fn′ [mol mol-1]
1.00
0.81
0.73
0.90
0.61
0.45
NH4+ [µg-N m-3]
5.6
8.7
4.4
18.8
12.6
8.2
(+54.3 %)
(-28.9 %)
(-33.0 %)
(-62.1 %)
NH3 [µg-N m-3]
7.5
0.4
0.2
0.6
0.1
0.0
Total ammonia [µg-N m-3]
13.2
9.1
4.6
19.5
12.7
8.2
(-30.7 %)
(-67.9 %)
(-34.8 %)
(-63.3 %)
adjGR [mol mol-1]
2.56
0.87
0.77
0.93
0.61
0.46
BC [µg m-3]
6.5
5.2
2.6
9.7
6.1
4.0
(-20.2 %)
(-63.3 %)
(-37.4 %)
(-63.9 %)
CO [ppbv]
904.4
737.6
440.2
1260.0
887.0
564.9
(-18.4 %)
(-53.8 %)
(-29.8 %)
(-56.8 %)
Key points
– Dominance of NO3- compared with SO42-
– Dominance of SO42- compared with NO3-
– Under 10 % conversion ratio for SO42-
– Approximately 30 % conversion ratio for SO42-
– Lower relative humidity
– Higher relative humidity of around 70 %
– Abundant NH3 supply above China
– No NH3 in the gas phase
– NH3-rich air mass maintaining neutralization of NO3-
– NH3-neutral conditions during transport, SO42- neutralized
Note: the parentheses indicate the multiplying
factors compared with the status over China. The status is averaged over 6 h
before the air mass leaves China, during the transport time from China to
Fukuoka above the ocean, and over 3 h before the air mass reaches Fukuoka.
Other NOy consists of NO, NO2, NO3, HNO2, N2O5,
and PANs.
During the type S pattern (Fig. 12), SNA concentrations were higher compared
with the type N pattern (Fig. 11), partly due to the slower motion of the
air mass compared with type N. SO42- concentration was around 2 µg-S m-3
over Shanxi Province to Henan Province (-72 to -36 h). It
increased slightly to 3 µg-S m-3 over Henan Province to Shandong
Province (-36 to -24 h) and subsequently increased to 3 µg-S m-3
over the ocean (Fig. 12b and Table 2). The aqueous-phase reaction
contributed to the production, as suggested by the variation in HO2
concentration. Over the oceans, the relative humidity was around 80 %, which is 20 %
larger than in the case of the type N pattern (Fig. 12a and Table 2). The Fs
conversion ratio was 0.3 when the air mass reached Fukuoka (Fig. 12f). The
increase in SO42- over Fukuoka compared with over China was
+67.2 % (Table 2). The behavior of gas-phase HNO3 and NH3
was similar in type N patterns (Figs. 12d and e); the NH3
concentration was high over China, and HNO3 gradually increased after
the air mass approached the ocean, partly because coarse-mode NO3-
was produced. However, the NH3 concentration was smaller compared with
type N patterns and remained near zero when the air mass passed over Henan
Province (Fig. 12e). The adjGR showed a slight NH3-rich status over China
(-54 to -36 h) and shifted to an NH3-neutral status before the air mass
left China (-36 h). At the same time, the ratio of NO3-,
indicated by Fn′, decreased to 0.45 (Fig. 12f and Table 2).
Similar to type N, the rates of decrease in total sulfate, total nitrate, and total ammonia
were generally consistent with the rates of decrease in normalized BC and
CO.
To summarize the key points for type N and S patterns, the behavior of
SO42-, NO3-, and HNO3 concentrations during transport
from China to Japan is shown in Fig. 13. The relative percentages of the
concentration of each species to the total concentration of SO42-,
NO3-, and HNO3 are shown. It is clear that NO3- was not
produced, whereas SO42- was produced during transport from China to
Fukuoka. Types N and S both showed the dominance of NH4NO3 (over
70 %) when the air mass was over China (-48 h). For the air mass close
to Fukuoka, because there was no NH3, NH4NO3 decomposed into
gas-phase NH3 and HNO3. Therefore, HNO3 concentration
increased as the air mass reached Fukuoka for types N and S. At this time,
the SO42- concentration was important in determining the NO3-
concentration. SO42- production (Figs. 11b and 12b) through an
aqueous-phase reaction was indicated by the HO2 concentration (Figs. 11c
and 12c), and Fs showed a large difference between types N (lower
than 0.1 during transport; Fig. 11f) and S (around 0.3 at Fukuoka;
Fig. 12f). Once
H2SO4 was produced via an aqueous-phase reaction, it reacted with
gas-phase NH3 to produce (NH4)2SO4, leading to the further
decomposition of NH4NO3. The ratio of NO3- (Fn′)
showed different behavior for types N (around 0.8 during transport;
Figs. 11f) and S (lower than 0.5 at Fukuoka; Fig. 12f). The transport pattern
under similar conditions over China was determined by the low SO42-
concentration maintaining a higher NO3- concentration at Fukuoka
(type N), or by SO42- production under a higher relative humidity
resulting in the dominance of SO42- with further NH4NO3
decomposition (type S).
The path analysis of the model results along trajectories TN and
TS. The percentages of each concentration relative to the sum of the
SO42-, NO3-, and HNO3 concentrations in equivalent
units are shown. The time axis indicates the backward time from Fukuoka. The
brown
and blue bars at the bottom are schematics of the trajectory location over
land and ocean.
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