京 都 大 学 防 災 研 究 所 年 報 第 50 号 C 平 成 19 年 4 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 50 C, 2007

Observational Indications of Indirect Acidification in Asia: Enhanced Deposition of
Nitric Acid Gas Expelled from the Aerosol Phase by Sulfate

Mizuo KAJINO*, Hiromasa UEDA**, Shinji NAKAYAMA** and Hirohiko ISHIKAWA*COE researcher, DPRI, Kyoto University**Acid Deposition and Oxidant Research Center, Niigata, JAPAN
Synopsis
Indirect acidification, i.e., enhancement of semi-volatile aerosol components deposition
associated with increased SO42- concentration has now been shown to occur in general air
pollution, analyzing Acid Deposition Monitoring Network in East Asia (EANET) monitoringdata collected at six stations in Japan. This effect was firstly detected as a result of volcanic
SO2 plumes (Satsumabayashi et al., 2004, Kajino et al., 2005). Results indicate that as SO42concentration increases, NO3- in aerosol phase is expelled, increasing the HNO3 gas
concentration. The deposition of HNO3 gas occurs at a high velocity, consequently accelerating
the deposition of NO3- despite a constant total (gas plus aerosol) nitrate concentration. In
western Japan, this indirect acidification is most influential from spring to autumn, when the
Asian continental outflow carries sulfate-rich contaminated air masses, but is not pronounced
in the air masses containing abundant sea-salt particles and related cation components inaerosols.
Keywords: acid deposition, sulfate, nitrate, dry deposition, wet deposition

1.

Introduction

Heterogeneous (gas and particles) and multiphase(gas and liquid phase) reactions play a major role intropospheric chemistry, such as aerosol and oxidant
formation,inducing
healthproblems
andenvironmental acidification (ex. Ravishankara, 1997;
Finlayson-Pitts and Pitts Jr., 1997; Jang et al., 2002;
Knipping and Dabdub, 2003; Laskin et al., 2003).
Indirect acidification, which involves heterogeneousand multiphase chemistry, is a process of aciddeposition acceleration associated with changes in
gas-aerosol partitioning of semi-volatile aerosol
components, such as nitrate, chloride and ammonium,
while the respective total concentrations in gas andaerosol phases do not change. Nitrate, chloride andammonium are thermodynamically partitioned into a
gas phase and aerosol (particulate) phase in theatmosphere. The partitioning depends on temperature,humidity and the presences of other aerosol
components. Focusing on acidic components,
equilibrium vapor pressures between gas phase andaerosol phase are HCl > HNO3 > H2SO4. When SO42concentration increases, NO3- and Cl- in aerosol
phase are expelled into gas phase, resulting inincreased concentrations (fractions) of gaseous HNO3
and HCl.
The deposition velocities of the highly reactive
gaseous HNO3 and HCl are high (Seinfeld and Pandis,1998). For example, measured dry deposition
velocities of HNO3 gas are 20 times higher than thoseof aerosols (Brook et al., 1997). As for wet deposition,
solution equilibrium Henry’s law constants of HNO3
and HCl are 2.1 x 105 and 727 mol/L/atm,
respectively. They are extremely large compared tothose of SO2 and NO2, 1.23 and 0.01 mol/L/atm,－ 165 －

respectively(Seinfeld
andPandis,1998).
Consequently, the deposition of nitrate and chloride
increases as the gas-phase fractions increase due toincreased SO42-, even when the total (gas plus
aerosol) concentrations do not change.
The indirect effect was prominent during extreme
phenomena, for example the substantial SO2
emissionsoftheMiyakejima
volcano(Satsumabayashi et al., 2004, Kajino et al., 2005).
The Miyakejima volcano, which has erupted
continuously since July 2000, has resulted in a hugeamount of SO2 emissions to the troposphere. One
year after the onset, SO2 emissions totaled 9 Tg,
amounting to half the anthropogenic SO2 emissions
from China, 20 Tg/yr. As the result, monthly mean
SO42- concentration increased by 2.5 times in Japan(Kajino et al., 2004). According to the ground based
observation of gas and aerosols at the Happo Ridgeobservatory, located 300 km north of the Miyakejima
volcano, the fractions of gaseous HNO3 and HCl
were above 95% in the Miyake-volcanic plumes,
while the contaminated air masses from thecontinental outflow contained fractions ofapproximately 40%. Consequently, bimonthly mean
NO3- and Cl- concentrations in precipitation (wetdeposition) in August and September 2000 increasedby 2.7 and 1.9 times, respectively, over the samemonths in 1999.
In the current study, we present the indirect
acidification effects under general air pollution
conditions, not as a result of an extreme event such as
the volcanic eruption, analyzing EANET (Acid
Deposition Monitoring Network in East Asia)
monitoring data. SOx emissions from continental
Asia are expected to increase in the future.
Deposition of NO3- and Cl- will be accelerated
through the indirect acidification mechanism in areas
downwind of large emission sources.

2. The monitoring network and a thermodynamic
equilibrium model
2.1 EANET monitoring network.
EANET began collecting data on a regular basis in
2001. Currently, 13 countries in East and South East
Asia participate, with a total of 47 wet deposition
monitoring stations. In 2002, two Japanesemonitoring stations began measuring concentrations

Fig. 1 Locations of 6 EANET monitoring stations inJapan. The numbers are identical to Table 1.
of gas and aerosol components for dry deposition,
increasing to the current 10 stations in Japan(EANET, 2000a). These stations monitor 7 or 14-day
accumulated concentrations of gaseous species(HNO3, HCl, NH3, SO2) and aerosol components
2-

-

-

+

+

2+

+

2+

(SO4 , NO3 , Cl , NH4 , Na , Mg , K , Ca ) using
the filter-pack method (EANET, 2003); 1-day or
7-day accumulated concentrations in precipitation
2+
+
2+
+
2+
(SO4 , NO3 , Cl , NH4 , Na , Mg , K , Ca ) using
precipitation collection and the ion chromatographymethod (EANET, 2000b); and, hourly SO2, NO, NO2,O3 concentrations, temperature and relative humidity.
In the present study, we used monitoring datacollected at 6 Japanese stations in EANET. As shownin Fig. 1, the stations are situated along the Japanarchipelago, representing various climate and air
quality features from the north to southwest. Table 1
presents the monitoring results for the 6 stations. At 4
stations, Tappi, Happo, Banryu and Yusuhara, data
were collected for two years from April 2003 to
March 2005. At Oki, data were collected for three
years beginning April 2002, and at Rishiri, for two
and a half years beginning July 2002.
2.2 Multi-component gas-aerosol equilibrium
model (SCAPE).
Gas-aerosol partitioning of the semi-volatilecomponents is predicted with a multi-component
thermodynamic equilibrium model using EANET－ 166 －

Table 1 Information and monitoring results from 6 EANET stations.

observation data. We employ the Simulating
Compositions of Atmospheric Particles at
Equilibrium (SCAPE) (Kim et al., 1993a, 1993b;
Kim and Seinfeld, 1995; Meng et al., 1995) model inthe present study. The SCAPE model calculates thethermodynamic equilibrium state among 9 inorganic
aerosol components, H2SO4, HNO3, HCl, NH3,
+
2+
+
2+
H2CO3, Na , Mg , K , Ca , under certain
temperature and relative humidity conditions andpredicts gas-aerosol partitioning of semi-volatilecomponents, water uptake and aerosol acidity. The
model is useful for predicting changes in gas-aerosol
partitioning of nitrate, chloride and ammonium undercertain hypothetical conditions, for example, if only
the SO42- concentration changes (see section 5.1), etc.

3. Indirect acidification mechanism
The indirect acidification mechanism is a processof acid deposition acceleration associated withchanges in the gas-aerosol partitioning ofsemi-volatile aerosol components, such as nitrate,
chloride and ammonium, while the respective total
concentrations in gas and aerosol phases aremaintained. As an example, Fig. 2 shows schematic
illustrations of the indirect acidification effects on
nitrate. The semi-volatile aerosol components such as
nitrate, chloride and ammonium are partitioned into a
gas phase and aerosol (particulate) phase in theatmosphere.
Thermodynamic
equilibriumisestablished for ammonium nitrate and ammonium
chloride as follows,－ 167 －

Fig. 2 Schematic illustration of the indirect acidification mechanism of nitrate.

NH 4 NO3 ( p) l NH 3 ( g )  HNO3 ( g ) ,

(1)
(2)

NH 4Cl ( p) l NH 3 ( g )  HCl( g ) ,where (p) and (g) indicate particulate (aerosol)and gaseous phases, respectively. The partitioning isdetermined by temperature, humidity and thepresence of other chemical components. Focusing on
acidic components, equilibrium vapor pressures
between the gas phase and aerosol phase are HCl >HNO3 > H2SO4. NO3- and Cl- in aerosol phase, forexample, are evaporated under the following
conditions,Cl  ( p ) o HCl ( n ) , NO 3  ( p ) o HNO 3 ( n ) , (3)2 NH 4 NO3 ( p)  H 2 SO4 o‫ޓޓޓޓޓޓޓ‬( NH 4 ) 2 SO4 ( p )  2 HNO3 (n)2 NH 4Cl ( p )  H 2 SO 4 o‫ޓޓޓޓޓޓޓ‬( NH 4 ) 2 SO 4 ( p )  2 HCl (n )NaCl ( p )  HNO 3 ( g ) o‫ ޓޓޓޓޓޓ‬NaNO 3 (p )  HCl (n )

.

,

,

(4)

(5)

(6)

Equation 3 reflects evaporation as temperature(T) increases. The gas phase fractions of nitrate andchloride increase in summer. SO42- is produced byatmospheric oxidizing reactions from SO2 taking
gaseous NH3, which forms ammonium sulfate
aerosols. When NH3 gas is exhausted, SO42- expels
NO3- and Cl-, changed from aerosol phase into gas
phase as described by Eqs. 4 and 5. Equation 6
indicates a reaction that HNO3 gas, deposited on
sea-salt particle surfaces, expels Cl- into gas phase.
The deposition velocities of the highly reactive
gaseous HNO3, HCl and NH3 are high (Seinfeld andPandis, 1998). As SO42- concentration increases,
fractions of gaseous HNO3 and HCl concentrations
increase, described in Eqs. 4 and 5, resulting inenhanced deposition of NO3- and Cl- even if the total(gas plus aerosol) concentrations (t-NO3, t-Cl) do notchange. In contrast, the basic component NH3 gas istaken into aerosol phase forming ammonium sulfate,resulting in a decrease in the gas phase fraction,
which may also contribute to environmental changes.
In the current paper, we focus only on increases
in wet deposition of NO3- due to increased SO42concentration. We analyze only wet deposition－ 168 －

because dry deposition amounts are not directly
measured by EANET. Dry deposition is discussed
later because of its pronounced effects on forests andfarmlands. We compare the nitrate to sulfate (N/S)
ratio of wet deposition with their atmosphericconcentrations, and discuss the indirect acidificationof NO3-. However, the same analysis is hardly
applicable for HCl’s as the process is more complex(a comparison of Cl/N, Cl/S, N/S ratios is needed).
The decline in the gas-phase fraction of ammonium
results in a decreased deposition flux of NH3 gas,
which may accelerate acidification. However, wet
deposition flux of NH4+ may increase due to thesubstantial nucleation scavenge of ammonium, andits contribution to acidification or neutralization is
not identified. Analysis of ammonium is also notdiscussed.

S ( IV )S ( IV )  nssS (VI ),[ S ] SO2
‫ޓޓޓޓޓޓ‬[ S ] SO2  [ S ] nss  SO 2 Fs {4

a ratio of tetravalent S and (tetravalent plus
hexavalent) anthropogenic S, assuming that all SO2 isanthropogenic. Low values of Fs indicate that SO2 issufficiently oxidized to SO42- during long-range
transport. Finally, the correlation coefficients R
between sets of variables are shown. DN5/S6/CN5/S6
indicates a ratio of N/S molar ratio in precipitation
and atmospheric concentration (D represents wet
Deposition),

DN 5 / S 6 / CN 5 / S 6 {4. Sampling Resultsd [ NO3 ]d [ SO42  ]u[ SO42  ] / 96

In this section, we present the indications of
indirect acidification occurrence, including increased
wet deposition of NO3-, using the EANET monitoring
data.
Table 1 summarizes information for the six
Japanese monitoring stations in EANET and theircorresponding monitoring data. The monitoring data,collected using the filter pack method, are analyzed
and mean concentrations of nss-SO4, t-NO3, t-Cl,
t-NH3 and Minerals in Pg/m3 with standard
deviations
areshown.
Nss-SO4
indicates
anthropogenic sulfate, which is defined as thedifference between total SO4 and sea-salt originatedSO4 concentrations (=0.251 × [Na+]). [ ] denotes
atmospheric concentrations in Pg/m3. [t-NO3], [t-Cl],[t-NH3] indicates total (gas plus aerosol) nitrate([HNO3]+[NO3-]), chloride ([HCl]+[Cl-]), ammonium([NH3]+[NH4+])
concentrations,
respectively.
[Minerals] indicates the total concentrations ofcations, namely Na+, Mg2+, K+ and Ca2+ (with the
exception of NH4+). Temperature (°C) and relative
humidity (%) are next. CnssS/N5 indicates molar ratiosof nss-SO4 and N(V) concentration, namely t-NO3 (Cimplies atmospheric Concentration). [nss-SO4]/[SO4]is an indicator of the anthropogenic air masses.
Pg-HNO3 indicates gas phase fraction of nitrate (Pimplies Partitioning). Fs is defined as,

(7)

,

(8)

‫ޓޓޓޓޓޓ‬[ HNO3 ]g / 63  [ NO3 ] p / 62

where d[ ] denotes concentrations in precipitation(Pmol/L), the subscripts g and p denote gas phaseand particulate phase, respectively. As themeasurement of atmospheric concentrations using thefilter pack method is on a 7- and 14-day basis, thedaily measured wet deposition amounts areaccumulated for each station over the same sampling
periods. Hourly data such as T and RH are also
averaged for each period.
The six stations generally have the following
features. (1) Rishiri and (2) Tappi represent relatively
clean marine atmospheres and the anthropogenic
indicators [nss-SO4]/[SO4] are less than 0.8 on
average. As the counterpart of NO3-, mineral
concentrations of Na+ and Mg2+ are high (> 2 Pg/m3),
resulting in low values of Pg-HNO3 (< 0.3). The stations
are located in the northern part of Japan, andtemperatures drop to below 0 °C in winter. In contrast,(3) Happo and (6) Yusuhara represent inland air
quality, mineral concentrations are less than 1Pg/m3
and [nss-SO4]/[SO4] is approximately 0.98, resulting
in high values of Pg-HNO3 (> 0.7). Happo is located in
the mountainous area of Central Japan, with analtitude of 1850 m MSL, and winter temperatures are
below 0 °C. (4) Oki and (5) Banryu, which arelocated in marine and coastal regions of western－ 169 －

Japan, respectively, reflect the influence of the Asian
continental outflows, resulting in relatively higher
values of [nss-SO4]/[SO4] (>0.85) and Pg-HNO3(0.3—0.4) than Rishiri and Tappi. [Nss-SO4] isgreater at Oki, Banryu and Yusuhara, which arelocated in western Japan where the anthropogenic
influence is pronounced and less at Rishiri, Tappi andHappo. No significant differences in RH are detectedamong the stations.
Pg-HNO3 shows positive correlations with T at all
stations as indicated by the direction in Eq. 3. The
correlation coefficient R between CnssS/N5 and Pg-HNO3,
indicates the expulsion as described in Eq. 4. At Oki,
Banryu and Yusuhara, located in western Japan where[nss-SO4] is greater than 3.5 Pg/m3, R isapproximately 0.7 due to significant expulsion of
NO3- by SO42- at the stations in western Japan where
the continental outflow is influential. In contrast, R issmall or near zero at other stations. High altitude andlow cation component concentrations at Happo resultin higher values of Pg-HNO3 (>0.90 for 33% of thesamples). Consequently, the correlation coefficient R
between CnssS/N5 and Pg-HNO3 is low (0.31), however
the tendency is apparent in a scattergram (not shown).
At Rishiri and Tappi, representing clean marine air
quality, Pg-HNO3 depends mainly on T and hardly on
nss-SO4, and the R between CnssS/N5 and Pg-HNO3 are–0.03, 0.04, respectively (Eq. 4 does not seem towork).
Positive correlations are obtained between Pg-HNO3
and DN5/S6/CN5/S6 at 4 stations, Oki, Banryu, Yusuhara,and Happo, corresponding to the stations where
positive correlations between CnssS/N5 and Pg-HNO3 are
obtained. Here DN5/S6/CN5/S6 represents thepartitioning of N(V)/S(VI) molar ratio between
deposition and the atmospheric concentrations
defined in Eq. 8. The higher value of DN5/S6/CN5/S6
indicates a greater N/S molar ratio in precipitation
than in atmosphere, namely, greater wet deposition
flux of N than of S. Accordingly, positive correlations
between Pg-HNO3 and DN5/S6/CN5/S6 indicate that thewet deposition flux of nitrate becomes larger as thegas phase fraction increases, which is indirect
acidification. Together with the positive correlations
between CnssS/N5 and Pg-HNO3, the expulsion ofparticulate NO3- by anthropogenic SO42- to the gas
phase and increased gas-phase HNO3 resulting in theacceleration of the wet deposition flux are indicatedat Oki, Banryu, Yusuhara and Happo.
At Rishiri and Tappi where no correlations aredetected between CnssS/N5 and Pg-HNO3, the analysis
using DN5/S6/CN5/S6 is nonsensical. The correlation
coefficients R between Pg-HNO3 and DN5/S6/CN5/S6 are
0.17 and -0.14, at Rishiri and Tappi, respectively, due
to the abundance of cation components originatingfrom sea-salt particles. In addition, the indirect effectis usually based on the calculation of vapor-liquid
equilibrium of semi-volatile components, howeverthese two stations are located in the northern part ofJapan where temperatures drop below 0 °C in winterand precipitation consists mostly of ice and snow, notliquid. Therefore, we selected samples underconditions that [nss-SO4]/[SO4] is greater than the
average, reflecting a smaller influence of sea-salt
particles, and mean temperature during sampling isabove 0 °C. The same analysis using the conditional
samples increased the correlation coefficients R
between CnssS/N5 and Pg-HNO3 from -0.03 to 0.39, 0.04
to 0.60, and R between Pg-HNO3 and DN5/S6/CN5/S6
increased from 0.17 to 0.73, -0.14 to 0.34,
respectively at Rishiri and Tappi. Consequently,
indirect acidification is evident under the substantial
influence of anthropogenic contamination, lower
influence of sea-salt particles and sufficiently hightemperatures for vapor-liquid equilibrium to beestablished.
Figure 3 illustrates scattergrams of sampling at(4) Oki (above) and (1) Rishiri (below) started in
2002 using the filter pack. Oki, located in westernJapan, is influenced by the pronounced Asian
continental outflow and the correlation coefficients R
are greater than at other stations. At Rishiri, locatedin northern Japan, the sea-salt effect is predominant
and the correlations were very weak, but areenhanced significantly by sample selection under theconditions mentioned above. Figure 3 shows
scattergrams between (a) CnssS/N5 and Pg-HNO3 (%), (b)Pg-HNO3 (%) and DN5/S6/CN5/S6, (c) Fs and DN5/S6/CN5/S6
at Oki. Figures 3 (d) – (f) present the same
information as (a) - (c) for Rihiri. The circles denoteall samples and the closed circles in (d) - (f) denotesamples under the conditions that T>0 and[nss-SO4]/[SO4]>average. The solid lines in (a) - (c)are regression lines of all samples, while dashed linesin (d) - (f) represent all samples (opened plus closed
circles). The solid lines in (d) - (f) are the regression－ 170 －

Fig. 3 Scattergrams of (a) CnssS/N5 and Pg-HNO3, (b) Pg-HO3 and DN5/S6/CN5/S6 and (c) Fs and DN5/S6/CN5/S6 at Oki. (d)– (f) are same as (a) – (c) for Rishiri.

－ 171 －

lines of the conditional samples (closed circles). In
Fig. 3 (d) and (a), Pg-HNO3 can be greater than 50%
under lower influence of sea-salt particles (closed
circles) when CnssS/N5 is less than 2 at Rishiri, while
Pg-HNO3 is smaller than 30% when CnssS/N5 is less than
2 at Oki. This is because t-NH3 concentration is low
at Rishiri (0.68 Pg/m3) on average, half that at Oki,
which can fix NO3- in aerosol phase. However, when
cations originating from sea-salt are abundant at
Rishiri (opened circles in Fig. 3d), Pg-HNO3 is smaller
than 20% although CnssS/N5 is greater than 4.
The Fs value is the indicator of an anthropogenic
air mass. For the relatively short-distance transport
inside Japan, the Fs value is greater than 0.6,
approximately, and smaller than 0.2 for long-range
transport
fromthecontinenttoJapan(Satsumabayashi et al., 2004). In Fig. 3 (c),DN5/S6/CN5/S6 is greater than 3 when Fs is smaller than
0.2 and the correlation coefficient R between them is–0.45. Time series diagrams are not shown in thecurrent paper, however the transport of thesulfate-rich contaminated continental air mass to Okifrom spring to autumn results in significant indirect
acidification and accelerated wet deposition of NO3-.
The same trend can be seen in Fig. 3 (f) at Rishiri
under a lower influence of sea-salt particles(R=-0.40). The correlation coefficients R between Fs
and DN5/S6/CN5/S6 at all six stations are shown in Table
1.
5. Discussion
5.1 Indirect acidification induced by future SOx
emission increases in Asia.
The Special Report on Emissions Scenarios(SRES) of the Intergovernmental Panel on Climate
Change (IPCC, 2000) describes that in all four
scenarios, A1, A2, B1 and B2, the rates of emissionsin Asia are increasing faster than those worldwide.
SOx emissions are estimated to peak between theyears 2020 and 2050, becoming twice as high as in2000. NOx emissions continue to increase after SOx
cease increasing, peaking after 2040 at more than
twice the present levels in the lowest estimation (B1)and by approximately 4 times in 2100 in the A2
scenario. SRES does not evaluate the increasing NHx
emission rates, which affect the gas-aerosol
partitioning substantially. Klimont et al. (2001)

estimated that NHx emissions will increase in East
Asia by 2030 to 1.7 times 1995 levels.
The SCAPE model simulates changes in
gas-aerosol partitioning of nitrate and ammonium asSO42-, t-NO3, and t-NH3 concentrations change. The
partitioning is simulated using the samples from all
six EANET stations as the model input. The averageand the median of the simulated Pg-HNO3 are 0.49 and
0.44, while those of the measured (monitored)
Pg-HNO3 are 0.41 and 0.29, respectively. The
correlation coefficient R of the modeled and themeasured Pg-HNO3 is 0.70. At Oki, the average and the
median of the modeled Pg-HNO3 are 0.44 and 0.31,
respectively, while those of the measured are 0.29
and 0.21, respectively. The modeled values are
greater than the measured ones and the correlation is
not very strong (R=0.52).
If only SOx emissions increase to twice current
levels on the continent (NOx and NHx emissions
remain unchanged) and SO42- concentrationstransported to Oki are increased by two as well, theaverage and the median of Pg-HNO3 using the SCAPE
model are 0.72 and 0.91, respectively. Roughestimation of DN5/S6/CN5/S6 can be made from Fig.
3(b). When SO42- concentration increases by two andwhen Pg-HNO3 increases from 30% to 70%,
DN5/S6/CN5/S6 increases by approximately two from
1.8 to 3.7 according to the regression line.
In equation 8 (DN5/S6/CN5/S6), when [SO42-] isdoubled, d[SO42-] is assumed to be doubled, while thetotal [HNO3]g/63+[NO3-]p/62 remains unchanged(only the partitioning changes). Consequently,“DN5/S6/CN5/S6 is doubled” indicates that “d[NO3-] isdoubled”. Although NOx emission is assumed
constant, the wet deposition of NO3- may double
through the indirect acidification mechanism when
SO42- concentration doubles.
Simply, but more realistically, based on the above
mentioned emission scenarios, the case that [SO42-],[t-NO3] and [t-NH3] concentrations increase by two is
examined. The average and the median of themodeled Pg-HNO3 at Oki is 0.57 and 0.61, respectively.
3 mol of NH4+ is required to balance 1 mol of SO42and 1 mol of NO3-. If those components increase
together at the same rate, Pg-HNO3 increases, although
the increasing ratio is smaller than the case in which[t-NH3] is stable.
Thus, future increases in SO42- concentration in－ 172 －

Asia can induce increased wet deposition of NO3- inregions downwind of large emission sources throughthe indirect acidification processes.
5.2 Other indirect acidification effects.
Increases in SO42- concentrations induce changes
in the gas-aerosol partitioning of semi-volatile
aerosol components, resulting in increases in gas
phase concentrations of HNO3 and HCl and aerosol
phase concentrations of NH4+. Differences in thedeposition velocities of gas and aerosols may change
the deposition amounts of those components,
subsequently. Increased wet deposition of NO3- isdetected using the above-mentioned analysis, but towhat extent does indirect acidification affect thedepositions of other components?
Dry deposition velocities depend not only on
components but also on meteorology and surface
conditions. The dry deposition velocity of particles
depends substantially on their size distribution.
Consequently, estimations of the dry deposition
velocities vary substantially across the targeted
regions, seasons and vegetations, and the parameters
used in each study. According to several
measurements, the dry deposition velocity of HNO3
gas is high (> 2cm/s) because it is highly reactive(Wesely and Hicks, 2000). The modeled dry
deposition velocities of aerosols over eastern North
America range from 0.1 to several cm/s (Zhang et al.,2001). The measured dry deposition velocities ofNO3- aerosol onto the forests, where the surface
resistance to dry deposition is low, are as large as
several cm/s (Erisman et al., 1997, Ruijgrok, et al.,1997), however, for HNO3 gas dry deposition
velocities may be larger on the same conditions.
Simultaneous measurements of HNO3 gas andaerosol dry deposition velocities onto the farmlands
indicate that the velocities of HNO3 areapproximately 20 times larger than those of aerosols(Brook et al., 1997). This condition, for example if
SO42- concentration increases and Pg-HNO3 increases
by two from 30% to 60%, result in a forty-foldincrease in dry deposition flux of HNO3 gas.
Nitrate deposition reduces plant diversity
worldwide and species richness of grasslands (Sala etal., 2000, Stevens et al., 2004). Measured dry
deposition amounts of HNO3 gas in Japanese forestsand farmlands are more than twice those of NO3-

aerosols (Matsuda et al., 2001). Anothermeasurement in Japanese cedar forests reported thatthe dry deposition amount of H+ is greater than thewet deposition amount of H+ throughout the year
(Takahashi et al., 2001). In certain regions, seasonsand vegetation, the effects of the indirect
acidification on increased dry deposition of NO3- andconsequent influence on vegetation are cause forconcern.
Few studies have focused on the deposition of
HCl gas. HNO3 gas, expelled by SO42- and depositedon aerosols, expels Cl- into the gas phase effectively(Eq. 6). At Rishiri and Tappi, representing cleanmarine air, the correlation between HNO3 gas
concentration and the gas-phase fraction of HCl,
Pg-HCl is strong (R>0.8). However, sea-salt originatedCl- such as NaCl is highly hygroscopic and act as
cloud condensation nuclei (CCN), meaning that thewet deposition flux of Cl- is largely due to nucleation
scavenging in clouds. Although, increases in Pg-HCl
cannot be shown to contribute to enhanced or
reduced wet deposition of Cl-, HCl gas is highly
reactive and dry deposition velocities may be as large
as those of HNO3 gas. As a result, the indirect effects
on the dry deposition of Cl- in forests and farmlands
are crucial. Chlorine emissions from sea-salt particles
substantially increase ozone concentrations in coastal
urban regions (Knipping and Dabdub, 2003), also
inducing vegetation damage.
NH3 gas is also highly reactive, resulting in a highdeposition velocity. As NH3 gas quickly reacts with
SO42- forming ammonium sulfate, the correlations
between [nss-SO42-] and [NH4+] are strong (R>0.8) at
the all six stations. Increased SO42- concentrations
result in decreased NH3 gas concentration and may
cause decreases in dry deposition flux of NH4+.
However, ammonium sulfate is also hygroscopic, so
the wet deposition flux of NH4+ may also increase.
In order to assess all the indirect acidification
effects mentioned in the present paper quantitatively,
further studies are necessary using three-dimensional
chemicaltransport
modelswithaccurate
parameterizations of dry deposition velocities andprecisely formulated wet deposition processes,
together with regional-scale long-term monitoringdata of dry and wet deposition.

－ 173 －

5.3 Effects on the radiation budget.
Optical properties of gas and aerosols are very
different. Adams et al. (2001) estimated that theradiative forcing of nitrate would be comparable tothat of sulfate in the year 2100 according to the SRES
A2 scenario when NOx emissions reach 4 times 2000
levels. Schaap et al. (2007) reported that even in
1995, in Europe during the winter when the aerosol
phase fraction of NO3- is large, the radiative forcingof nitrate amounts to half that of sulfate.
Consequently, future changes in the gas-aerosol
partitioning of nitrate may induce substantial change
in the radiative forcing of aerosols.

chemical composition measured by the filter packmethod and in precipitation sampled on a daily basis,
may originate from different air masses (it is still
meaningful that certain correlations were found in thecurrent analysis). Thus, precise wet deposition fluxes
and wet scavenging coefficients from thismeasurement could not be derived. Further
observational studies would be interesting,investigating relationships between gas-aerosol
partitioning and the wet scavenging coefficients ofnitrate derived from measurements with finer time
resolutions (ex., 6-hour simultaneous sampling of gas,
aerosols and precipitation).
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6. Conclusions
Analyzing EANET monitoring data, indirect
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deposition associated with increases in SO42- wasrevealed under general air pollution conditions. The
indirect acidification is most influential from spring
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