Monitoring the SO 2 concentration at the summit of Mt. Fuji and a comparison with other trace gases during winter

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd004428, 2004 Monitoring the SO 2 concentration at the summit of Mt. Fuji and a comparison with other trace gases during winter Yasuhito Igarashi, 1 Yosuke Sawa, 1 Katsuhiro Yoshioka, 2 Hidekazu Matsueda, 1 Kenji Fujii, 3 and Yukiko Dokiya 4 Received 8 December 2003; revised 2 March 2004; accepted 8 June 2004; published 3 September [1] An SO 2 continuous monitor consisting of a commercially available pulsed UV fluorescence instrument with zero and span gas calibration was installed at the summit of Mt. Fuji (3776 m asl) in September The system produces data with a time resolution comparable with other trace gases. The instrumental feasibility was tested onsite, and the SO 2 concentration level at the summit was thereafter routinely observed. The present detection capability of the system, expressed in terms of the critical level (L c, definition by International Union of Pure and Applied Chemistry and International Organization for Standardization), was estimated to be about 0.05 ppbv. Thus it was difficult to observe the temporal change of very low background SO 2. However, the system is satisfactory for observing episodic transport of SO 2, particularly during winter. No high SO 2 episodes were observed during summer, in contrast to winter. One extraordinary episode was observed in late October 2003, the only one attributable to the Miyake-jima SO 2 volcanic plume. High SO 2 episodes were more evident (with longer duration and higher concentration level) in February 2003 among the winter months observed. Typical February conditions were determined using backward trajectory, a surface weather map, and other indicators. A comparison of the temporal changes in SO 2, CO, and 222 Rn concentrations in the winter months suggests that these gases in the free troposphere over Japan may have been transported together in most cases from the same source regions somewhere in the Asian continent. The correlation between SO 2 and 222 Rn in such episodes may also suggest a short timescale for transport from the source to Mt. Fuji of within a few days. The chemical time series data of SO 2 at Mt. Fuji is important for understanding the free tropospheric chemical nature, such as the Asian outflow over the North Pacific. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); 0368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; 0370 Atmospheric Composition and Structure: Volcanic effects (8409); 0394 Atmospheric Composition and Structure: Instruments and techniques; KEYWORDS: SO 2, air concentration, UV fluorescence, free troposphere, Mt. Fuji, transport Citation: Igarashi, Y., Y. Sawa, K. Yoshioka, H. Matsueda, K. Fujii, and Y. Dokiya (2004), Monitoring the SO 2 concentration at the summit of Mt. Fuji and a comparison with other trace gases during winter, J. Geophys. Res., 109,, doi: /2003jd Introduction [2] Chemical observations at mountainous sites can provide information regarding the transport and transformation of minor and trace chemical species that have climatological 1 Geochemical Research Department, Meteorological Research Institute, Ibaraki, Japan. 2 Shimane Prefectural Institute of Public Health and Environment Science, Shimane, Japan. 3 Kansai Environmental Engineering Center Co. (KANSO), Osaka, Japan. 4 Edogawa University, Chiba, Japan. Copyright 2004 by the American Geophysical Union /04/2003JD and environmental impacts in the free troposphere. There is only a limited number of sites available for chemical observation in Japan: Mt. Tateyama [Osada et al., 2003; Kido et al., 2001], Mt. Norikura [Osada et al., 2002], and Mt. Fuji. [3] Mt. Fuji (35.4 N, E; 3776 m asl; see Figure 1) is the highest mountain in Japan. Its summit is positioned in the free troposphere for most of the year, serving as a suitable platform for observations of atmospheric chemistry. The mountain has a nice conical profile, and the summit is relatively free of mountain and valley winds. Long-range transport of pollutants from the Asian continent attributable to westerly winds in the free troposphere can be assessed at the summit. The installation of a background observatory for the WMO pollution monitoring program at the summit 1of21

2 Figure 1. Location of Mt. Fuji and active volcanoes in Japan that are emitting SO 2. has been examined [Kawamura, 1974]; unfortunately, the plan never came to fruition. Nakazawa et al. [1984] conducted CO 2 measurements at the summit using an NDIR analyzer in 1981, but this did not extend longer than four months. There were several years with no monitoring activity until Dokiya et al. [1993], at the Meteorological College at that time, started their study of precipitation chemistry at Mt. Fuji in They later repeated a shortterm campaign observation of gas, aerosol, fog, and precipitation for major ionic species in summer [Dokiya et al., 1995, 2001; Hayashi et al., 2001; Sekino et al., 1997; Tsuboi et al., 1994]. The Japan Meteorological Agency (JMA) has maintained a weather station at the summit with electricity and some work areas, and attendant staff are present for meteorological observations throughout the year. This advantage made more members to join the group, and thus we have continued summer campaign observations on chemical species in the ambient air since 1997 [Hayashi et al., 2001; Murakami et al., 2002; Naoe et al., 2003]. One disadvantage of Mt. Fuji observations is that no machinepowered commute or logistics are available on the mountain except a bulldozer in summer months. [4] Tsutsumi et al. [1994, 1998] and his collaborators at the Meteorological Research Institute (MRI) have continued their observations of ambient O 3 at the summit since 1992 to investigate background O 3 in the middle troposphere. They have found diurnal O 3 concentration changes at the summit of Mt. Fuji smaller than those at Mauna Loa, suggesting that the summit of Mt. Fuji is less affected by the boundary layer air than Mauna Loa. Therefore it is possible to argue that the time series of chemical species in the ambient air at the summit of Mt. Fuji mostly exhibits the spatial distribution of the species in the free troposphere. Later, Tsutsumi and Matsueda [2000] commenced continuous CO observations at the summit. Our collaboration is further expanding to include continuous 222 Rn observations at the summit [Yoshioka, 2002]. Investigations into the minor and trace chemical species in the free troposphere are important present and future tasks from the standpoint of global impacts. Our preliminary comparisons of such measurements are presented in this paper. [5] One of our concerns is the concentration level of SO 2, the precursor species for aerosol sulfate, in the free troposphere. Aerosol sulfate is an important species, along with SO 2, in terms of radiative effects (radiative forcing) and acidification. Intergovernmental Panel on Climate Change (IPCC) [2001] acknowledged that much uncertainty remains in the understanding of sulfate aerosols in the atmosphere, including the sources of precursor gases and the gas-to-aerosol conversion processes. Many chemical transport model studies [e.g., Barrie et al., 2001; Barth et al., 2000; Chin et al., 1996, 2000a, 2000b; Feichter et al., 1996, 1997; Golombek and Prinn, 1993; Huebert et al., 2001; Langner and Rodhe, 1991; Pham et al., 1995; Rasch et al., 2000] have been carried out to assess the sulfur cycle in the atmosphere and to evaluate the radiative forcing [Feichter et al., 1997; IPCC, 2001]. However, the model itself involves many uncertainties. For example, Pham et al. [1995, p. 26,088] reported that in this respect, long-term (climatological) measurements at remote sites are best suited to validate the model. Measurements of sulfur species in the free troposphere and near the tropopause are clearly needed. A study of long-term chemical time series is also important. Huebert et al. [2001] stated in their model study on free tropospheric sulfate at Mauna Loa that we strongly argue that time series on the order of a decade or more for reactive and soluble species are needed for testing the chemical transport model. Comparisons of the observation data with the model results, as well as model performance intercomparisons [Barrie et al., 2001], are of great significance to improve our current understanding of atmospheric sulfur. Despite their importance, SO 2 observations appear to have been conducted at only a limited number of mountainous sites situated under free tropospheric conditions, such as Mauna Loa, Hawaii [Luria et al., 1992] and Jungfraujoch, Switzerland [Filliger et al., 1994]. [6] Mt. Fuji is situated downwind of industrial emissions from the Asian continent, and thus the direct influence of the Asian outflow can be observed. The Asian continent may have been the largest source area of SO 2 (over 10 Tg S in 1987 and about 13 Tg S/y in 1995) in the Northern Hemisphere [Akimoto and Narita, 1994; Streets et al., 2001] in the 1990s. Ten Tg S is about 10% of the annual global emission of gaseous sulfur to the atmosphere, which is currently considered to be around 100 Tg S [IPCC, 2001]. Several atmospheric research projects, such as the NASA Pacific Exploratory Mission [Hoell et al., 1996; Thornton et al., 1997] and the PEACAMPOT observations [Hatakeyama et al., 1995], have been conducted to better understand the Asian outflow. A high concentration of SO 2 was unsurprisingly observed in those airplane observations. 2of21

3 However, while a large observation area is covered by flight observations and thus the aerial distribution can be determined, the data is inevitably limited to snapshots. [7] There are natural sources of SO 2 in our environment aside from anthropogenic sources. Volcanic activity emits a large amount of SO 2 (13 Mt/y, equal to 6.5 Tg S/y) into the air [e.g., Bluth et al., 1993]. Emissions from active volcanoes in the Japanese islands are also likely to be observed at the summit of Mt. Fuji, with a change in the prevailing wind reflecting any air mass change. The Japanese archipelago is volcanic, and thus there are several active volcanoes in Japan that emit SO 2 into the troposphere over Japan (see Figure 1). Fujita et al. [1992] estimated the annual emission of SO 2 from the volcano in Japan to be about 0.55 Tg S/y under the quiescent condition. This value may be beyond the annual industrial emission of SO 2 in Japan after late 1980s (less than 0.5 Tg S/y) [Fujita, 1996; Streets et al., 2001]. The Oyama volcano (813 m asl) at Miyake-jima Island situated off the south coast of Honshu Island (34.1 N, E; see Figure 1), started its eruption in July An enormous amount of SO 2, on an average of 42 kt/day (7.7 Tg S/y) [Kazahaya et al., 2003; Shinohara et al., 2003], corresponding to three-quarters of the discharge of the Asian continent and greater than the usual total global volcanic discharge, was emitted during the year The emission rate gradually decreased to a 10 kt/day (1.8 Tg S/y) level after late 2002 [Kazahaya et al., 2003; Shinohara et al., 2003], but SO 2 emission apparently still continues. Naoe et al. [2003] detected the Miyake-jima plume at the summit of Mt. Fuji by performing aerosol observations during intense observation campaigns in the summer of However, it remains uncertain to what extent the SO 2 plume from the volcano currently affects the SO 2 concentration levels in the free troposphere over Japan. In addition, the plume does not always extend beyond the marine boundary layer [Japan Meteorological Agency (JMA), 2004]. [8] Only Sekino et al. [1997] has reported the SO 2 concentration level at the summit of Mt. Fuji, and only for the summer period. They used an alkaline-impregnated filter coupled with ion chromatography as the analytical method. Therefore the time resolution of the observation was not fine, only 4 hours. It is advantageous to observe temporal changes of a chemical species with a greater time resolution in the atmosphere to compare the temporal trends with those of other trace gases to understand their transport. There are several methods available for measuring low-level SO 2 in air with a finer time resolution. An intercomparison of seven independent state-of-the-art analytical techniques for low-level SO 2 in ambient air (the GASIE project) was reported by Luther and Stecher [1997]. These include aqueous chemiluminescence, modified commercial pulsed UV fluorescence (very similar to the present method), isotope dilution gas chromatography/mass spectrometry (MS), mist chamber/ion chromatography (IC), diffusion denuder/chemiluminescence, HPLC/fluorescence, and carbonate-impregnated filter/ic [Stecher et al., 1997]. Of these, a dry method free from liquid agents is favorable at mountainous sites. The MS technique has excellent sensitivity, ranging from a single to a few tens pptv [Arnold et al., 1997; Thornton et al., 1997, 2002; Hanke et al., 2003]. However, the MS technique requires special expertise. Also, an automated instrument is preferable under the existing variable conditions at Mt. Fuji weather station. Hence we selected a UV fluorescence instrument as a realistic option. [9] An SO 2 continuous monitoring system that uses a currently available commercial UV fluorescence instrument, similar to the one employed by Luke [1997] and Luria et al. [1992], was installed at the summit of Mt. Fuji during September We expected this to be a first step toward clarification of the spatial distribution and concentration level of SO 2 in the free troposphere over Japan. However, the background level of SO 2 at the summit of Mt. Fuji was found to be so low that it could not be determined with that system. High SO 2 concentration episodes have been frequently found in the time series data despite this sensitivity limitation, and these episodes evidently reflect large-scale transport. [10] The basic performance of the system is reported in this paper as well as the temporal changes of SO 2 concentrations obtained from October 2002 to October 2003; its sporadic nature is also described. This is the first report of a relatively extended time series of SO 2 in the free troposphere over Japan. The data are compared with those obtained by previous summer observation campaigns conducted at the summit of Mt. Fuji [Sekino et al., 1997] and other reported data. A comparison of winter data with the other trace gases and with the meteorology is also provided. 2. Experiment 2.1. SO 2 Observations [11] The low-level SO 2 continuous monitoring system is composed of the primary UV fluorescence SO 2 monitor (Thermo Electron (TECO), 43C-Trace Level), an air compressor (Jun-Air Co., OF M) for producing zero-level (zero) gas, and a mixer (TECO) for preparing diluted SO 2 standard gas. The manufacturer claims that the Model 43C-TL extends the pulsed fluorescence technique and reflective optic design to measure SO 2 at levels never before achieved, down to a 0.1 ppbv level (see thermo.com/ethermo/cma/pdfs/product/productpdf_ pdf ). The system components were connected with tetra fluorocarbon and/or stainless steel tubing (Swagelok, USA) to minimize the deposition of SO 2 gas onto the inner wall of the tube and connectors. The UV fluorescence SO 2 monitor was calibrated onsite with standard SO 2 gas (about 1 ppmv with N 2 balance gas in a 47 L cylinder at 110 atm/cm 2, gravimetrically produced by Taiyo-Toyo Co., Japan) and zero gas. The manufacturer suggests that SO 2 standard gas at this concentration level be stable for a year or more. The zero gas is also generated onsite by passing compressed air taken inside the room through a heatless air dryer (TECO model 6643) using zeolite and activated aluminum oxide as adsorbent for the water vapor and a zero gas scrubber (TECO) filled with Purafil (oxidizing adsorbent, potassium permanganate coated aluminum oxide, 250 g) and activated charcoal (150 g). The zero gas is then processed for the mixer by means of a mass flow controller (TECO). The zero gas is directly introduced to the SO 2 monitor when the zero calibration is carried out. The standard SO 2 gas is diluted 100-fold by the zero gas at the mixer and then introduced to the SO 2 monitor when the span calibration is carried out. In the standard calibration, the diluted SO 2 standard gas is injected into the SO 2 3of21

4 monitor for 1 hour and then rinsed by zero air for 30 min or 1 hour. Zeroing is performed by injecting the zero air for 1 hour. Thus zero and span calibrations are conducted respectively every 11 and 23 hours to achieve the optimal data quality. These calibration times were set by multiples of 12 minus 1 hour to avoid data absence for the same time zone of a day. The span calibration was done onsite and no pressure or temperature corrections were made. The instrumental setting of the data running average time was set at 300 sec to remove any high-frequency instrumental noise. This setup was used to follow the results obtained by Luke [1997]. SO 2 raw data was accumulated in one-minute intervals by a personal computer (PC) through an RS- 232C port. It could be run by PC control without operator attendance once the system was manually calibrated (initializing the TECO 43C-TL) and the control parameters were set. However, the run itself was checked daily by the operator at the Mt. Fuji weather station. The instrumental parameters, such as the inner temperature, UV lamp intensity, and inner pressure, as well as the SO 2 data, were simultaneously stored on an MO disk and the HD. [12] The system was installed at the first building of the Mt. Fuji weather station along with other measurement instruments. The air inlet is composed of a 120 mm outer diameter stainless steel duct about 20 m long. The air intake is about 3 m above the ground on the eastern side of the building. The most frequent wind direction at the summit of Mt. Fuji is west [Muraki, 1974]; therefore the building shields the inlet from the heavy ice deposition during winter to spring that could cause plugging of the inlet. The inlet pipe was originally used for high-volume aerosol sampling and the area speed of the airflow is very slow, about 1 cm/sec. Therefore the loss of SO 2 by deposition onto the inner wall of the pipe caused by turbulence is minimized. The air is introduced to the system inside the room by a tetra fluorocarbon tube (1/4 inch, about 3 m long) through a fluorocarbon membrane filter (f 47 mm) to remove the aerosol and is then passed through the hydrocarbon kicker using a permeation tube, which removes aromatic hydrocarbon compounds that could potentially interfere with the UV fluorescence measurements [Luke, 1997]. The sample air is not dried since the room temperature is always higher than the outside temperature. [13] The system was installed at Mt. Fuji weather station during an intense summer observation campaign in September 2002 to investigate its feasibility. Some minor troubles occurred in the first stage of the test monitoring, such as drift of the lamp intensity, due to an electronic circuitry problem. The SO 2 continuous monitoring system was operated on a routine basis from the middle of October 2002 until the end of February 2003 (referred to as winter data), when the circuitry problem reoccurred. No machinepowered commute is available during winter months, and therefore the system was repaired and restarted at the end of May The SO 2 data obtained from June 2003 to October 2003 are referred to as summer data in this paper CO and 222 Rn Observations [14] The temporal change of SO 2 was compared with temporal changes in the CO and 222 Rn concentrations for the winter data set. Ambient CO was measured by a commercially available gas filter correlation CO monitor (TECO model 48-C). Detailed information regarding the CO measurement is presented elsewhere [Tsutsumi and Matsueda, 2000]. The monitor was calibrated with CO standard gas onsite, and its precision (1s) was reported to be about 5 ppbv at the 100 ppbv level. The detection capability of the CO system was sufficient for monitoring the temporal variation of CO at the summit of Mt. Fuji. The Radon-222 monitor (Ohyo Koken Kogyo Co., Ltd., Tokyo) uses electrostatic collection of radon progeny ions by high voltage within a 17 L hemispheric decay chamber, followed by an alpha spectrometric measurement. Details are described in the literature of the primary inventor [Iida et al., 1996]. It is claimed that the detection limit is 0.3 Bq/m 3 (two sided 95% confidence level; 2s) and the precision is about 11% at a 5 Bq/m 3 level [Iida et al., 1996]. Both instruments are PC-controlled Trajectory Analysis [15] An isentropic backward trajectory analysis computer program developed by one of the present authors (Y. S.) was applied in this study to backtrack the transport of air parcels observed at the summit of Mt. Fuji. The program has been used effectively and with credibility in studies at our institute [e.g., Matsueda et al., 1998; Sawa et al., 2004]. Backward trajectories were computed by using JMA operational meteorological objective analysis data ( , 18 layers, and time resolution of 6 hours) from the location at the summit of Mt. Fuji. Five trajectories were calculated together; one started from Mt. Fuji (central point) and other four were set by a 0.5 degree deviation from the central point in latitude or longitude (ensemble trajectory) to improve the validity of the trajectory analysis. The program basically calculates the isentropic trajectory, and thus the horizontal movement is credible. However, vertical motions of the air mass are not fully expressed, which is a drawback. Therefore a widely used program, the HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) model, developed by NOAA ARL [Draxler and Rolph, 2003; Rolph, 2003], was used as a reference for a comparison with the present backward trajectory analysis. The HYSPLIT model uses a modeled vertical velocity scheme; it can depict the vertical motion of the relevant air parcel, which offers an advantage over our trajectory model. [16] The HYSPLIT model was also used to produce the forward trajectory to analyze one extraordinary SO 2 episode observed in late October Forward trajectories were calculated in the analysis to investigate the possible transport of the Miyake-jima volcanic plume to the summit of Mt. Fuji. 3. Results and Discussion 3.1. Basic Performance of the SO 2 Monitoring System Signal Stability [17] Figure 2 reveals the zero signal stability under onsite conditions during November Zero air should flow at least 10 min after the SO 2 standard gas (about 9 ppbv) injection to achieve a zero level. It took less than 5 min of switching the sample air with the zero gas to achieve the zero level. The system response for the SO 2 concentration change was thus somewhat rapid. [18] Figure 3 depicts the signal solidity against the change in ambient conditions. The data obtained during the last 10 min of the zero airflow (from the data set described 4of21

5 Figure 2. Signal response of the SO 2 monitor. (a) Mode switch from SO 2 standard gas to zero air. (b) Mode switch from sample air to zero air. above) were used for the analysis since the inner temperature changed drastically through the day (up to 15 C) because the observation room is not adequately air conditioned. The UV fluorescence measurement may have been susceptible to this temperature change. However, the signal was not seriously affected by the change in temperature. The slope of the signal drift against the temperature was ppbv/ C. Any signal drift caused by the temperature variation would be less than 0.01 ppbv since the temperature of the optical cell was well controlled (within 1 C) against room temperature changes. The intensity of the Xe UV flash lamp fluctuation range was about 2.5% and within a range in which the response of the zero air did not change to a large extent. The signal variation caused by the change of pressure inside the optical cell may be statistically significant. This signal variation is due to the change of the light path caused by the pressure change. The extent of the signal variation was 0.02 ppbv for a pressure change of 20 hpa, based on an evaluation using the regression curve for the signal dependency on the pressure. The 26-year average of monthly atmospheric pressure at the summit of Mt. Fuji ranged from to hpa (statistics by the Gotemba base of Mt. Fuji weather station). This figure indicates that the pressure change of 20 hpa is close to the upper boundary, and thus the signal change caused by the pressure change would be negligible. As described later, the fluctuation of zero air signals was greater than those (1s is 0.03 ppbv) because those fluctuations were combined and affected the signal solidity. [19] Figure 4 indicates the effectiveness of the span and zero calibration during the November 2002 continuous measurement. Figure 4a presents the raw SO 2 data of zero air (last 10 min) without the span and zero calibration (the initial system). The frequency distribution exhibits tailing for the minus direction and the symmetry appears disturbed. The same data set yielded a better symmetrical distribution with the span and zero calibration (Figure 4b). The distribution was closer to Gaussian Detection Capability and Data Precision in the SO 2 Measurement [20] The detection capability of the current SO 2 system can be evaluated from the frequency distribution of the zero air data thus far obtained (Figure 4b). A definition equivalent to those that appeared in previous SO 2 reports was employed here since there has been confusion in descriptions of the detection capability of chemical measurement methods [Currie, 1999] (however, in principle, the detection limit should be obtained by hypothesis testing [Currie, 1999]). The 1.645s (L c, critical level of the chemical measurement process, defined by IUPAC [Inczédy et al., 2000] and ISO [International Organization for Standardization (ISO), 1997, 2000]; type I error of 5%, corresponding to a one-sided 95% confidence level) of the zero signal data set was calculated to be ppbv. The approximate detection limit was argued to be around 30 pptv with the modified TECO 43S employed by Luke [1997, p. 16,260], which was obtained as 3s of 3 to 4 adjacent 25 min averages of the baseline signal. His definition is not very clear but he argued that his results were in good agreement with detection limits obtained by Luria et al. [1992]. They employed TECO 43S as well and measured SO 2 in air at Mauna Loa, Hawaii, for one year beginning December The minimum detection limit they obtained at a one-sided 95% confidence level (the same as the aforementioned L c ) was ppbv. Our result was slightly inferior to the results by Luria et al. [1992] and Luke [1997]. However, considering the severe ambient conditions at Mt. Fuji weather station, such as the vigorous room temperature change, the present detection capability of the SO 2 monitoring system can be regarded as a practical accomplishment. This detection capability is not sufficient to verify the actual background level of SO 2 in the free troposphere, as is discussed later. Nonetheless, one of the best technologies for measuring SO 2 at low-levels within the array of practical choices is the present UV fluorescence measurement, with its ease of operation, high time resolution, freedom from laborious experimental procedures and liquid chemicals, and cost performance. [21] The precision of the SO 2 measurement can also be evaluated from Figure 4b. The fluctuation of the zero air data may be accounted for by the fluctuation in all the measurement parameters inherent to the TECO SO 2 monitor. Thus the signal fluctuation for the zero air can be presumed as the upper limit of the precision; the SO 2 concentration data would include ±0.03 ppbv errors (1s) at most Time Series SO 2 Data and Data Analysis Protocol [22] A cursory look at the SO 2 data obtained at the summit of Mt. Fuji reveals that the signal level of the zero 5of21

6 Figure 3. Signal dependence on parameters of the SO 2 monitor. (a) Inside temperature. (b) UV lamp intensity. (c) Pressure inside the photo cell. air was almost the same as that of the sample air, with the exception of episodic peaks. The frequency distribution of all the SO 2 air concentration data (1 min) obtained in November 2002 is plotted in Figure 5 to clarify this point. It is apparent that the most frequent data was around 0.1 ppbv (mode level). This value is comparable with the present detection limit (L D, 3.29s defined by IUPAC and ISO) and is less than the quantification limit (L Q, 10s defined by IUPAC and ISO) of the present SO 2 monitoring system (0.3 ppbv). This indicates that the sensitivity of the present system is insufficient to measure the extremely low background level of the ambient SO 2 concentration at the summit of Mt. Fuji, situated in the free troposphere. However, many elevated SO 2 episodes can be seen in the time series of the data. The SO 2 concentrations (1 hour median) exceeded several ppbv levels in those events (see later sections). [23] The median values of the 1 min data were used to obtain the hourly data. Minus concentrations are often obtained since the present system provides electronic signals without any SO 2 in the sample air. The negative data (below the detection limit) should not be treated the same importance as the positive data in the statistical analysis; simple averaging of the SO 2 concentrations should be avoided. It may even provide unrealistic one, such as when much of the data reveals minus concentrations. Data processing of the median filter is recommended to handle data sets containing negative data (below the detection limit). [24] Data obtained within 5 min after the mode switch (shift) from the zero air to sample air were discarded since it takes at least 5 min for the system response to reach equilibrium, as described earlier. Therefore the median values used for the data analysis are medians of 55 or 60 min data (hereafter considered the 1 hour median value). The zero air rinse duration was set for 30 min after the standard gas injection during the winter months of SO 2 observation, and thus 25 min data were available for data analysis. The rinse time was changed to 1 hour during the summer months because it was thought that 25 min data were insufficient as hourly data. The 25 min data in the winter data set were all discarded in the present time series analysis. [25] Diesel-engine-powered generators provide electricity for outages and emergencies such as lightning strikes at Mt. Fuji weather station. The exhaust from these generators could occasionally contaminate the surrounding air with SO 2. A comparison of the operation record of the power generator with the SO 2 time series revealed this contamination. No apparent concentration increase was found in the winter data, indicating that the SO 2 time series during winter was free from local contamination by human activity at the site, probably due to the high wind speed. However, coincidences between operation of the power generator and bulldozer cargo activity and SO 2 spikes were noted in the summer data. Therefore the coincidental SO 2 spikes (usually lasting less than 20 min) were carefully removed manually by hourly pieces. Only a few spikes were removed from the winter data set, while several spikes were removed from the summer data set Basic Features of the SO 2 Concentration at the Summit of Mt. Fuji [26] Figure 6 reveals the temporal changes of the 1 hour median SO 2 concentrations at the summit of Mt. Fuji (top) from the middle of October 2002 to the end of February 2003 (Figure 6a; the winter data) and from June 2003 to October 2003 (Figure 6b; the summer data) along with the wind data in the lower panel. There is a clear contrast between the high SO 2 concentration events, in which the 6of21

7 Figure 4. Frequency distribution of the SO 2 monitor output for zero air. (a) Without zero and span calibrations. (b) With zero and span calibrations. concentration could reach a few ppbv levels, and the low background concentration Diurnal Change [27] It is difficult to measure the lowest level of SO 2 at the summit of Mt. Fuji; nonetheless, it appears that basically no diurnal change exists in the SO 2 time series. The 1 min data were plotted for the timescale of a day to confirm this. No appreciable diurnal changes, such as a high (low) in the day (night) time, can be reported. This pattern of diurnal change could occur when the upslope wind brings polluted air from the lower layer during daytime [Kido et al., 2001; Osada et al., 2002, 2003]. This diurnal change in the chemical time series is typical at middle-altitude sites. There are three possible explanations for the absence of diurnal changes in the data: (1) The detection capability of the current system is not sufficiently high to distinguish the diurnal change of the very low background level SO 2 concentrations in the free troposphere, (2) the summit of Mt. Fuji is relatively free from mountain and valley winds because of its slender shape and high wind speed, particularly during winter months, as seen in the lower panel of Figure 6, and (3) the SO 2 concentrations in the free troposphere are controlled primarily by episodic transport. We cannot currently determine which is the major factor. Radon-222 will be useful to evaluate the uplift of the boundary layer air by mountain and valley winds, which is a future task. [28] Luria et al. [1992] had to remove data affected by upslope winds at Mauna Loa Observatory. They also had to consider the possible consequence of the volcanic plume from Kilauea. The summit of Mt. Fuji seems to be free from such effects, which is one advantage. In addition, no gases evolving from the crater are found. However, we need a more sensitive measurement technique to assess the very low level of background SO 2 in the free troposphere. The summit of Mt. Fuji serves as one of the best platforms for atmospheric chemistry, along with Mauna Loa and Jungfraujoch (for reference, e.g., Feb2004_Meetings/IGOS/report.pdf ) Comparison of the Present Data With Other Reports and Seasonal Trends [29] Table 1 compares the monthly median SO 2 data with those obtained by previous summer observation campaigns [Sekino et al., 1997] conducted at the summit of Mt. Fuji and other high-altitude sites and with literature regarding SO 2 concentrations above the boundary layer summarized by Warneck [2000], and the references therein. The present SO 2 concentrations provided in the table are the median of 1 min data, which almost certainly reflect the background levels in the lower free troposphere over Japan in each month. The present median will be of some statistical significance due to the central limit theorem, since a substantial amount of one-minute data are involved. Therefore the tendency of the summer low and winter high may reflect the general trend of SO 2 in the free troposphere over Japan, despite the insufficient sensitivity. This can also be noted in the standard deviations (SDs), which increased as the winter proceeded due to the intensity enhancement of the high SO 2 episodes in winter over those in late autumn. The SD was generally smaller in summer months and larger in winter, except for October The highest SD, in October 2003, was due to one extraordinary SO 2 episode in that month, which is described later. The summer SO 2 concentration (0.04 ppbv) obtained by Sekino et al. [1997] is in good accordance with our summer data. Their summer Figure 5. Frequency distribution of the 1-min SO 2 concentration in air observed at the summit of Mt. Fuji in November of21

8 Figure 6. Time series of the 1-hour median SO 2 concentration observed at the summit of Mt. Fuji and wind speed and direction (a) during October 2002 to February 2003 and (b) during June 2003 to October value is lower than our winter data, which also accords with the seasonal trend indicated in our time series data. However, both the natural and industrial source intensities have changed during the decade; a simple comparison has become complex. [30] The levels of the present SO 2 data are generally comparable to the reported SO 2 background values in the free troposphere over various locations. Hatakeyama et al. [1995] flew over the East China Sea, the Yellow Sea and the Sea of Japan and measured the SO 2 concentration using an alkaline-impregnated filter/ic in October They reported that 25 out of 48 samples collected at 0.5 to 3 km altitudes had SO 2 concentrations beyond their detection limit of 0.03 ppbv. The range of SO 2 concentrations was from 0.05 to 0.74 ppbv and the average concentration at a 3 km altitude was 0.15 ppbv (n = 13). The value of 0.15 ppbv is comparable with the median SO 2 concentrations we recorded at Mt. Fuji. Thornton et al. [1997] reported a higher value, which may be attributable to an encounter with the SO 2 plume transport, as described in a later section. [31] Seasonal trends are also clearly displayed in Figure 6. Many SO 2 spikes were seen from October 2002 to February 2003 (the winter data), while only a few spikes were observed from June 2003 to September 2003 (the summer data). It should be particularly noted that no appreciable spikes existed from August to early September This is most likely because the Pacific high covers the Japanese islands during summer and relatively clean maritime air was transported. Seasonal trends in the prevailing winds also explain the seasonal air mass changes. The west sector of the wind direction basically prevails all year at the summit of Mt. Fuji. The WSW-N direction prevailed in the winter months (depicted in the lower panel of Figure 6a), while the S-WSW direction was frequently observed in the summer months (depicted in the lower panel of Figure 6b). The monthly average wind speed ranged from 7.2 to 15.7 m/sec (statistics by the Gotemba base of Mt. Fuji weather station). The wind speed was substantial (above 20 m/sec) in the winter months, while the wind became moderate (often below 10 m/sec) during the summer months. This climatology coupled with the seasonal changes (mostly anthropogenic) of the SO 2 source and sink terms composes the SO 2 seasonal trends observed at the summit of Mt. Fuji. It is noteworthy that only one extraordinary episode could probably be attributed to the volcanic plume, as described in the next section Greatest SO 2 Episode in October 2003 and the Influence of Volcanic SO 2 [32] The greatest SO 2 episode was recorded on 28 October 2003 (see Figure 6b) and reached 14 ppbv (37 ppbv) in 1 hour median (1 min) data. The detailed hourly time series of the SO 2, wind direction and speed are depicted in Figure 7 to clarify the cause of this striking episode. The wind began to change its direction from SW to S-SE around dawn, ultimately slowing to about 10 m/s from 2200 to 2400 UTC 8of21

9 Table 1. Comparison of Present SO2 Data Obtained at the Summit of Mt. Fuji With the Literature Values Reference Location Altitude Month/Season SO 2 Concentration, ppbv Remarks Present study summit of Mt. Fuji 3776 m October ± 0.10 median of 1 min data November ± 0.26 December ± 0.21 January ± 0.38 February ± 0.79 June ± 0.22 July ± 0.26 August ± 0.23 September ± 0.11 October ± 1.19 Sekino et al. [1997] summit of Mt. Fuji 3776 m July August ± 0.02 average of 4 hour data July ± 0.16 median of 4 hour data Filliger et al. [1994] Jungfraujoch, Switzerland 3580 m 1994 about 0.2 annual average level Luria et al. [1992] Mauna Loa, Hawaii 3400 m December 1988 to November 1989 <0.04(ND) 0.1 range of monthly median of 1 h data Hatakeyama et al. [1995] northwest Pacific Rim km October range of average Thornton et al. [1997] Sea of Japan, N 2 4 km February March range of data From Warneck [2000] Maroulis et al. [1980] continental United States 5 6 km 0.16 ± 0.10 Pacific Ocean, 57S 37N ± no significant interhemispheric gradient Georgii and Meixner [1980] Bay of Biscane 1 12 km ± one-flight average Meixner [1984] continental Europe above 4 km ± three-flight average Andreae et al. [1990] Amazon Basin 3 5 km dry season ± free troposphere Amazon Basin 3 5 km wet season ± free troposphere Luria et al. [1990] North Atlantic, offshore United States 2.5 km 0.11 ± 0.05 Bermuda 2.5 km ± 0.04 Berresheim et al. [1990] west of Tasmania 2 3 km ± five-flight average Thornton et al. [1993] North Atlantic, offshore United States 5 km ± free troposphere North Atlantic, offshore Brazil 5 km ± free troposphere Wolz and Georgii [1996] South America 2 8 km ± south of 40S 55S 65N, 0 120W 6 9 km ± average of 47 points 9of21

10 Figure 7. (top) Temporal variation of SO 2 concentration and (bottom) wind speed and wind direction during the course of the peak SO 2 episode in late October (JST = UTC + 9 hour), while an abrupt increase in the SO 2 concentration simultaneously occurred. Miyake-jima Island is located in this direction (170 km southeast of Mt. Fuji; see Figure 1). The Oyama volcano on the island erupted in July 2000 [Kazahaya et al., 2003; Naoe et al., 2003; Shinohara et al., 2003]. [33] Surface weather charts (Figure 8a), a 700 hpa level weather chart (Figure 8b), infrared satellite images (Figure 9), surface wind information (Figure 10), backward trajectories (Figure 11) and HYSPLIT forward trajectories (Figure 12) were collected to analyze the episode. [34] The surface weather chart (Figure 8a) does not explain the transport of the Miyake-jima volcanic plume very well. However, the infrared satellite image (Figure 9) provided a picture in which convective activity was covering the main island of Honshu at that time. The edge of the high system, situated off the east coast of Hokkaido, was covered by clouds. The image is not helpful for determining the height of the clouds. The wet region is designated by dots in the weather chart at the 700 hpa level (Figure 8b). The wet region is where the difference between the temperature and the dew temperature was less than 3 degrees, approximating cloud coverage at that level. This figure illustrates the wet region on 28 October 2003, just off the south to east coast of the middle of the main island of Honshu that was associated with the southerly wind, covering Miyake-jima Island. This suggests that convective air motion would develop at that time in the region, most likely due to a small local low not evident in the surface weather chart (Figure 8a) but suggested by a dimple in the contour line. This convection activity could have certainly uplifted the Miyake-jima plume to the higher-level atmosphere. The Figure 8. (a) Surface weather chart of Far East Asia at 0000 UTC, 28 October (b) The 700 hpa weather chart over Japan at 0000 UTC, 28 October The wet region (T T d 3 ) is dotted and the relevant region is circled. 10 of 21

11 Figure 9. Infrared satellite image of Japan at 0000 UTC, 28 October surface winds depicted in Figure 10 suggest that a cyclonic wind system prevailed over the Kanto plain and off the south coast at 1900 UTC on 27 October 2003 (0400 JST, 28 October), a few hours before the Miyake-jima volcanic plume reached the summit of Mt. Fuji. The wind that prevailed at the 700 hpa level over the relevant region at that time was southerly to southwesterly, as indicated in Figure 8b. This proves that convergence would have occurred around the relevant region. The uplifted Miyake-jima volcanic plume was then directed toward the north. [35] Figure 11 depicts the backward trajectory starting from the summit of Mt. Fuji on 27 and 28 October 2003 in UTC in the region of 25 N to40 N and 120 E to 150 E. The ensemble trajectory confirms the precision of the analysis. Numbers 1 and 2 in the figure indicate the position of the air parcels one and two days prior to the starting time. The SSW trajectories were computed for the morning of 28 October Although the direction is not in full accordance with that of Miyake-jima island, southerly winds certainly brought the volcanic plume from Miyakejima Island to Mt. Fuji. The HYSPLIT model can produce a forward trajectory, so it was used to obtain more information about the transport of the Miyake-jima volcanic plume. It can calculate three different trajectories in an altitude at once. Trajectories starting from 500 m, 813 m (top of the volcano) and 1200 m asl (maximum height of the plume, see below) are projected in Figure 12. All of them were bound in a northwest to north direction from Miyake-jima, passing over the main island of Honshu. The lowest altitude trajectory struck Mt. Fuji within a few hours from the start. A significant wind shear apparently existed during the episode; the higher in altitude the air parcel was directed, the more northward its movement (clockwise rotation). [36] The following scenario can be proposed by summarizing all the related information described above. The Miyake-jima volcanic plume was vertically diffused by turbulence (probably due to low atmospheric stability) to a lower altitude with cyclonic airflow near the surface, and this part of the plume was transported northwest toward the Mt. Fuji region. Convective air flow, which was not well depicted even in the HYSPLIT forward trajectory, occurred around the region, and the plume passed by the summit of Mt. Fuji. It is suspected that the local low center may have passed near the region when the maximum SO 2 concentration was recorded (2200 UTC, 27 October), since the wind speed drastically slowed down at the time and the wind direction changed as well (Figure 6b). The timescale was minimal since the spatial scale of the episode was small (several hours), an aspect that differs from the episode described in section References to the Miyake-jima Volcanic Plume Observed at Mountainous Sites [37] Naoe et al. [2003] noted the coincidental enhancement of SO 2 4 (maximum in 4 hour average: 4 mg m 3 ) and SO 2 (maximum in 4 hour average: 5 ppbv) during an observation campaign at the summit of Mt. Fuji in September This occurred at some stage in the passing of a typhoon around Kyushu Island (west of Japan) to the Sea of Japan, when the humid maritime air was brought to the summit of Mt. Fuji by southerly and easterly winds. These winds conveyed the volcanic SO 2 from Miyake-jima Island. They further proved this transport by using a backward trajectory analysis. Abundant SO 2, ranging from 5 to 200 kt/day with an average of 42 kt/day (7.7 Tg S/y) Figure 10. Surface winds in the relevant region in Japan at 1900 UTC, 27 October 2003 (JST = UTC + 9 hours). Arrows indicate the direction tendency. 11 of 21

12 Figure 11. Backward trajectories from Mt. Fuji from 27 to 28 October Time unit is UTC. [Kazahaya et al., 2003; Shinohara et al., 2003], was emitted from the island during the year Naoe et al. [2003] conducted an observation at some point during this intense emission period. The emission rate gradually decreased to a 10 kt/day (1.8 Tg S/y) level after late 2001 [Kazahaya et al., 2003; Shinohara et al., 2003]. While the emission rate of the Miyake-jima SO 2 has decreased, the intensity of the SO 2 episode we observed was greater than the one noted by Naoe et al. [2003]. This may be due to a difference in the transport process. A southeasterly wind played a role in the transport of the Miyake-jima volcanic plume in both cases. The E-S sector of the wind direction is indicated by mesh in the lower panels of Figures 6, 7, 13, and 14. The daily prevailing wind direction of this sector was not often revealed in the winter data, but was frequently noted in the summer data, particularly in June, from the end of July to the middle of August, and after mid-september [38] However, the high SO 2 episode was not specifically noted in the time series. This may be because the plume height did not reach the altitude of the summit of Mt. Fuji (close to 4 km asl). A report from JMA [2004] on earthquakes and volcanoes described the maximum plume height from Miyake-jima Island since October 2002 as being about 1200 m. This suggests that plume uplift to the middle free troposphere would rarely occur without convection. This may have resulted in the Miyake SO 2 influence remaining in the lower troposphere over the North Pacific region without convective activity after late We may need to consider the conversion of SO 2 to aerosol during transport in summer; the timescale would be too short to expend all the SO 2 due to the distance between Mt. Fuji and Miyake-jima Island (170 km). Osada et al. [2002, 2003] noted their findings of the possible influence of the Miyake-jima volcanic plume. In their 2002 report, they described an enhanced volume concentration of accumulated mode particles (0.3 to 1.0 mm) in summer at Mt. Tateyama (located in central Japan, 36.6 N, E, 2450 m asl) after 2000, which is attributable to the Miyake-jima volcanic plume. In their 2003 report, they addressed the high nss-so 4 2 observed in their filter samples collected at Mt. Norikura (located in central Japan, 36.1 N, E, 2700 m asl) during September A backward trajectory analysis indicated possible transport of the Miyake-jima volcanic plume to Mt. Norikura. However, their observation sites were lower than the summit of Mt. Fuji (below 3 km asl). This altitude difference may be critical. In addition, the plume height had been elevated to more than 1200 m prior to October 2002 because of the eruptive activity. The influence of SO 2 plumes from other volcanoes would be negligible at the summit of Mt. Fuji since the emission from Mt. Oyama at Miyake-jima Island is exceptionally voluminous compared to others (more than equivalent to the total annual emissions from volcanoes prior to its eruption, as stated previously) Comparison of the SO 2 Time Series With Those of Other Trace Gases in Winter [39] Figure 13 compares the temporal changes in SO 2, CO, and 222 Rn during February Carbon monoxide has 12 of 21

13 Figure 12. Forward trajectories starting from Miyake-jima Island at 2100 UTC, 27 October 2003 obtained using the HYSPLIT model. Starting heights are 500, 813 (mountain top), and 1200 m asl. an anthropogenic origin, which may share a common source with SO 2, while 222 Rn has a lithospheric origin. The SO 2 enhancement was generally accompanied by increases in CO and 222 Rn concentrations, though the CO and 222 Rn increases were not always accompanied by an SO 2 concentration increase. This may suggest common origins for SO 2 and CO, possibly fossil fuel combustion at the surface. [40] One may assume that this correlation is partly due to volcanic influence, since Japan has many active SO 2 -emitting volcanoes. Hirabayashi [1990] reported that volcanic gas contains many species, such as HF (0.06 6), HCl (0.4 11), SO 2 (1.5 47), CO 2 (15), H 2 ( ), CO (0.02), CH 4 (0.34), and NH 3 (no datum); the figures in parenthesis indicate the annual global emission (Tg/y) described in the reference. The estimate of volcanic emission of SO 2 is still uncertain; in total 13 Mt/y of SO 2 (6.5 TgS/y) would be emitted into the atmosphere [Bluth et al., 1993]. Radon-222 would certainly be emitted along with those gases; however, its inventory is not estimated in our incomplete literature survey. Emission of 222 Rn from a volcano is usually disregarded [e.g., World Meteorological Organization (WMO), 2004]. We assume that the annual emission of CO reported by Hirabayashi [1990] is certain, and it is minimal compared to the total source intensity of CO in the atmosphere, on the order of a few thousand Tg(CO)/y [Brasseur et al., 1999; IPCC, 2001; Seinfeld and Pandis, 1998; Warneck, 2000]. No textbook includes volcanic activity as a source of CO in the atmosphere. The average CO/SO 2 molar ratio (which is equal to the volume ratio) in volcanic gas would be , based on the above mentioned literature values. In conclusion, the coincidental increase of SO 2, CO, and possibly 222 Rn should not be 13 of 21

14 Figure 13. (top) Temporal changes of SO 2, CO, and 222 Rn concentrations and (bottom) wind speed and wind direction at the summit of Mt. Fuji during February The CO and 222 Rn concentrations are 1-hour averages, while the SO 2 concentration is the 1-hour median. volcanic at all. In addition, the Miyake-jima plume was not transported to the summit of Mt. Fuji, except for the one extraordinary episode cited in the previous section. [41] While the residence time of SO 2 and CO appears mostly dependent on the OH radical concentration, SO 2 is more rapidly oxidized to SO 4 2 in the atmosphere (residence time ranging from 0.6 to a few days) [IPCC, 2001; Warneck, 2000] compared to CO (average residence time in the troposphere is about two months) [Warneck, 2000]. Therefore the correlation between SO 2 and CO may also Figure 14. (top) Temporal variation of the SO 2 concentration and (bottom) wind speed and wind direction during the course of Episode 1 in February of 21

15 Figure 15. Backward trajectories from Mt. Fuji during 18 and 20 February Time unit is UTC. suggest rapid transport of the polluted boundary layer air to the free troposphere in the winter season. The fact that SO 2 enhancement is always accompanied by a 222 Rn concentration increase also reinforces the theory, since 222 Rn has a physical half-life of 3.8 days. In contrast, the reasons that CO and 222 Rn concentration enhancement is not always accompanied by SO 2 concentration enhancement may be the oxidation of SO 2 and/or gas scavenging by cloud processes. We must acquire SO 2 4 concentration data along with the SO 2 data, which is a future task. A combination of simulation model analysis and meteorological data analysis is important to clarify the transport processes Transport of Polluted Air From the Asian Continent in Winter Months [42] A case study was conducted regarding the Asian outflow to the Pacific region, a significant concern in the science community [e.g., Hatakeyama et al., 1995; Hoell et al., 1996; Kaneyasu et al., 2000; Thornton et al., 1997]. The Asian winter monsoon is vigorous in the winter season because of the active Siberian high, and 15 of 21

16 Figure 16. The 700 hpa weather chart over Japan on (a) 18 February 2003, (b) 19 February 2003, (c) 20 February 2003, and (d) 21 February Time unit is UTC. polluted air is transported over Japan to the Pacific. One particular episode, during February (episode 1), was selected and analyzed since that SO 2 enhancement episode was more evident than in other observed months. Figure 14 depicts an enlarged version of the time series for Episode 1 to enable close scrutiny. Episode 1 recorded the highest 1 hour median (1 min) value of SO 2 concentration, 6 ppbv (16 ppbv), in the winter data, and continued for almost two and a half days. Such episodes were frequently observed in November and December 2002 and January 2003; however, the SO 2 concentration enhancement in the episode tended to be higher and last longer during February 2003 (see Figure 6). The temporal change of the SO 2 concentration in Episode 1 was associated with CO and 222 Rn concentration enhancement. This fact proves that the polluted boundary layer air was brought up to a height of 3 4 km above sea level and transported. [43] An isentropic backward trajectory analysis was performed for the episode to clarify the source region and air mass transport. Figure 15 depicts the 10-day backward trajectory starting from each date and time in UTC. Five trajectories were calculated, as stated previously. Although the trajectories were backtracked for ten days, the SO 2 lifetime would be shorter than a few days and the area that the trajectory passed over in the last few days would be the most likely source area of the SO 2 observed during the episode. The trajectories passed over Siberia, Mongolia, central and northeastern China, and the Korean peninsula during Episode 1. Possible effects of the volcanic plume from Japan seem unlikely because of the prevailing wind system during the episode (see Figure 14). The 700 hpa level weather chart also clarifies the prevailing wind system of the W-N sector on the relevant days at an altitude of 3 km asl (Figure 16), reinforcing the explanation that the SO 2 plume was transported from continental sources. However, our backward trajectory program has a drawback. The program is based on an isentropic assumption; the trajectory in this case did not reach the boundary layer over the continent. Therefore the HYSPLIT model was run for the backward trajectory analysis to further confirm the advection and convection processes in the currently relevant episode. One trajectory is shown in Figure 17 for the air parcel that reached the summit of Mt. Fuji at 0600 UTC (15 JST) 20 February 2003, when the SO 2 spike was almost at its 16 of 21

17 Figure 17. Backward trajectories starting from Mt. Fuji at 0600 UTC, 20 February 2003 obtained using the HYSPLIT model. Starting heights are 3500, 3776 (mountain top), and 4000 m asl. peak. Trajectories starting from 3500 m, 3776 m, and 4000 m asl were computed. The HYSPLIT trajectories, particularly the 4000 m trajectory, backtracked horizontally, similar to our trajectories from the same starting time. The HYSPLIT trajectories generally exhibited good accordance with our trajectory results; this coincidence also supports the credibility of our own trajectory model that uses different wind fields than the HYSPLIT model. The HYSPLIT trajectories indicate a possible influence on the vertical motion from the boundary layer air within Japan on the previous day (19 February). Only the 4000 m trajectory remained over central China around 1000 m above ground for longer than five days prior to 15 February. This may be important because the area would be most polluted not only by SO 2 but also by NO x, CO, and black carbon, etc. Thus the source region supposition may be validated not by the local one. [44] The surface weather in the relevant region was as follows during Episode 1 and its preceded periods (Figures 18 and 19): a weak low was located in Inner Mongolia on 16 February, and on the next day it moved eastward into northern China. This low appears to have played a role in the transport of the SO 2 peak observed on 20 February at the summit of Mt. Fuji. Another small low was situated off the south coast of Honshu Island on 18 February that was moving eastward; it developed over western North Pacific on the next day. Also, the small aforementioned low moved to the Sea of Japan off the north coast of Honshu Island and developed, moving eastward on 20 February. The central area of the Japanese islands was in a trough on that day. Japan was then covered by a high system moving in from the continent on 21 February. Episode 1 occurred during the preceding period of winter monsoon caused by a 17 of 21

18 Figure 18. Surface weather chart of the Asia and Pacific regions on (a) 16 February 2003 and 17 February Time unit is UTC. characteristic meteorological pattern of a western high and eastern low. This air flow pattern is captured well in the satellite image (Figure 20). The westerly was prevailing over the Japanese islands on February, while the wind system changed to northerly on 20 February, which is depicted in Figure 20b as a bunch of cloud lines. Although Episode 1 is consecutive, the transport mechanism of the SO 2 peak on February and that of 20 February may appear to differ. The wind stalled twice on 20 February with direction changes from the W sector to the N sector. This phenomenon is similar to events during the Miyake-jima volcanic episode. The wind speed and direction were fairly constant during February. It appears that the transport of polluted air mass from the continent is controlled by synopticscale weather. [45] The spatial scale of the air mass for the episode may be simply determined by the sum of the wind speed and lapse time product (SWSLTP) during the episode. It was about 2800 km from 1400, 18 February to 2200, 20 February, the period when the SO 2 concentration was exceeded 0.3 ppbv (except for two hours in the early morning of 19 February). This clearly contrasts with the Miyake-jima SO 2 episode described in section 3.4, in which the SWSLTP was about 430 km (from 0200 to 0900, 28 October 2003), one order of magnitude smaller (see Figures 7 and 14). The spatial scale of the estimated air mass may yield the source location, local or remote. The SWSLTP would naturally be smaller when the source is local, since the effect of diffusion is minimal. This also indicates that the presently reported SO 2 -rich air mass came from a source region in the Asian continent rather than the Japanese islands. [46] A similar pattern of transport of polluted air to the North Pacific by a synoptic-scale weather cycle was previously described by Kaneyasu et al. [2000]. They reported that a polluted air mass formed under anticyclonic conditions over the continent and persisted for a few days, with the inversion layer likely playing a role, and then was transported sporadically to the east by the development of a low around Japan. A post cold front air mass [Cooper et al., 2001] may correspond to that polluted air. This pattern of airflow, coupled with convective flow, may cause upward transport of the polluted boundary layer air to the free troposphere over Japan and result in long-range transport to the North Pacific in wintertime. Thornton et al. [1997, 28,492] also indicated that often these higher concentrations are related to frontal activity that provides uplift from the surface. In some cases the transport of SO 2 at 1 to 3 km extended over 1500 km from the western rim countries. The spatial scale thus reported is in good agreement with our result. We may need to perform a CTM model study to describe this transport process of the polluted air mass in more detail, which would be a future step. However, the chemical time series of SO 2 at Mt. Fuji could contribute to our knowledge regarding the actual features of the Asian outflow of polluted air in the free troposphere over the North Pacific. 4. Summary [47] 1. An SO 2 continuous monitor using a commercially available pulsed UV fluorescence instrument with zero and span gas calibration was installed at the summit of Mt. Fuji in September 2002, and SO 2 in the free troposphere has been measured since then. A total of ten months data are presented here. The system produces time resolution data that is comparable to other trace gas measurements of O 3, CO, and 222 Rn. [48] 2. The measurement capability of the SO 2 monitoring system was tested onsite. The critical level (L c, definition by IUPAC and ISO) was estimated to be around 0.05 ppbv. Thus the detection capability of the current SO 2 system was insufficient for monitoring very low background temporal changes of SO 2 in the free troposphere. [49] 3. However, episodic transport of SO 2 was frequently observed at the summit of Mt. Fuji. This was more remarkable during the winter months than during the summer months. The seasonal change in air mass transport, coupled with that in SO 2 source and sink terms, most likely resulted in the winter high and summer low seasonal trends in the SO 2 time series obtained. [50] 4. One extraordinary SO 2 episode was recorded in late October A meteorological analysis indicated that the Miyake-jima volcanic SO 2 plume was the cause. Except 18 of 21

19 Figure 19. Surface weather chart of Far East Asia on (a) 18 February 2003, (b) 19 February 2003, (c) 20 February 2003, and (d) 21 February Time unit is UTC. for this atypical event, no influence of volcanic SO 2 at the summit of Mt. Fuji can be confirmed. Miyake-jima SO 2 evidently does not usually affect the middle of the free troposphere over Japan, or presumably over the North Pacific region, because of the relatively low plume height since late [51] 5. A comparison of the temporal changes in SO 2, CO, and 222 Rn during winter suggests that these gases in the free troposphere over Japan may have a common source over the Asian continent and common transport processes. The correlation between SO 2 and 222 Rn concentrations may suggest rapid transport of polluted air from the source region to the free troposphere at the summit of Mt. Fuji within a few days. [52] 6. Data analysis coupled with meteorology suggested possible transport of the polluted boundary layer air to the free troposphere in winter months, which may correspond to the Asian outflow. Further data analysis may reveal such processes in more detail. [53] 7. The importance of the chemical time series in the free troposphere over Japan is strongly emphasized as a means of identifying the anthropogenic impact on the air quality of the free troposphere over the North Pacific region. The summit of Mt. Fuji is an excellent platform for atmospheric background monitoring. Figure 20. Infrared satellite image of Japan at (a) 0400 UTC, 19 February 2003 and (b) 0600 UTC, 20 February of 21