Regional background ozone and carbon monoxide variations in remote Siberia/East Asia

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 18, NO. D1, 428, doi:1.129/21jd1412, 23 Regional background ozone and carbon monoxide variations in remote Siberia/East Asia Pakpong Pochanart and Hajime Akimoto Frontier Research System for Global Change, Yokohama, Japan Yoshizumi Kajii Department of Applied Chemistry, Tokyo Metropolitan University, Tokyo, Japan Vladimir M. Potemkin and Tamara V. Khodzher Limnological Institute, Siberian Branch, Russian Academy of Science, Irkutsk, Russia Received 22 October 21; revised 1 March 22; accepted 29 April 22; published 15 January 23. [1] Continuous measurements of O 3 and CO were made during at Mondy, a remote mountain site in East Siberia, in order to quantify their mixing ratios and their climatology in the background troposphere of continental Eurasia. The seasonal cycles of O 3 and CO show the spring maximum-summer minimum similar to that previously reported in the remote Northern Hemisphere. The influences of Siberian forest fires on the variations of CO mixing ratios at Mondy were observed both on a local and a regional scale during spring 1997 and fall 1998, respectively. We further evaluate the possible impact of European pollution export to the remote atmosphere of Siberia using trajectory analysis. It was found that the O 3 and CO mixing ratios in the air masses transported from Europe are higher than those from Siberia and high-latitude regions for most of the year. The medians of O 3 and CO mixing ratios associated with the European air masses are 44.2 and 134 ppb, respectively, in comparison with 42.7 and 128 ppb in the Siberian air masses, and 41. and 11 ppb in the high-latitude air masses. The residence time analysis of air masses transported from the European continent indicates that CO mixing ratios significantly decrease with longer transport time of air masses from Europe, while rapid air motion retains higher CO mixing ratios in every season due to the admixture of polluted European air into the continental background air during air mass transport over Eurasia and photochemical loss by OH. Because of a shorter lifetime in summer, CO mixing ratios decrease at a rate of 6 7 ppb per day, while they decrease at a rate of 2 4 ppb per day in winter and spring. The similar trend is found for O 3 but only in summer, at a rate of 2 3 ppb per day. From this analysis, we are able to identify that European pollution exerts an influence, though not very strong, on the background O 3 and CO at Mondy in remote Siberia/East Asia. INDEX TERMS: 322 Atmospheric Composition and Structure: Constituent sources and sinks; 345 Atmospheric Composition and Structure: Pollution urban and regional (35); 365 Atmospheric Composition and Structure: Troposphere composition and chemistry; 368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; KEYWORDS: Biomass burning, European emission, Intercontinental transport, Trajectory analysis Citation: Pochanart, P., H. Akimoto, Y. Kajii, V. M. Potemkin, and T. V. Khodzher, Regional background ozone and carbon monoxide variations in remote Siberia/East Asia, J. Geophys. Res., 18(D1), 428, doi:1.129/21jd1412, Introduction [2] In a global and regional perspective, the background region for atmospheric pollutants would be referred to as the area where the atmosphere is not directly perturbed by local and regional anthropogenic sources. Atmospheric observations in such a background region are very important as a reference for evaluating the anthropogenic impact of other regions. In the Northern Hemisphere, however, such regions Copyright 23 by the American Geophysical Union /3/21JD1412 are very limited due to the extensive human activities in most of the continental area. The possible background region thus defined may include the central Pacific, middle Atlantic, and central Eurasia. Among these three regions, atmospheric chemistry measurements in the continental eastern Eurasia are very scarce in contrast to the central Pacific and middle Atlantic, where substantial amounts of data have been obtained by both ground-based and aircraft measurements [see e.g., Fehsenfeld et al., 1996; Hoell et al., 1996, 1997, 1999; Penkett et al., 1998, and references therein]. It is known that even measurements of trace species, especially ozone and carbon monoxide, in the ACH 7-1

2 ACH 7-2 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA Pacific and the Atlantic are sometimes unavoidably influenced by the long-range transport of air pollutants on the hemispherical scale [Parrish et al., 1993, 1998; Jaffe et al., 1997]. For O 3 and CO measurements in remote Siberia/East Asia, only a ground observation at a midlatitude Chinese WMO station at Waliguan (J. Tang, personal communication) and train measurement by trans-siberian railway [Crutzen et al., 1998] have been carried out. No other continuous measurement at a remote site in East Siberia has been reported so far to our knowledge. [3] O 3 and CO data in this area are of extreme interest from at least two points of view. One is that this region defines the inflow boundary condition to the East Asia Pacific rim region [Akimoto et al., 1996; Kajii et al., 1997; Pochanart et al., 1999]. Most of the O 3 /CO monitoring sites in East Asia Pacific rim region are located downwind of the large scale anthropogenic emission regions of China, Korea, and Japan [Chan et al., 1998; Kajii et al., 1998; Nagao et al., 1999; Narita et al., 1999; Pochanart et al., 1999, 22; Tsutsumi and Matsueda, 2; Tanimoto et al., 2; Lam et al., 21]. Data obtained at remote sites in East Asia Pacific rim region are known to be interpreted as a perturbation to the East Siberian background air mass by the addition of anthropogenic emissions to a different extent depending on the pathway of air mass [Nagao et al., 1999; Pochanart et al., 1999; Lam et al., 21]. This is of particular concern since East Asia is regarded as one of the largest and most rapidly developing regions in the world [Akimoto and Narita, 1994]. Emissions from large-scale anthropogenic activities in this region are predicted to continue growing in the next decades [Brasseur et al., 1998; Aardenne et al., 1999]. The enormous amounts of O 3 precursors from these increased emissions could significantly affect the atmosphere of East Asia on a hemispherical scale and could potentially enhance O 3 and CO levels in this region [see, e.g., Elliott et al., 1997; Brasseur et al., 1998]. Thus, in order to evaluate the quantitative relation between O 3 increase and precursor emissions in this region, it is necessary to study the East Asian continental background concentrations of O 3 and CO prior to a regional contamination. Another interest is from the point of recently emerging topic of the trans-eurasian intercontinental transport of air pollutants from Europe to East Asia [Wild and Akimoto, 21]. The data in East Siberia would be very useful to evaluate the influence of European outflow. [4] In this work, we report for the first time the 3-year continuous data set of O 3 and CO at Mondy, a remote mountain station in East Siberia. In addition, enhanced levels of O 3 and CO due to the local/regional-scale biomass burning in Siberia are also reported. 2. Observations and Method 2.1. Monitoring Site [5] The O 3 and CO data used in this work were obtained from multiyear continuous monitoring at Mondy observatory site (51 39N, 1 55E, 26 m above sea level) in central Eastern Siberia. The site is solar-terrestrial observatory station in the remote mountain ranges near a borderline of Russia-Mongolia southwest of Lake Baikal as shown in Figure 1. The nearest major city, Irkutsk, is about 25 km to the east with a population of 6,. There are no other Figure 1. Mondy Mongolia Russia China Geographical location of Mondy. major cities within a 5-km radius from Mondy except small villages which spread lightly all over the region. The nearest residence to the observatory site is Mondy village which is located downhill at a distance of 15 km at 8 m asl elevation. A population of Mondy village is about 22. This area of Siberia is known to be one of the very lightly populated areas of the world. No major anthropogenic pollutant, neither from point sources nor mobile sources, has been found to strongly affect this area. Crutzen et al. [1998] defined Siberia as a pristine region where surface ozone levels should be close to their natural level. Mondy, thus, ideally represents one of the first true representative continental remote stations for monitoring background atmospheric trace components in the unpolluted region of Siberia/East Asia O 3 and CO Measurements [6] Measurement of O 3 at Mondy has been conducted continuously since October 1996 and CO measurement has been carried out from March Mixing ratios of O 3 and CO were measured using an UV absorption analyzer (Dylec Model 17-AHJ) and a nondispersive infrared spectrometer (Kimoto Model 541), respectively. The similar criteria for instruments calibration and data handling were applied as for our other monitoring sites in East Asia. Details can be found elsewhere [Pochanart et al., 1999; Tanimoto et al., 2]. In brief, O 3 analyzer was calibrated by a primary O 3 standard analyzer (TEII Model 49 Ps). During O 3 measurement, a change in zero level of O 3 analyzer was constantly checked using a zero air generator (TEII Model 111). As a calibration at site could not be carried out on a frequent basis, the analyzer was regularly cross-checked against three more of the same model O 3 analyzers, one in the laboratory and two at other sites in East Asia, which were initially set to the same conditions as the Mondy instrument, and constantly calibrated. The CO instrument was set to automatically sample ambient air for 4 min following by 2 min of zero air in one hour cycle. The analyzer was

3 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH 7-3 calibrated using certified CO standard gas (Nippon Sanso, 1.2 ppm and/or 1.8 ppm), manually every day on a presence of staff and automatically once every 1 15 days when the site is left unattended. The detection limit of the O 3 measurements is about the same as at the other sites, 1 ppb with the overall uncertainty of 5% at a mixing ratio of 2 ppb [Tanimoto et al., 2]. The detection limit for CO, defined as three times the standard deviation of baseline fluctuation over 4 min period, can reach 2 3 ppb under the best circumstances but normally it is up to 1 ppb, which gives 1% overall uncertainties of hourly averaged CO data at 1 ppb mixing ratio. The raw data used in this work are 1-hour averaged mixing ratios. Examples of time series plots of the 1-hr averaged mixing ratios of O 3 and CO during 1-year period from July 1997 to June 1998 are shown in Figure 2. For most of the year, the raw data at Mondy do not reveal any strong influence from local/regional pollution (Figures 2a 2l). Only for a few cases when the site experienced a rare pollution episode do the Mondy data show strong O 3 and CO fluctuations, e.g., the plumes from local forest fires which were observed in the area during March April 1997 (Figure 2m, see details later in text). We discard data from such events and do not further use them in routine data processing (e.g., calculation of daily average and monthly average). All times reported in this work are local winter times (+8 hours ahead of UT) except for trajectory calculation, which is at and 12 UT Trajectory Analysis [7] In order to identify the origins of the air masses arriving at Mondy and investigate the influence of the long-range transport on O 3 and CO characteristics in Siberia, the 1 days backward isentropic air mass trajectory analysis has been conducted for two years periods during The trajectory model, developed at National Institute for Environmental Studies ( NIES), is the same as that used in our previous works [Pochanart et al., 1999, 21]. The basic meteorological data set is the ECMWF/ TOGA Basic Level III-A Upper Air provided by European Center for Medium-Range Weather Forecasts with the spatial resolution of 2.5 degrees in longitude and latitude and 15 levels in vertical resolution from 1 to 1 hpa. We used clusters of five trajectories in one run, one started at the exact location and the other four being displaced by ±.5 degree of latitude and longitude, to reduce the uncertainty in trajectory calculation [Pochanart et al., 1999]. Trajectories were calculated twice a day with the initial level at 2 m elevation and the time step of 1 hour. 3. Results and Discussion 3.1. The Typical Air Mass Transport Patterns Arriving at Mondy [8] From the trajectory analysis, we see that most of the air masses (>8%) at Mondy arrived from western section as a result of rapid air circulation in the midlatitudes of the Northern Hemisphere. The trajectory studies at 1 E longitude line by Newell and Evans [2] showed similar results (Mondy is coincidentally located at the exact longitude of 1 55E). Trajectories are known to be good indicators of large-scale flow and can be useful for studying the potential regional sources of O 3 and CO but a caution must be made here that uncertainties exist in trajectory model and any trajectory should not be viewed as the exact pathway of an air mass [Harris and Oltmans, 1997; Liu et al., 1999; Harris et al., 2]. We further classified the typical types of air mass reaching Mondy into four groups according to their origins and transport pathways, namely Europe (EU), Siberia (SI), High-latitude (HL), and Southwest (SW) air masses. Figure 3 shows examples of the typical EU, SI, and HL trajectory categories during early spring and summer. The EU category is for the air masses originating in or passing through Europe (defined here as the west of Ural mountain at 6 E longitude) before arriving at Mondy (Figures 3a and 3b). The SI category is for long-range transport of air mass within Siberia (east of Ural Mountain) for the whole period of 1 days (Figures 3c and 3d). The HL air masses originated in the high-latitude and Arctic polar region (higher than 75 N) and were transported southward to Mondy (Figures 3e and 3f ). The EU, SI, and HL categories, respectively, account for 27, 19, and 15% of all air masses arriving at Mondy for two years periods in 1997 and 1998 with the total numbers of 146 runs (73 trajectories). The altitudes of transport courses, as can be seen in the figure, normally varied from 2 6 km. The median value of the trajectory maximum altitude is about 68 hpa (3.2 km). We found only a few cases that air masses are directly transported from the higher level of troposphere, as the example is shown in Figure 3f. In total, only 4.4% of all air masses descend from a level above 45 hpa (around 6.3 km). The SW type (not shown) is the air mass transported with a much shorter range, usually less than several hundred kilometers from southwest direction. It significantly counts for 18% of the total frequency but most of the air masses in this type dropped to the trajectory based level (955 hpa) in the early days of backward calculation and were not further used in interpretation. The rest, 21%, is of unclassified type and other minor types. Most of the unclassified type comprises the mix pattern of the typical types showing dispersion into different directions of the five trajectories. The minor types are the transports with distinct pattern but infrequently found, such as air masses transport from the coastal regions of East Asia, or across the pole from Alaska. The percentages of minor transport types are very low and not considered significant for statistical analysis General Behaviors of O 3 and CO at Mondy O 3 Seasonal Cycle [9] The O 3 seasonal variation observed at Mondy during late is shown in Figure 4a. The O 3 mixing ratios increase gradually from fall and winter to reach a maximum in April May, 52.9 ppb on average, and then decrease to a minimum, 38.1 ppb on average, during the summer fall. In winter, its mixing ratios are almost constant and the lowest variability is observed (also see, Figures 2f 2h). In summer, O 3 exhibits the highest variability despite the low averaged mixing ratios. This variability is due to the diurnal and temporal change of summertime O 3 (Figures 2a, 2b, and 2l). The 3-year annual averaged mixing ratio is 43.5 ± 5.2 ppb. The averaged mixing ratios of O 3 (and CO) for each season are shown in Table 1. The interannual variations of O 3 at

4 ACH 7-4 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA Mondy, excluding O 3 data influenced by local/subregional fires in spring 1997, are not found very large for each season. The averaged O 3 mixing ratios during winter, summer, and fall in each year are almost constant. A slightly higher interannual variation is found in spring. [1] The spring maximum of O 3 at Mondy which resembles the seasonal cycles of O 3 observed at many clean sites in the Northern Hemisphere is a well-known phenomenon [Oltmans and Levy, 1992, 1994; Laurila and Lättilä, 1994; Derwent et al., 1994, 1998; Solberg et al., 1997; Laurila, 1999]. In East Asia, most of the apparent O 3 seasonal cycles also show a spring maximum and summer minimum [Tsuruta et al., 1989; Sunwoo et al., 1994; Kajii et al., 1998; Nagao et al., 1999; Tanimoto et al., 2; Pochanart et al., 22] but this apparent seasonal cycle is a result of a circulation change: low O 3 from Pacific maritime high pressure system during summer and high O 3 from continental outflow in other seasons [Ogawa and Miyata, 1985; Pochanart et al., 1999]. Mondy is located in the middle of Siberia where this continental outflow forms, and is hardly influenced by such continental-marine air mass exchange. The trajectory analysis confirmed the long residence times of air masses over the Eurasian continent during their transport to the site for most seasons. A recent review concerning spring maximum can be found elsewhere [Monks, 2]. Our interest is that the seasonal cycle of O 3 at Mondy is among the first evidences reported in remote Siberia and, importantly, it represents the true remote continental background O 3 behavior in the midlatitudes of the Northern Hemisphere. It augments the fact that the continental background O 3 seasonal cycle in the region where data are missing of Siberia/East Asia shows a similar O 3 seasonal pattern as those observed elsewhere in the remote Northern Hemispheric midlatitudes. It is noted that significantly higher O 3 mixing ratios are observed at Mondy than at most European background sites [Laurila and Lättilä, 1994; Derwent et al., 1994, 1998; Simmonds et al., 1997; Solberg et al., 1997; Laurila, 1999; Pochanart et al., 21] because the background O 3 in northern/central Europe is a result of the long-range transport of low O 3 mixing ratios in marine air masses from the Atlantic Ocean [Derwent et al., 1998; Pochanart et al., 21]. [11] During summer and early fall, the averaged mixing ratios of O 3 at Mondy are lowest. It was proposed that the photochemical loss process in the very clean atmosphere can be responsible for the summer minimum of O 3 observed at some high-latitude sites in Europe [Solberg et al., 1997]. At Mondy, we suggest that the low summer O 3 mixing ratios are due to the O 3 sink by surface deposition and the more active vegetation in summer. It has been pointed out that the natural environment such as the huge forest can act as a net sink for O 3 [Kirchhoff, 1988] although most of the observable cases take place in the tropics where suitable conditions for photochemical activities (high temperature, high humidity, and high solar radiation) exist. From a 3D-Global CTM study, a short photochemical lifetime of O 3 and a lower transport flux of O 3 during summer than spring and fall are found in the remote Eurasia [Wild and Akimoto, 21]. Their results agree with Mondy O 3 behavior, implying that the lower background troposphere of the continental Eurasia can be asinkofo 3 in summer. [12] It is noteworthy that some recent results from O 3 measurements in the upper troposphere by the commercial airliners show the summer O 3 maximum over Siberia, which is the opposite pattern to summer O 3 minimum at Mondy [Brunner et al., 21; Stohl et al., 21]. Stohl et al. [21] analyzed O 3 data from MOZAIC program using trajectory statistics and suggested that summer O 3 maximum in the upper troposphere can be connected to a possible large O 3 source, most likely the biomass burning, in the Central Asian/Eurasian continental boundary layer in summer. Brunner et al. [21] explain that a broad maximum of O 3 in the upper troposphere observed during NOXAR program is associated with the seasonal cycle in tropospheric photochemical activity. A discrepancy between these results and Mondy data would be due to the tropospheric levels of O 3 measurements. Mondy is a surface site at 2 km asl and can be strongly affected by surface O 3 deposition. Large variations of O 3 mixing ratios at Mondy in summer (see Figures 2a, 2b, 2l, and 4a) imply that both O 3 sources and sinks are strong but its averaged levels indicate that O 3 sinks dominate over the sources compared to other seasons. The upper troposphere of Central Asia/Eurasia, however, is a different situation. Once O 3 and its precursors, presumably emitted by forest fires, are convectively uplifted to the higher level of troposphere, their photochemical lifetimes extend as there is no strong O 3 deposition in such level and the photochemical activities could become very effective. Thus, a summer O 3 maximum in the upper troposphere over Siberia can be possibly observed in contrast with the summer minimum at surface levels CO Seasonal Cycle [13] As shown in Figure 4b, CO mixing ratios decrease from a maximum in spring to a minimum in summer fall and increase successively during winter similarly to O 3 mixing ratios. The 3-year monthly averaged CO mixing ratios range between 9 and 174 ppb with an annual average of 127 ± 2 ppb. While both O 3 and CO show the spring maximum and summer minimum, phase shift of Figure 2. (opposite) Time series plots of 1 hr averaged ozone mixing ratio (solid line) and CO mixing ratio (grey circle) at Mondy for one year period from (a) July 1997 to (l) June Plots for (m) April 1997 demonstrate enhanced ozone and CO mixing ratios by local forest fires in spring 1997 (see details in text). Note that the scale and symbols for ozone and CO in (m) are different from (a) to (l) to highlight the mixing ratios variation. The thick solid line and thin solid line in (m) represent ozone and CO mixing ratios, respectively. The horizontal bars in (h) and (i) give the examples of the corresponding times when different categories of air masses arrive at site (see Figure 3.): black (European air mass), grey (Siberian air mass), and white (High-latitude air mass).

5 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH Jul Aug 2-Aug Sep 2-Sep Oct Nov Dec 2-Dec a) July Jul 3-Jul b) August Aug 4-Aug c) September Sep 4-Sep 5-Sep 6-Sep 7-Sep d) October Nov 3-Nov 4-Nov 2-Oct 3-Oct 4-Oct 5-Oct 6-Oct e) November 1997 f) December Dec 4-Dec 5-Dec 6-Dec 7-Dec 8-Dec 9-Dec 1-Dec 11-Dec 12-Dec 13-Dec 14-Dec 15-Dec 16-Dec 4-Jul 5-Jul 6-Jul 7-Jul 8-Jul 9-Jul 1-Jul 11-Jul 12-Jul 13-Jul 14-Jul 15-Jul 16-Jul 17-Jul 18-Jul 5-Aug 6-Aug 7-Aug 8-Aug 9-Aug 1-Aug 11-Aug 12-Aug 13-Aug 14-Aug 15-Aug 16-Aug 17-Aug 18-Aug 19-Aug 2-Aug 5-Nov 6-Nov 7-Nov 8-Nov 9-Nov 1-Nov 11-Nov 12-Nov 13-Nov 14-Nov 15-Nov 16-Nov 17-Nov 18-Nov 19-Nov 2-Nov 21-Nov 7-Oct 8-Oct 9-Oct 1-Oct 11-Oct 12-Oct 13-Oct 14-Oct 15-Oct 16-Oct 17-Oct 18-Oct 19-Oct 2-Oct 21-Oct 22-Oct 23-Oct 8-Sep 9-Sep 1-Sep 11-Sep 12-Sep 13-Sep 14-Sep 15-Sep 16-Sep 17-Sep 18-Sep 19-Sep 2-Sep 21-Sep 22-Sep 23-Sep 24-Sep 17-Dec 18-Dec 19-Dec 2-Dec 21-Dec 22-Dec 23-Dec 24-Dec 25-Dec 26-Dec 27-Dec 28-Dec 29-Dec 3-Dec 31-Dec 19-Jul 2-Jul 21-Jul 22-Jul 23-Jul 24-Jul 25-Jul 26-Jul 27-Jul 28-Jul 29-Jul 3-Jul 31-Jul 21-Aug 22-Aug 23-Aug 24-Aug 25-Aug 26-Aug 27-Aug 28-Aug 29-Aug 3-Aug 31-Aug 22-Nov 23-Nov 24-Nov 25-Nov 26-Nov 27-Nov 28-Nov 29-Nov 3-Nov 25-Sep 26-Sep 27-Sep 28-Sep 29-Sep 3-Sep 24-Oct 25-Oct 26-Oct 27-Oct 28-Oct 29-Oct 3-Oct 31-Oct

6 ACH 7-6 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA Ozone mixing r atio (pb) Jan Feb Mar Apr May Jun Apr 2-Apr g) January Jan 3-Jan h) February Feb 3-Feb i) March Mar 3-Mar j) April Apr 3-Apr k) May May 3-May l) June Jun 3-Jun 4-Jan 5-Jan 6-Jan 7-Jan 8-Jan 9-Jan 1-Jan 11-Jan 12-Jan 13-Jan 14-Jan 15-Jan 16-Jan 17-Jan 18-Jan 4-Feb 5-Feb 6-Feb 7-Feb 8-Feb 9-Feb 1-Feb 11-Feb 12-Feb 13-Feb 14-Feb 15-Feb 16-Feb 17-Feb 18-Feb 4-Mar 5-Mar 6-Mar 7-Mar 8-Mar 9-Mar 1-Mar 11-Mar 12-Mar 13-Mar 14-Mar 15-Mar 16-Mar 17-Mar 18-Mar 4-Apr 5-Apr 6-Apr 7-Apr 8-Apr 9-Apr 1-Apr 11-Apr 12-Apr 13-Apr 14-Apr 15-Apr 16-Apr 17-Apr 18-Apr 4-May 5-May 6-May 7-May 8-May 9-May 1-May 11-May 12-May 13-May 14-May 15-May 16-May 17-May 18-May 19-May 4-Jun 5-Jun 6-Jun 7-Jun 8-Jun 9-Jun 1-Jun 11-Jun 12-Jun 13-Jun 14-Jun 15-Jun 16-Jun 17-Jun 18-Jun m) April 1997 Forest fire plumes 3-Apr 4-Apr 5-Apr 6-Apr 7-Apr 8-Apr 9-Apr 1-Apr 11-Apr 12-Apr 13-Apr 14-Apr 15-Apr 16-Apr 17-Apr 18-Apr 19-Apr 2-Apr 19-Feb 2-Feb 21-Feb 22-Feb 23-Feb 24-Feb 25-Feb 26-Feb 27-Feb 28-Feb 19-Jan 2-Jan 21-Jan 22-Jan 23-Jan 24-Jan 25-Jan 26-Jan 27-Jan 28-Jan 29-Jan 3-Jan 31-Jan 19-Mar 2-Mar 21-Mar 22-Mar 23-Mar 24-Mar 25-Mar 26-Mar 27-Mar 28-Mar 29-Mar 3-Mar 31-Mar 19-Apr 2-Apr 21-Apr 22-Apr 23-Apr 24-Apr 25-Apr 26-Apr 27-Apr 28-Apr 29-Apr 3-Apr 2-May 21-May 22-May 23-May 24-May 25-May 26-May 27-May 28-May 29-May 3-May 31-May 19-Jun 2-Jun 21-Jun 22-Jun 23-Jun 24-Jun 25-Jun 26-Jun 27-Jun 28-Jun 29-Jun 3-Jun 21-Apr 22-Apr 23-Apr 24-Apr 25-Apr 26-Apr 27-Apr 28-Apr 29-Apr 3-Apr

7 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH 7-7 Figure 3. Typical air mass transport patterns arriving at Mondy. Isentropic trajectories shown are 1- day backward. Symbols are shown with 12-hr intervals. The upper panel represents the corresponding vertical motion. The transport patterns shown are the examples for (a) European transport (boundary layer), (b) European transport (free troposphere), (c) Siberian transport (west Siberia), (d) Siberian transport (east Siberia), (e) High latitude and polar transport (free troposphere), and (f ) High latitude and polar transport (upper level of free troposphere). the maximum period is observed. Normally, the maximum of CO seasonal cycle appears around March, one month earlier than for O 3. The spring maximum of CO shows about a factor of two higher CO mixing ratios than in summer fall minimum as can be seen in Figure 4b and Table 1. The difference of CO mixing ratios between spring and summer at Mondy can be explained partially by a destruction process with OH radicals that favors summer [Novelli et al., 1992, 1998a; Holloway et al., 2]. The patterns of seasonal cycle of background CO mixing ratios at remote sites in the middle latitudes of the Northern Hemisphere do not show strong differences [Novelli et al, 1992, 1998a, 1998b]. [14] However, it should be noted here that the CO mixing ratios during summer fall at Mondy are significantly lower than those previously reported in the middle latitudes of the Northern Hemisphere. In direct comparison with other of our monitoring sites (using the same instrumental system) in East Asia, apparently higher CO mixing ratios are observed in Japan [Kajii et al., 1998; Narita et al., 1999; Pochanart et al., 1999; Tanimoto et al., 2] during fall, winter, and spring which can be interpreted as a direct effect of largescale emissions in East Asia. The nearest NOAA/CMDL CO monitoring site to Mondy at Ulaan Uul, Mongolia (UUM, 44 N, 111 E, 914 m asl) [Novelli et al, 1998a; Holloway et al., 2] shows slightly higher CO mixing ratios, by about 1 2 ppb. From trace gas measurements using the trans- Siberian railroad during Trans-Siberian Observations on Chemistry of the Atmosphere 2 (TROICA 2) campaign in Siberia/Russia in summer 1996, Crutzen et al. [1998] found the lowest surface CO level of near 11 ppb over the sparsely populated area in west Siberia [see also Bergama-

8 ACH 7-8 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA 8 7 a) Mondy ozone Sep-96 Jan-97 May-97 Sep-97 Jan-98 May-98 Sep-98 Jan-99 May-99 Sep Month 3 b) Mondy CO Sep-96 Jan-97 May-97 Sep-97 Jan-98 May-98 Sep-98 Jan-99 May-99 Sep Month Figure 4. Seasonal variations of (a) ozone and (b) CO mixing ratios at Mondy. The solid squares indicate monthly averaged mixing ratios. The 5th percentile, 25th percentile, median, 75th percentile, and 95th percentile are presented by the lower bar, lower box edge, centerline, upper box edge, and upper bar, respectively. Different years are separated by different colors.

9 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH 7-9 Table 1. Seasonal Averaged O 3 and CO Mixing Ratios at Mondy During a Mixing Ratio, ppb Year Hours Winter (Dec. Feb.) Spring (March May) Summer (June Aug.) Fall (Sept. Nov.) 1997 O ± ± ± ± O ± ± ± ± O ± ± ± ± CO ± ± ± ± CO ± ± ± ± CO ± ± ± ± 15 a Indicated values are average ±1 standard deviation. Hours indicates total hours of valid data from each year. Only winter data are not continuous, taken from January, February, and December of the same year. schi et al., 1998]. This value is about 15 ppb higher than summer fall averaged CO mixing ratios at Mondy. These low CO mixing ratios would probably reflect the location of the site where the elevation represents closely the free troposphere, is less affected by the CO emission within the boundary layer, and is very far from regional anthropogenic sources of CO Evidence of Siberian Forest Fires at Mondy [15] Recently, the biomass burning and forest fires in the midlatitude of the Northern Hemisphere have gained more attention in the field of global biomass burning. The large areas of burning in middle and high latitudes of the Northern Hemisphere (including Siberia, China, Canada and Alaska) were verified [Carhoon et al., 1994; Levine et al., 1995; Tanimoto et al., 2; Wotawa and Trainer, 2; Wotawa et al., 21]. It was suggested that these boreal forest fires could significantly enhance the mixing ratios of O 3 and CO in the region downwind from the fire [Phadnis and Carmichael, 2]. In East Asia, the Siberian forest fires were suspected to cause a shift of CO spring maximum to the later period based on isotopic CO observations and model calculation at Happo, Japan [Kato et al., 2]. In addition, trajectory analysis implied that the Siberian forest fires may have caused higher CO mixing ratios during spring and summer 1998 at Happo [Kato et al., 22]. The direct capture of an elevated level of CO mixing ratios resulting from Far Eastern Siberian Forest fires was observed at Rishiri Island in northern Japan during summer fall 1998 [Tanimoto et al., 2]. [16] Mondy observatory site experienced the interference from forest fires in Siberia to the O 3 and CO monitoring both on a local/subregional scale and a regional scale. In spring 1997, forest fires occurred in the area around Mondy and sometimes they could be detected just within a visible distance from the site. These fires started from late March and lasted for several weeks. Abrupt changes of CO and O 3 mixing ratios at Mondy were usually observed when the fire plumes reached the site. The time series plots of 1-hour averaged O 3 and CO mixing ratios in Figure 2m show such an example in April The arrows indicate the periods when forest fire plumes were transported downwind to the site and abruptly elevated CO mixing ratios were noticed. O 3 mixing ratios also appeared to be influenced by these forest fires though not as distinct as CO mixing ratios. At the beginning of April when the fires were located far from the site, encounters of the plumes show O 3 increase simultaneously with CO as a result of photochemical production by fire emissions (see examples during 4 5, 9 1 April in Figure 2m) but in late April when the fires moved closer to the site, O 3 decrease was often seen with the abrupt increase of CO as a result of titration by NO emitted from fires (22 23, April, in Figure 2m). For comparison, Figure 2j shows the O 3 and CO mixing ratios in April In spring 1998, the local forest fires were not directly observed in the area and CO mixing ratios at Mondy remained constant for most of time. [17] In contrast, although the satellite advanced very high resolution radiometer (AVHRR) revealed very active fire spots over large areas of Siberia from spring to fall of 1998 [Kato et al., 22], we do not find a direct influence of those fires on CO mixing ratios at Mondy during spring and early summer. Enhancement of CO mixing ratios at Mondy was not observed in spring and early summer of 1998 (unfortunately, we lost CO data from the middle of July and the whole month of August 1998). However, during fall 1998 CO at Mondy shows about 4 ppb higher mixing ratios than its baseline levels in 1997 and 1999 (see Table 1 and Figure 4). This CO enhancement is likely associated with the extensive Siberian forest fires in The intense Siberian forest fires in 1998 were suggested to burn up larger areas in the Far East Siberia in summer fall than areas in central and east Siberia [Kato et al., 22]. Unlike the local/subregional fires of spring 1997 which were noticed together with the abrupt changes of CO signal in a short timescale, the impact of large-scale Siberian forest fires in 1998 would rather be seen as the increase of CO baseline levels as in case of CO data during fall 1998 due to the long range transport and atmospheric mixing in a regional scale. The CO anomaly at Mondy in 1998 agrees very well with the recent reports of CO enhancement in the Extratropical Northern Hemisphere [Wotawa et at., 21] and in the high-latitude sites [Röckmann et al., 22] by the large-scale forest fires in the Northern Hemisphere during summer and fall of Although the forest fires observation in the area around Mondy and the AVHRR data do not seem to be entirely compatible in 1998, the detection of forest fires in Siberia in this work supports the contention that the fires, burning areas, their emissions, and their impact on a background troposphere of the remote Siberia on both local and regional scale could be substantial and varied among different regions of Siberia and times of the year Influence From Transport of European Anthropogenic Emissions Toward Siberia/East Asia [18] Based on trajectory analysis, the frequency of air masses transported from Europe to Mondy is about 27%

10 ACH 7-1 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA Table 2. Monthly Averaged O 3 and CO Mixing Ratios Classified by Trajectories a European Air Masses Siberian Air Masses High-Latitude Air Masses Month O 3 CO O 3 CO O 3 CO Jan. 4.8 ± ± ± ± ± ± 13 Feb. 46. ± ± ± ± ± ± 11 March 47. ± ± ± ± ± ± 1 April 51.8 ± ± ± ± ± ± 11 May 53.4 ± ± ± ± ± ± 9 June 46.9 ± ± ± ± ± ± 1 July 43.3 ± ± ± ± ± ± 15 Aug ± ± ± ± ± ± 12 Sept ± ± ± ± ± ± 1 Oct. 4.5 ± ± ± ± ± ± 17 Nov ± ± ± ± ± ± 16 Dec ± ± ± ± ± ± 16 a Indicated values are average ±1 standard deviation in a unit of parts per billion mixing ratios. O 3 and CO mixing ratios were calculated from January 1997 to December 1998, and July 1997 to December 1998, respectively. annually. This value is comparable with the trajectory studies by Newell and Evans [2]. Considering that the contribution of EU air masses is relatively large, the ability of regional scale anthropogenic emissions from Europe to pollute the atmosphere of Siberia/East Asia is of interest. Europe is one of the three largest source regions in the world and the influence of this large-scale anthropogenic emission within its own territory is well-recognized [see, e.g., Derwent et al., 1998; Ryall et al., 21; Pochanart et al., 21, and references therein]. However, the study of the impact of European emission transport toward other regions based on observational data received little attention, probably due to a deficiency of data in the region downwind. In this section, the following three analysis techniques were employed in order to identify this potential influence from European pollution export. First, we applied a clustering analysis of O 3 and CO data in different air mass regions based on trajectories. Second, a probability distribution frequency of these trajectory categorized O 3 and CO data was studied. The third technique concerns a residence time analysis of air masses leaving Europe. Details and results from the three approaches are presented below Cluster Analysis [19] Hourly averaged O 3 and CO data were paired with the corresponding trajectory categorized EU, SI, and HL air masses for each month during These were done regardless of the air mass vertical distribution in each group. As more than 95% of air masses are transported below 45 hpa, a stratospheric influence is not thought to be a major concern for CO and O 3 data analysis at Mondy. Seasonal variations of O 3 and CO associated with different air mass regions are displayed in Table 2. The O 3 and CO are shown as monthly averaged mixing ratios. Seasonal variations of O 3 and CO in all three categories show the spring maximum-summer minimum, an analogous pattern to the overall seasonal cycle. This implies that seasonal cycles of O 3 and CO in Siberia do not significantly depend on the different air masses regions, which are in contrast to the East Asian sites [Pochanart et al., 1999, 21; Lam et al., 21] and the European sites [Derwent et al., 1994, 1998; Simmonds et al., 1997]. The annual averaged O 3 mixing ratios in EU, SI, and HL air masses, derived from monthly averages, are 44.3 ± 4.1, 42.4 ± 4.4, and 42.2 ± 3.8, ppb, respectively. Slightly higher O 3 mixing ratios are observed in EU air masses than in SI and HL air masses, by about 2 ppb annually and 3 ppb during spring summer. For a given error range (±1 standard deviation) one might argue that the differences of O 3 mixing ratio in the three air mass regions are not significantly large. However, for the intercontinental scale transport, these values are comparable with the impact of trans-pacific transports of Asian O 3 to the US from a model study [Jacob et al., 1999]. For CO mixing ratios, the results also do not reveal large difference among the three categories. The annual averages in EU, SI, and HL air masses, derived from monthly values, are 127 ± 18, 128 ± 17, and 122 ± 13, ppb for 1998, respectively. [2] From the seasonal variations of O 3 and CO in different air mass regions, the impact of long-range transport of polluted air masses from Europe may not be clearly seen. However, it is generally agreed that this cluster analysis will be useful only if there is a distinct difference in magnitude of the contribution from the different source regions (or trajectories). When the differences in magnitude among categories of trajectory are small, showing monthly averaged values might not prove to be useful. In case of Mondy, a very long distance between the possible sources (Europe) and the site could also prevent any clear evidence of pollution transport episode. The next approach was presented to confirm this possibility of European pollution affecting Siberia/East Asia Probability Distribution Study [21] While the monthly averaged mixing ratios of O 3 and CO do not reveal a large difference for EU, SI, and HL air masses, investigation of a distribution of 1-hour averaged data in each air mass region could provide some useful information about long range transport of the European pollution to Siberia/East Asia. Figure 5 shows the frequency distribution of O 3 and CO mixing ratios at Mondy. As CO monitoring started in March 1997 and was influenced by the local forest fires in spring 1997, the data used were from July 1997 to December The histograms show more numbers of O 3 and CO data with higher mixing ratios in EU air than in SI and HL air. O 3 mixing ratios in the three air mass regions appear to be a normal distribution with the highest numbers of data in EU air. Below 35 ppb of O 3,the amounts of data for each air mass region are approximately the same but the numbers of data in EU air increase notably at a higher than 35 ppb of O 3. CO data show broader

11 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH a) Europe Siberia High Latitude 6 b) Amount of Data (hr) Mean 99% 95% 75% 5% 25% 5% 1% c) Europe Siberia High Latitude EU Europe d) SI Siberia HL High Latitude Air mass category 99% 95% Amount of data (hr) Mean 75% 5% 25% 5% 1% EU Europe SI Siberia HL High Latitude Air mass category Figure 5. Frequency distribution of 1-hr averaged ozone and CO mixing ratios in different air mass regions during July 1997 to December 1998: (a) ozone histogram, (b) ozone box chart, (c) CO histogram, and (d) CO box chart. Numbers in (b) and (d) indicate data percentiles. distribution but their probability distribution functions still appear to be a normal distribution. From CO frequency distribution plots, the numbers of CO data below 12 ppb are about the same for each air mass region but significantly increase in EU air for CO higher than 12 ppb. As shown in Figure 5b, at 5th percentile (median) of the entire data in EU, SI, and HL group, the O 3 mixing ratios are 44.2, 42.7, and 41. ppb, respectively. The corresponding CO mixing ratios in Figure 5d are 134, 128, and 11 ppb. [22] Cumulative probability distributions for each group are further plotted as shown in Figure 6 for the entire data and seasonally separated data. For the entire data set, the higher O 3 and CO mixing ratios in EU air at most cumulative percentages are noticed. The same feature is also found with the seasonal data. Discarding the upper and lower ends of the data (<5th percentile and >95th percentile), O 3 and CO at any percentile in EU air show significantly higher mixing ratios than those in SI and HL air masses in most seasons. The orders from the highest mixing ratios appear to be EU, SI, and HL air, respectively. The largest difference of O 3 mixing ratios among air regions is observed in summer, suggesting that the EU air mass is more photochemically active than the SI and HL air in this season of the year. For CO, the largest difference is found during fall but summer and winter also show the significant difference, especially between EU and HL air. The only exception is seen in spring when CO in HL and SI air show higher mixing ratios than EU air at the same percentiles. At the moment, we still have no evident explanation for the higher springtime CO transported by HL air mass but it is

12 ACH 7-12 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA Cumulative percentage All data - Europe All data - Siberia All data - High Latitude Cumulative percentage All data - Europe All data - Siberia All data - High Latitude Cumulative Percentage Summer - Europe Summer - Siberia Summer - High Latitude Cumulative Percentage Summer - Europe Summer - Siberia Summer - High Latitude Cumulative Percentage Fall - Europe Fall - Siberia Fall - High Latitude Cumulative Percentage Fall - Europe Fall - Siberia Fall - High Latitude Cumulative Percentage Winter - Europe Winter - Siberia Winter - High Latitude Cumulative Percentage Winter - Europe Winter - Siberia Winter - High Latitude Spring - Europe Spring - Siberia Spring - High Latitude Cumulative Percentage Spring - Europe Spring - Siberia Spring - High Latitude Cumulative Percentage

13 POCHANART ET AL.: REGIONAL BACKGROUND OZONE AND CO IN SIBERIA ACH 7-13 generally known that the high CO mixing ratios are often found in the Arctic region during late-winter and early spring as a result of transport of anthropogenic pollutants from northern Europe into the polar vortex [Novelli et al., 1998a]. For SI air mass, the forest fires during spring 1998 in western and central Siberia might induce the higher CO baseline. These two events, in combination with an uncertainty due to fewer hours of valid data (spring data were from 1998 only), may bring about higher CO mixing ratios in HL and SI air masses during spring, respectively. [23] Based on this frequency distribution, it can be concluded that for most seasons of the year the EU air masses reveal higher O 3 and CO mixing ratios than the SI and HL air masses, likely due to the long-range transport of pollution from Europe although the magnitude of mixing ratios differences is not very large. We have provided here the evidence that Europe pollution does have influence on the background O 3 and CO at Mondy. In the following part, we further investigate this possible influence of European pollution using trajectory with residence time analysis Residence Time Analysis of Air Mass From Europe [24] As mentioned, EU air masses are those originating in or passing through Europe before arriving at Mondy. The relationship between transport times of air masses after leaving Europe until arriving at Mondy and CO and O 3 mixing ratios in the EU air region is investigated. We applied a technique similar to that described by Pochanart et al. [21] to CO and O 3 mixing ratios at Mondy. This simple technique assumes that for the same group of EU air mass, one would expect to see a difference or gradient of trace gases mixing ratios between those by a rapid air mass transport from Europe and a slowly transported one if an export of European pollution and a long range transport are the only contributing factors for those trace gases variations. [25] Europe in this investigation is defined as the areas between west of Ural mountain (6 E longitude) and the west coast of the European continent. These large areas cover the western and eastern European countries, and parts of Russia and FSU countries. The averaged transport times of the clusters of 5 trajectories after leaving this European wall until arriving at Mondy (or to say so, the residence times over the remote Eurasian continent) were then calculated. Data from July 1997 to December 1998 are applied. This one and a half year period gives total numbers of 198 trajectory clusters in which 293 clusters were from Europe. With a removal of about 1% possible outliers, 268 clusters are used in calculation. The trajectories are then matched with 12 hours of CO and O 3 mixing ratios at the times air masses arrived at Mondy on a seasonal basis. It must be noted that because of the rather limited amounts of trajectories (about 6 7 per season), we do not further stratify whether the air masses come from the boundary layer or free troposphere of Europe. A general impression is that the free tropospheric motion shares a larger fraction in total transport frequencies (see also, Newell and Evans [2]) but the altitudes of trajectory courses are not found to be very high. It is found that 53% of the EU air masses descend from levels between 4 and 7 hpa, and 44% are from the levels below 7 hpa. On average, the maximum elevation of EU air masses transport routes during 1-day period is about 3.9 ± 1.4 km. Examples of the relationship between CO mixing ratios and averaged transport times of the air mass clusters after leaving Europe in summer and winter are shown in Figure 7. [26] It can be seen that CO mixing ratios gradually decrease when it takes longer time for air masses to reach Mondy. The rapid transport of air masses from Europe results in higher mixing ratios of CO. This appears to be a common feature for CO in every season. We also see the same trend for O 3 in some seasons. To further simplify the relationship between trace gases and residence times, we grouped the transport times of EU air masses after leaving Europe into unit of days, namely, 1 day, 2 days,...,upto8 days. These transport times were then plotted against the averaged mixing ratios of the corresponding CO and O 3 data in each group. The results are shown in Figure 8. The CO mixing ratios exhibit a significant correlation with the transport time of air masses from Europe in every season. For O 3 mixing ratios, the correlation is seen only in summer and spring. CO is known to be good anthropogenic tracer [Parrish et al., 1998]. Decrease of CO mixing ratios over longer transport times of air masses from Europe primarily reflects a dilution of European polluted air, high in CO, into the well-mixed background troposphere of lower CO over Eurasia, and the photochemical loss of CO assuming that no additional CO from other emission sources is advected into air during transport of the polluted EU air to Mondy. This admixture of CO in European air into continental background air would occur on the timescale of days. [27] Linear regressions have been plotted to estimate CO decreasing rates. In Figure 8, we compared the results from a simple linear regression presented as dotted lines with those of Reduced Major Axis (RMA) regression presented as solid lines. It was suggested that the evaluation of atmospheric traces data would be better achieved using the RMA analysis [Ayers, 21]. In our case, the results between two methods do not show a strong difference. The slope represents the CO decreasing rate and the Y axis intercept gives the CO mixing ratio when the air masses leave Europe, and presumably represents the CO mixing ratio under European regionally polluted condition. [28] For the estimated CO in European regionally polluted condition (Y axis intercept), its mixing ratios are lowest in summer, about 136 ppb, and increase to highest in spring, 168 ppb. As no ground-based CO monitoring around this European wall was reported, we cannot verify whether these estimated values properly represent the European regionally polluted CO mixing ratios. In comparison with spectroscopic column CO at Zvenigorod (37 E), Russia [Yurganov et al., 1997] and CMDL/NOAA surface European sites [Novelli et al., 1998a; Holloway et Figure 6. (opposite) Cumulative frequency distribution of ozone and CO in different air mass categories. The entire data set (July 1997 to December 1998) and seasonal data sets are shown. Europe, Siberia, High Latitude denote the air masses from these regions, respectively. Summer, fall, winter, and spring are for June August, September November, December February, and March May, respectively.