5.1. Episodes of High-ΔCO Events
 In the statistical analyses described in the following sections, we only used air masses in which ΔCO values were greater than 30 ppbv (i.e., air masses clearly influenced by anthropogenic sources over the northeastern part of China). A threshold value of 30 ppbv was subjectively chosen by assuming a symmetrical distribution of ΔCO values in background air masses around the ΔCO = 0 line (“background relationship” line in Figures 7a–7d). Air masses in which ΔCO > 30 ppbv consisted of 30% of “continental air masses,” and this fraction is higher at 0–2 km as compared with that at 2–7 km. Locations where these air masses were sampled are shown in Figure 1. After this selection, 19–42 and 9–29 data points remain in “continental” and “Chinese” air masses, respectively, in each of the 0–1, 1–2, and 2–4 km altitude ranges, while only 2 and 1 data points remain in these two air masses in the 4–7 km range. Results at 4–7 km likely do not represent the average in the western Pacific region. Uncertainties in the estimation at 4–7 km altitude range are discussed in section 5.5.
 In Figure 8a, a scatter plot between CO and NOy is shown using all of the “continental air mass” data. As seen in this figure, enhancements in NOy are positively correlated with those in CO, suggesting that these NOy enhancements were due to anthropogenic emissions. The enhancement was greatest at altitudes below 2 km. A clear positive correlation between NOy and CO, suggesting anthropogenic influences, was also found in the lower and middle troposphere over the western Pacific in February–April 1994, during PEM-West B [Koike et al., 1997].
 In Figure 8c, a scatter plot between ΔNOy and ΔCO is shown for “continental air mass” data. The relationship between ΔNOy and ΔCO is similar to that between NOy and CO (Figure 8a) because background values of individual data are not very different. The ΔNOy and ΔCO are positively correlated; however, the slope is lower than the emission ratio, suggesting that anthropogenically emitted nitrogen compounds had been removed from the air masses before they were sampled from the P3-B aircraft. Ratios of increases, ΔNOy/ΔCO, are generally higher at altitudes below 2 km as compared to those at higher altitudes, suggesting removal of NOy during the vertical transport. On the other hand, ratios of increases in air masses in which ΔNOy and ΔCO values are very high (ΔNOy > 6 ppbv and ΔCO > 400 ppbv) are similar to those of other air masses. These air masses are considered to have been influenced by anthropogenic sources more strongly, however the removal rate is similar to those of less-influenced air masses.
 It is noted that there are some data which have greater ΔNOy/ΔCO ratios than the ratio. This is likely due to inhomogeneity in emission ratios and/or influences from Japanese and/or South Korean emissions. Most of these data points are found in air masses in which ΔCO < 30 ppbv and therefore, they were discarded in the following statistical analyses as described in section 5.1.
 In Figure 8b, a scatter plot between CO and SOx is shown using “continental air mass” data. A clear correlation was found between the two values, especially at altitudes below 2 km. A positive correlation is also seen in a scatter plot between ΔCO and ΔSOx (Figure 8d). As for NOy, the slope was lower than the emission ratio, ESO2/ECO, suggesting the removal of SOx. Air masses sampled at 2–7 km appear to belong to two groups. In one of the groups, the slope of the increase is similar to that of the 0–2 km data, while very little increase of SOx is found in the other group. These two groups of air masses were sampled during different flights (mostly from flights 13 and 19); however, many of these air masses had been transported from below 800 hPa over the northeastern part of China. Consequently, the observed difference in the transport efficiency was largely due to differences in the transport processes, in which the degree of loss of SO2 and/or nssSO42− was different (more discussion on flight 19 data is given in section 5.4.).
5.3. Vertical Profile of ε(NOy)
 Average ΔCO and ΔNOy values in “continental air mass” data were calculated within each altitude range (0–1, 1–2, 2–4, and 4–7 km), and the transport efficiency, ε(NOy), was calculated using these averages ( and ). In this calculation, we only used data with ΔCO > 30 ppbv. We took the ratio of the average rather than using the median value of individual ratios because events in which ΔCO and ΔNOy were large should have a greater impact on the overall efficiency of the transport. It has been reported that the majority of transport of pollutants from the Asian continent to the Pacific Ocean is generally the result of episodic events rather than continuous steady flow [Bey et al., 2001; Liu et al., 2003; Jaffe et al., 1999]. We note that the choice of the threshold value of ΔCO (>30 ppbv) is not very sensitive to the results because the averages are used instead of using the medians.
 A vertical profile of the average transport efficiency in the “continental air” is shown in Figure 9a (Table 3). In this figure, the transport efficiency calculated using only “Chinese air masses” is also shown. “Chinese air masses” were those that passed over China at altitudes below the 800-hPa level (boundary layer) within 5 days prior to measurement without passing over South Korea or Japan at these low altitudes, as described in section 3.1. We note that when air masses in which ΔCO > 30 ppbv are selected, Chinese air masses in each altitude range consist of 20–70% of the continental air masses. As seen in Figure 9a, a transport efficiency of 20–40% was found at 0–2 km. “Chinese air masses” at 0–2 km were mostly sampled over the Yellow Sea, and they were sampled within 24 hours after they had left the coast of China. The efficiency estimated in this study indicates that 60–80% of NOy had already been removed in the source region before these air masses left the continent, although additional loss could have also occurred during this short transport over the ocean. The efficiency tends to be lower at higher altitudes, and values of about 15% were found at 2–7 km. The lower efficiency in the free troposphere suggests that HNO3 and NO3− were removed by wet deposition during the vertical transport because precipitation is often accompanied by convection and synoptic-scale air mass lifting in association with cold frontal systems, which is known as the warm conveyer belt (WCB). The removal process is examined more quantitatively below.
Figure 9. Vertical profiles of the transport efficiency for (a) NOy, defined in equation (1) and (b) SOx (=SO2 + nssSO42−), defined in equation (2). Values of ΔCO, ΔNOy, and ΔSOx were averaged (only using air masses in which ΔCO > 30 ppbv) before taking their ratios. Emission ratios, given in Table 2 were used. Horizontal bars indicate uncertainties in the estimations (50% range).
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Table 3. Transport Efficiency of Reactive Nitrogena
|4–7 km||0.18 (0.18)||0.004||0.001||0.14 (0.11)|
|2–4 km||0.13 (0.13)||0.007||0.001||0.13 (0.08)|
|1–2 km||0.21 (0.18)||0.013||0.025||0.26 (0.25)|
|0–1 km||0.38 (0.28)||0.048||0.16||0.31 (0.26)|
 In Figure 10a, vertical profiles of median values of HNO3/NOy and NO3-/NOy in “continental air masses” are shown. These values are also given in Table 4. For this calculation, we only used data with ΔCO > 30 ppbv. As seen in this figure, HNO3 + NO3− accounts for 50% and 30% of NOy at 0–1 and 1–2 km, respectively. Henry's law equilibrium and laboratory studies of HNO3 uptake on ice surfaces indicate that HNO3 is ∼100% fractionated into the condensed phase in both warm and glaciated clouds [Mari et al., 2000, and references therein]. One-dimensional model calculations for tropical continental deep convection suggest that about 80% of HNO3 was scavenged during vertical transport [Mari et al., 2000]. Similarly, very low NO3−/NOy values of 0.007 in the free troposphere, as compared with those observed in the boundary layer of 0.1–0.4, indicate that nitrate aerosols were also scavenged quite efficiently during vertical transport. In fact, ammonium nitrate (NH4NO3), which is generally seen in accumulation mode aerosol and which is considered to have been measured by the NOy instrument, is expected to be efficiently scavenged by cloud droplets. Assuming 100% removal of HNO3 and NO3− aerosols in the boundary layer during vertical transport into the free troposphere, the factor of 2 reduction of ε(NOy) estimated in this study can be explained. Formation of HNO3 from NOx during and after vertical transport retains the HNO3/NOy ratios in the free troposphere, while very low NO3−/NOy ratios in the free troposphere suggest that the formation of nitrate aerosol was likely limited because an insufficient amount of NH3 was available to neutralize the acidity.
Figure 10. (a) Vertical profile of the partitioning within the NOy species (P-3B data). Median values and 67% ranges (horizontal bars) in “continental air masses” (and ΔCO > 30 ppbv) are shown. (b) Vertical profile of nssSO42−/SOx ratio (P-3B data). Median values and 67% ranges (horizontal bars) in “continental air masses” (and ΔCO > 30 ppbv) are shown. Individual data points are also shown.
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Table 4. Partitioning Within NOy and SOx (P-3B)a
|2–4 km||0.051 (0.028–0.078)||0.46 (0.36–0.64)||0.23 (0.12–0.33)||0.0078 (0.0015–0.0723)||0.49 (0.01–0.87)|
|1–2 km||0.064 (0.036–0.083)||0.39 (0.30–0.52)||0.17 (0.10–0.23)||0.12 (0.00–0.49)||0.37 (0.21–0.83)|
|0–1 km||0.12 (0.09–0.17)||0.34 (0.25–0.38)||0.091 (0.046–0.146)||0.42 (0.26–0.74)||0.56 (0.45–0.71)|
 The transport efficiency of NOy during the TRACE-P period was also studied by Miyazaki et al. . Although a single value was used for the background levels of NOy and CO in their analysis, generally similar results were obtained: 30% and 10–20% in the boundary layer and free troposphere, respectively. Synoptic-scale WCB and convection were found to be important mechanisms for the transport of anthropogenic reactive nitrogen to the free troposphere. The transport efficiency of NOy from North America was studied by Stohl et al.  using North Atlantic Regional Experiment (NARE) 1996 and 1997 data. They used CO and Lagrangian tracers as a reference and found that only 3–5% of NOy emissions reached altitudes above 3 km. D. D. Parrish et al. (Relation of NOy and CO concentrations in the free troposphere: Fraction and composition of NOy transported in polluted air masses lofted from the North American continental boundary layer, submitted to Journal of Geophysical Research, 2003) also studied the relationship between NOy and CO using five field campaigns over the North America (summer season) and the western North Atlantic Ocean (spring and fall) and the NOy transport efficiencies in the free troposphere are estimated to have been 10–30%.
 Vertical profiles of the partitioning of reactive nitrogen (Figure 10a and Table 4) show that PAN was the dominant species in “continental air masses” (ΔCO > 30 ppbv). Because of the long lifetime of PAN at low temperatures in the free troposphere, PAN would be transported farther downwind and eventually produce NOx after descending to lower altitudes [e.g., Moxim et al., 1996; Koike et al., 1997]. Consequently, this fraction is considered as a potential source of NOx. Direct transport of NOx, however, was found to be very limited. The NOx/NOy ratio was 0.06–0.12 and 0.02–0.05 at 0–2 km and 2–7 km, respectively (Figure 10a and Table 4). When ε(NOy) values in Table 3 (Figure 9a) are used, the transport efficiency of NOx, ε(NOx), was calculated to be about 1–5% and 0.5% in the boundary layer and free troposphere, respectively. This result means that only 0.5% of anthropogenically emitted NOx remained in the form of NOx when these air masses were transported into the free troposphere. Values of NO3−/NOy have large variability at 0–2 km, partly because the NO3− amount in aerosol is generally limited by the availability of NH3. When median values of NO3−/NOy are used, the formation efficiency of NO3−, ε(NO3−), was calculated to be 3–16% and 0.1% in the boundary layer and free troposphere, respectively. This low yield in the free troposphere is due to the efficient scavenging of nitrate aerosol, as described above.
 A scatter plot between ε(NOy) and NOx/NOy is shown in Figure 11a. As seen in this figure, these two quantities are positively correlated. Part of the correlation is due to their vertical profiles; however, a correlation is seen even within the boundary layer data. Because the loss of HNO3 and NO3− results in a higher NOx/NOy ratio, a positive correlation cannot be simply explained by this process. When an air mass remained or stagnated within the boundary layer for a longer time period, conversion from NOx to HNO3 by gas-phase reaction with OH and nighttime heterogeneous reaction through N2O5 proceed further, leading to a lower NOx/NOy ratio. Concurrently, the loss of HNO3 and NO3− results in a lower ε(NOy). Furthermore, during vertical transport, we may expect an efficient conversion of NOx to HNO3 due to the relatively large amounts of OH from H2O and wet surface area for heterogeneous reaction. The results presented in this study suggest that the transport efficiency was higher in air masses that spent less time in the boundary layer, though a very limited fraction of emitted NOx can still be brought into the free troposphere.
Figure 11. Scatter plot between (a) NOx/NOy and transport efficiency of NOy, and (b) transport efficiencies of NOy and SOx (P-3B data). Air masses in which ΔCO > 30 ppbv within “continental air masses” are used.
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5.4. Vertical Profile of ε(SOx)
 Vertical profiles of the average transport efficiency of SOx (=SO2 + nssSO42−), ε(SOx), in the “continental air masses” and “Chinese air masses” are shown in Figure 9b (Table 5). As for ε(NOy), averages of ΔSOx and ΔCO in high-ΔCO air masses (ΔCO > 30 ppbv) were calculated ( and ), and their ratio was divided by the emission ratio given in Table 2. As a result, the transport efficiency at 0–2 km was estimated to be 25–45%. “Chinese air masses” at 0–2 km were mostly sampled over the Yellow Sea, and they were sampled within 24 hours after they had left the coast of China, as described above. The efficiency estimated in this study indicates that 55–75% of SOx had already been removed in the source region before these air masses left the continent. The transport efficiency of 15% at 2–7 km is lower than that at 0–2 km, suggesting that SO2 and nssSO42− were removed by wet deposition during vertical transport. One-dimensional model calculations for tropical continental deep convection suggest that about 30% of SO2 was scavenged during vertical transport [Mari et al., 2000]. Although this value is lower compared to the value of 80% for HNO3 due to the low Henry's Law constant, nssSO42− particles are considered to have been scavenged quite efficiently within the cloud and precipitation leading to lower ε(SOx) values in the free troposphere.
Table 5. Transport Efficiency of SOxa
|4–7 km||0.15 (0.14)||0.015||0.06 (0.06)|
|2–4 km||0.18 (0.16)||0.087||0.14 (0.06)|
|1–2 km||0.27 (0.24)||0.10||0.38 (0.38)|
|0–1 km||0.45 (0.29)||0.25||0.43 (0.42)|
 A vertical profile of median values of nssSO42−/SOx ratio is shown in Figure 10b (and Table 4). At 0–1 km altitude, nssSO42−/SOx ratios of about 0.6 were observed during each of the four fights, suggesting that the conversion from SO2 to SO42− had taken place quite efficiently near the source region. Ratios of nssSO42−/SOx of about 0.3–0.7 were obtained from the ground-based measurements at Cheju Island in the East China Sea [Carmichael et al., 1997] and Taean station in Korea facing the Yellow Sea [Kim et al., 2001], which also suggest rapid conversion within the boundary layer. On the other hand, at 2–4 km, nssSO42−/SOx ratios differ significantly between the flights. In air masses sampled over Japan Sea during flight 19, nssSO42−/SOx ratios were quite low (Figure 10b), while the ε(SOx) values were higher than those of the other flights. During this flight, the CO, SO2, and NOy values reached 300–400 ppbv, 5–9 ppbv, and 2–4 ppbv, respectively, at 3 km, suggesting a significant impact from anthropogenic emissions. Because ε(SOx) values at 2–4 km during this flight were close to those at 0–1 km, the observed low nssSO42−/SOx ratios might have been due to a limited formation of nssSO42− particles, rather than the removal of nssSO42− particles. When a median value of nssSO42−/SOx is used with the transport efficiency of SOx, a formation efficiency of sulfate aerosol is estimated to be 10–25% and 2–9% at 0–2 km and 4–7 km, respectively. The efficiency in the free troposphere was higher than that of nitrate aerosol in the same altitude rage (ε(NO3−) = 0.1%) likely because sulfate aerosols can be formed more easily in the free troposphere after removal during vertical transport.
 A scatter plot of the transport efficiencies for NOy and SOx is shown in Figure 11b. As seen in this figure, they are positively correlated, even within each altitude range. This result suggests that loss processes of NOy and SOx proceeded simultaneously.
5.5. Comparison With ε(NOy) and ε(SOx) Values Derived From DC-8 Data
 In this study, the P-3B aircraft data were used exclusively. However, because measurements of NOx, PAN, HNO3, SO2, and SO42− (particles with diameters smaller than 2.7 μm) were also made onboard the NASA/DC-8 during TRACE-P, we performed the same analyses using the DC-8 data set obtained in the study area defined for the present study (30°N–42°N and 124°E–140°E). For NOy values, we summed the DC-8 observations of NOx, PAN, and HNO3. In HNO3 measurements, particulate nitrate of diameters smaller than 1–2 μm is considered to have been measured with gas phase HNO3. When we plot a CO-CO2 scatter plot using Halon 1211 and OCS as markers of Chinese urban emissions, very similar results to those shown in Figures 7a–7c are obtained (not shown). The “background relationship” derived from the P-3B also fits the DC-8 data set well. This is partly because the same research groups made measurements onboard the two aircraft for CO, CO2, and species obtained from whole air sampling. In fact, very good agreement between the two sets of measurements was found during the P-3B/DC-8 intercomparison flights [Eisele et al., 2003].
 Vertical profiles of ε(NOy) and ε(SOx) values derived from DC-8 “continental air” and “Chinese air” are shown in Figures 9a and 9b, together with the P-3B results. DC-8 “Chinese air masses” were selected using the same definition as that for P-3B data using air mass trajectories. These results are also summarized in Tables 3 and 5. As seen in the figure, most of the results obtained from measurements from the two aircraft agreed to within 50%. When only the transport efficiencies of SO2 are compared, a similar degree of agreement was found at 0–2 km, although the P-3B values in the boundary layer and free troposphere were found to be lower and higher, respectively. Disagreement by a factor of 3 was found between the P-3B and DC-8 SO2 measurements during the P-3B/DC-8 intercomparison flights [Eisele et al., 2003]. However, the agreement in the estimates indicates that the results obtained in this study are robust.
 As described in section 5.1, only two samples of air masses, in which ΔCO > 30 ppbv, were used for P-3B data analyses at 4–7 km (continental air masses), and therefore they do not represent the average in the western Pacific region. A number of data with ΔCO > 30 ppbv is small at this altitude range, primarily due to a limited number of uplifting events. When a criterion of ΔCO > 5 ppbv is used instead, 7 data (10% of the total number of data at 4–7 km) remain and ε(NOy) and ε(SOx) of 22 and 20% are obtained, suggesting a range of uncertainties in our estimations. Measurements at altitudes above 4 km were made more frequently by the DC-8. Although data above 7 km were also used for the DC-8 statistics, the agreement with the DC-8 estimates (within 25% and 85% for NOy and SOx, respectively) derived using a greater number of data (32 data) supports the validity of the results obtained from the P-3B data at 4–7 km (Tables 3 and 5).
5.6. Export Flux and Budget
 The transport efficiencies ε(NOy) and ε(SOx) defined in equations (1) and (2) were determined for individual air masses. The value of ε(NOy) was derived from the number density of observed NOy in one air mass divided by the expected NOy number density in that air mass assuming that there is no loss after anthropogenic input. Consequently, the efficiency can be 100% in individual air masses. In the next step, we estimated a time-averaged eastward flux of NOy and SOx across the 130°E meridional plane at 30°N–40°N during the TRACE-P period. Because of the limited air mass sampling frequency, only fractional contributions from various altitudes are considered. To estimate these quantities, we first examined an eastward flux of CO. The export flux fraction of CO is determined in this study as follows.
where Fair is the eastward flux of air molecules at 130°E meridional plane at 30°N–40°N derived using ECMWF data. Here γ is the occurrence frequency of the events in which ΔCO > 30 ppbv were observed from the P-3B aircraft. For , averaged values in the continental air masses (ΔCO > 30 ppbv) were used. These three quantities were determined as a function of altitude and calculations were made below 7 km, where P-3B data are available. Note that the occurrence frequency of pollution transport events (events in which ΔCO > 30 ppbv) is highly dependent on where and when P-3B aircraft measurements were made, although only the relative frequency among the various altitude ranges was used for our estimation. Consequently, the flux estimated here should be considered as a case study for air masses which we sampled during TRACE-P. Using equation (3), it was found that 30% of CO export took place through the boundary layer (0–2 km) and 70% was transported through the free troposphere (2–7 km). Although the size of the ΔCO value was similar at altitudes below 4 km, higher wind speeds resulted in a greater flux in the free troposphere. The CO flux within each 1-km layer was suggested to be greatest in the 2–4-km altitude range. This result is consistent with the GEOS-CHEM model for the PEM-West B period (February–March 1994) shown by Bey et al. . Liu et al.  also showed similar results for the TRACE-P period. When we attempted to estimate the absolute value of the averaged eastward flux of CO instead of the fractional contributions, values of 1–3 × 10−9 mol cm−2 s−1 were obtained at altitudes below 4 km. This is again in reasonable agreement with the results obtained by Liu et al. .
 The export flux fraction of NOy is determined in the similar way.
The denominator is the expected total flux of NOy molecules that could have crossed the meridional plane, assuming that there is no loss after receiving anthropogenic emissions. The numerator is the observed flux of NOy molecules that actually crossed the meridional plane within each altitude range (continental air masses in which ΔCO > 30 ppbv). The export flux fraction can be considered as a weighted average of the transport efficiency for which air flux and occurrence frequency of pollution transport event are used as a weighting function. Using equation (4), the export flux fraction of NOy at 0–7 km during TRACE-P was estimated to be 18%. About half (8%) of the export flux took place through the boundary layer and the other half (10%) took place through the free troposphere. Similarly, the export flux fraction of SOx at 0–2 km and 2–7 km was estimated to be 10% and 12%, respectively. The fact that export flux fractions were similar between the two altitude ranges for NOy and SOx is in contrast to the case of CO, in which the fraction in the free troposphere was greater as described above. The free tropospheric fraction was less significant for NOy and SOx because loss during the vertical transport resulted in lower ε(NOy) and ε(SOx) values. The flux in the free troposphere is important for long-range transport, due to greater wind speeds and less-frequent removal, while the flux in the boundary layer suggests an impact on neighboring countries, such as Japan.
 The budget of NOy and SOx (total export flux versus emissions) over the East Asia region is also examined here. During TRACE-P, predominant westerly flow generally brought air masses influenced by anthropogenic emissions over the East Asia into the western Pacific region. Liu et al.  show using the GEOS-CHEM model that eastward fluxes of Asian CO is strongest at 30–45°N in the boundary layer and at 20–35°N in the lower free troposphere. The former mostly reflects anthropogenic emissions, while the latter includes contributions from both anthropogenic and biomass burning emissions. When we assume that a majority of air masses influenced by anthropogenic emissions is transported to 30–40°N (and sampled from the P-3B aircraft) or an export flux fraction is similar at other latitude ranges, we can use an estimate of export flux fraction as a budget estimate. This assumption may not be very unreasonable, because a total flux of CO at 30–40°N at 130°E meridional plane described above agrees with the total emission of CO at 25°N–50°N and 100°E–130°E (except for emissions from South Korea) within 20%. However considering the limitation of air mass sampling in space and time during TRACE-P, this budget analysis should be considered as a case study as for the flux analysis. The results of the export flux fraction obtained above suggest that of total number of molecules of NOx and SO2 emitted over the northeastern part of China, only 18 and 22% were transported out from the Asian continent into the western Pacific in air masses which we sampled during TRACE-P.
 Budget of NOy and SOx in the East Asia region has been estimated in various numerical model studies [e.g., Bey et al., 2001; Chin et al., 2000; Tan et al., 2002]. Usually, the total export flux from a selected domain is calculated from the difference between the total source and total sink. Bey et al.  showed using the GEOS-CHEM model for the PEM-West B period (February–March 1994) that about 70–80% of NOx emitted in Asia was lost within the domain by deposition of HNO3, and the net export flux was 20–30%, although the contribution from biomass burning in the southern part of China was also included in their model calculations. Chin et al.  showed using the GOCART model that the export flux fraction of anthropogenic SOx from eastern Asia was about 16%, with 2/3 of it as SO2. Nearly half of the emitted SO2 is lost by dry-deposition to the surface. Tan et al.  showed using their three-dimensional (3-D) model calculations for the late winter/early spring period that about 50% of the anthropogenic SOx emitted over East Asia was removed from the continental source regions. About 30% is further removed within the neighboring ocean (region west of about 155°E), and 20% is exported out of their model domain. The vast majority of the exported SOx was in the form of sulfate aerosol. These estimates are generally consistent with those obtained in this study.
 Uncertainties in the estimation of transport efficiency are affected by uncertainties in the emission data, biogenic sources of CO2 [Vay et al., 2003] which is not considered in this study, measurements imperfections, background relationships, the choice of the threshold value of ΔCO (30 ppbv), and loss of NOy and SOx in background air masses. Also, the two assumptions were made in our analyses: first, CO, CO2, NOy, and SOx values in background air masses have a linear relationship, and second, the ratio of enhancements due to anthropogenic emissions is always the same. It is hard to evaluate individual uncertainties in a consistent manner, however we estimated an overall uncertainty in our estimation of transport efficiency to be on the order of 50% (horizontal bars in Figures 9a and 9b).
 Some specific sources of uncertainty are described here. First, Palmer et al.  shows using an inverse model analysis that the anthropogenic emissions of CO in China estimated by Streets et al.  is likely to be underestimated by 30%. When CO emissions are increased by 30% in our analyses, the transport efficiencies and the export flux fractions (equation (4)) increase by about 30%.
 Second, emissions from Japan and South Korea are examined. Emission ratios of CO, CO2, and NOx in Japan and South Korea are very different from those in China (Table 2), as described in section 3.2. Consequently, when an air mass is influenced by emissions over South Korea or Japan, the ratio of increase, ΔNOy/ΔCO and ΔSOx/ΔCO, will be different from those expected from Chinese emissions. However, as shown in Figures 9a and 9b, results obtained using “continental air masses” generally agreed with those using “Chinese air masses”. As explained in section 3.1, a more strict criterion on the air mass trajectories was applied to select “Chinese air masses,” suggesting the robustness of the current estimates.
 Third, in situ sources of reactive nitrogen, lightning NO production and aircraft emissions, are examined. Lightning during the TRACE-P period was limited over India and Southeast Asia to latitudes lower than 30°N [Fuelberg et al., 2003], and their contributions to the budget of reactive nitrogen in the present study area are therefore considered small. Influences from aircraft emissions are also considered to be small at altitudes below 7 km [e.g., Koike et al., 2000]. Mixing of stratospheric air mass can increase ΔNOy values, however results obtained using “Chinese air masses” (originated from below the 800-hPa level) are considered to have been influenced little.
 Fourth, transport of NOy and SOx in coarse mode particles is examined. As described in section 2, particulate matter with diameters greater than about 1 μm was not sampled either by aerosol composition measurement (PILS) or the NOy measurement. Because of this size cut, the transport efficiencies estimated in this study are limited for gas phase and fine particles. It has been reported, however, that a significant amount of nitrate was contained in coarse mode aerosols sampled over the western Pacific [e.g., Chen et al., 1997; Kim and Park, 2001]. If nitrate in these particles originated from anthropogenic NOx emission, we have underestimated the total transport amount (gas + fine particles + coarse particles). Although coarse particles may not be very important for long-range transport due to their shorter residence time, they can play an important role in budget of reactive nitrogen. Three-dimensional model calculations for the East Asia region for March 1994 period also suggest that in the presence of mineral dust particles and sea-salt aerosols, a significant amount of HNO3 is partitioned onto these particles because carbonate in the dust particles and chloride in sea-salt aerosols are volatile and easily replaced by nitrate [Song and Carmichael, 2001]. During TRACE-P, coarse mode particles were also collected from the DC-8 [Dibb et al., 2003]. Analyses using these aerosol data indicate that air masses with elevated levels of dust particles (high nssCa2+ levels) are generally mixed with pollutants because these air masses often passed over the northeastern part of China [Jordan et al., 2003]. As a result of an efficient uptake of HNO3 by the dust particles, particulate NO3− constituted 50% of the total NO3− (gas phase HNO3 + particulate NO3−) on average in these air masses [Jordan et al., 2003; Dibb et al., 2003]. Because HNO3 constitute 10–20% of NOy (Figure 10a and Table 4), we could have underestimated the total transport efficiency of NOy by 10–20%. Chemical conversion of SO2 to sulfate on mineral dust particles may also take place [Xiao et al., 1997]. Ground based measurements indicate that although most of the sulfate has been found on accumulation mode particles [Chen et al., 1997], significant amounts were found in the coarse mode during heavy dust events [Kim and Park, 2001]. Though no major dust events occurred during TRACE-P, some signatures of uptake of SO2 on dust particles were found from the DC-8 measurements [Jordan et al., 2003]. More studies are needed in order for us to better understand the budget of nitrate and sulfate over East Asia.
 Finally, the approach using a single background value is examined, because it has been used in various studies [e.g., Parrish et al., 1991; Stohl et al., 2002; Miyazaki et al., 2003]. When we use background values of CO and NOy of 129 ppbv and 157 pptv, which are the minimum values at 0–2 km in the study area, a transport efficiency of NOy for air masses used for the present study (ΔCO > 30 ppbv in “continental air masses”) at 0–2 km and 2–7 km is estimated to be 25–35% and 20%, respectively. This is well within the uncertainties in our estimate. Because the variability of the CO was large, the choice of a background CO level was not very critical during TRACE-P. However, if we use CO2 as a reference, an error in the estimate will be large. This is because changes in CO2 due to anthropogenic emissions were generally comparable to those caused by background air mass mixing.