Springtime trans-Pacific transport of Asian pollutants characterized by the Western Pacific (WP) pattern

Atmospheric Environment 147 (2016) 166e177

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Springtime trans-Pacific transport of Asian pollutants characterized by the Western Pacific (WP) pattern Ja-Ho Koo a, Jaemin Kim b, Jhoon Kim a, Hanlim Lee c, Young Min Noh d, Yun Gon Lee b, * a

Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea Department of Atmospheric Sciences, Chungnam National University, Daejeon, South Korea c Department of Spatial Information Engineering, Pukyong National University, Busan, South Korea d International Environmental Research Centre, Gwangju Institute of Science & Technology (GIST), Gwangju, South Korea b

h i g h l i g h t s
Trans-Pacific transport is largely enhanced during the positive phase of WP (WPþ).
Dipole structure becomes intensified over the North Pacific during WPþ pattern.
Cyclonic wave breaking during WP pattern inhibits the springtime aerosol transport.
WP pattern seems useful to diagnose the transport of East Asian aerosols in spring.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2016 Received in revised form 1 October 2016 Accepted 6 October 2016 Available online 6 October 2016

Springtime trans-Pacific transport of Asian air pollutants has been investigated in many ways to figure out its mechanism. Based on the Western Pacific (WP) pattern, one of climate variabilities in the Northern Hemisphere known to be associated with the pattern of atmospheric circulation over the North Pacific Ocean, in this study, we characterize the pattern of springtime trans-Pacific transport using longterm satellite measurements and reanalysis datasets. A positive WP pattern is characterized by intensification of the dipole structure between the northern Aleutian Low and the southern Pacific High over the North Pacific. The TOMS/OMI Aerosol Index (AI) and MOPITT CO show the enhancement of Asian pollutant transport across the Pacific during periods of positive WP pattern, particularly between 40 and 50 N. This enhancement is confirmed by high correlations of WP index with AI and CO between 40 and 50 N. To evaluate the influence of the WP pattern, we examine several cases of trans-Pacific transport reported in previous research. Interestingly, most trans-Pacific transport cases are associated with the positive WP pattern. During the period of negative WP pattern, reinforced cyclonic wave breaking is consistently found over the western North Pacific, which obstructs zonal advection across the North Pacific. However, some cases show the trans-Pacific transport of CO in the period of negative WP pattern, implying that the WP pattern is more influential on the transport of particles mostly emitted near ~40 N. This study reveals that the WP pattern can be utilized to diagnose the strength of air pollutant transport from East Asia to North America. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Trans-Pacific transport Western Pacific Air pollutants Dipole structure

1. Introduction Trans-Pacific transport of Asian pollutants is a representative atmospheric pattern whereby East Asian pollution affects the global atmosphere (Wilkening et al., 2000). This pattern appears yearround but is generally strongest in the springtime (Liang et al.,

* Corresponding author. E-mail address: [email protected] (Y.G. Lee). http://dx.doi.org/10.1016/j.atmosenv.2016.10.007 1352-2310/© 2016 Elsevier Ltd. All rights reserved.

2004; Holzer et al., 2005). Previous studies have examined transPacific transport, focusing on the spatiotemporal pattern of various air pollutants such as Asian desert dust (e.g. Gong et al., 2003), black carbon (e.g., Hadley et al., 2007), carbon monoxide (CO; e.g., Turquety et al., 2008), non-methane hydrocarbons (e.g., Jaffe et al., 2003), ozone (O3; e.g., Wang et al., 2006), nitrogen compounds (e.g., Walker et al., 2010), sulfur compounds (e.g., Van Donkelaar et al., 2008), mercury (e.g., Strode et al., 2008), PAHs and pesticides (e.g., Killin et al., 2004; Genualdi et al., 2009). These

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pollutants traverse the Pacific Ocean and affect western North America in general, but may reach as far as eastern North America (Wu et al., 2015) or move a full circuit around the globe (Uno et al., 2009). Long-range transport of air pollutants has a significant impact on the air quality of North America (Verstraeten et al., 2015), and even reflects the extent of international economic connection between North America and East Asia (Lin et al., 2014). To examine the spatiotemporal characteristics of trans-Pacific transport, a number of intensive field campaigns have been executed. For example, the Aerosol Characterization ExperimentAsia (ACE-Asia) campaign in spring 2001 observed the eastward transport of airborne dust from the Chinese desert and compared to model outputs (e.g., Gong et al., 2003). Also, the Intercontinental Chemical Transport Experiment-B (INTEX-B) in spring 2006 monitored the chemical composition above the North Pacific Ocean based on in-situ and satellite measurements to identify the air masses transported over the Pacific Ocean (e.g., McKendry et al., 2008). Based on the various measurements and model simulations obtained from these intensive field campaigns, the generation, evolution, and enhancement of air masses that undergo transPacific transport have been better understood: for example, the
, general pathway of transport (Heald et al., 2006; Luan and Jaegle 2013), altitude differences according to the transported substance (Nam et al., 2010; Yu et al., 2012) or emission source regions (Eguchi et al., 2009), meteorological patterns related to dust plume generation (Zhao et al., 2006; Lee et al., 2015), and quantitative influence on air quality in North America (Goldstein et al., 2004; Fairlie et al., 2007; Yu et al., 2012). One of the main questions related to trans-Pacific transport is which atmospheric conditions play a role as a main driving force for trans-Pacific transport. Several studies indicated that a north-south dipole structure of air systems seems related to strong transport over the Pacific Ocean (Holzer et al., 2005; Liang et al., 2005; Nam et al., 2010). When this dipole structure, composed of the Aleutian Low (north) and Pacific High (south), intensifies, air mass advection from the western Pacific Ocean tends to be zonally strengthened, and more Asian pollutants reach North America (Rodionov et al., 2007; Song et al., 2008). The warm conveyor belt (WCB) system is considered to be another atmospheric pattern enabling East Asian pollutants to enter the strong westerlies (Cooper et al., 2004; Zhang et al., 2011). Since these large-scale meteorological systems can be associated with climate variability, the relationship of transPacific transport to teleconnections was also examined (Liang et al., 2005; Liu et al., 2005; Gong et al., 2006). Interannual variation of trans-Pacific transport (Reidmiller et al., 2009) may also involve the long-term variation of climate patterns. However, these studies were mostly based on simple correlation analyses, or just mentioned a possibility of a mutual connection. Therefore, deeper analysis and explanation seems required to answer the unresolved question of which climate pattern is the dominant influence. In this study we characterize the springtime trans-Pacific transport of Asian pollutants based on the Western Pacific (WP) pattern. The WP pattern is one of the large-scale teleconnection patterns above the North Pacific Ocean, exemplifying the latitudinal variation of atmospheric patterns (Wallace and Gutzler, 1981). The WP pattern has been considered an important factor in describing the spatial variation of zonal winds over the Pacific Ocean (Linkin re, 2010), meteorology in East Asia (Choi and Nigam, 2008; Rivie and Moon, 2012; Cheung and Zhou, 2016), and the chemical environment of the regional atmosphere (Koo et al., 2014). Our analysis shows that this WP pattern easily depicts the occurrence and enhancement of trans-Pacific transport, and proves this idea using the long-term satellite measurements. We also perform another evaluation by comparing the strong and weak transport cases reported in the previous research based on the phase of the WP

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pattern. This combination of typical long-term features and patterns of various cases will be useful to connect the previous findings consistently and to suggest the possible relationship of typical trans-Pacific transport cases to the large-scale variability. This will be probably interesting information even for investigating the effect of climate change to the pattern of long-range transport pattern. 2. Data description In this study, we use satellite measurements for the detection of Asian pollutants, and reanalysis datasets for the analysis of largescale atmospheric patterns. We mainly focus on the features in April because the emission and production of absorbing aerosols in East Asia (e.g., desert dust and carbonaceous aerosols) is generally high during April and the pattern of atmospheric circulation in April is still distinct from wintertime (Strong and Davis, 2008). In the case studies (Section 3.2), we will also treat some additional cases that occurred in May. To examine the transport pathway of Asian pollution in spring, we use the Aerosol Index (AI) from Total Ozone Mapping Spectrometer (TOMS) and Ozone Monitoring Instrument (OMI) measurements (Torres et al., 1998, 2007). TOMS, onboard the Nimbus-7 (from November 1978 to May 1993) and Earth Probe (EP) (from July 1996 to December 2005) satellites, had provided a continuous observation record for longer than two decades. OMI, onboard the Aura satellite, has continued this record since 2005. AI is estimated based on the wavelength dependence of reflectivity between the two specific ultraviolet (UV) channels (Herman et al., 1997; Torres et al., 2007), showing the extent of absorption and scattering by the particle phase. Therefore, transport analysis of UV-absorbing particles, such as dust particles and some carbonaceous aerosols, has been conducted by using AI (e.g., Song et al., 2008). Since dominant types of springtime aerosol in East Asia are dust and black carbon particles (e.g., Lee et al., 2010), AI can be utilized to investigate the general pattern of aerosol transport from East Asia to North America. In this study, we concurrently used the version 8 TOMS AI and version 3 OMI AI, which together cover about recent three decades, and are comparable to each other. The data produced in the periods of satellite replacement between NIMBUS-7, EP, and OMI are not considered in this study because of potential instability issues. Concretely, the data from 1994 to 1996 are not considered due to the replacement between Nimbus-7 and EP TOMS, and data from 2002 to 2004 are also not considered due to the possible calibration problems in EP TOMS (Kiss et al., 2007) and the replacement of TOMS by the OMI instrument. Additionally, level 3 data set of MODerate resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) is simply used to compare with AI. We also use the Measurements Of Pollution In The Troposphere (MOPITT) carbon monoxide (CO) data to compare with results from AI. MOPITT measurements started in 2000 (Deeter et al., 2003; Bowman, 2006), mostly overlapping with the period of OMI measurement. MOPITT produces both a column and vertical profile of CO. In this study version 6 CO column is utilized. Owing to an instrumental issue on May 2001 (Bowman, 2006), retrieved CO values before and after this date are not quite consistent. Thus we only use the MOPITT CO after 2001. Also we only consider the monthly mean data in this study because daily representative MOPITT CO data set shows a poorer spatial coverage, and is not useful to see the transport pattern of air pollution on a daily scale. The WP pattern is one of typical teleconnections appearing over the North Pacific. As well described in the website of NOAA Climate Prediction Center (CPC) (http://www.cpc.ncep.noaa.gov/data/ teledoc/wp.shtml), this pattern is generally characterized three

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oscillational components: two main centers located in the Kamchatka Peninsula and western North Pacific, and one more small centers in southwestern United States. WP index, indicating the phase of WP pattern, is calculated based on the rotated principal component analysis (RPCA). Detail methodology is described and discussed in Barnston and Livezey (1987) and NOAA CPC website. In this study, we use the monthly and daily WP indices provided from the NOAA CPC and PSD data archive. A monthly WP index is produced by applying the RPCA method to standardized 500 hPa height anomalies and normalizing to the means and standard deviations in the base period, from 1950 to 2000. A daily WP index is also based on the 500 hPa height and estimated by utilizing the mean of ensemble forecasts from the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory (ESRL)/Physical Science Division (PSD) Global Ensemble Forecast System (GEFS) reforecast2 data set (Hamill et al., 2013). We perform the composite analysis of AI and CO for the periods of extremely positive or negative phase of the WP pattern (WPþ and WP-) utilizing April WP index (Fig. 1) since 1979 (Section 3.1). Considering the different periods of TOMS, OMI, and MOPITT measurements, respective 5 years are selected for WPþ and WP- cases during the TOMS measurement period (1979e2001), and 3 years are selected during the OMI (2005e2015) and MOPITT (2002e2015) period. The selected years for each measurement period are summarized in Table S1. For the case studies (Section 3.2), composite analysis is performed based on the daily WP index corresponding to the period of each case. For the analysis of Northern Hemispheric meteorological patterns, we use the National Centers for Environmental Prediction Department of Energy (NCEP-DOE) reanalysis II dataset (Kanamitsu et al., 2002). A number of previous works revealed that trans-Pacific transport tends to occur in the free troposphere (Heald et al., 2006; Yu et al., 2012); therefore, we mainly focus on the 700 hPa level meteorology and examine the mean and anomaly of geopotential height and wind pattern. Each daily anomaly is estimated based on the daily mean climatology from 1979 to 2015.

3. Results and discussions In this section, we first examine how trans-Pacific transport differs according to the phase of the WP pattern, and how it typically relates to the large-scale circulation pattern over the Pacific Ocean. Then we look into several cases, which were reported in previous studies, to evaluate the influence of WP variations and

relevant meteorology patterns on the extent of trans-Pacific transport.

3.1. Trans-Pacific transport related to the WP pattern Gong et al. (2006) showed that there is a positive correlation between the WP index and the amount of simulated Asian dust loading, suggesting the possible connection between the WP pattern and dust transport over the Pacific Ocean. If certain regions experience the transport of air pollutant associated with the WP pattern, we can find correlations between the WP index and AI or CO information over those regions. For each grid point of satellite measurements (1  1 ), then we calculate the correlation coefficients (R) of April WP index with April mean TOMS AI (Fig. 2a), OMI AI (Fig. 2b), and MOPITT CO column (Fig. 2c). Positive correlations generally appear over the north China and North Pacific regions; particularly, a zonal band of high correlations (R > 0.5) can be found between 40 and 50 N latitudes. AI also shows moderately high correlations with AOD similarly (Fig. S2), illustrating that high radiative-absorbing aerosols contribute to the high amount of particles transported over the North Pacific. This feature shows the distinct impact of the WP pattern on springtime transport of air pollutants across the North Pacific Ocean. While AI pattern shows higher correlations, CO shows weaker but broader correlations with the WP index. Although the possible connection to the WP pattern was reported in part (Gong et al., 2006), previous work did not clearly survey why the WP pattern has such an impact on aerosol transport and whether this correlation implies a real significant relationship or is just coincidence. Therefore, here we examine and compare the spatial distribution of AI and meteorology information according to the WP pattern. For the comparison of atmospheric conditions between two different extreme cases, analysis of the contrast between two mean patterns generally can provide the useful information, called a composite analysis. To perform a composite analysis for this study, we first selected the years showing extremely positive or negative phase of WP pattern (WPþ and WP-) in April and compared these subsets against each other. For the selected years of WPþ and WP(Table S1), composite differences of April mean AI between WPþ and WP- (AI for WP þ minus AI for WP-) are shown during the TOMS and OMI periods, and also for MOPITT CO periods (Figs. 3 and S1). TOMS AI, OMI AI, and MOPITT CO consistently show the enhancement between 40 and 50 N latitudes over East Asia, the North Pacific Ocean, and western North America during the

Fig. 1. (a) April mean WP index from 1979 to 2015. Crosses show years with no AI observations or poor quality of AI.

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Fig. 2. Correlation coefficients between WP indices and April mean (a) TOMS AI (1979e2001), (b) OMI AI (2005e2014), and (c) MOPITT CO (2002e2014). If the number of available data is smaller than 50% of the maximum number of available data, the correlation at that grid is not estimated.

WP þ period. Again, it seems that certain atmospheric features associated with the WP þ pattern play a role as a driving force for springtime trans-Pacific transport over the northern Pacific. This composite difference of AI and CO between the WPþ and WPperiod seems clear for the period of TOMS measurements (Fig. 3a), but a little smaller during periods of OMI and MOPIIT measurements (Fig. 3b and c). As shown in Fig. 1, this discrepancy is probably because the WP þ pattern is not much strong in recent years

(i.e., the periods of OMI and MOPITT measurements) compared to past years (i.e., the periods of TOMS measurements), similar to a recent finding that the influence of WP pattern seems varied between 1990s and 2000s (Koo et al., 2014). It has been known that the phase change of WP pattern relates to the atmospheric fluctuation over the North Pacific Ocean. Typically, the Aleutian Low system can be intensified during the WP þ periods (Linkin and Nigam, 2008). And for the strong

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Aleutian Low case, storm track patterns tend to move across the North Pacific, reaching to the western area of North America (Rodionov et al., 2007). This large-scale atmospheric movement seems worth scrutinizing for understanding the influence of WP pattern on trans-Pacific transport in the free troposphere. Thus we looked into the composite differences of 700 hPa wind patterns (wind speed and direction) and geopotential height between the WPþ and WP- cases for TOMS (1979e2001), OMI (2005e2015), and MOPITT (2002e2015) period (Figs. 4 and S3). For all periods,

700 hPa geopotential height is lower near the polar area (around 60 N) but higher over the North Pacific (around 40 N) during WP þ years, illustrating the intensified dipole structure between Aleutian low and Pacific high. Due to the cyclonic circulation over the low pressure system in the north and anti-cyclonic circulation over the high pressure system in the south, westerly advection tends to be much accelerated between this north-south dipole, resulting in fast zonal advection to North America. Thus, 700 hPa zonal wind speed over the Pacific can be largely enhanced around

Fig. 3. Composite difference of April mean (a) TOMS AI, (b) OMI AI, and (c) MOPITT CO between WPþ and WP- years.

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50 N during WP þ years, as confirmed in Figs. 4 and S4. As shown in Figs. 2 and 3, we can also confirm that this wind-enhanced region is where both AI and CO show high enhancements during WP þ years, illustrating the moderately high connection between the transport of pollutants and the atmospheric circulation affected by the WP pattern. Several previous works also supposed that the strength of north-south dipole structure relates to the extent of springtime trans-Pacific transport from East Asia to North America (Liang et al., 2005; Nam et al., 2010). But here we revealed that the strength of dipole structure is actually fluctuating according to the phase of the WP pattern, and identified that the region of 40e50 N latitudes is the dominant pathway of the trans-Pacific transport related to the WP pattern. It means that we can use the WP pattern as useful information to diagnose the strength of trans-Pacific transport of springtime air pollutants from East Asia. In the next section, we will clarify this influence of WP pattern based on the detailed analysis of several known cases. 3.2. Case study To confirm the relationship between the WP pattern and the extent of transport across the Pacific, we again examine several events carefully reported in four different previous studies: Wilkening et al. (2000), Gong et al. (2003), McKendry et al. (2008), and Shen et al. (2014). From Wilkening et al. (2000) and Shen et al. (2014), we choose a single trans-Pacific transport case from each study. From Gong et al. (2003) and McKendry et al. (2008), we find

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two cases from each study: one case showing strong transport, and the other not showing trans-Pacific transport in spite of strong emissions in East Asia. Consequently, we examine a total of six events: four transported and two non-transported events, named such as 1998, 2001A, 2001B, 2006A, 2006B, and 2010 based on the years when these cases were observed. First we compare the daily WP index for these 6 cases (Fig. 5). Each case is composed of four days during the major events and two days before and after the event respectively, totaling an eight-day time series in total. Fig. 5 clearly indicates that high positive WP index can be found consistently for all trans-Pacific transport cases (1998, 2001A, 2006A, and 2010), while negative WP patterns generally appear for the cases not showing trans-Pacific transport (2001B and 2006B), similar to our findings in Section 3.1. We will more look into each case using daily AI and atmospheric circulation patterns to better describe why trans-Pacific transport intensifies only in the WP þ period. 3.2.1. Cases in AprileMay 2001 As previously mentioned, the ACE-Asia campaign was performed to figure out the emission sources and transport patterns of Asian dust during the springtime in 2001. Gong et al. (2003) analyzed two dust storms occurred from Gobi and Taklimakan deserts, resulting in a trans-Pacific transport case of dust generated in 7 April 2001 and a non-transport case in the end of April 2001. The possible reasons behind the non-transport case were explained in their study based on the different patterns of free tropospheric circulation. Similarly, we compare these two cases: the transport

Fig. 4. Composite difference of 700 hPa geopotential heights (GPH700) and wind patterns (WIND700) (left), and 700 hPa zonal wind speed (U700) (right) compared between WPþ and WP- during the TOMS observation period (1979e2001) (top, a and b), the OMI observation period (2005e2015) (middle, c and d), and the MOPITT observation period (2002e2014) (bottom, e and f).

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Fig. 5. 8-Day time series of daily WP index for the selected transported (black) and non-transported (red) cases across the Pacific Ocean in the spring of (a) 1998, (b) 2001, (c) 2006, and (d) 2010. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

case (2001A) from 6 to 13 April and the non-transport case (2001B) from 30 April to 7 May 2001. Fig. 6 shows the spatial distribution of TOMS AI, 700 hPa geopotential height and 700 hPa wind pattern for 2001A and 2001B cases. The contrast between northern low and southern high pattern clearly appears during 2001A (Fig. 6c). But this north-south structure of geopotential height becomes weak during 2001B; geopotential height becomes highly enhanced over the western North Pacific area higher than 40 N. The enhanced geopotential height intrudes into the northern latitudes, inducing a regional trough and ridge system (Fig. 6d). This low trough and high ridge pattern of geopotential height looks like a cyclonic wave-breaking re (2010) indicated that a stronger cyclonic pattern. Actually Rivie wave-breaking pattern could be found over the western North Pacific when the WP index is negative, which is applicable to this 2001B case. Comparison between 2001A and 2001B (Figs. 5b and 6) shows that the dipole structure during the WP þ period facilitates the zonal advection across the Pacific Ocean, but northward development of higher geopotential height during the WP- period destroys the dipole shape and weakens the westerly, resulting in the absence of trans-Pacific transport of Asian dust. Composites of anomaly patterns of 700 hPa geopotential height (Fig. 6e and f) also clarify that the north-south dipole structure over the North Pacific Ocean disappears when higher geopotential height over the western North Pacific intensifies during the WP- period. 3.2.2. Cases in April 2006 We also selected both a transport and a non-transport case in April 2006 when the INTEX-B field campaign was performed. McKendry et al. (2008) examined large Asian dust emissions during the INTEX-B campaign. They found that the dust generated in 16 April 2006 was transported to western Canada, but the dust generated in 24e25 April 2006 did not reach the Canadian west

coast because of inauspicious meteorology. These two cases also have opposite phases of WP (Fig. 5c). Again, we compare two cases: a transport case (2006A) from 15 to 22 April and a non-transport case (2006B) from 24 April to 1 May 2006. The OMI AI distribution illustrates a stronger transport pattern during 2006A than 2006B (Fig. 7a and b), but the strength during 2006A seems weaker than in 2001A. In spite of a WP þ pattern, a small cyclonic wave-breaking pattern, which usually occurs during the WP- period, is detected over the western North Pacific during the 2006A case (Fig. 7c), probably the reason for weaker transport than the 2001A case. Nonetheless, moderate trans-Pacific transport occurs because the perturbation of high geopotential height seems relatively small. However, the area of high geopotential height in the 2006B case is much wider, extending to the northern latitude region above 70 N (Fig. 7d). Since East Asian deserts, the source of dust emissions, are generally located higher than 40 N, this vast cyclonic wave-breaking pattern strongly inhibits the zonal advection of Asian dust. Anomaly patterns of wind and geopotential height (Fig. 7e and f) also reveal that the atmospheric pattern in 2006A is close to the north-south dipole structure, producing a larger westerly than the climatology. However, this is not applicable to the 2006B case due to the strong blocking pattern over the western North Pacific. 3.2.3. Cases in April 1998 and April 2010 Two additional trans-Pacific transport cases are also examined to support conclusions about the influence of the WP pattern. Wilkening et al. (2000) presented the dust transport case generated from the Gobi desert on 19 April 1998. The consecutive movement of this dust event was monitored by the Sea-Viewing Wide Field-ofView Sensor (SeaWiFS) satellite measurements (Husar et al., 2001). Similar to the result in (Wilkening et al., 2000), AI clearly shows the movement of this dust plume across the Pacific region (Fig. 8a). This

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Fig. 6. Transported (8e11 April 2001, left) and non-transported (2e5 May 2001, right) cases of Asian dust during the ACE-Asia campaign in 2001, showing the 4-day composite patterns of AI (a and b), 700 hPa geopotential height and wind pattern (c and d), and their anomalies (e and f).

case from 19 to 26 April 1998 also relates to the positive WP index (Fig. 5a) and clear north-south dipole pattern of geopotential height (Fig. 8c and e). Shen et al. (2014) examined the trans-Pacific transport pattern of black carbon based on the third phase of High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) pole-to-pole observations (HIPPO-3) campaign. Instead of dust particles, they detected a high amount of black carbon over the eastern North Pacific area. In the case from 7 to 14 April 2010, the WP index is also highly positive (Fig. 5d), the spatial pattern of AI shows distinct transport across the Pacific (Fig. 8b), and a dipole structure is clearly located over the North Pacific (Fig. 8d and f). Both 1998, and 2010 cases demonstrate that the atmospheric circulation pattern under the WP þ phase is a favorable condition for trans-Pacific transport of dust and carbonaceous particles generated in East Asia. 3.2.4. Trans-Pacific transport in the WP- pattern The case studies presented so far, particularly the comparison between transport and non-transport cases in AprileMay 2001 and April 2006 (Figs. 6 and 7), clarify how the WP pattern affects transPacific transport of radiative absorbing aerosols. As discussed, the north-south dipole structure over the western North Pacific Ocean generally appears in the WP þ pattern, providing the pathway of

rapid zonal advection across the North Pacific Ocean. In contrast, we can consistently find the occurrence of cyclonic wave breaking over the western Pacific area in the WP- pattern, interrupting the zonally rapid transport of air pollutants. Nevertheless, we found that some reported trans-Pacific transport cases happened in the WP- pattern. In this section, we study these cases and inspect our understanding. Eguchi et al. (2009) explored a case of trans-Pacific dust transport that occurred from 5 to 15 May 2007 using satellite measurements and model simulation. They performed a budget analysis that revealed that a third of Asian dust flux entering the western Pacific reached North America. Despite the existence of trans-Pacific transport, however, our analysis (Fig. S5) shows that WP indices were all negative values. Also the extent of AI seems quite small in this period. We investigate the variation of mean AI in the 40e50 N latitude range (Fig. S6) from west (~120 E) to east Pacific (~150 W), the area of high zonal wind (Fig. 4). Fig. S6 actually illustrates the extent of particle transport in 2007 is similar to the cases 2001B and 2006B, confirmed as non-transport cases in Section 3.2.1 and 3.2.2. The spatial pattern of 700 hPa geopotential height also shows the cyclonic breaking feature in the western North Pacific, also quite similar to the non-transport case in 2006 (2006B). Strictly speaking, this 2007 case hardly looks a strong trans-Pacific transport compared with other transport case

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Fig. 7. Transported (19e22 April 2006, left) and non-transported (27e30 April 2006, right) cases of Asian dust during the INTEX-B campaign in 2006, showing the 4-day composite patterns of AI (a and b), 700 hPa geopotential height and wind pattern (c and d), and their anomalies (e and f).

such as 1998, 2001A, 2006A, and 2010, therefore is not against our analysis results underlining that the cyclonic wave breaking occurs and blocks the trans-Pacific transport during the WP- periods. During the springtime of 2002, there was another aircraft-based field campaign studying pollution transport over the North Pacific Ocean, called the 2002 Intercontinental Transport and Chemical Transformation (ITCT-2K2) campaign. Using the data obtained in this campaign, a case of trans-Pacific transport of CO observed on 5 May 2002 has been discussed in detail. For example, Cooper et al. (2004) accounted for the role of WCB as an important mechanism of intercontinental pollution transport, highlighting the entrainment of air pollutants by the WCB. We are unable to examine how much absorbing aerosol and CO was emitted and transported for this case because of the poor quality of TOMS AI in 2002 and MOPITT CO in a daily scale, but we were able to ascertain the negative WP pattern in this period (Fig. S7a). Again, the 700 hPa geopotential height pattern in this case displays the cyclonic breaking pattern similar to cases 2001B, 2006B, and Eguchi et al. (2009). Weak transport of CO may have been feasible due to the eastward shift of cyclonic wave breaking, but the case during ITCT2K2 does not confirm to the other cases' conditions for strong trans-Pacific transport. Actually Cooper et al. (2004) mentioned that a WCB does not seem to be the main factor accounting for direct and rapid intercontinental transport of Asian pollution to

North America. But anyway they detected the high CO transport near the western region of North America during the year of WPpattern, then we may need think about the difference of transPacific transport pattern between Asian dust and CO. Associated with the comparison between dust and CO transport from East Asia, additional interesting discussion is found in Reidmiller et al. (2009), about the inter-annual variation of longrange transport from East Asia to the western coast of United States. They contrasted opposing transport patterns between two years: CO transport is substantially enhanced during spring 2005, whereas the transport of dust seems strong during spring 2006. They compared the model outputs and reanalysis meteorology between 2005 and 2006, and concluded that the combined effect of year-to-year change of emissions and synoptic patterns caused this opposite transport pattern. Comparing the monthly WP index between spring 2005 and 2006, we can see clear contrast between 2005 and 2006: an all-negative WP pattern in spring 2005, but positive WP index in spring 2006 (Fig. S8). Accompanying higher AI over the North Pacific Ocean and enhanced zonal wind speed are also confirmed in spring 2006 (Fig. S9). Therefore, strong dust transport in spring 2006 (WP þ period) looks reasonable based on the view of our study: the impact of the WP pattern. For spring 2005 showing the WP- pattern, the unusually intense biomass burning in southern Asia seems to have facilitated stronger CO

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Fig. 8. Two more selected transport cases of Asian pollutants, 21e24 April 1998 from Wilkening et al. (2000) (left) and 9e12 April 2010 from Shen et al. (2014) (right) showing the 4-day composite patterns of AI (a and b), 700 hPa geopotential height and wind pattern (c and d), and their anomalies (e and f).

transport (Reidmiller et al., 2009). Probably the WP- pattern can be a better condition for the air mass pathway from South Asia because rapid zonal advection from East Asia to North America is generally located at a lower latitude during a WP- period (Figs. S4 and S10). This indicates that the location of emissions source should be carefully considered for the analysis of the influence from the WP pattern. The effect of the WP pattern in the 40e50 N latitude region (Fig. 1) reveals the connection to the air pollutants emitted from the similar latitude region. Most of East Asian dust particles are emitted from the Gobi and Taklimakan desert located around 40 N, and the large amounts of black carbon and CO are also produced from the polluted area in the middle of China around 40 N (e.g., the latitude of Beijing city is 39.9 N). However, large amount of CO can be also generated from wildfires or biomass burning events over the southern part of Asia located in the tropical region (<30 N). Air pollutants from this southern region can be more affected by the rapid zonal wind located in the lower latitudes, which appears much more readily during WP- periods. As shown in Reidmiller et al. (2009), strong trans-Pacific transport during a WPperiod seems possible if there is an irregularly high emission in the low latitude region. In sum, trans-Pacific transport of air pollutants from East Asia can be found in the negative WP pattern. Typically the strength of

pollutant transport across the Pacific Ocean is quite weak during WP- periods compared to the cases exhibiting the WP þ pattern. Transport cases in the WP- pattern discussed in this section (Cooper et al., 2004; Eguchi et al., 2009; Reidmiller et al., 2009) are also basically weak themselves. Based on our investigation of some examples, however, CO transport seems somewhat affected by the WP- pattern, while enhanced particle transport consistently occurs during WP þ periods. We suppose that the difference feature between CO and particles in the WP- pattern probably relates to the latitudinal difference of emission source area; Asian dust is mostly generated over ~40 N, but CO can be emitted in the lower latitude region (Southeast Asia) partly in addition to the mid-latitudes. Nam et al. (2010) indicated that the pattern of CO enhancement is broader than the AOD enhancement, confirmed in Figs. 2 and 3. This feature also probably relates to the latitude range of emission source. Either entrainment of air pollutants through a WCB or high emissions from a southern source region is also another possible situation that we need to consider more. Namely, consideration of various factors is still required for the comprehensive understanding of trans-Pacific transport cases. But once there is a high particle emissions in the mid-latitude region (i.e., ~40 N) of East Asia, we can find an enhancement of particle transport over the North Pacific during the WP þ period consistently. Therefore, at least we may be able to utilize WP index, which

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is one of widely known teleconnections, as a simple proxy to predict the influence of trans-Pacific transport of particles generated in East Asia. However, our analysis for the counter examples in this section (Cooper et al., 2004; Reidmiller et al., 2009) also indicates that the effect of WP pattern should be considered more carefully for the transport of other trace gases such as CO. Inspecting more details, such as the different contribution of regional pollutants according to the location of emissions sources (latitudinal difference, in particular), will be necessary for better usage of the WP index. Since we do not rule out the possible relationship of transPacific transport to other climate patterns, further study to find potential connections with other large-scale climate variability will also be required. This approach will be helpful to figure out why each teleconnection shows different correlation patterns as reported in previous studies (Liang et al., 2005; Gong et al., 2006).

2008), the connection with WP pattern means that the extent of trans-Pacific transport can also be sensitive to the climate change. This large-scale view will lead to the better discussion for the characteristic of trans-Pacific transport in addition to the view focused on local emission and chemistry. Despite the practical advantage of using the WP index for this purpose, we do not presume that the WP pattern is the only dominant factor. As discussed in this study, we also need to resolve the gap of transport pattern between the Asian particle and CO, revealing that the WP pattern cannot perfectly explain all transport cases occurred over the North Pacific. Considering the variability of transporting pathway (e.g., relationship to the WCB) or the wide range of emissions source region, it seems still necessary to improve the limitation of our idea and to find out other influential mechanisms for better understanding of trans-Pacific transport events.

4. Summary and conclusion Acknowledgements This study has investigated the impact of WP teleconnection patterns on the trans-Pacific transport of Asian air pollutants. We identified a high correlation between satellite-derived aerosol index and CO columns with the WP index, particularly in 40e50 N latitudinal bands, implying the enhancement of trans-Pacific transport when the WP index becomes positive. The analysis of meteorology over the North Pacific showed that the north-south dipole structure intensifies during a WP þ period and becomes a driving force to induce strong zonal advection of air pollutants from East Asia to North America. Enhancement of free tropospheric zonal wind speed in 40e50 N also clarified the impact of WP pattern on the trans-Pacific transport. The influence of WP pattern was evaluated through the examination of known cases reported during previous measurement campaigns. All transport cases consistently related to the well-developed dipole structure over the North Pacific Ocean occurred in the positive phase of the WP pattern. In contrast, we confirmed that the intensified cyclonic wave breaking appears over the western North Pacific when the WP pattern is negative. It seems that this cyclonic breaking pattern weakens the dipole structure and interrupts the transport of air pollutants to North America. Analysis of known cases also indicated the trans-Pacific transport of Asian particles becomes feeble during WP- period. However, some reported cases (cases in spring 2002 and 2005) showed the enhanced trans-Pacific transport of CO during the WP- pattern. Considering that the emission source and transport pathway of CO is also located in the lower latitude, the WP pattern has limited influence on CO transport but larger impact on the transport of Asian particles generated and moved in 40e50 N. Despite this limitation, the WP pattern still seems a simple and useful metric enabling an estimate the extent of springtime trans-Pacific transport. To avoid exaggeration of findings based on only several specific cases or oversight of small but meaningful signals, in this study we tried a comprehensive approach combining the understanding of general feature obtained from the long-term satellite measurements and the comparison of various situations from reported cases in a short-term scale. As a result, we can reach a simple mechanism, WP pattern, which can explain the extent of transPacific transport generally well: enhanced transport during WP þ pattern due to the rapid zonal wind between north-south dipole structure, and weakened transport during WP- pattern due to the impediment by the cyclonic wave breaking. Since WP pattern has a potential connection with other large-scale patterns such as ~ o and Southern Oscillation (ENSO; Yeh and Kirtman, the El Nin 2004), East Asian Winter Monsoon (EAWM; Cheung and Zhou, re, 2010), and 2016), Pacific-North American pattern (PNA; Rivie even other surface meteorological properties (Linkin and Nigam,

This research was supported by the GEMS program of the Ministry of Environment, Korea and the Eco Innovation Program of KEITI (2012000160002), and by the Korea Meteorological Administration Research and Development Program under Grant KMIPA 2015-5170. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.atmosenv.2016.10.007. References Barnston, A.G., Livezey, R.E., 1987. Classification, seasonality, and persistence of lowfrequency atmospheric circulation patterns. Mon. Weather Rev. 115, 1083e1126. Bowman, K.P., 2006. Transport of carbon monoxide from the tropics to the extratropics. J. Geophys. Res. 111, D02107. http://dx.doi.org/10.1029/2005JD006137. Cheung, H.H.N., Zhou, W., 2016. Simple metrics for representing East Asian winter monsoon variability: urals blocking and western Pacific teleconnection patterns. Adv. Atmos. Sci. 33, 695e705. Choi, K.-S., Moon, I.-J., 2012. Influence of the Western Pacific teleconnection pattern on Western North Pacific tropical cyclone activity. Dynam. Atmos. Oceans 57, 1e16. Cooper, O.R., Forster, C., Parrish, D., Trainer, M., Dunlea, E., Ryerson, T., Hübler, G., Fehsenfeld, F., Nicks, D., Holloway, J., de Gouw, J., Warneke, C., Roberts, J.M., Flocke, F., Moody, J., 2004. A case study of transpacific warm conveyor belt transport: influence of merging airstreams on trace gas import to North America. J. Geophys. Res. 109, D23S08. http://dx.doi.org/10.1029/ 2003JD003624. Deeter, M.N., Emmons, L.K., Francis, G.L., Edwards, D.P., Gille, J.C., Warner, J.X., Khattanov, B., Ziskin, D., Lamarque, J.-F., Ho, S.-P., Yudin, V., Attie, J.-L., Packman, D., Chen, J., Mao, D., Drummond, J.R., 2003. Operational carbon monoxide retrieval algorithm and selected results for the MOPITT instrument. J. Geophys. Res. D14, 4399. http://dx.doi.org/10.1029/2002JD003186. Eguchi, K., Uno, I., Yumimoto, K., Takemura, T., Shimizu, A., Sugimoto, N., Liu, Z., 2009. Trans-pacific dust transport: integrated analysis of NASA/CALIPSO and a global aerosol transport model. Atmos. Chem. Phys. 9, 3137e3145. Fairlie, T.D., Jacob, D.J., Park, R.J., 2007. The impact of transpacific transport of mineral dust in the United States. Atmos. Environ. 41, 1251e1266. Genualdi, S.A., Killin, R.K., Woods, J., Wilson, G., Schmedding, D., Simonich, S.L.M., 2009. Trans-Pacific and regional atmospheric transport of polycyclic aromatic hydrocarbons and pesticides in biomass burning emissions to western North America. Environ. Sci. Technol. 43, 1061e1066.
, L., Horowitz, L., Cooper, O., Goldstein, A.H., Millet, D.B., McKay, M., Jaegle Hudman, R., Jacob, D.J., Oltmans, S., Clarke, A., 2004. Impact of Asian emissions on observations at trinidad head, California, during ITCT 2K2. J. Geophys. Res. 109, D23S17. http://dx.doi.org/10.1029/2003JD004406. Gong, S.L., Zhang, X.Y., Zhao, T.L., McKendry, I.G., Jaffe, D.A., Lu, N.M., 2003. Characterization of soil dust aerosol in China and its transport and distribution during 2001 ACE-Asia: 2. Model simulation and validation. J. Geophys. Res. 108, 4262. http://dx.doi.org/10.1029/2002JD002633. Gong, S.L., Zhang, X.Y., Zhao, T.L., Zhang, X.B., Barrie, L.A., Mckendry, I.G., Zhao, C.S., 2006. A simulated climatology of Asian dust aerosol and its trans-Pacific transport. Part II: interannual variability and climate connections. J. Clim. 19, 104e122. Hadley, O.L., Ramanathan, V., Carmichael, G.R., Tang, Y., Corrigan, C.E., Roberts, G.C.,

J.-H. Koo et al. / Atmospheric Environment 147 (2016) 166e177 Mauger, G.S., 2007. Trans-Pacific transport of black carbon and fine aerosols (D < 2.5mm) into North America. J. Geophys. Res. 112, D05309. http://dx.doi.org/ 10.1029/2006JD007632. Hamill, T.M., Bates, G.T., Whitaker, J.S., Murray, D.R., Fiorino, M., Galarneau, T.J., Zhu, Y., Lapenta, W., 2013. NOAA's second-generation global medium-range ensemble reforecast dataset. Bull. Am. Meteor. Soc. 94, 1553e1565. Heald, C.L., Jacob, D.J., Park, R.J., Alexander, B., Fairlie, T.D., Yantosca, R.M., Chu, D.A., 2006. Transpacific transport of Asian anthropogenic aerosols and its impact on surface air quality in the United States. J. Geophys. Res. 111, D14310. http:// dx.doi.org/10.1029/2005JD006847. Herman, J.R., Bhartia, P.K., Torres, O., Hsu, C., Seftor, C., Celarier, E., 1997. Global distribution of UV-absorbing aerosols from Nimbus 7/TOMS data. J. Geophys. Res. 102, 16911e16922. Holzer, M., Hall, T.M., Stull, R.B., 2005. Seasonality and weather-driven variability of transpacific transport. J. Geophys. Res. 110, D23103. http://dx.doi.org/10.1029/ 2005JD006261.
, S., Gill, T., Husar, R.B., Tratt, D.M., Schichtel, B.A., Falke, S.R., Li, F., Jaffe, D., Gasso Laulainen, N.S., Lu, F., Reheis, M.C., Chun, Y., Westphal, D., Holben, B.N., Gueymard, C., McKendry, I., Kuring, N., Feldman, G.C., McClain, C., Frouin, R.J., Merrill, J., DuBois, D., Vignola, F., Murayama, T., Nickovic, S., Wilson, W.E., Sassen, K., Sugimoto, N., Malm, W.C., 2001. Asian dust events on April 1998. J. Geophys. Res. 106, 18317e18330. Jaffe, D., McKendry, I., Anderson, T., Price, H., 2003. Six ‘new’ episodes of transPacific transport of air pollutants. Atmos. Environ. 37, 391e404. Kanamitsu, M., Ebisuzaki, W., Woollen, J., Yang, S.-K., Hnilo, J.J., Fiorino, M., Potter, G.L., 2002. NCEP-DOE AMIP-II reanalysis (R-2). Bull. Am. Meteor. Soc. 83, 1631e1643. http://dx.doi.org/10.1175/BAMS-83-11-1631. Killin, R.K., Simonich, S.L., Jaffe, D.A., DeForest, C.L., Wilson, G.R., 2004. Transpacific and regional atmospheric transport of anthropogenic semivolatile organic compounds to Cheeka Peak Observatory during the spring of 2002. J. Geophys. Res. 209, D23S15. http://dx.doi.org/10.1029/2003JD004386. Kiss, P., J
anosi, I.M., Torres, O., 2007. Early calibration problems detected in TOMS Earth-Probe aerosol signal. Geophys. Res. Lett. 34, L07803. http://dx.doi.org/ 10.1029/2006GL028108. Koo, J.-H., Wang, Y., Jiang, T., Deng, Y., Oltmans, S.J., Solberg, S., 2014. Influence of climate variability on near-surface ozone depletion events in the Arctic spring. Geophys. Res. Lett. 41, 2582e2589. http://dx.doi.org/10.1002/2014GL059275. Lee, J., Kim, J., Song, C.H., Kim, S.B., Chun, Y., Sohn, B.J., Holben, B.N., 2010. Characteristics of aerosol types from AERONET sunphotometer measurements. Atmos. Environ. 44, 3110e3117. http://dx.doi.org/10.1016/ j.atmosenv.2010.05.035. Lee, Y.G., Ho, C.-H., Kim, J.-H., Kim, J., 2015. Quiescence of Asian dust events in South Korea and Japan during 2012 spring: dust outbreaks and transport. Atmos. Environ. 114, 92e101.
, L., Jaffe, D.A., Weiss-Penzias, P., Heckman, A., 2004. Long-range Liang, Q., Jaegle transport of Asian pollution to the northeast Pacific: seasonal variations and transport pathways of carbon monoxide. J. Geophys. Res. 109, D23S07. http:// dx.doi.org/10.1029/2003JD004402. Liang, Q., Jaegle, L., Wallace, J.M., 2005. Meteorological indices for Asian outflow and transpacific transport on daily to interannual timescales. J. Geophys. Res. 110, D18308. http://dx.doi.org/10.1029/2005JD005788. Lin, J., Pan, D., Davis, S.J., Zhang, Q., He, K., Wang, C., Streets, D.G., Wuebbles, D.J., Guan, D., 2014. China's international trade and air pollution in the United States. P. Natl. Acad. Sci. U. S. A. 111, 1736e1741. http://dx.doi.org/10.1073/ pnas.1312860111. Linkin, M.E., Nigam, S., 2008. The North Pacific oscillation e West Pacific teleconnection pattern: mature-phase structure and winter impacts. J. Clim. 21, 1979e1997. Liu, J., Mauzerall, D.L., Horowitz, L.W., 2005. Analysis of seasonal and interannual variability in transpacific transport. J. Geophys. Res. 110, D04302. http:// dx.doi.org/10.1029/2004JD005207.
, L., 2013. Composite study of aerosol export events from East Asia Luan, Y., Jaegle and North America. Atmos. Chem. Phys. 13, 1221e1242. McKendry, I.G., Macdonald, A.M., Leaitch, W.R., van Donkelaar, A., Zhang, Q., Duck, T., Martin, R.V., 2008. Trans-Pacific dust events observed at whistler, British Columbia during INTEX-B. Atmos. Chem. Phys. 8, 6297e6307. Nam, J., Wang, Y., Luo, C., Chu, D.A., 2010. Trans-Pacific transport of Asian dust and CO: accumulation of biomass burning CO in the subtropics and dipole structure of transport. Atmos. Chem. Phys. 10, 3297e3308. Reidmiller, D.R., Jaffe, D.A., Chand, D., Strode, S., Swartzendruber, P., Wolfe, G.M.,

177

Thornton, J.A., 2009. Interannual variability of long-range transport as seen at the Mt. Bachelor observatory. Atmos. Chem. Phys. 9, 557e572. re, G., 2010. Role of Rossby wave breaking in the west Pacific teleconnection. Rivie Geophys. Res. Lett. 37, L11802. http://dx.doi.org/10.1029/2010GL043309. Rodionov, S.N., Bond, N.A., Overland, J.E., 2007. The Aleutian Low, storm tracks, and winter climate variability in the Bering Sea. Deep-Sea Res. 54, 2560e2577 pt II. Shen, Z., Liu, J., Horowitz, L.W., Henze, D.K., Fan, S., Levy II, H., Mauzerall, D.L., Lin, J.T., Tao, S., 2014. Analysis of transpacific transport of black carbon during HIPPO3: implications for black carbon aging. Atmos. Chem. Phys. 14, 6315e6327. Song, S.-K., Kim, Y.-K., Shon, Z.-H., Lee, H.W., 2008. Influence of meteorological conditions on trans-Pacific transport of Asian dust during spring season. J. Aerosol Sci. 39, 1003e1017.
, L., Jaffe, D.A., Swartzendruber, P.C., Selin, N.E., Holmes, C., Strode, S.A., Jaegle Yantosca, R.M., 2008. Trans-Pacific transport of mercury. J. Geophys. Res. 113, D15305. http://dx.doi.org/10.1029/2007JD009428. Strong, C., Davis, R.E., 2008. Variability in the position and strength of winter jet stream cores related to northern hemisphere teleconnections. J. Clim. 21, 584e592. Torres, O., Bhartia, P.K., Herman, J.R., Ahmad, Z., Gleason, J., 1998. Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation: theoretical basis. J. Geophys. Res. 103, 17009e17110. http:// dx.doi.org/10.1029/98JD00900. Torres, O., Tanskanen, A., Veihelmaann, B., Ahn, C., Braak, R., Bhartia, P.K., Veefkind, P., Levelt, P., 2007. Aerosols and surface UV products from Ozone Monitoring Instrument observations: an overview. J. Geophys. Res. 112, D24S47. http://dx.doi.org/10.1029/2007JD008809. Turquety, S., Clerbaux, C., Law, K., Coheur, P.-F., Cozic, A., Szopa, S., Hauglustaine, D.A., Hadji-Lazaro, J., Gloudemans, A.M.S., Schrijver, H., Boone, C.D., Bernath, P.F., Edwards, D.P., 2008. CO emission and export from Asia: an analysis combining complementary satellite measurements (MOPITT, SCIAMACHY, and ACE-FTS) with global modeling. Atmos. Chem. Phys. 8, 5187e5204. Uno, I., Eguchi, K., Yumimoto, K., Takemura, T., Shimizu, A., Uematsu, M., Liu, Z., Wang, Z., Hara, Y., Sugimoto, N., 2009. Asian dust transported one full circuit around the globe. Nat. Geosci. 2, 557e560. http://dx.doi.org/10.1038/NGEO583. Van Donkelaar, A., Martin, R.V., Leaitch, W.R., Macdonald, A.M., Walker, T.W., Streets, D.G., Zhang, Q., Dunlea, E.J., Jimenex, J.L., Dibb, J.E., Huey, L.G., Weber, R., Andreae, M.O., 2008. Analysis of aircraft and satellite measurements from the Intercontinental Chemical Transport Experiment (INTEX-B) to quantify longrange transport of East Asian sulfur to Canada. Atmos. Chem. Phys. 8, 2999e3014. Verstraeten, W.W., Neu, J.L., Williams, J.E., Bowman, K.W., Worden, J.R., Boersma, K.F., 2015. Rapid increases in tropospheric ozone production and export from China. Nat. Geosci. 8, 690e695. http://dx.doi.org/10.1038/ NGEO2493. Walker, T.W., Martin, R.V., van Donkelaar, A., Leaitch, W.R., MacDonald, A.M., Anlauf, K.G., Cohem, R.C., Bertram, T.H., Huey, L.G., Avery, M.A., Weinheimer, A.J., Flocke, F.M., Tarasick, D.W., Thompson, A.M., Streets, D.G., Liu, X., 2010. TransPacific transport of reactive nitrogen and ozone to Canada during spring. Atmos. Chem. Phys. 10, 8353e8372. Wallace, J.M., Gutzler, D.S., 1981. Teleconnections in the geopotential height field during the Northern Hemisphere winter. Mon. Weather Rev. 109, 784e812. Wang, Y., Choi, Y., Zeng, T., Ridley, B., Blake, N., Blake, D., Flocke, F., 2006. Late-spring increase of trans-Pacific pollution transport in the upper troposphere. Geophys. Res. Lett. 33, L01811. http://dx.doi.org/10.1029/2005GL024975. Wilkening, K.E., Barrie, L.A., Engle, M., 2000. Trans-Pacific air pollution. Science 290, 65e67. Wu, Y., Han, Z., Nazmi, C., Gross, B., Moshary, F., 2015. A trans-Pacific Asian dust episode and its impacts to air quality in the east coast of U.S. Atmos. Environ. 106, 358e368. Yeh, S.-W., Kirtman, B.P., 2004. The north Pacific oscillation e ENSO and internal atmospheric variability. Geophys. Res. Lett. 31, L13206. http://dx.doi.org/ 10.1029/2004GL019983. Yu, H., Remer, L.A., Chin, M., Bian, H., Tan, Q., Yuan, T., Zhang, Y., 2012. Aerosols from overseas rival domestic emissions over North America. Science 337, 566e569. Zhang, Y., Tao, S., Ma, J., Simonich, S., 2011. Transpacific transport of benzo[a]pyrene emitted from Asia. Atmos. Chem. Phys. 11, 11993e12006. Zhao, T.L., Gong, S.L., Zhang, X.Y., Blanchet, J.-P., McKendry, I.G., Zhou, Z.J., 2006. A simulated climatology of Asian dust aerosol and its trans-Pacific transport. Part I: mean climate and validation. J. Clim. 19, 88e103.