Influences of the Long-Range Transport of Biomass-Burning Pollutants on Surface Air Quality during 7-SEAS Field Campaigns

The transport of aerosols relies primarily upon air flow for conveyance; however, the air flow pattern is dominated by large-scale circulation conditions. One mission of the 2013 7-SEAS/BASELInE (Seven SouthEast Asian Studies/Biomassburning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment) was to capture/confirm the downwind effect on the surface air quality due to the long-range transport of Southeast Asia biomass-burning (SEA BB) pollutants. This phenomenon was first discovered during the 2010 Dongsha experiment and directly observed by a lidar system at Hengchun in southern Taiwan during 7-SEAS/BASELInE. Through three-dimensional structural analysis, it was found that the sinking motion behind the upper-level active short wave trough is the major mechanism that enhances subsidence along the cold surge leading edge. In turn, the enhanced subsidence could bring the long-range transport of the SEA BB pollutants down to the surface. Furthermore, the HYSPLIT backward air trajectories helped identify the SEA BB pollutants in the mid-troposphere, while the fine-resolution WRF model simulation combined with dual-polarization lidar observations demonstrated the evolution of the brought-down aerosols process. An additional significant finding of this study is that the upper-level ridge-trough short wave within 20°–35°N was very active during spring 2013, highlighting the inter-annual variability of the long-range transport of SEA BB pollutants.


INTRODUCTION
The 2010 Dongsha pilot experiment (Lin et al., 2013) in the northern region of the Seven SouthEast Asian Studies (7-SEAS; Reid et al., 2013) project was the first field campaign conducted to better understand the marine boundary layer and lower free troposphere pollution environment due to the northern Southeast Asian (SEA) biomass-burning (BB).Lin et al. (2009) proposed that a mountain leeside trough over Indochina was the mechanism for uplifting near-surface tracers from biomass-burning source regions into the westerlies 3 km above for further downstream transport.The downwind effect of the long-range transport of tracers was confirmed by observed peak ozone concentrations around 4 km over northern Taiwan from ozone sondes, but surface air quality impacts were not considered.Several recent observational and modeling studies (e.g., Fu et al., 2012;Cheng et al., 2013;Chi et al., 2016;Chuang et al., 2016) have demonstrated that the long-range transport of biomass-burning pollutants from SEA were detected at Mt. Lulin in central Taiwan and subsided along the west side of the Central Mountain Range to the surface.
The transport of SEA BB pollutants from high altitudes to the surface at Hengchun on the southern tip of Taiwan was unexpectedly discovered during the Dongsha experiment (Lin et al., 2013;Yen et al., 2013).Yen et al. (2013) hypothesized that southwesterly flow confluence within the boundary layer coupled with a well-organized convergent center over a thermal low might induce a distinct ascending motion to uplift SEA BB pollutants to the lower troposphere for subsequent transport downwind.The authors further suggested that subsidence along the cold surge leading edge could bring the SEA BB pollutants down to the surface and affect the surface air quality in southern Taiwan.Cheng et al. (2013) further demonstrated using numerical simulations that a thermal low, together with high terrain in the northern SEA, was the uplifting mechanism.Their simulation results also confirmed the long-range transport of SEA BB pollutants reaching Mt.Lulin, yet did not address the deposition processes to the surface.
In the follow-on 2013 7-SEAS/BASELInE (Biomassburning Aerosols & Stratocumulus Environment: Lifecycles and Interactions Experiment) field campaign (Tsay et al., 2016), a substantial effort was made to combine various in-situ and remote sensing instruments, including lidar, for deployment to the Hengchun supersite in Southern Taiwan to directly observe the brought-down BB phenomenon.Additional supersites for 7-SEAS/BASELInE were located near the BB source region in Doi Angkhang, Thailand, and along the SEA BB transport pathway in Son La and Yen Bai in Vietnam (Tsay et al., 2016).Observational data from this campaign has been used in recent studies focused on aerosols and chemistry near BB source regions (Lee et al., 2016;Nguyen et al., 2016;Popovicheva et al., 2016), long-range transport issues (Chi et al., 2016;Chen et al., 2016;Chuang et al., 2016) and even cloud condensation nuclei comparisons between BB source regions over Thailand and downwind receptor areas in Taiwan (Hsiao et al., 2016).In this paper, we report on the possible unique mechanism to bring the long-range transport of SEA BB pollutants down to the surface using data collected during 7-SEAS/BASELInE.Chuang et al. (2016) pointed out several aerosol sources could affect the surface air quality in Taiwan under different flow scenarios, although our current and previous efforts focus specifically on situations under the influence of cold surge events, leaving other possible mechanisms for future research endeavours.

METHODS
Surface weather charts from the Central Weather Bureau (CWB) of Taiwan provided the synoptic environment overview for the Asian region in this study.Meteorological and air pollutants measurement data collected at the Hengchun supersite were used to illustrate the time evolution for the observed brought-down SEA BB pollutants case.The European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis interim 40 (ERA-interim, resolution of 1° longitude × 1° latitude) data (Simmons et al., 2006;Dee et al., 2011) were analyzed to evaluate both the horizontal and vertical dynamic circulations for the observed case.The lidar system (Table 1) was adopted to observe the vertical profiles of aerosol particles during the field experiment.This compact lidar employs a Nd:YAG laser at wavelength 532 nm (Lotistii LS-2134U) to detect Rayleigh/Mie backscattering from atmospheric molecules, aerosol, and cloud particles from near surface to 8 km in the daytime and 16 km in the nighttime.Lidar backscatter profiles are calculated using Klett's method (Klett, 1985) with a fixed lidar ratio of 70 sr, where aerosols are assumed to be polluted continental aerosol (Omar et al., 2009).The backscattering ratio BR is defined as the ratio of total atmospheric backscatter coefficient (including air molecules, aerosol, and cloud) to backscatter coefficient of air molecules.Backward air trajectories generated from National Oceanic and Atmospheric Administration (NOAA) Hybrid Single-Particle Lagrangian-Integrated Trajectory (HYSPLIT) model (Draxler and Rolph, 2003) were employed to help reveal the aerosol source origins.
In order to demonstrate the detailed local time-height evolution of air motions for this case study, a WRF model simulation with 2 km × 2 km resolution was also performed.In this study, the WRF (Ver 3.6.1)model with Yonsei University scheme (YSU) planetary boundary layer scheme was adopted.The cloud microphysics used in this simulation was the single-Moment 6-Class Microphysics scheme.The Rapid Radiative Transfer Model (RRTMG) was used for both long-wave and short-wave radiation schemes.The initial and boundary conditions for WRF were obtained using data sets of the Global Forecast System from the National Center for Environmental Prediction (NCEP-GFS) 0.5° × 0.5° analysis data at six-hour intervals.Two nested domains were constructed with spatial grid resolutions of 6 km and 2 km, which contained 91 × 91, and 181 × 226 grid points in the north-south and east-west directions, respectively.Both domains had 45 vertical levels, and the model top was set at 10 hPa.To ensure that the meteorological fields were well simulated, the four-dimensional data assimilation (FDDA) scheme was activated in coarse domain using the NCEP-GFS analysis data.

RESULTS AND DISCUSSION
During the 2010 7-SEAS/Dongsha Experiment, it was proposed that SEA BB pollutants aloft were brought down  (Yen et al., 2013).This hypothesis regarding the mechanism by which the long-range transport of SEA BB pollutants significantly affects the surface air quality over the downwind areas was further qualitatively confirmed by using the HYSPLIT particle dispersion model.Fortunately, a brought-down BB pollutants case was observed during 2013 7-SEAS/BASELInE, enabling us to revisit the process mechanism in greater detail.

Synoptic Conditions
At 14LST (06Z) on 24 March 2013, a surface cold front was located over southern Taiwan along the leading edge of a mild East Asian cold surge anticyclone (Fig. 1(a)).The cold front shifted eastward with time as the anticyclone and accompanying cold surge progressed southeastward (Figs.1(b)-1(d)).The leading edge of the cold surge anticyclone reached the Hengchun supersite by 22 LST on 24 March as evident by decreasing temperatures and wind speeds, along with increasing relative humidity values displayed in the observed meteorological data around this time (Figs.2(a) and 2(b)).During this period, the concentrations of PM 10 , PM 2.5 , EC and OC (Figs. 2(c) and 2(d)) also increased in conjunction with the passage of cold surge leading edge over Hengchun through roughly 05 LST on 25 March, followed by an increase in wind speed.The strong prevailing northeasterly wind persisted for about one day until the cold surge began to exit the region and weaken in strength.

Large-Scale Environment
To better understand the role played by the aforementioned cold surge, we examined the horizontal and vertical evolution of the large-scale circulation system over East Asia during this period.Streamlines depict the atmospheric circulation at 925 hPa (Fig. 3(a)) and exhibit a cold surge couplet (Lau and Lau, 1984) with a continental surface High over East Asia along the eastern seaboard and a surface low centered over the northern Japan Sea at 14LST on 24 March 2013.Taiwan resided at the confluent area where the surface cold front was located (Fig. 1(a)).Interestingly, the vertically northward tilting structure for  the classic middle latitude frontal system was apparent in the local North-South circulation depicted by the latitudeheight cross section of (v D , -ω) at 120°E (Fig. 3(i): where v D is meridional divergent wind and -ω is vertical velocity).Forced ascent at mid and upper levels ahead of the short wave trough, combined with low-level convergent flow in the vicinity of the surface cold front (Fig. 3(e)) led to a broad region of upward motion between 25°N and 30°N (Fig. 3(i)).By 20 LST, cold northwestwerly flow spread across Taiwan (Fig. 3(b)) as the cold front continued eastward (Fig. 1(b)).Around this time, subsidence above the cold surge leading edge intensified and shifted southward from 40°N to 30°N (Fig. 3(j)).Furthermore, a divergent center (Fig. 3(f)) induced by subsidence of cold, heavy air within the cold surge anticyclone emerged around 40°N coincident with the surface High over East Asia (Fig. 3(b)).The anticyclone-cyclone cold surge couplet maintained southeastward motion with north-northeasterly cold surge flow and increasing surface divergence reaching Taiwan around 02LST on 25 March (Figs.3(c) and 3(g)).In addition to the cold surge's southeastward migration, the leading edge subsidence widely expanded its north-south coverage from 20°N to 30°N, coincident with the path of the longrange transport of SEA BB pollutants.This downward motion provided a pathway by which the SEA BB pollutants could potentially be brought down to the surface.As time progressed, the southeastward moving cold surge anticyclone strengthened in intensity with strong subsidence evident between 30-40°N, while its leading edge gradually weakened and disappeared by 08LST on 25 March (Figs. 3(d), 3(h), and 3(l)).
The upper level horizontal and local East-West vertical circulations were also constructed to further examine the three-dimensional features for this cold surge (Fig. 4).The  local East-West circulation was depicted by the longitudeheight cross section of (u D , -ω) at 22°N (Fig. 4(i): where u D is longitudinal divergent wind) where the Hengchun supersite is located.At 14 LST on 24 March, a ridgetrough short wave feature was located between 95°-110°E and within 20°-35°N at 500 hPa (Fig. 4(a)).Increasing vorticity between the ridge and trough zone induced upper air convergence and sinking motions through atmosphere between 95°-110°E (Fig. 4(i)).This subsidence aloft likely limited the uplift motion beneath and forced the air to flow eastward in the lower troposphere.Weak rising motion was evident east of 110°E, downwind of the short wave trough axis (Fig. 4(i)).By 20LST on 24 March, the short wave trough at 500 hPa had migrated toward 116°E (Fig. 4(b)) with rising motion east of this location and sinking motion to the west that gradually extended to the boundary layer (Fig. 4(j)).Through 02LST on 25 March when the short wave trough migrated toward 122°E (Figs.4(c) and 4(g)), the sinking motion behind the trough apparently coupled with the intensified subsidence along the cold surge leading edge (Fig. 3(k)) and evidently led to significant downward vertical motion within a narrow corridor extending from upper troposphere to low levels near 120°E (Fig. 4(k)).We surmise that the long-range transport of SEA BB pollutants were eventually brought down to the surface due to this downward motion.By 08LST on 25 March, the short wave trough had migrated further eastward toward 130°E (Figs. 4(d) and 4(h)), along with the region of sinking motion, such that the transport of SEA BB pollutants down to the surface at Hengchun would have likely been greatly diminished.

Lidar Observations
Many studies (e.g., Chen et al., 2007;Burton et al., 2015;He and Yi, 2015) have successfully demonstrated that dual-depolarization lidar can detect non-spherical particles such as dust aerosols and smoke particles by examining the linear particle depolarization ratio (δ), defined as the ratio of perpendicular to parallel backscatter intensity.Furthermore, the depolarization ratio for biomass-burning is about 6-11% (Wandinger et al., 2002;Murayama et al., 2004).By adopting these conventions, vertical profiles of lidar backscattering ratio (BR) and δ observed at the Hengchun supersite are displayed in Fig. 5.A distinct two layer structure appeared in the BR around 00LST on 25 March, 2013 (Fig. 5(a)), with high near-surface BR values within the marine atmospheric boundary layer (MABL), and a region of smaller BR values also visible in the mid-troposphere (~1.5-4 km).These features are similar to those observed at Dongsha (Wang et al., 2013) given that these two islands are surrounded by the oceans and located along the pathway of the long-range transport of SEA BB pollutants.Source  regions for aerosol arriving at Hengchun were identified using HYSPLIT model backward trajectories (Fig. 6), with near-surface aerosol coming from the western Pacific within the MABL, while the mid-troposphere aerosol originated from the Indochina Peninsula and followed the long-range transport pathway.A time-height plot of horizontal flow and vertical velocity above the Hengchun site generated from the WRF model simulation (Fig. 7) showed in greater detail the development and downward progression of subsidence behind the passing short wave trough beyond roughly 14 LST on 24 March, 2013.Note that Hengchun, located at the southwest end of the passing cold front (Fig. 1), experienced both warm sector southwesterly return flow from subtropical high and cold sector northwesterly from East Asian continental high (Figs. 3 and 7).Sinking motion aloft before/around 00LST on 25 March likely restricted the MABL aerosol deduced from the lidar BR (Fig. 5

CONCLUSIONS
During the 2010 7-SEAS Dongsha pilot experiment, it was discovered that descending motion induced by the cold surge subsidence likely brought mid-tropospheric SEA BB pollutants down to the surface at Hengchun in southern Taiwan.Based upon this surprising finding, one of the missions of the 7-SEAS/BASELInE's full-scale field campaign in 2013 was to capture this downwind effect on the surface air quality due to the long-range transport of  SEA BB pollutants.Fortunately, the 7-SEAS/BASELInE campaign was well designed and enabled us to directly observe and confirm this phenomenon at Hengchun through lidar observations.Through three-dimensional structural analysis, it was also found that the sinking motion behind the upper-level active short wave trough (Figs. 4(c), 4(g), and 4(k)) seemed to be the major mechanism that enhanced the subsidence along the cold surge leading edge (Fig. 3(k)).We surmise that this enhanced subsidence brought the long-range transport of Southeast Asian biomass burning pollutants down to the surface.HYSPLIT backward air trajectories helped identify the SEA BB pollutants in the mid-troposphere, while the fine-resolution WRF model simulation, combined with evidence from the particle depolarization lidar observations, demonstrated the time evolution of the brought-down aerosols process.
Comparisons between the springtime synoptic flow patterns of this case and those of several others from the 2010 7-SEAS Dongsha experiment seem to reveal that more active upper-level short wave activities between 95°-110°E and 20°-35°N were present in 2013 than 2010.Composites from selected 2010 cases show a cold surge couplet with a continental surface high over East Asia and a surface low centered over southeast Japan (Fig. 8 Ongoing work shows that the spring of 2010 was abnormal while that of 2013 was close to normal when compared to the 35-year (1979-2013) springtime average of large-scale circulations.It remains to be determined whether the active short wave is common during the normal year or not, yet climate variability could obviously play an important role in the long-range transport of SEA BB over Southeast Asia during springtime.Therefore, an investigation of inter-annual variation of long-range transport of SEA BB is being pursued and will be reported on in the near future.

Fig. 2 .
Fig. 2. Time series from 00LST on 24 March through 00LST on 26 March, 2013 for (a) Temperature and Relative Humidity, (b) Wind Direction and Wind Speed, (c) PM 10 and PM 2.5 , and (d) EC and OC.
(a)) to below 1km.Meanwhile, the downward motion began to gradually transport the mid-tropospheric SEA BB aerosols downward as evident by a coincident downward progression of positive lidar δ values (Fig. 5(b)).By 02LST on March 25, low-level subsidence associated with the southeastward advancing cold surge (Fig. 7) largely suppressed the MABL aerosol feature (Fig. 5(a)), whereas the extension of the positive δ values towards the surface strongly suggests mid-tropospheric aerosols continued to be brought down to near the surface (Fig. 5(b)).

Fig. 5 .
Fig. 5. Time series of lidar profiles of (a) backscattering ratio and (b) linear particle depolarization ratio at Hengchun supersite from 00LST on 24 March to 00LST on 26 March, 2013.White regions in panel (a) likely denote signal attenuation due to clouds.

Table 1 .
Specification of the dual-polarization lidar system deployed at the Hengchun supersite.