Taehwa Research Forest: A receptor site for severe pollution events in Korea during 2016

During the May-June 2016 International Cooperative Air Quality Field Study in Korea (KORUS-AQ), light synoptic meteorological forcing facilitated Seoul metropolitan pollution outflow to reach the remote Taehwa Research Forest (TRF) site and cause regulatory exceedances on 24 days. Two of these severe pollution events are thoroughly examined. The first, 25 occurring on 17 May 2016, tracks transboundary pollution transport exiting eastern China and the Yellow Sea, traversing the Seoul Metropolitan Area (SMA), and then reaching TRF in the afternoon hours with severely polluted conditions. This case study indicates that although outflow from China and the Yellow Sea were elevated with respect to chemically unperturbed conditions, the regulatory exceedance at TRF was directly linked in time, space, and altitude to urban Seoul emissions. The second case studied, occurring on 09 June 2016, reveals that increased levels of biogenic emissions, in combination with 30 amplified urban emissions, were associated with severe levels of pollutions and a regulatory exceedance at TRF. The case studies are assessed with multiple aircraft, model (photochemical and meteorological) simulations, in-situ chemical sampling, and extensive ground-based profiling at TRF. These observations clearly identify TRF and the surrounding rural communities as receptor sites for severe pollution events associated with Seoul outflow, which will result in long-term negative effects to both human health and agriculture in the affected areas. 35 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c © Author(s) 2019. CC BY 4.0 License.

to 11 June 2016. NOx (which is predominantly NO2 during daytime) can rapidly form O3 in the presence of VOCs and favorable meteorology. The two largest sources within the South Korea NOx emissions inventory are mobile/vehicular emissions (41.7%) and road transport and "other" mobile sources (20.0%) [Lee et al., 2011].
The prevalence of vehicular emissions is apparent in the urban environment ( Figure 1c) with NOx amounts at OLY frequently an order of magnitude greater than those at rural TRF. Herman et al., 2018 has further shown during the  AQ study that the difference between TRF (also referred to as Taehwa Mountain) and OLY in columnar NO2 can be as much as 3.0 DU (Dobson Units). The concentrations of surface O3 at TRF during stagnant flow regimes far exceeded those at OLY ( Figure 1d) on several days during the study, indicating O3 formation within the SMA is VOC limited, due to the overabundance of NOx. However O3 formation can rapidly increase as the distance from NOx sources increases. Recent chemical simulations performed during the KORUS-AQ period by Miyazaki et al., 2018 also reveal that observed boundary 10 layer O3 can be as much as 30 ppbv different with dynamic and stagnant flow regimes. Supporting work at TRF has also shown that fast oxidation rates (Kim et al., 2018) and overall oxidation capacity (Jeong et al., 2018) at TRF can exacerbate severe pollution events.
The differences in daily maximum hourly O3 between the TRF and OLY sites vary from day to day, but the sites had mean and 1-hr maximum concentrations of 86.1 ± 21.9 ppbv and 80.4 ± 17.5 ppbv, respectively. The South Korean national 15 standard for O3 is 100 ppbv (parts-per-billion by volume) for a 1-hr average and 60 ppbv for an 8-hr average (http://eng.me.go.kr/eng/web/index.do?menuId=253). Both sites exceeded the 8-hr standard 24 times between 10 May and 10 June 2016. However, TRF exceeded the national 1-hr standard 11 times, whereas OLY exceeded on only 3 days. These daily and diurnally varying O3 amounts during the KORUS-AQ study at TRF are described with back-trajectories (2.1), synoptic meteorology (2.2) and balloon-borne profiles from TRF (3.1.1). Since the evolution of plume composition is a critical 20 component in understanding severe O3 exceedances, two contrasting case studies (17 May (3.2) and 09 June (3.3)) of the 11 TRF 1-hr exceedances are examined with aircraft and ground-based measurements including O3 lidar and ceilometer backscatter profiles. These are complemented by a photochemical box model used to calculate net O3 production and are used distinguish urban/industrial emissions from one of mixed urban and biogenic origins (4).

Back-trajectories
To understand variations in the air mass history as it is advected towards TRF, 4D (time, height, latitude, longitude) back-trajectories were simulated for every day during the KORUS-AQ study ( Figure 2). The back-trajectory calculations were performed using the Lagrangian FLEXible PARTicle dispersion model (FLEXPART, http://flexpart.eu;Brioude et al., 2013), 30 driven by the WRF (Weather Research and Forecasting) model meteorology at 3 km spatial resolution. For this simulation, thousands of "air parcels" were released at 15:00 KST (Korean Standard Time; UTC -9 hrs), and their spatial and vertical Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c Author(s) 2019. CC BY 4.0 License. (Figure 2b) trajectory locations were followed back in time for 6 hrs. Chaotic processes like turbulence or convection were applied in a stochastic manner to each parcel. At hourly intervals the concentration of parcels in each cell of a regular grid was calculated, thus providing the best statistical estimate of the air mass location and altitude prior to reaching TRF.
Red trajectory paths in Figure 2 are used to identify air masses that were associated with the 11 O3 exceedance days at TRF (based on the South Korean National Standard of 1-hr standard of 100 ppbv in Figure 1d). Two of these days, 17 May 5 and 09 June (detailed in section 4), are shown in yellow and orange, respectively. Conversely, days where the daily 1-hr O3 at TRF did not exceed 100 ppbv are shown in blue. Days that exceeded the 1-hr standard had generally shorter trajectory paths, indicating they were associated with more stagnant conditions and weaker synoptic forcing. The altitudes of exceedance day air masses (Figure 2b) were also much closer to the surface (thus closer to ground level anthropogenic and biogenic emissions) as compared to non-exceedance days. Exceedance days at TRF were also associated with air masses that flowed through or 10 near the southern portion of the SMA, where they were more likely to interact with local anthropogenic and industrial emission sources [Iqbal et al., 2014].

Geopotential Height Anomalies
The light synoptic forcing, winds were generally light (e.g. 1-2 ms -1 observed at TRF) and westerly in the morning and afternoon, changing to calm winds at TRF after 14:00 KST. Based on the back-trajectory (Figure 2), the light westerly winds in the morning transported pollutants from the western portion of the SMA to the eastern portion. As the flow became more quiescent 20 in the afternoon, local emissions were pooled in the south-eastern portion of the SMA and then continued directly into the rural forested area near TRF. This synoptic system also ushered in warm (24° C and 27° C at 15:00 KST at TRF and OLY, respectively) and cloud-free conditions throughout the day. With this synoptic meteorology, unimpeded solar radiation, and ample O3 precursor emissions from SMA, TRF reached a maximum hourly O3 value of nearly 120 ppbv ( Figure 1d) at 17:00 KST, well above the 100 ppbv South Korean national standard. 25 In contrast, on 09 June a weak 500 hPa ridge existed over South Korea, the Yellow Sea, and eastern China with a +50 m height anomaly. Under this multi-day weak synoptic forcing, the back trajectory indicates recirculation of the air mass throughout the Korean peninsula, rather than extended zonal transport. The air mass appears to have tracked near Seoul on the previous day, followed by recirculation back to the densely forested region south and west of TRF. Light and northwesterly surface winds occurred in the morning and afternoon (e.g. 2-3 ms -1 at TRF), decreasing and becoming more southerly in the 30 late afternoon (e.g. less than 0.5-1 ms -1 at TRF after 12:00 KST). Throughout the day, the light north-westerly transported pollutants from the northwest to the southeast within the SMA. As the flow reduced in the afternoon and became more southerly, local fresh (as well as aged/recirculated) emissions were pooled throughout the southern portion of the SMA, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c Author(s) 2019. CC BY 4.0 License.
yielding adequate time to interact with the rural forested area near TRF. This system was also associated with much warmer (27° C and 31° C at 15:00 KST at TRF and OLY, respectively) conditions than 17 May, favoring increased emissions of BVOCs, such as isoprene . With weakly forced synoptic meteorology, TRF reached a maximum hourly O3 value of nearly 110 ppbv, exceeding the 1-hr 100 ppbv South Korean national standard.
3 Case Studies of Pollution Transport to TRF 5

Methods
Vertical profiles of key atmospheric chemical constituents measured via aircraft and from ground based platforms (Table 1) during two representative case studies are used to better understand trans-boundary and local transport effects from urban regions to the rural landscape. To assess the transport and evolution of urban emissions impacting TRF, airborne measurements of O3, NO2, CO (carbon monoxide), SO2 (sulfur dioxide), isoprene (C5H8), and toluene (C6H5-CH3) were 10 collected. Downwind plume chemistry is also further investigated and fingerprinted using the onboard DC-8 observations and an explicitly constrained 0-D photochemical box model simulation. Constituent profiles at TRF were also measured using ground-based instrumentation such as O3 lidar (Sullivan et al., 2016;, electrochemical cell (ECC) O3-sondes, and aerosol backscatter. Surface observations of O3, NO2, toluene, and isoprene at TRF are also presented.

Ozonesonde Profiles at TRF
A total of 34 O3-sondes were released from TRF throughout KORUS-AQ from 10 May to 12 June. Afternoon soundings (13:30 to 16:30 KST) of O3 (top panel, Figure 4) and temperature (bottom panel, Figure 4) illustrate day-to-day variability in the first 3km ASL. There were large disparities in boundary layer O3 throughout the campaign period. From 10-16 May, concentrations were mostly between 70-80 ppbv, which were associated with cooler temperatures and higher synoptic 25 wind speeds. However, by the early afternoon of 17 May, a stagnant high-pressure system located over the Yellow Sea (c.f. Figure 3a) introduced a warmer air mass, calmer winds, and clearer skies. Through most of the campaign until 3 June, a similar meteorological setup persisted, providing favorable conditions for rapid O3 production to more than 120 ppbv. On 4 and 6 June, intermittent shower activity events limited O3 production. However, by 9-10 June another high-pressure system (c.f. The Hanseo University King Air conducted a sampling pattern that included a near ground level approach in Seoul and westbound leg out towards the Yellow Sea (green panel, Figure 5), a southbound leg directly over the Yellow Sea at two altitudes (orange panel, Figure 5), a returning eastbound leg (magenta panel, Figure 5) and then finally a northbound returning leg towards Seoul (cyan panel, Figure 5). During this pattern, measurements of O3, NO2, CO and SO2 capture long-range pollution transport across the Yellow Sea. Eastern China is densely populated with coal-fired power plants, which are strong 15 emitters of NOx, SO2 and particulate matter [Zhao et al., 2008]. Carbon monoxide and SO2 are longer lived species with lifetimes on the order of 1-2 months [Miyakazi et al., 2012] and 1-2 days [He et al., 2012], respectively, and are used to support the interpretation of trans-boundary pollution transport. During the KORUS-AQ study period, Huang et al., 2012 has further used CO to evaluate chemical transport models and assess transboundary impacts on the Korean peninsula.
During the low level approach to Seoul near 08:55 KST (denoted with dashed black line, Figure 5) chemical sampling 20 of low O3 (20-40 ppbv), high NO2 (20-40 ppbv), high CO concentrations (500-700 ppbv) and high SO2 (6-10 ppbv) are observed in the first 500 m ASL. These indicate morning urban emissions (and subsequent O3 titration) and because of the proximity to the surface level, these pollutants are largely associated with local SMA vehicular and industrial morning emissions. As the aircraft moves westward at 1000 m ASL, it samples a much cleaner air mass, but reaches a plume of polluted air near 09:20 KST, associated with increased concentrations of O3 (to 90 ppbv), CO (to 500-600 25 ppbv) and SO2 (to 6-8 ppbv). In conjunction with the 48-hr back trajectory, this is likely the outflow of aged industrial emission from Eastern China [Zhao et al., 2008] that has been transported over the Yellow Sea. Similar concentrations of these species are observed during the southbound leg at 500 m ASL, indicating that the vertical distribution of pollutants is relatively well mixed in the polluted air mass from 500-1000m ASL.
During the eastbound leg at 1500 m ASL, the aircraft samples mostly clean air (similar to the outbound leg with O3 30 near 60 ppbv), with the exception of a pollution plume at 10:55 KST. As the aircraft returns to the Korean peninsula, it observes relatively cleaner conditions until it approaches the southern portion of the SMA near 11:30 KST. As it descends Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c Author(s) 2019. CC BY 4.0 License. within the SMA, it encounters a rapid increase in concentrations of all species, with pronounced increases in concentrations of NO2 (to 15-25 ppbv), CO (to 600-800 ppbv), and SO2 (to 4-6 ppbv).
In summary, the westbound in-situ observations indicate transport of polluted air across the Yellow Sea towards South Korea. The largest chemical perturbations (e.g. 15-40 ppbv in NO2; 6-10 ppbv in SO2, 200-300 ppbv in CO) during the flight pattern were spatially correlated with local emission sources during the initial ground level approach and the final transect 5 nearing the SMA. The southbound transect indicated on this day that the background level of O3 is near 60 ppbv and a 20-30 ppbv enhancement in O3 is observed at 500m over the water. However, the increases in NO2 are chemically responsible for rapid O3 production, which were observed in near negligible amounts during the trans-boundary transects. Toluene, a reactive aromatic and industrial VOC, is a useful tracer for urban anthropogenic emissions because it is a highly reactive O3 precursor with a chemical lifetime on the order of a day. Toluene is a dominant VOC throughout the SMA and contributes to nearly 60.7% of the total VOC emissions [Lee et al., 2011]. Isoprene, a BVOC and derivative of photosynthesis, is largely associated with deciduous trees (e.g. oak, which accounts for 85% of broadleaf trees in South Korea 20 [Lim et al., 2011]). Isoprene can enhance photochemical O3 production, is emitted almost entirely during the daytime, can form additional oxidative byproducts, and is released more abundantly with increased temperatures. Previous results from Kim et al., 2014 indicate that isoprene accounts for most of the midday hydroxyl radical (OH) reactivity (11-15 KST) at TRF and can rapidly increase O3 production rates.

Aircraft Analyses
Within the SMA and below 0.5 km ASL, there is a significant chemical perturbation as compared to the free 25 tropospheric concentrations of NO2 (40-50 ppbv, Figure 6c) and toluene (5-7 ppbv, Figure 6c). These increases both lead to increases in modeled P(O3) (10-20 ppbv/hr, Figure 6f). Isoprene is mostly less than 0.3 ppbv during this sampling. The concentrations of NO2 have increased by 10-20 ppbv since the afternoon Hanseo University King Air sampling ( Figure 5), indicating a persistent reservoir of reactive nitrogen coming from SMA throughout the day. Ozone remains mostly between 70-80 ppbv (Figure 6b) during the initial descent into Seoul, indicating emissions are fresh enough that rapid O3 production 30 has not occurred yet (which is further corroborated with the P(O3) model output).
As the aircraft moves towards TRF, it samples various spatial chemical gradients. For example, NO2 and toluene concentrations decrease to near 20-30 ppbv and near 2-3 ppbv, respectively. An increase in isoprene concentrations to 0. 5-0.8 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c Author(s) 2019. CC BY 4.0 License. ppbv corresponds to the forested region southeast of Seoul. Directly over TRF, O3 concentrations are increased as compared to those near Seoul by 5-10 ppbv, indicating that downwind O3 production has increased with diluted levels of NOx. The aircraft passes TRF and continues its eastward descent to 0.3 km ASL; O3 concentrations markedly increase to between 110-125 ppbv and P(O3) rapidly increases to between 20-32 ppbv/hr. Although the aircraft samples nearly negligible concentrations of isoprene and NO2, toluene concentrations have increased and are comparable to those sampled near Seoul (3-4 ppbv). This 5 is a strong indicator of an aged urban air mass containing highly reactive O3 precursors impacting rural sites. In summary, trans-boundary pollution transport was observed via the Hanseo University King Air ( Figure 5) on 17 May, but locally emitted O3 precursors can be confidently attributed as a catalyst for the highest levels of boundary layer O3 production observed near TRF.

Ground-Based Observations at TRF
To fingerprint and quantify the transported pollution reaching TRF, diurnally resolved observations are presented in As the aircraft ascended out of the boundary layer prior to reaching Seoul, O3 remained above 100 ppbv to near 2000 m ASL. Mixing heights are 500m deeper than on 17 May, presumable which as a result of warmer temperatures and greater 25 convective mixing. As the DC-8 ascended out of the overpass south of Seoul, NO2 concentrations reached 20-30 ppbv, toluene reached 3-6 ppbv and isoprene exceeded 1.2 ppbv (nearing the peak concentration measured via the DC-8 during the entire campaign). This air mass was also associated with lower values of O3 and P(O3) as compared to the forest plume south of TRF, between 75-90 ppbv and 10-20 ppbv/hr, respectively, indicating the anthropogenic and biogenic emissions were still fresh.
Similar to 17 May, as the aircraft leaves urban Seoul and approaches TRF there is a strong spatial gradient in nearly all chemical 30 constituents. At TRF, O3 and P(O3) increase to over 120 ppbv and 30 ppbv/hr, respectively, while NO2 decreases to 5-10 ppbv, indicating O3 production was rapidly occurring and impacting rural sites downwind of Seoul.

Aircraft Analyses: NASA B-200
On 09 June 2016, the NASA B-200 performed a morning and afternoon raster (Figure 9) of the greater SMA from 12:00-14:00 KST and 14:00 -16:00 KST, respectively. This yields a unique view of the concentrations and chemical transport of NO2 throughout the SMA during the afternoon hours. During the 12:00-14:00 KST sampling, there is a clear maximum in NO2 slant columns in the south and west of Seoul. Afterwards, the 14:00-16:00 KST measurements show the advection of NO2 5 (and presumably other urban pollutants) eastward and southward throughout the SMA. During the 14:00-16:00 flight, large NO2 column amounts extend to the southeastern portion of the SMA, with enhanced levels of NO2 reaching the edge of TRF.

Ground-Based Observations at TRF
To fingerprint and quantify the chemical transport reaching TRF, diurnally resolved observations are presented in 10 12:00 to 19:00 KST: Boundary layer concentrations of O3 and aerosols are well mixed with steady growth in O3 from 12:00 until 16:30 KST, when a rapid influx of O3 and aerosol occurs. This occurs after a significant positive perturbation in isoprene, while preceding an abrupt increase in NO2 and toluene. The O3 peak closely corresponds to the O3 sampled via the 20 DC-8 south and west of TRF, and in conjunction with the back-trajectory (Figure 2), it appears O3 was likely advected through TRF during this time.
19:00 to 23:00 KST: As solar radiation declines, aloft concentrations of O3 near 100 ppbv mixing to 2000 m ASL are observed, indicating a stable residual layer persisted into the evening and likely impacted the next day's O3 composition (e.g. Figure 4 indicates O3 at TRF on 10 June exceeded >100 ppbv). As surface O3 quickly decayed to near 20 ppbv after 19:00 25 KST, it corresponded to increases in NO2 from near 15 to 20-30 ppbv and toluene from 8 to 10 ppbv, which points to the incoming pollution plume captured with the GeoTASO observations (bottom panel, Figure 9), indicating TRF was impacted

Aircraft Observation and P(O3)
Although two case studies are presented to illustrate the contrasting types of pollution influences at TRF, it is important to assess how representative events were for the entire KORUS-AQ study. To do this, the chemical observations of isoprene 5 ( Figure 11, left panel) and toluene (Figure 11, right panel) on-board the NASA DC-8 are presented for all remaining afternoon flights (exact dates can be found here: http://www-air.larc.nasa.gov/missions/korus-aq/). Data was used when the DC-8 aircraft was below 1.5 km ASL within 1-degree latitude and longitude of the TRF site. They are compared with the photochemical box model results in order to illustrate the relative contribution of VOCs and BVOCs on O3 production during the campaign.
In both panels of Figure 11, the case studies chosen (17 May -red, 09 June -blue) appear to be accurate representations of 10 "typical" pollution events seen at TRF as recorded by all other days (black dots).
For isoprene, 17 May had concentrations centered around 0.5 ppbv and were associated with P(O3) values between 5-15 ppbv/hr. However, 09 June concentrations of isoprene were well over 0.5 ppbv, extending to over 2.0 ppbv. These were associated with P(O3) values in excess of 20 ppbv/hr and nearing 35 ppbv/hr, indicating that biogenic emissions contributed more to net O3 production on this day than on 17 May. The 09 June case yielded nearly the highest isoprene-driven O3 15 production rates during the campaign.
For toluene, 17 May appears to have several focused regions of toluene nearing 7 ppbv mostly associated with P(O3) at or below 15 ppbv/hr. However, there is a subset of toluene with concentrations between 3-5 ppbv that are associated with P(O3) rates between 20-35 ppbv/hr. This is indicative of the aged urban plume associated with O3 photolysis reaching the area east of TRF. On 09 June, concentrations of toluene are nearly all below 5 ppbv and are grouped much closer together. This is 20 indicative of a much more well-mixed air mass (which was also suggested with the Geo-TASO observations). Although 9 June was associated with a larger contribution of isoprene driven O3 production, it had a similar concentration of toluene as 17 May, indicating that both contrasting high pollution events (and P(O3)) were associated with high levels of urban pollutants.

GSFC O3 Lidar Derived Campaign Average
NASA GSFC O3 lidar profiles at TRF during all flight days during the KORUS-AQ campaign ( Figure 12) can be used to 25 derive a diurnal campaign average (c.f. Sullivan et al., 2015b). The early morning low O3 feature is prominent in the composite figure, as well as the aloft residual O3 concentrations we have linked to trans-boundary transport between 65-75 ppbv above 500 m ASL. As solar radiation and convective mixing increase in the late morning hours (after 11:00 KST), surface concentrations of O3 better correlate with concentrations measured aloft from the lidar. In the afternoon hours, O3 increases at a rate of 5 ppbv hr -1 between 500-1500m ASL and peak O3 is observed between 17:00-18:00 KST. The difference between 30 aloft and surface concentrations in the evening can be linked to a decoupling of the surface layer and rapid depletion and titration of surface O3 from local NOx emissions. This process isolates the polluted aloft residual layer during night-time hours Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-1328 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 14 January 2019 c Author(s) 2019. CC BY 4.0 License. that will potentially affect downwind locations on the following morning. This confirms that during the May-June time period, the late day peak O3 occurrence is a persistent feature and is more enhanced than at the surface.

Conclusions
As part of the KORUS-AQ study, nearly continuous chemical measurements at the surface and within the first 3km ASL have 5 quantified several of the key pollution features (e.g. residual layer O3 entrainment and late-day O3 maxima) responsible for O3  Figure 12), suggesting late day O3 increases occur frequently at TRF, and that rural sites in this region may be experiencing long term negative effects of O3. Because threshold health effects, mortality rates, and crop yield analyses have been historically calculated using only surface measurements [Lee et al., 2000;Kim et al., 2004;Wang and Mauzerall (2004), Ghim et al., 2007], Figure 12 also indicates that these analyses may be underestimating the extent of 15 the negative impacts of high-O3 at TRF and its surrounding rural areas.
These results clearly demonstrate that Korea is subject to highly (aged) polluted air masses that cross the Yellow Sea but are exacerbated by domestic pollution produced near SMA. This emphasizes a reevaluation of domestic emission controls, in particular reactive aromatics such as toluene (e.g. toluene not only contributes to the P(O3) shown herein, but also contributes to 9% of modeled secondary organic aerosol over SMA [Nault et al., 2018]). Organic aerosol formation has also been recently 20 investigated during the KORUS-AQ study period to estimate relationships between in situ observations and satellite derived products (e.g. formaldehyde, (Liao et al., 2019)). These findings are in line with the detailed Rapid Science Synthesis Report (espo.nasa.gov/sites/default/files/documents/KORUS-AQ-ENG.pdf) that provides findings from the KORUS-AQ study which are intended to be useful for policy makers as they develop air quality mitigation strategies and continue to identify specific emission sources that should be targeted for reduction. Direct observations of free tropospheric O3 were rarely observed below 25 60 ppbv during the KORUS-AQ study period, indicating that the baseline conditions of which South Korean national regulatory standards are predicated on, are trending (Cooper et al., 2016) towards a regime where they are increasingly unattainable. In order to further assess trans-boundary pollution, emission sources and plume evolution, there has been an international effort to launch the Geostationary Environmental Monitoring Spectrometer (GEMS) to provide hourly measurements of key pollutants (e.g. O3, NO2, SO2 and particulate matter) over the Korean peninsula and the Asia-Pacific 30 region. The KORUS-AQ analyses offer an exemplary synergistic approach on how to collect the statistics required by the regulatory agencies of Korea to improve air quality in both urban and rural settings.