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In this present study, an aerosol effective height (AEH) retrieval algorithm has been developed using the O4 air mass factor (AMF) at 477 nm from the hyperspectral Ozone Monitoring Instrument (OMI). We investigated the magnitude of change in topographical and seasonal O4 vertical column density (VCD) in Northeast Asia and evaluated its effect on AEH retrieval accuracy using our AEH retrieval algorithm. In addition, the effect of a temperature-dependent cross-section for O4 (TDCS) on Look Up Table (LUT)-based AEH retrieval accuracy was quantified. A comparison between the retrieved AEH and those from the NIES lidar network for the period from January 2005 to June 2009, applying both the TDCS and the seasonal and topographical O4 VCDs, resulted in a root mean square error (RMSE) of 0.44 km for both smoke and dust aerosols. However, when both a TDCS (an O4 absorption cross-section at a single temperature of 293 K; SCS) and a single O4 VCD value were applied to the LUT, the RMSE for both aerosol types was calculated to be 0.52 km (0.51 km), which implies that TDCS contributes most to AEH retrieval accuracy when accurate O4 VCDs are applied to the LUT. For smoke aerosols only, both TDCS and multiple O4 VCD (SCS and single O4 VCD) applications had RMSE values of 0.46 km (0.66 km). The retrieved AEHs were additionally compared with satellite-based lidar measurements. We also investigated the effects of uncertainties in our algorithm input data (e.g., O4 VCD, TDCS, AOD, and surface reflectance) on AEH retrieval error using synthetic radiances. Large errors can be caused by uncertainties in O4 VCD and AOD. In particular (0.4 ≤ AOD

It is important to estimate the effects of the long-range transport of atmospheric pollutants for efficient and effective strategies to control air quality. In this study, the contributions of trans-boundary transport to the mean concentrations of SO 2 , NO 2 , CO, and PM 10 in Seoul, Korea from 2001 to 2014 were estimated based on the conditional potential source contribution function (CPSCF) method. Eastern China was found to be the major source of trans-boundary pollution in Seoul, but moderate sources were also located in northeastern China. The contribution of long-range transport from Japan was negligible. The spatial distributions of the potential source contribution function (PSCF) values of each pollutant showed reasonable consistency with their emission inventory and satellite products. The PSCF values of SO 2 and PM 10 from eastern China were higher than those of NO 2 and CO. The mean concentrations of SO 2 , NO 2 , CO, and PM 10 in Seoul for the period from 2001 to 2014 were 5.34, 37.0, and 619.1 ppb, and 57.4 4 μg/m 3 , respectively. The contributions of long-range transport to the mean concentrations of SO 2 , NO 2 , CO, and PM 10 in Seoul were 0.74, 3.4, and 39.0 ppb, and 12.1 μg/m 3 , respectively, which are 14%, 9%, 6%, and 21% of the mean concentrations, respectively. The annual mean concentrations of SO 2 and NO 2 followed statistically significant increasing linear trends (0.5 and 1.6 ppb per decade, respectively), whereas the trends in the annual mean concentrations of CO and PM 10 were statistically insignificant. The trends in the ratio of the increased concentrations associated with long-range transport to the annual mean concentrations of the pollutants were statistically insignificant. However, the results indicate that the trans-boundary transport of SO 2 , NO 2 , CO, and PM 10 from eastern China consistently affected air quality in Seoul over the study period (2001–2014). Regionally, the effects of the long-range transport of pollutants from Beijing and Harbin-Changchun on air quality in Seoul have become more significant over this period.

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 long-term 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.

To better understand air quality issues in South Korea, it is essential to identify the main contributors of air pollution and to quantify the effects of transboundary transport. In this study, geostationary satellite measurements were used to assess the effects of aerosol transport on air quality in South Korea. This study proposes a method to define the long-range transport (LRT) of aerosols into the Korean Peninsula using remote sensing obervations and back-trajectories and estimates the LRT effects on air quality in Seoul using in-situ particulate matter (PM) measurements. Aerosol optical depths (AODs) are obtained from the Geostationary Ocean Color Imager (GOCI), and the back-trajectories are from the National Ocean and Atmospheric Administration (NOAA) HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model. For LRT events, satellite observations showed high AOD plumes over the Yellow Sea, a pathway between Eastern China and South Korea, and the movements of aerosol plumes transported to South Korea were also detected. PM2.5 concentrations, PM10 concentrations, and AOD during LRT increased by 52%, 49%, and 81%, respectively, relative to their average values for 2015–2018. To quantitatively characterize the LRT of aerosols, the effects of LRT on PM2.5 concentrations were estimated for each PM concentration category. The contribution of LRT to PM2.5 concentrations was estimated to be 33% during 2015–2018. When high concentrations of PM2.5 were observed in Seoul, they were likely to be associated with LRT events.

In this paper, we introduce a new method to estimate the change in mean mixing ratio of a target species at a receptor site due to the contribution of the long-range transport (CLRT). We applied our method to determine inter-annual and inter-seasonal variations in the CLRT of SO2 in Seoul, a major megacity in northeast Asia, during the period from 2001 to 2010. The major potential source areas of SO2 for the 2001–2010 period were located in East China according to the potential source contribution function (PSCF) maps. The CLRT of SO2 in Seoul was estimated to range from 0.40 to 1.03 ppb, which accounted for 8–21% of the ambient mean SO2 mixing ratio in Seoul. The inter-annual variations of estimated CLRT of SO2 was well correlated with those of the total emissions in China during the period of 2001–2008 (R = 0.85). We found that both local emissions from around Seoul and long-range transport from East China, especially the Shandong peninsula, affected the SO2 mixing ratio in Seoul throughout the decade of study. The CLRT of SO2 in Seoul increased after 2007 even though the total emissions of SO2 by China have been decreasing since 2006. The CLRT of SO2 in Seoul was high in spring and winter, which can be attributed to enhanced SO2 emissions in East China during these seasons and a dominant westerly wind. The CLRTs of SO2 accounted for 15, 11, 4, and 12% of the seasonal mean SO2 mixing ratio in spring, summer, fall, and winter, respectively. The uncertainty ranged from 24 to 62% of the estimated CLRT values.

The contribution of dust and non-dust aerosols to the lidar observed total backscattering was estimated from the linear particle depolarization ratios (δp) under the assumption of externally mixed aerosols. During intensive observation period, the contribution rate of the non-dust aerosol optical depth (AOD, τ) to total AOD increased from 30% on 22 October to 82% on 30 October, which implies that large amount of non-dust particles was transported along with Asian dust storm. Using independent aerosol optics and profiles for dust and non-dust, for the first time, instantaneous aerosol direct radiative forcing (ADRF) of the Asian dust plumes on atmosphere-surface system was quantified. The average ADRF by the dust particles was −31.5 ± 16.1 (−66.3 ± 20.2) W m−2 at the surface, −16.5 ± 8.8 (−11.2 ± 9.2) W m−2 at the TOA, respectively. The forcing efficiency, defined as the aerosol forcing per unit τ (532 nm), for dust (non-dust) particles was −124.6 ± 12.2 (−209.4 ± 59.1) W m−2/τ532 at the surface and −64.9 ± 7.8 (−35.1 ± 28.0) W m−2/τ532 at the TOA, respectively. Though the contribution of non-dust optical depth to total τ was smaller than the contribution of optical depth from dust to total optical depth, the non-dust particles contributed larger to the radiative forcing at the surface than the dust radiative forcing. This result demonstrates that increased Asian dust radiative forcing can be largely attributed to the presence of non-dust particles mixed into Asian dust layers rather than the radiative forcing by pure dust particles. We also found that non-dust particles play a significant role in stabilizing the dust layer by increasing the radiative heating rates within dust layers.

In order to investigate seasonal and diurnal variation of primary organic carbon (POC) and secondary organic carbon (SOC) concentrations in a megacity, hourly measurements of particulate and gaseous pollutants were carried out in Seoul from January to December 2010. The EC Tracer Method (ECTM) and the Multiple Regression Method (MRM) have been used to estimate seasonal and diurnal concentrations of POC and SOC concentrations. Annual mean SOC concentrations estimated by ECTM (SOCECTM) and MRM (SOCMRM) accounted for 14.61 and 17.21% of TOC concentrations, respectively. Seasonal patterns in SOCMRM were comparable to those of SOCECTM, but the annual average SOCMRM was about 15% greater than that of SOCECTM. In spring, however, a large discrepancy was observed between SOCECTM and SOCMRM, which is thought to be due to a high ozone concentration and primary TOC/EC ratio. Regarding the annual mean diurnal characteristics, POC concentration showed peaks around 10:00 and 00:00 local time that were also observed in diurnal variations of TOC and EC concentrations. Annual mean SOC concentration, however, showed peaks at around 15:00. In the morning over all seasons, we found discrepancies between SOCECTM and SOCMRM due to overestimated SOCECTM concentration. The diurnal variations in SOC concentrations were found to have seasonal characteristics. The diurnal pattern of SOC concentration in spring was similar to that in autumn, and SOC concentrations in all seasons with the exception of winter showed a peak at around 15:00. In summer, however, the SOC concentration peak at around 15:00 was greater by 70%, 81%, and 54% than the peaks seen in spring, autumn, and winter, respectively, which could be explained by the high ozone concentration and strong UV radiation in summer. From 10:00 to 15:00 in summer, the average increase rates in SOCECTM and SOCMRM were 0.39 and 0.24 μg m−3 h−1, respectively. In winter, negligible diurnal variations of estimated SOC concentrations demonstrate that SOC formation is less active than in other seasons. The high concentration level of mean SOC in winter could be attributed to a low mixing height or stagnant atmospheric condition.

Topographic Target Light scattering Differential Optical Absorption Spectroscopy (ToTaL-DOAS) measurements were performed in a street canyon in Gwangju, Korea (35°10′51.43″N, 126°52′53.74″E) during a period of 10 days in October 2010. NO2 slant volume mixing ratios (VMRs) at the lowest elevation angle (EL) of 1°, as obtained from ToTaL-DOAS measurements, were validated via comparisons with collocated in situ ground data, showing good agreement within a scatter range of 20% during the entire measurement period, and within a scatter range of 15% when cloudy days were excluded. For the first time, based on inversion calculations using ToTaL-DOAS data, we retrieved NO2 vertical distributions that consist of five layers (from 6 to 118 m) for three measurement days. We obtained generally decreasing NO2 VMRs with altitude, whereas increased NO2 VMRs at two elevated layers (6–19 m and 19–33 m) were observed on 26 and 27 October. We obtained a coefficient of determination (R2) of 0.87 (0.61) between slant VMRs at 1° EL (retrieved NO2 VMRs in the layer from 6 m to 19 m) and in situ data, with the scatter of the correlation being within the 15% (10%) range. The relation between retrieved NO2 VMRs at 6–19 m and slant VMRs at 1° EL yielded an R2 value of 0.97, with the scatter of the correlation being within the 5% range. The results demonstrate that the ToTaL-DOAS technique is a useful tool in identifying the temporal and vertical characteristics of NO2 VMRs in an urban street canyon with a complex geometry.

Total column ozone (TCO) amounts observed by the Dobson spectrophotometer from 1985 to 2008 in Seoul and vertical ozone profiles obtained with the ozonesonde from 1995 to 2007 in Pohang were used to investigate the relationship between the occurrence of Secondary Ozone Peak (SOP) in the Upper Troposphere/Lower Stratosphere (UT/LS) layer and the enhancement in TCO over the Korean Peninsula. Based on Hybrid Single-Particle, Lagrangian Integrated Trajectory (HYSPLIT) simulations, the advection of a northern mid-latitudinal ozone-rich airmass from the northwest is closely related to SOP occurrences, which consequently lead to enhancements in the amount of ozone in both the UT/LS and the total column at stations in Korea. In addition, both the frequency of the northwesterly advection and the northern mid-latitudinal ozone amount are revealed to affect the amount of ozone in the UT/LS and total column by up to 7 DU. The relationship between the SOP occurrence frequency and the long-term TCO trend was investigated with observed data collected in all seasons. The UT/LS ozone enhancements, which are largely affected by SOP occurrences, are considered to be positively related to the TCO trend from 1985 to 2008 over the Korean Peninsula.

A measurement method combining multi-axis differential optical absorption spectroscopy (MAX-DOAS) and a transmissometer (TX) is introduced as a means of retrieving surface trace gas mixing ratios in ambient air. The combined measurement method was utilized to derive surface NO2 mixing ratios from 27 March to 11 May 2007 in Seoul, Korea. To convert the differential slant column density (DSCD) to the volume mixing ratio (VMR), the light path length (LPL) along the MAX-DOAS line of sight was derived using the light extinction coefficient and Angstrom exponent data obtained by a TX and sunphotometer, respectively. Temporal variations of the NO2 VMRs at the 0–1 km layer obtained from radiative transfer model (RTM) simulations coupled with MAX-DOAS data show similar patterns, but with reduced magnitudes, to the ground level data and those of the combined MAX-DOAS and TX measurements at 0.08 km. The NO2 VMRs retrieved by the combined measurement were in agreement with those obtained from the RTM simulations coupled with MAX-DOAS data and the in-situ measurements within 40 and 50%, respectively. The coefficient of determination (R2) of 0.75 was obtained between the combined measurement data sets and those of the RTM simulations coupled with MAX-DOAS data whereas that between the combined measurement data sets and those of the in-situ measurements was 0.53. The coefficient of determination (R2) between the data sets derived from the RTM simulations coupled with MAX-DOAS data and those of the in-situ measurements was 0.67 with the scatter of the correlation within the 50% range.

Aerosol optical and microphysical parameters from severe haze events observed in October 2005 at Gwangju, Korea (35.10°N, 126.53°E) were determined from the ground using a multi-wavelength Raman lidar, a sunphotometer, and a real-time carbon particle analyzer and from space using satellite retrievals. Two different aerosol types were identified based on the variability of optical characteristics for different air mass conditions. Retrievals of microphysical properties of the haze from the Raman lidar indicated distinct light-absorbing characteristics for different haze aerosols originating from eastern and northern China (haze) and eastern Siberia (forest-fire smoke). The haze transported from the west showed moderately higher absorbing characteristics (SSA = 0.90 ± 0.03, 532 nm) than from the northern direction (SSA = 0.96 ± 0.02). The organic/elemental carbon (OC/EC) ratio varied between 2.5 ± 0.4 and 4.1 ± 0.7.

The East Asian countries have been affected by atmospheric gaseous pollutants (in particular SO2) transported from the Asian continent as well as Asian dust storms. For investigation of the impact of these anthropogenic trace gases on local air quality in Korea, ground-based measurements using a Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) system and in situ gas analyzers as well as synoptic meteorological data and scattered sunlight spectra obtained by the satellite-borne instrument, Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) launched on board of Environmental Satellite (ENVISAT) in March 2002, were utilized to retrieve SO2 and trace its transport from the Asian continent to Korea in May 2005. The ground-based measurements were carried out in the region of interest, at Korea Global Watch Observatory (KGAWO) in Korea. Plumes of high SO2 over Chinese industrial areas and their transport to the Korean peninsula were observed in SCIAMACHY data in the period of 21–26 May 2005. Highly increased SO2 was measured by the MAX-DOAS system and in situ gas analyzer in the period of 27–29 May 2005 at KGAWO. These observations are supported by the meteorological results that the air-masses picking up these high SO2 plumes while passing over the Chinese industrial and metropolitan areas were transported to the Korean peninsula. The tropospheric SO2 VCDs over these Chinese industrial and metropolitan areas ranged up to 1.4×1017 mol cm−2 in SCIAMACHY data. These SO2 plumes resulted in increased SO2 surface levels of up to 7.8 ppbv (measured by an in situ gas analyzer) at KGAWO.