Diurnal variation of aerosol optical depth and PM<sub>2.5</sub> in South Korea: a synthesis from AERONET, satellite (GOCI), KORUS-AQ observation, and the WRF-Chem model

Abstract

Abstract. Spatial distribution of diurnal variations of aerosol properties in South Korea, both long term and short term, is studied by using 9 AERONET (AErosol RObotic NETwork) sites from 1999 to 2017 and an additional 10 sites during the KORUS-AQ (Korea–United States Air Quality) field campaign in May and June of 2016. The extent to which the WRF-Chem (Weather Research and Forecasting coupled with Chemistry) model and the GOCI (Geostationary Ocean Color Imager) satellite retrieval can describe these variations is also analyzed. On a daily average, aerosol optical depth (AOD) at 550 nm is 0.386 and shows a diurnal variation of 20 to −30 % in inland sites, which is larger than the AOD of 0.308 and diurnal variation of ±20 % seen in coastal sites. For all the inland and coastal sites, AERONET, GOCI, and WRF-Chem, and observed PM2.5 (particulate matter with aerodynamic diameter less than 2.5 µm) data generally show dual peaks for both AOD and PM2.5, one in the morning (often at ∼08:00–10:00 KST, Korea Standard Time, especially for PM2.5) and another in the early afternoon (∼14:00 KST, albeit for PM2.5 this peak is smaller and sometimes insignificant). In contrast, Ångström exponent values in all sites are between 1.2 and 1.4 with the exception of the inland rural sites having smaller values near 1.0 during the early morning hours. All inland sites experience a pronounced increase in the Ångström exponent from morning to evening, reflecting an overall decrease in particle size in daytime. To statistically obtain the climatology of diurnal variation of AOD, a minimum requirement of ∼2 years of observation is needed in coastal rural sites, twice as long as that required for the urban sites, which suggests that the diurnal variation of AOD in an urban setting is more distinct and persistent. While Korean GOCI satellite retrievals are able to consistently capture the diurnal variation of AOD (although it has a systematically low bias of 0.04 on average and up to 0.09 in later afternoon hours), WRF-Chem clearly has a deficiency in describing the relative change of peaks and variations between the morning and afternoon, suggesting further studies for the diurnal profile of emissions. Furthermore, the ratio between PM2.5 and AOD in WRF-Chem is persistently larger than the observed counterparts by 30 %–50 % in different sites, but spatially no consistent diurnal variation pattern of this ratio can be found. Overall, the relatively small diurnal variation of PM2.5 is in high contrast with large AOD diurnal variation, which suggests the large diurnal variation of AOD–PM2.5 relationships (with the PM2.5 ∕ AOD ratio being largest in the early morning, decreasing around noon, and increasing in late afternoon) and, therefore, the need to use AOD from geostationary satellites to constrain either modeling or estimate of surface PM2.5 for air quality application.

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

“…The in situ observations indicate concentrations were above 100 ppbv until 18:30, indicating polluted conditions persisted well into the evening at the surface and even longer aloft. This later afternoon buildup and transport is also identified with results from Lennartson et al (2018) during the KORUS-AQ study, which indicate TRF had consistently higher aerosol optical depth (AOD) values of near 0.4-0.6 in the morning, decreasing throughout the day and eventually rising again in the early evening at 15:00-16:00 KST.…”

“…This emphasizes a reevaluation of domestic emission controls, in particular reactive aromatics such as toluene (e.g., toluene not only contributes to the P (O 3 ) 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 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 (https://kr.usembassy.gov/wp-content/ uploads/sites/75/2017/07/KORUS-AQ-RSSR.pdf, last access: 27 March 2019) 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.…”

New Era of Air Quality Monitoring from Space: Geostationary Environment Monitoring Spectrometer (GEMS)

“…A long-term decreasing trend in AOD over East Asia beginning in 2011 has been identified (e.g., Kim et al 2017). Diurnal variations of aerosol properties are captured well by GOCI (Lennartson et al 2018). Diurnal variations of AODs usually occur with human activity cycles, outbreak of wildfires, and long-range…”

Validation, comparison, and integration of GOCI, AHI, MODIS, MISR, and VIIRS aerosol optical depth over East Asia during the 2016 KORUS-AQ campaign

“…Because the distribution of AOD is non-Gaussian and skewed towards low values, AOD evaluation is difficult using simple statistical techniques. Thus, the metrics applied here consist of the number of matched and co-located data points (N ), Pearson's linear correlation coefficient (R), the root-mean-square error (RMSE), the mean bias (MB) error, and the fraction within the expected error of MODIS Collection 5 DT land AOD [f within EE DT or f ; EE DT = ±(0.05 + 0.15× AERONET AOD)], as suggested by Levy et al (2007). The range of EE DT consists of upper and bottom linear lines and becomes wider as AOD increases, which reflects increasing AOD uncertainties with AOD in general.…”

“…During the campaign, GOCI aerosol optical properties were retrieved and provided in near real time to support air quality forecasting, determination of the flight plan for aircraft measurements to detect heavy pollution plumes, and data assimilation using near-real-time chemical transport model simulations Saide et al, 2014). Also, the GOCI AOD was used to evaluate a Japanese non-hydrostatic icosahedral atmospheric model (NICAM) AOD and to analyze diurnal variation in AOD and PM 2.5 over the Korean Peninsula (Goto et al, 2019;Lennartson et al, 2018). Since most aerosol analysis and application studies using satellites have still been demonstrated by using LEO satellite products such as MODIS, new aerosol products from GEO should be verified in terms of quality and spatiotemporal coverages through comparison with LEO satellite measurement.…”