Diurnal variation of aerosol optical depth and PM2.5 in South Korea

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 sites from 1999 to 2017 and an additional 10 sites during the KORUS-AQ field campaign in May and June of 2016. The extent to which WRF-Chem model and the GOCI satellite retrieval can describe these variations is also analyzed. In daily average, Aerosol Optical Depth (AOD) at 550 nm is 0.386 and shows a diurnal variation of 20 to −30 % in inland sites, respectively larger than the counterparts of 0.308 and ±20 % in coastal sites. For all the inland and coastal sites, AERONET, GOCI, WRF-Chem, and observed PM 2.5 data consistently show dual peaks for both AOD and PM 2.5 , one at ∼10 KST and another ∼14 KST. In contrast, Angstrom 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 of Angstrom Exponent from morning to evening, reflecting overall decrease of particle size in daytime. To statistically obtain the climatology of diurnal variation of AOD, a minimum of requirement of ∼2 years of observation is needed in coastal rural sites, twice more than the urban sites, which suggests that diurnal variation of AOD in urban setting is more distinct and persistent. While Korean GOCI satellite retrievals are able to consistently capture the diurnal variation of AOD, WRF-Chem clearly has the deficiency to describe the relatively change of peaks and variations between the morning and afternoon, suggesting further studies for the diurnal profile of emissions. Furthermore, the ratio between PM 2.5 and AOD in WRF-Chem is persistently larger than the observed counterparts by 30–50 % in different sites, but no consistent diurnal variation pattern of this ratio can be found. Overall, the relative small diurnal variation of PM 2.5 is in high contrast with large AOD diurnal variation, which suggests the large diurnal variation of AOD-PM2.5 relationships, and therefore, the need to use AOD from geostationary satellites for constrain either modeling or analysis of surface PM 2.5 for air quality application.