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. Spatial distribution of diurnal variations of aerosolproperties in South Korea, both long term and short term, is studied byusing 9 AERONET (AErosol RObotic NETwork) sites from 1999 to 2017 and anadditional 10 sites during the KORUS-AQ (Korea–United States Air Quality) fieldcampaign in May and June of 2016. The extent to which the WRF-Chem (WeatherResearch and Forecasting coupled with Chemistry) model and the GOCI(Geostationary Ocean Color Imager) satellite retrieval can describe thesevariations 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 inlandsites, 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, andWRF-Chem, and observed PM 2.5 (particulate matter with aerodynamicdiameter less than 2.5  µ m) data generally show dual peaks for both AODand PM 2.5 , one in the morning (often at ∼08 :00–10:00 KST, KoreaStandard Time, especially for PM 2.5) and another in the early afternoon( ∼14 :00 KST, albeit for PM 2.5 this peak is smaller andsometimes insignificant). In contrast, Angstrom exponent values in allsites are between 1.2 and 1.4 with the exception of the inland rural siteshaving smaller values near 1.0 during the early morning hours. All inlandsites experience a pronounced increase in the Angstrom exponent from morning toevening, reflecting an overall decrease in particle size in daytime. Tostatistically obtain the climatology of diurnal variation of AOD, a minimumrequirement of ∼2  years of observation is needed incoastal rural sites, twice as long as that required for the urban sites, which suggests thatthe diurnal variation of AOD in an urban setting is more distinct and persistent.While Korean GOCI satellite retrievals are able to consistently capture thediurnal variation of AOD (although it has a systematically low bias of 0.04on average and up to 0.09 in later afternoon hours), WRF-Chem clearly has adeficiency in describing the relative change of peaks and variations betweenthe morning and afternoon, suggesting further studies for the diurnalprofile of emissions. Furthermore, the ratio between PM 2.5 and AOD inWRF-Chem is persistently larger than the observed counterparts by 30 %–50 %in different sites, but spatially no consistent diurnal variation pattern of thisratio can be found. Overall, the relatively small diurnal variation ofPM 2.5 is in high contrast with large AOD diurnal variation, whichsuggests the large diurnal variation of AOD–PM 2.5 relationships (withthe PM 2.5 ∕  AOD ratio being largest in the early morning, decreasing aroundnoon, and increasing in late afternoon) and, therefore, the need to use AODfrom geostationary satellites to constrain either modeling or estimate ofsurface PM 2.5 for air quality application.

atmospheric chemistry and physics

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