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.
